The Brain Is Contained Within the Cranium, and Constitutes the Upper, Greatly Expanded Part of the Central Nervous System

Home , Lobes of the brain, Posterior cortex

The brain is contained within the cranium, and constitutes the upper, greatly expanded part of the central nervous system.

Henry Gray (1918)

Looking through the gray outer layer of the cortex, you can see a mass of white matter. At the center is a cluster of large nuclei, including the basal ganglia, the hippocampi, the amygdalae, and two egg-shaped structures at the very center, barely visible in this figure, the thalami. The thalami rest on the lower brainstem (dark and light blue). You can also see the pituitary gland in front (beige), and the cerebellum at the rear of the brain (pink). In this chapter we will take these structures apart and re-build them from the bottom up.

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The brain

OUTLINE

1.0 Introduction 127 3.2 Output and input: the front-back division 143 1.1 The nervous system 128 3.3 The major lobes: visible and hidden 145 1.2 The geography of the brain 129 3.4 The massive interconnectivity of the cortex and thalamus 149 2.0 Growing a brain from the bottom up 133 3.5 The satellites of the subcortex 151 2.1 Evolution and personal history are expressed in the brain 133 4.0 Summary 153 2.2 Building a brain from bottom to top 134 5.0 Chapter review 153 3.0 From ‘ where ’ to ‘ what ’ : the functional 5.1 Study questions 153 roles of brain regions 136 5.2 Drawing exercises 153 3.1 The cerebral hemispheres: the left-right division 136

1.0 INTRODUCTION found. While knowledge of the brain is constantly expanding, we will focus on the basics. Our brains give us our biggest evolutionary edge. Cognitive neuroscience inevitably focuses on the Other large mammals have bigger muscles and greater cortex, often considered to involve the ‘ highest level ’ speed, but humans have an exceptionally big and flex- of processing. The cortex is only the outer and visible brain, specialized for excellent vision and hear- ible portion of an enormous brain, one that has devel- ing, language and social relationships, and for manual oped over hundreds of millions of years of evolution. control and flexible executive control. Human brains The word ‘ cortex ’ means bark, since that was how it make culture and technology possible. appeared to early anatomists. While the cortex is vital In this chapter, we look at the structure of the brain, for cognitive functions, it interacts constantly with while in the coming chapters we will cover its func- major ‘ satellite ’ organs, notably the thalamus, basal tions – how it is believed to work. It is important to ganglia, cerebellum, hippocampus, and limbic regions, understand that brain anatomy is not a static and among others. The closest connections are between settled field: new and important facts are constantly the cortex and thalamus, which is often called the tha- being discovered. On the microscopic and nanoscopic lamo-cortical system for that reason. In this core system levels, whole new classes of neurons, synapses, con- of the brain, signal traffic can flow flexibly back and nection patterns, and transmitter molecules have been forth, like air traffic across the earth.

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The major lobes of cortex are comparable to the make the connections between cortical neurons look earth’s continents, each with its population centers, nat- white to the naked eye. ural resources, and trade relations with other regions. While cortical regions are often specialized, they are 1.1 The nervous system also densely integrated with other regions, using web- like connections that spread throughout the cortex and The brain is part of the nervous system which per- its associated organs. This outer sheet is called the gray vades the human body. The two main parts are the matter from the way it looks to the naked eye. It is the central nervous system (CNS), which includes the outer ‘ skin ’ of the white matter of cortex which appears brain and the spinal cord, and the peripheral nerv- to fill the large cortical hemispheres, like the flesh of a ous system (PNS), which contains the autonomic and fruit. However, this is only appearance. In fact, the gray peripheral sensory and motor system ( Figure 5.1 ). matter contains the cell bodies of tens of billions of Together the CNS and PNS provide a dynamic and neurons that send even larger numbers of axons in all massive communication system throughout all parts directions, covered by supportive myelin cells that are of the body, with a hub at the brain that is accessed filled with white lipid molecules. These white myelin through the spinal cord. We will focus in this chapter sheaths of cortical neurons are so pervasive that they on one part of the CNS, the brain ( Figure 5.2 ).

Peripheral nervous Central nervous system (PNS) system (CNS)

Brain Cranial nerve

Spinal Spinal cord nerve

FIGURE 5.1 The central and peripheral nervous systems. FIGURE 5.2 Parts of the central nervous system include the Source : Standring, 2005. spinal cord and the brain. Source : Standring, 2005.

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In this chapter, we will focus on two sensory and hearing have been most studied in the human input systems within the brain: vision and hearing. brain. We will focus on two output systems, speech Although there are other sensory input systems, such and hand-arm control, again because they have been as olfaction (smell) and somatosensory (touch), vision the target of much study. Throughout this chapter on the brain, we will describe the anatomy of the brain and brain regions and we will also highlight the function they serve. We will begin with discussing the many levels of analysis that we can take in describing the brain – from large-scale regions such as cerebral hemispheres and cortical lobes, to finer-scale classifi- cations, such as cortical layer topography.

1.2 The geography of the brain Let’s begin with the large-scale brain areas and work our way down to a finer analysis. First, to state the rather obvious, the brain is located in the human head, as depicted in Figure 5.3 . We can look at the brain at different geographical levels – from continents to countries, states, and cities. Thus, we have several levels of detail. The first dis- tinct geographical regions are the two cerebral hemispheres, which are entirely separate, joined through a complex connective region called the corpus callosum. We will discuss the hemispheres in more detail later in the chapter: the question of why we have two separate hemispheres in the brain has long intrigued scientists and philosophers alike. Next , we have the cortical lobes (Figure 5.4 ): there are four lobes in each hemisphere. Beginning at the FIGURE 5.3 The location of the brain in the head, showing a midsagittal view of the right hemisphere.

FIGURE 5.4 The four major lobes of the cortex are visible from a lateral view of the brain. Here we show a view of the left hemisphere with the frontal lobe (purple) at the anterior of the brain, the parietal lobe (orange) posterior to the frontal lobe at the superior aspect of the brain, the temporal lobe (blue) posterior to the frontal lobe and inferior to the parietal lobe, and the occipital lobe (yellow) posterior to both the parietal and temporal lobes. Just below the occipital lobe is the cerebellum (green), which is not part of the cortex but is visible from most aspects of the brain. Source : Squire et al ., 2003.

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FIGURE 5.5 Some important landmarks of the brain in the left hemisphere from a lateral perspective (left panel) and a midsagittal perspective (right panel). Source : Standring, 2005.

1 2 4 7A 3 5 6 4 8 6 B 5 7 1 7 2 8 9 31 3 24 40 23 9 32 19 43 19 46 39 26 42 10 33 29 18 44 18 10 45 30 27 22 17 25 37 11 17 12 19 47 37 34 18 21 11 38 28 38 35 20 36 20 FIGURE 5.7 41 The Brodmann classification of regions in the 52 right hemisphere, shown in a midsagittal view.

FIGURE 5.6 The Brodmann classification of regions in the left hemisphere, shown in a lateral view. Areas 41 and 52 are indicated We can see the major lobes with the naked eye, by lines. Some areas, like the insula and auditory region, are tucked along with their hills and valleys, the gyri and sulci. away behind the temporal lobe. Some of these are so important that we will call them landmarks – we need to know them to understand everything else. In Figure 5.5 , we show some of the front or anterior part of the brain (shown on the left major landmarks that brain scientists have long used side of Figure 5.4 ), we see the frontal lobe . Immediately to identify regions in the brain. These landmarks are behind the frontal lobe, at the top or superior part of widely used today when discussing the results of neu- the brain, we find the parietal lobe. Below, or inferior roimaging studies. to, the parietal lobe and adjacent to the frontal lobe, At a more microscopic level of description, we have we find the temporal lobe. At the back or posterior part the Brodmann areas , the numbered postal codes of the of the brain, we find the occipital lobe . We will discuss cortex. When the surface layers of cortex are carefully the anatomical features and cognitive function of these studied under a microscope, small regional differences lobes later in the chapter. can be seen in the appearance of cells in the layers and

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their connections. Those subtle differences were first AREA 17 23 μm23μm described by Korbinian Brodmann in 1909, and are therefore known as Brodmann areas, each with its own 1 unique number (Figure 5.6 shows a lateral view of Brodmann areas in the left hemisphere, and Figure 5.7 a medial (midsagittal) view of the Brodmann areas in the right hemisphere). About 100 Brodmann areas 2/3 are now recognized, and it is therefore convenient to take this as a rough estimate of the number of specialized regions of the cortex. The Brodmann areas cor- 4A respond well to different specialized functions of the cortex, such as the visual and auditory areas, motor 4B cortex, and areas involved in language and cognition. They are essentially the postal codes of the brain. They 4Cα range in size from a few square inches – the primary visual cortex, for example, is about the size of a credit card – to the small patch of Brodmann area 5 at the 4Cβ top of the somatosensory cortex. Notice that in Figure 5.6, with the brain facing left, neighboring Brodmann areas are colored to show 5 their major functions including vision, hearing, olfaction, motor control, Broca’s area (speech output), 6A and Wernicke’s area (speech perception and comprehension). This figure will be used as a reference map 6B throughout this book. We can focus even more specifically by observ- WM ing hypercolumns, columns, and single neurons. FIGURE 5.8 At this fine level of resolution the current standard The six major layers of cortex in cross section. The figure shows three columns in Area 17, also called V1, the first is the Talairach coordinates (Talairach and Tournoux, visual projection area to the cortex. Source : Squire et al ., 2003. 1988), which is used in functional brain imaging. The Talairach system can be compared to the map coordinates of the world, as shown on a GPS locator. They dimensions. While human brains vary a great deal, indicate the street addresses of the brain. much as human faces do, the Talairach system allows It helps here to refer back to Figure 4.3, p. 99. The different brains to be mathematically ‘ squeezed into a fine red lines show the axes of a three-dimensional shoebox ’ , so that different brains can be compared in a coordinate system. On the upper left, we see the single framework. medial view of the right hemisphere, looking to the Let ’s continue our description of the geography of left (see the small head inset for orientation). In this the brain with a look at the fine structure of the cor- image, the horizontal red line always runs between tex. The visible outer brain consists of a large thin the pineal body (not visible), and the small cross- sheet only six cellular layers thick (Figure 5.8 ), called section of the anterior commissure – one of the tiny the cortex (meaning ‘ bark ’ , like the outside bark of white fiber bridges that run between the two sides of a tree). This sheet is called the gray matter from the the brain. The three-dimensional zero point (0, 0, 0) way it looks to the naked eye. Not all cortex has six of the coordinate system is always at the front of these layers; only the giant mammalian cortex does, and is two points. This allows all three dimensions to be therefore sometimes called ‘ neocortex ’ (That is, the defined with good reliability. Notice the three stand- new cortex, because it only emerged 200 million years ard perspectives on the brain: the medial view ago!) Older regions of cortex are also found in rep- (midsagittal), the horizontal or axial, and the coro- tiles, like salamanders, for example, such as the lim- nal (crown-shaped) cross-slice. This software display bic cortex, which we will discuss later in this chapter. allows any point in the brain to be specified pre- That region has five cortical layers and is sometimes cisely, by entering numbers for the points in three referred to as ‘ paleocortex ’ .

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The six horizontal layers of cortex are organized in The geography analogy is useful, but the brain, like cortical columns, vertical barrel-shaped slices (Figure 5.8). the world, is a dynamic place. New streets are built These often contain closely related neurons, such as and old ones move or are rebuilt. Houses and their visual cells that respond to different orientations of residents appear and disappear. Until about a decade a single light edge in just one part of the visual field. ago, it was believed that neurons did not change in the Columns may be clustered into hypercolumns, which adult brain, but we now know of a number of ways may be part of an even larger cluster. Thus, cortex has in which neurons continue to grow, migrate, connect, both a horizontal organization into six layers, and a disconnect, and die, even in the healthy mature brain. vertical one, into columns, hypercolumns, and eventu- The brain is never frozen into a static rock-like state. ally entire specialized regions. The visual cortex of the These dynamic aspects of the brain can be seen macaque monkey is often used as an animal model to even at the level of the six layers of cortex. Let’s take study vision. Human visual cortex looks quite simi- another look at the six layers of the cortex, this time lar. Note that there are six layers, with numbering (in using a schematic drawing of the layers and their Roman numerals) beginning at the top with layer I inputs and outputs ( Figure 5.9 ). Notice that some cor- and progressing down to layer VI (Figure 5.9). tical neurons send their axons to the thalamus, while

FIGURE 5.9 A schematic drawing of the six layers of cortex, the gray matter. Note that some cortical neurons send their axons to the thalamus, while others receive input from thalamic neurons. Ipsilateral ϭ same side of the cortex; Contralateral ϭ opposite side.

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others receive input from thalamic neurons. Millions 2.0 GROWING A BRAIN FROM THE of cortical nerve cells go to the opposite hemisphere BOTTOM UP (the contralateral hemisphere), while many others project their axons to the same hemisphere (the ipsilateral side). However, the densest connections are to 2.1 Evolution and personal history are neighboring neurons. Cortical layer I consists largely expressed in the brain of dendrites (input fibers) that are so densely packed We usually see the brain from the outside, so that the and interconnected that this layer is sometimes called cortex is the most visible structure. But the brain grew a ‘ feltwork ’ , a woven sheet of dendrites. The neurons and evolved from the inside out, very much like a in this drawing (Figure 5.9) are called ‘ pyramidal ’ tree, beginning from a single seed, then turning into because their bodies look like microscopic pyramids. a thin shoot, and then mushrooming in three direc- They are embedded in a matrix of glial cells, which tions: upward, forward, and outward from the axis of are not shown here. These connection patterns in cor- growth. That point applies both to phylogenesis – how tex undergo major change in human development species evolved – and ontogenesis – how the human and through-out the lifespan (see Chapter 15 for more brain grows from the fetus onward. discussion of this).

White matter Grey matter (fiber tracts) (outer cell bodies of cortex)

Cerebellum

Fluid ventricles Basal ganglia (light blue) (green)

Thalami Hippocampi (light brown)

Brainstem Amygdalas (pons) (orange)

FIGURE 5.10 Growing the brain from the bottom up. If you can memorize these basic shapes, you will have a solid framework for understanding the brain. Notice how the brain builds on the brainstem, with the thalami on top as major input hub. The hippocampi and amygdalas are actually nestled inside each of the temporal lobes. The light blue fluid ventricles have no neurons, but provide the brain’s own circulatory system. The basal ganglia can be thought of as the output hub of the system. A great deal of traffic flows back to the cortex as well. Source: Baars and Fu, with permission.

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FIGURE 5.11 Diagram of the evolution of the mammalian brain. The forebrain evolves and expands along the lines of the three basic neural assemblies that anatomically and biochemically reflect ancestral commonalities with reptiles, early mammals, and late mammals. Source : Adapted from MacLean, 1967, with kind permission. FIGURE 5.12 Do you really need a cortex? A structural brain scan (MRI) from a 7-year-old girl who had a surgical removal of her left hemisphere at age 3 for Rasmussen’s encephalitis. Such surger- ies can save children’s lives if they are performed early enough. The mature brain reveals that pattern of growth Because the brain is highly flexible at this age, the language capac- and evolution. It means, for example, that lower ity has shifted to the right hemisphere. Notice, however, that her regions like the brainstem are generally more ancient brainstem and thalami are intact. The brainstem is crucial to life than higher regions, such as the frontal cortex. Basic functions, and cannot be removed. She is able to play and talk, and has mild right side motor impairment. Source : Borgstein and survival functions like breathing are controlled by Grotendorst, 2002. neural centers in the lower brainstem, while the large prefrontal cortex in humans is a late addition to the basic mammalian brain plan. It is located the farthest mushroom over the course of evolutionary time. The upward and forward in the neural axis ( Figure 5.11 ). neuraxis – the spinal cord and brain – grows from tiny Thus, local damage to prefrontal cortex has little cellular clumps, then forward into a slender cylindri- impact on basic survival functions, but it can impair cal shoot, and then thickening centrifugally to form sophisticated abilities like decision-making, self- the spinal cord, covered by an approximate mush- control, and even personality. room shape. In the womb, the embryonic brain devel- ops into an S shape, and then the neocortex covers the 2.2 Building a brain from bottom to top older regions. We can follow the brain from bottom to top to show Because the brain involves hundreds of millions of structures that are normally hidden by the head of the years of evolutionary layering on top of older layers, mushroom. We encourage you to draw these succes- the more recent levels hide the older ones. That is par- sive levels of the great tower of the brain (Figure 5.10). ticularly true for the fast-ballooning neocortex in pri- Unlike most other mammals, humans stand mates and humans, called the ‘ new cortex’ because upright, and therefore bend their eyes and cortices at it is more recent, and has six layers rather than the a right angle forward. That is why the upper direc- four or five layers of the reptilian and early mamma- tion of the human brain is both called ‘ dorsal ’ , mean- lian brain. The brain therefore grows a little bit like a ing ‘ toward the back ’ and also ‘ superior ’ , meaning

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Edinger-Westphal nucleus

Oculomotor nucleus

Trochlear nucleus Trigeminal mesencephalic nucleus

Trigeminal motor nucleus Trigeminal main sensory nucleus Abducens nucleus Trigeminal spinal nucleus Facial motor nucleus

Superior Salivatory nuclei Inferior Dorsal cochlear nucleus Superior Dorsal vagal motor nucleus

Nucleus ambiguous Nucleus of tractus solitarius Lateral Hypoglossal nucleus

Dorsal cochlear Inferior Efferent cranial nerve nuclei Medial

Vestibular nuclei Afferent cranial nerve nuclei

FIGURE 5.13 Detailed anatomy of the brainstem and pons. Notice that all the major input-output pathways of the brain emerge here, either flowing down the spinal cord, or out through narrow openings in the cranium. Vision, hearing, olfaction, and taste use cranial nerves as major pathways. Touch and pain perception in the head do the same. The brainstem also controls vital functions like breathing and heart rate. (Afferent ϭ input to cortex; efferent ϭ output from cortex). Source : Standring, 2005.

upward. The other directions are called ‘ ventral ’ , Next , we have the thalamus – actually, they are the ‘toward the belly’ , and also ‘ inferior ’ , meaning down- thalami, because there are two of them, one in each ward. We have a double vocabulary for the human hemisphere (Figure 5.14). The two egg-shaped thalami brain, an important point to understand in order to form the upper end of the brainstem. The thalami are avoid getting confused. the great traffic hubs of the brain. They are also inti- In this section, we will ‘ grow ’ a brain, beginning mately connected with each great hemisphere. at the bottom with the older regions of the brain and Immediately below and in front of each thala- layering on until we come to the newest part of the mus is a cluster of nuclei called the hypothalamus . It is brain, the neocortex. We begin with the brainstem and connected with the pituitary gland, often called the pons which are at the bottom or ‘ oldest ’ section of the ‘ master gland ’ of the body ( Figure 5.15 ). Together, brain. the hypothalamus and pituitary are an extraordinar- The brainstem ( Figure 5.13 ) is continuous with the ily important neurohormonal complex. Many types spinal cord. Its upper section, the pons, has nerve of physiological homeostasis are monitored by the fibers that connect the two halves of the cerebellum. hypothalamus. When hypothalamic neurons detect a The brainstem and pons form a major route from the deviation from the proper blood level of oxygen, they spinal cord to the brain. Some basic functions such as trigger increased breathing – such as the sigh you control of breathing and heart rate are controlled here. might make after reading intensively in a cramped

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olfactory surface replace their cells, just as the rest of the body does. The ventricular stem cells are believed to be a source of these new neurons. Next up are the basal ganglia, literally, the clumps at the bottom of the brain ( Figure 5.19 ). There is one outside of each thalamus. The elegant shield-like Thal structure with outward radiating tubes is called the putamen. Looping over each is another artistic struc- GP Put ture called the caudate nucleus. This ‘ shield and loop ’ structure is fundamentally important for control of Cd movement and cognition. Notice that the basal ganglia are located outside of the thalami. Finally , we can mount the two hemispheres on top of the lower levels of the brainstem, thalami, hippocampi and amygdala, ventricles and basal ganglia (Figure 5.20). So you should not be surprised when you carve away the cortex to see deeply buried, more ancient brain structures appear in the excavation. One final note on ‘ growing ’ the brain: we present a FIGURE 5.14 Transparent overview of the thalamus in the bottom view of the brain in order to show you some center of each hemisphere, and the basal ganglia looking like a ‘ shield brain regions that are difficult to see otherwise (Figure and loop ’ on the outer side of each thalamus. Source : Ohye, 2002. 5.21 ). You will notice the optic nerve linking the eyes, for example, to the cortex. So there you have the brain, shown ‘ growing ’ from position. The hypothalamus also has crucial emotional ancient areas to the neocortex in the two hemispheres. functions. Now let’s take a look at the functional significance of Seated on top of the thalami like a rider on a horse are these brain areas in human cognition. In this discus- the two hippocampi , one on each side (Figures 5.10, 5.17). sion, we will proceed in a ‘ top down ’ fashion, begin- Each hippocampus is nestled inside of a temporal lobe ning with the two hemispheres, moving through the on each side, as we will see later on. But it is impor- major lobes, and then on to the subcortical ‘ satellites ’ tant to see the doubled structure of two hippocampi. of the brain. As we have seen, the hippocampus plays a major role in transferring experiential information to longer-term memory, and in retrieving episodic memories as well. It is also involved in spatial navigation. 3.0 FROM ‘ WHERE ’ TO ‘ WHAT ’ : THE Near the tip of each hippocampal loop is an FUNCTIONAL ROLES OF BRAIN almond-shaped structure called the amygdala , which REGIONS plays a starring role in emotions and emotional association ( Figure 5.18 ). We have discussed the many levels of analysis with The next level up is deceiving. It looks like a neu- which to understand brain structure and shown where four ventricles ral structure but is not. It is the , of which the major brain areas are located. Let’s work through you can see the right and left one (Figure 5.10). The the brain now, beginning with the neocortex and end- ventricles are small cavities containing a circulating with the brainstem, and discuss the functional ing fluid that is separate from the bloodstream. This roles they play in human cognition. brain-dedicated circulatory system descends into the spinal cord through a tiny tube called the aqueduct, and the fluid of the ventricular system is therefore 3.1 The cerebral hemispheres: the called the cerebrospinal fluid. The ventricular walls left-right division have recently been found to be sites for neural stem cells, much to the surprise of many scientists. It was The two mirror-image halves of the cortex have puz- long believed that neurons could not be replaced dur- zled people for centuries. Why are there two hemi- ing life, but certain regions like the hippocampus and spheres? If we have but one mind, why do we have

09_P375070_Ch05.indd 136 1/29/2010 4:08:50 AM 3.0 FROM ‘ WHERE ’ TO ‘ WHAT ’ : THE FUNCTIONAL ROLES OF BRAIN REGIONS 137 Hypothalamohypophysial portal system : Source Standring et al ., Paraventricular nucleus Supraoptic nucleus Optic chiasma Acidophil Anterior lobe Basophil Chromophobe Hypophysial vein (to dural sinuses) Primary capillary plexus in the upper infundibulum receives releasing and inhibitory neuroendocrine factors from hypothalamic nuclei axon terminals Portal veins carry neuroendocrine factors to the adenohypophysis Secondary capillary plexus The hypothalamus (highlighted with dark blue circle) is a cluster of nuclei located immediately below and in front of each thalamus. The of each is a cluster of nuclei located immediately below and in front The hypothalamus (highlighted with dark blue circle)

Axon terminal

Mammillary body the posterior lobe Capillary plexus of Hypothalamic nuclei exteroceptive stimuli The trabecular artery Posterior or neural lobe respond to emotional and Inferior hypophysial artery connects the superior and Superior hypophysial artery inferior hypophysial arteries FIGURE 5.15 Hypophysial vein (to dural sinuses) , 2008, Chapter 21, Figure 21.11. 21.11. , 2008, Chapter 21, Figure Anatomy Gray’s hypothalamus is important in regulating physiological and emotional processes and is closely connected with the pituitary gland. and is closely connected with the pituitary gland. physiological and emotional processes hypothalamus is important in regulating

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FIGURE 5.16 We begin ‘ growing ’ the brain with the brainstem and pons. Source : Baars and Fu. FIGURE 5.18 The amygdalas are situated just in front of the tip of each hippocampus. Source : Baars and Fu.

Anterior limb of internal capsule Body of caudate nucleus

Head of caudate nucleus Thalamus

Anterior Posterior

Putamen Tail of caudate nucleus

Amygdala Posterior limb of internal capsule

FIGURE 5.19 Side view of the basal ganglia, with the ‘ shield FIGURE 5.17 Schematic drawing of the hippocampi. Source : and loop ’ formed by the putamen and caudate nucleus, respectively. Baars and Fu. Source : Standring, 2005.

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Corpus callosum

FIGURE 5.20 The cerebral hemispheres are shown mounted FIGURE 5.22 A cut-away of a three-dimensional magnetic above the brainstem and other subcortical bodies. Source : Baars and Fu. resonance image showing the location of the corpus callosum – a white fiber arch extending horizontally from the anterior of the brain to the posterior, forming a fiber link between the two hemispheres. Source : Mark Dow, University of Oregon, with permission.

Because he believed that the soul must be a unitary whole, he looked for at least one brain structure that was not doubled, and finally decided on the small pineal gland at the back of the brainstem. There he believed the soul resided – roughly what we mean by subjective experience. Unfortunately for Descartes, when microscopes became powerful enough to exam- ine the tiny pineal gland in detail, it also turned out to have two symmetrical halves, roughly mirror images of each other. How do the two hemispheres ‘ talk ’ to each other? The answer lies in the fiber tract that runs from the front to the back of the brain, linking the two hemispheres. FIGURE 5.21 A view of the brain from below showing the medial temporal lobe and optic tracts. Source : Baars and Fu. 3.1.1 The corpus callosum

two hemispheres? Sir Charles Sherrington (1947) The hemispheres are completely separate, divided wrote: by the longitudinal fissure that runs between the two hemispheres from the anterior (front) to the posterior This self is a unity . . . it regards itself as one, oth- (back) part of the brain. The link between the hemi- ers treat it as one. It is addressed as one, by a name to spheres is provided by the corpus callosum , a large arch which it answers. The Law and the State schedule it as of white matter ( Figure 5.22 ). The number of axons one. It and they identify it with a body which is consid- traveling between the two hemispheres is estimated ered by it and them to belong to it integrally. In short, at more than 100 million. The corpus callosum has fib- unchallenged and unargued conviction assumes it to be ers that project between the hemispheres in an orderly one. The logic of grammar endorses this by a pronoun way, with regions in the anterior portion connecting in the singular. All its diversity is merged in oneness. similar brain areas in the frontal lobes and regions in The philosopher Rene Descartes, for example, was the posterior portion connecting similar brain areas in dumbfounded by the doubled nature of the brain. the occipital lobe.

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L. R. Visual Visual field field

R. and L. Auditory Optic L. and R. Auditory chiasma

LEFT HEMISPHERE RIGHT HEMISPHERE

Almost non-verbal Verbal Musical Linguistic description Geometrical Mathematical Spatial comprehension Sequential Temporal synthesis Analytical The non-speaking hemisphere

R. Stereognosis L. Stereognosis

R. Visual field L. Visual field

Divided corpus callosum

FIGURE 5.23 A top view of the two hemispheres. Schematic drawing of the two halves of the cerebral cortex, showing some major functions of the right and left hemispheres. Note the massive bridge of the corpus callosum connecting the two sides. The eyes on top focus on converging lines in order to enable stereoscopic depth perception. Source: Standring, 2005.

The role of the two hemispheres in human cogni- is also crossed over, with the left hemisphere controlling tion and the mind-brain has been the subject of exten- the right hand, and the right controlling the left hand sive study, and we are still unfolding the subtle and ( Figure 5.24 ). While the left and right hemispheres have not-so-subtle differences in the roles that the mir- some different functions, the corpus callosum has some ror-image hemispheres play in perception, language, 100 million fibers, constantly trafficking back and forth, thought, and consciousness. There are some hemi- which serves to integrate information from both sides. spheric differences that are fairly well understood, The time lag between the two hemispheres working such as crossover wiring. Many aspects of sensory on the same task may be as short as 10 ms, or one-hun- and motor processing entail the crossing over of input dredth of a second (Handy et al., 2003). Therefore, when (sensory) or output (motor) information from the left the great information highway of the corpus callosum is side to the right, and vice versa ( Figure 5.23 ). intact, the differences between the hemispheres are not For example, each optic nerve coming from the retina very obvious. But when it is cut, and the proper experi- is split into a nasal half (on each side of the nose), which mental controls are used to separate the input of the crosses over to the opposite side of the brain, and a lat- right and left half of each eye’s visual field, suddenly eral half, which proceeds to the same side (ipsilaterally). major hemispheric differences become observable. Only the olfactory nerve, which is a very ancient sen- The question of the perceived unity of the world sory system, stays on the same side of the brain on its continues to interest scientists. The most spectacular way to cortex. The cortical output control of the hands finding in that respect has been the discovery that the

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Cranial nerves and motor system Reflexes Sensation Coordination

FIGURE 5.24 The pattern of cortical control over regions of the body. Notice that sensation and cortical motor control pathways cross over in the brain. Simple reflexes do not cross over, and coordination involves interaction between both sides. Source: Standring, 2005.

corpus callosum can be cut in humans, without chang- their patients believed that cutting some 100 million fib- ing the perceived unity of the world and of the self. ers in the corpus callosum had no noticeable effect at all! Indeed, for many years such callosectomies (separa- It is a dramatic illustration of the capacity of the brain tion or cutting the corpus callosum) were performed to to adapt to quite severe damage – to fill in the missing improve uncontrollable epilepsy. A complete slicing of details of the experienced world by means of eye move- the corpus callosum is called a callosotomy; more com- ments, for example. More careful study, however, has mon, today, is a partial cut of only the regions of the provided evidence that a complete slicing, a callosot- two hemispheres that spread epileptic seizure activity. omy does have subtle but long-lasting effects and so a This partial cut is called a callosectomy. Doctors and partial resection, or callosectomy, is preferred.

FRONTIERS OF COGNITIVE NEUROSCIENCE

Synesthesia

Imagine a world of magenta Tuesdays, tastes that have shapes, and wavy green symphonies. One in a hundred otherwise ordinary people experience the world this way, in a condition called synesthesia – the fusion of different sense experiences. In synesthesia, stimulation of one sense triggers an experience in a different sense. For example, a voice or the sound of music are not just heard but also seen, tasted, or felt as a touch. Synesthesia is a fusion of different sensory perceptions: the feel of sandpaper might evoke the musical sound of F-sharp, a symphony might be experienced in blue and gold colors, or the concept of February might be experienced as being located above one’s right shoulder. Most FIGURE 5.25 David Eagleman, PhD, Department of synesthetes (people with synesthesia) are unaware that Neuroscience, Baylor College of Medicine, Houston, Texas, USA their experiences are in any way unusual.

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Synesthesia comes in many varieties, and having one type gives you a high chance of having a second or third type. Experiencing the days of the week in color is the most common kind of synesthesia, followed by colored letters and numbers. Other common varieties include tasted words, colored hearing, number-lines perceived in three dimensions, and the personification of letters and numerals. Synesthetic perceptions are involuntary, automatic, and generally consistent over time. Moreover, synesthetic perceptions are typically intrinsic, meaning that what is sensed is something like a simple color, shape, or texture, rather than something that is a thought association. Synesthetes don’t say, “ This music makes me experience a vase of flowers on a restaurant table.” It just happens to them. Synesthesia seems to be the result of increased cross- talk among sensory areas in the brain – like neighboring countries with porous borders on the brain’s map. Synesthesia has fascinated laypersons and scientists alike with its wealth of sensory amalgamations, but only recently has it been appreciated how the brains of such individuals yield surprising insights into normal brain function. Although synesthesia has been explored in behav- ioral and neuroimaging experiments, its genetic basis remains unknown. My laboratory group realized that synesthesia is an ideal condition for genetic analysis, FIGURE 5.26 In a common form of synesthesia, months for three reasons: (1) synesthesia clusters in families and days of the month can have both colors and specific spa- and appears to be inherited; (2) synesthetic perception tial configurations. Source: Cytowic and Eagleman, 2009 in results from increased cross-talk between neural areas, Wednesday is Indigo Blue: Discovering the Brain of Synesthesia . which suggests a set of candidate genes; and (3) a bat- Cambridge: MIT Press. tery of tests developed in our lab allows for confident identification of real synesthetes, not just people who have free associations to their experiences. We therefore are performing a large-scale genetic in these families. Understanding the genetic basis of study, called a family linkage analysis, to map the synesthesia yields insight into the way normal brains gene(s) that correlate with color synesthesias. To this are wired. And it demonstrates that more than one kind end, we have developed a battery of tests to clearly iden- of brain – and one kind of mind – is possible. tify synesthetes; that is, to distinguish them from control Synesthesia affects the brain wiring of one in several subjects. These tests are offered free to the research com- hundred people, making it far more common than origi- munity at www.synesthete.org . Several families with nally thought, and far more important scientifically than multiple synesthetes have provided pedigrees, and we a mere curiosity. Other evidence suggests that we may have harvested DNA samples from over 100 people in all be synesthetic to some extent – but the majority of us these families. A genomewide scan is identifying the remains unconscious of the sensory fusions going on in most probable genetic region responsible for synesthesia our brains.

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Central Primary somatosensory sulcus Somatosensory association

Visual association

Primary visual

FIGURE 5.27 A view of functional areas in some of the sensory regions of the cortex. The central sulcus is seen separating the frontal lobe from the parietal lobe. Immediately posterior to the central sulcus is the primary somatosensory area. The Sylvian fissure is also Sylvian Primary Wernicke’s area called the lateral fissure. Source : Squire et al ., fissure auditory 2008.

3.2 Output and input: the front-back division extends to the parietal and temporal lobes. The auditory cortex is located in the temporal lobe but also The cortex is a folded sheet of gray matter that would extends to the parietal lobe. Somatosensory areas are measure roughly 2 feet by 2 feet (60 cm by 60 cm) if located in the parietal lobe. Taste and smell regions are it were unfolded. To fit within the skull, the cortex is located at the bottom of the temporal lobes. This ‘ back folded into hills (gyri) and valleys (sulci). The cortex of the brain’ large region, encompassing three cortical contains four lobes that are visible from the outside lobes, is not simply a site for processing sensory infor- and two large regions that are not visible. Before we mation. It is also the region of cortex for associative discuss the functions of these regions, let’s take a look processes, where information from the various senses at another major division of the brain: the front-back is ‘ bound together ’ for higher order processing. Think division of the cerebral cortex. In order to understand about watching a movie – these association areas will this division, you will need to be able to locate some help you understand how to relate what you are hear- landmarks in the brain. In Figure 5.27 , see if you can ing to what you are seeing on the screen. Much of this locate the central sulcus that runs vertically between type of processing occurs in the parietal lobe, and we the frontal lobe and the parietal lobe. To locate it, look will discuss this important lobe in more detail in the for the region labeled ‘ primary somatosensory ’ . The next section. These association regions are largest in central sulcus is just in front of, or anterior to, this primates and largest of all in humans. region. The second landmark to look for is the Sylvian The motor – or output – regions of the cortex are fissure. It runs more or less horizontally from the fron- located in the frontal lobe, anterior to the central sul- tal lobe posterior, separating the temporal lobe from cus and the Sylvian fissure. Look again at Figure 5.27 the parietal and frontal lobes. and locate the region labeled ‘ primary somatosen- The sensory – or input – regions of the cortex are sory ’ , just posterior to the central sulcus. Although it located posterior to the central sulcus and the Sylvian is not labeled on this figure, the primary motor region fissure, in the parietal, temporal, and occipital lobes. is in the frontal lobe, just across the central sulcus and These lobes contain the visual cortex, auditory cortex, anterior to the somatosensory regions in the pari- and somatosensory cortex, where information coming etal lobe. The close physical connection between the from the eyes, ears, and body is processed. The visual so matosensory cortex and the motor cortex allows for cortex, for example, begins in the occipital lobe but a tight coupling between the senses of touch, pressure,

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Trunk

Neck Hip

Head

Shoulder Arm Leg Elbow Forearm

Wrist Hip Hand

Trunk

Wrist Knee Elbow Little Hand Shoulder Ring

Ankle Little Middle Foot Toes Ring Index Toes Middle Thumb Index Eye Thumb Nose Neck Genitalia Brow Face Upper lip Face

Eyelid and eyeball

N Lips

Lips O

Z I

L Lower lip A Teeth, gums, and jaw

Jaw C

Tongue O

Swallowing N V

O I Ton gue T A N V N I O L I A AT S Pharynx IC ST MA Intra-abdominal

FIGURE 5.28 Drawing of the somatosensory homunculus, showing the representation of body areas in the cortex. Note that some FIGURE 5.29 Drawing of the motor homunculus, showing the body areas, such as the face, have a disproportionately larger repre- representation of body areas in motor cortex. Note that some body sentation than other areas, such as the trunk. Source : Standring, 2005. areas, such as the face, have a disproportionately larger representation than other areas, such as the trunk. Source : Standring, 2005.

and pain and the action or motor system. In fact, there and surgeons could electrically stimulate the exposed is an intricate mapping of the body that is reflected in cortical surface and ask their awake patients about similar ways in the somatosensory region located just their experiences as a result. Their discoveries have posterior to the central sulcus and its corresponding largely stood the test of time. Exploration by electri- motor region located just anterior. This intricate map- cal stimulation was medically necessary in order to ping is a representation of areas of the body: the dif- know where to operate in the brain while minimizing ferent regions of the body are not equally represented damage to functional regions in patients. In the case in these cortical regions; some areas, such as the face of the sensory homunculus (somatosensory), local and hands, have quite a disproportionately large rep- stimulation would evoke feelings of touch in the cor- resentation and other regions, such as the center of the responding part of the body. Stimulation of the motor back, have a disproportionately small representation. homunculus would evoke specific body movements, Consider how much more sensitive your fingertips but interestingly, patients would deny a sense of own- are to touch, pressure, and pain than, say, the small of ership of those movements. When Penfield would ask, your back. The representational map in cortex reflects ‘ Are you moving your hand?’ when stimulating the this differing sensitivity. There are two maps of the hand region, a patient might say, ‘ No, doctor, you’re body: one is in somatosensory cortex and a very simi- moving my hand’ . If, however, the surgeon moved lar one is in motor cortex ( Figures 5.28 and 5.29 ). perhaps a centimeter forward to the pre-motor strip, These two body maps or homunculi ( ‘ little men ’ ) stimulation would evoke a reported intention to move were first discovered by the pioneer neurosurgeon one’s body, without a sense of being externally con- Wilder Penfield at the University of Montreal in the trolled. It is a fundamental distinction, which we will 1950s and 1960s. Penfield’s team was the first to stim- return to later. ulate the cortex of awake patients, which is possible The essential point here is that the central sulcus because the cortical surface contains no pain receptors. is an important landmark to know. Not only does it Therefore, local anesthetic applied to the incision was separate the sensory and motor homunculi but, more enough to dull the pain of the removal of the scalp, broadly, the central sulcus separates the more sensory

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half of cortex (posterior), from the frontal half (anterior). Posterior cortex contains the projection regions of the major sense organs – vision, hearing, touch, smell, taste. In contrast, frontal cortex is involved in action control, planning, some working memory functions, language production, and the like. In a sense, the posterior half deals with the perceptual present, while the anterior half tries to predict and control the future.

3.3 The major lobes: visible and hidden We have used the analogy of the geography of the brain. In this setting, the major lobes can be viewed as FIGURE 5.30 large continents in brain geography. While each is sep- Basic brain directions. Because the human brain is rotated 90 degrees forward from the spinal cord (unlike most mam- arate from the other and has its own local functions mals and reptiles), it has two sets of labels. The dorsal direction is also and anatomical features, each is part of the whole, the called superior, the ventral is also called inferior, and rostral, roughly brain, and thus is united and intimately linked to total the same as frontal and caudal, is sometimes called posterior. To brain function. The four ‘ continents ’ of the brain are simplify, just use plain language, like front, back, upper, and lower. Source : Standring, 2005. shown in Figure 5.30 and include the frontal, parietal, temporal, and occipital lobes. In this section, we will discuss their functional roles in cognition. Two other part of the brain. The prefrontal cortex is a large cortical major regions, not visible from the exterior view of the region, taking up an estimated one-third of the entire brain, play important roles in cognition and we will area of cortex. What is the prefrontal cortex specialized describe those as well. for and why is it so uniquely a human region? The prefrontal cortex is specifically needed for:

3.3.1 Frontal lobe ● initiating activities ● planning The massive frontal lobe is the site for motor planning ● holding critical information ready to use (an aspect and motor output. As we mentioned, the motor areas of working memory) are tightly connected to the somatosensory regions ● changing mental set from one line of thinking to with similar homunculus maps representing body another areas. These motor functions that are present in the ● monitoring the effectiveness of one’s actions human brain are present in most mammalian brains ● detecting and resolving conflicting plans for action in a similar way. But the frontal lobe in humans is far ● inhibiting plans and actions that are ineffective or larger than in non-human primates or any other crea- self-defeating. ture. What other functions does the frontal lobe per- form and how is its role unique in humans? This list shows how important the prefrontal cortex is The frontal lobe has been termed the ‘ organ of civi- to human cognition. Many anatomists believe that pre- lization’ (Luria, 1966). The regions of the frontal lobe frontal cortex is largest in humans, and distinguishes that have earned this term are primarily in the pre- our species from other primates. In addition, the prefrontal cortex. The prefrontal cortex is located on the frontal cortex has regions for emotional and person- medial, lateral, and orbital surfaces of the most ante- ality processes as well as social cognition – knowing rior portion of the frontal lobe ( Figure 5.31 ). ‘ how to behave’ for example. On the lateral convexity, Prefrontal cortex is the non-motor part of frontal cor- interposed between the dorsolateral prefrontal and the tex. Notice that prefrontal cortex is the most forward ventral portion of premotor cortex, is Broca’s area. This part of the frontal cortex. The term ‘ prefrontal ’ is some- area is involved in the abstract mediation of the verbal what confusing, but it means ‘ at the front of the fron- expression of language, a uniquely human function. tal cortex’ . There are no obvious boundary markers for The frontal lobe, then, is far larger in humans than prefrontal cortex, which is defined instead by a set of other primates and has developed many new functions projections from the thalamus. Nevertheless, prefron- and processes for dealing with human activities such tal cortex is perhaps the most distinctively ‘ cognitive ’ as language, thought, and executive control of higher

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FIGURE 5.31 Left: An activation map rendered on a three-dimensional magnetic resonance image showing regions in the medial prefrontal cortex. Right: How to find the prefrontal cortex. The entire frontal cortex is in front of the central sulcus, the vertical fold that runs from the top of the cortex down to the temporal lobe. Locate the central sulcus in this figure. The two purple gyri (hills) immediately in front of the central sulcus are called the motor and premotor cortex. The reddish-purple patch in front of that is called the supplementary motor cortex. However, the three shades of yellow in the frontal third of the whole cortex is prefrontal cortex, often considered the most ‘ cognitive ’ part of the brain. Source : Harenski and Hamann, 2006.

order processes. A second lobe that has also evolved to However, it is thought to be the site for multisensory be much larger in humans is the parietal lobe and we integration. will see what functions it performs in the next section. 3.3.3 Temporal lobe 3.3.2 Parietal lobe The temporal lobe is the region where sound is pro- As we noted earlier, the anterior region of the pari- cessed and, not surprisingly, it is also a region where etal lobe holds the somatosensory cortex. However, auditory language and speech comprehension systems the parietal lobe is not just a somatosensory region in are located. The auditory cortex is located on the upper humans, much as the frontal lobe is not just a motor banks of the temporal lobe and within the Sylvian fis- region. One important function of the parietal lobe is sure. Just posterior to the auditory cortex is Wernicke’s multiple maps of body space. What does ‘body space’ area for speech comprehension. But the temporal lobe mean exactly? Think about sitting in a chair at a table is not only a sound and language processing region. and looking down at your hands. Your eyes bring sen- The middle sections of the temporal lobe are thought sory input to your brain about where your hands are to contain conceptual representations for semantic in respect to your body, but there are other inputs tell- knowledge. More inferior and posterior temporal lobe ing you where you hands are as well (which is why areas are more finely tuned for representing visual you know where your hands are even if your eyes are objects and include the fusiform face area. closed). Your imagined hand position will be from your own perspective, or the egocentric perspective. 3.3.4 Occipital lobe Now imagine a friend sitting across the table from you and conjure up where your hands are from his or her The occipital lobe, at the very posterior region of cortex, perspective. How do you do this? It is easy to accom- is home to visual cortex. Most of visual cortex is hidden plish and regions in the parietal lobe are where this within the calcarine fissure. The visual system occupies type of processing take place ( Figure 5.32 ). a large area within the occipital lobe that extends ante- Posterior and inferior to the somatosensory region is rior to the parietal and temporal lobes. New techniques an area termed the inferior parietal lobe or IPL. The func- provide the ability to ‘ inflate ’ these cortical regions to tional significance of this region is still being elucidated. remove the folds and allow us to see the functional

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(a) (b)

FIGURE 5.32 Schematic of some of the multisensory functions of the parietal lobe. The sight and sound of the bell are combined by neurons in the parietal cortex, using a ‘ map ’ of the space surrounding the body (egocentric space). Source : Beauchamp, 2005.

10 INSULA 19 AI

AII 21

FIGURE 5.33 An actual human brain, showing the insula just 20 above and hidden behind the temporal lobe. Source : Standring, 2005. FIGURE 5.34 A cut-away view of the left hemisphere reveal- ing the insula, which is not visible from a lateral view. ‘ Insula ’ visual regions that are normally tucked into the cal- means ‘ island ’ because of this appearance when the brain is dis- carine fissure and difficult to see on a brain scan. sected. Source : Standring, 2005.

3.3.5 The insula and Sylvian fissure especially important: the insula and the Sylvian fissure. Like a large tree, the cortex has grown to cover up large When the temporal lobe is gently pulled away from the parts of itself, as we can see by inflating the cortex rest of cortex, a new hidden world appears. This region mathematically and spreading it into a flat sheet. Two is called the ‘ insula ’ , or ‘ island ’ , because it appears like of the areas that are hidden by the expanding cortex are a separate island of cortex ( Figures 5.33 and 5.34 ). The

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FIGURE 5.35 The medial temporal lobe (MTL) – the midline regions seen from the bottom. This is the ancient ‘ smell brain ’ which is now surmounted by a massive ‘ new ’ cortex in higher mammals. It is therefore difficult to see from the outside, but it still retains many essential functions, including encoding conscious events into memories (episodic memories). Source : Buckley and Gaffen, 2006.

insula is not often seen in popular accounts of the brain, plane, is home to somatosensory cortex that wraps but it involves hundreds of millions of neurons and around and under the top section of the fissure. quite a wide expanse of cortical surface. Neurological evidence suggests that it may be involved in ‘ gut feel- 3.3.6 The medial temporal lobe ings’ like the conscious sense of nausea and disgust. But the insula is so large that it probably has multiple func- The medial temporal lobe (MTL) is actually part of the tions. There does seem to be good convergent evidence temporal lobe, but its function and anatomy dif- that interoception – feelings of one’s inner organs – may fer strikingly and it is typically referred to as a sepa- be one of the major roles of the secret island of cortex. rate structure. The MTL is home to the hippocampi One researcher suggests that: ‘ In humans, . . . the and related regions that are associated with memory right anterior insula, . . . seems to provide the basis for functions ( Figure 5.35 ). There are many regions in the the subjective image of the material self as a feeling (sen- MTL, including a region called the limbic area. The tient) entity, that is, emotional awareness ’ (Craig, 2005). word ‘ limbus ’ means ‘ boundary ’ , and true to its name, The Sylvian fissure is a very large sulcus that runs there is a great deal of debate about the proper bound- in a roughly horizontal direction from the frontal lobe, aries of this region. You will occasionally see the entire between the parietal and temporal lobes, ending near complex of hippocampus, amygdala, and limbic cor- the junction of the parietal, temporal, and occipital tex being called the ‘ limbic system ’ . All these terms lobes. The anatomy of the fissure differs widely across have their uses, and it is just important to be aware of individuals and also between hemispheres. Tucked what is intended. inside the Sylvian fissure, on the upper banks of the The upper arc is called the cingulate gyrus ( ‘ cingu- superior temporal gyrus, is the supratemporal plane . lum’ means belt or sash as in ‘ cinch ’ ), which is nestled This region is called a plane because is it a somewhat between the corpus callosum and the cingulate sulcus flat bank of cortex extending from the lateral surface (Figure 5.36). The front half of this region generally into the medial regions. The supratemporal plane is lights up in brain scans during tasks that involve con- home to primary and secondary auditory cortex as flicting stimuli or responses, a very important aspect well as parts of Wernicke’s area for speech comprehen- of executive function. In the classical Stroop effect, for sion. The upper bank of the Sylvian fissure, adjacent example, there is a conflict between the color of words to the parietal lobe and opposite the supratemporal and the meaning of the same words. The front half of

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Cingulate sulcus intertwined not only with each other, but also with the satellites of the subcortex in the massively interconnected brain. Sprouting from the cells in the grayish layers of cortex are billions of axons, the output fibers from nerve 2 cells, and dendrites, which provide electrical input to 3 each cell. When white support cells, called the mye- 1 lin, wrap around those fibers, they look like a white mass to the naked eye, and are therefore called the Hippocampus white matter. The whole giant structure of the cortex Rhinal sulcus Collateral sulcus is shaped much like a superdome stadium, with two giant halves, each filled with billions of cables going Amygdala in all directions, centered on a thalamic hub nestled

FIGURE 5.36 The medial temporal lobe and cingulate gyrus in the middle of each hemisphere (Figure 5.37). The (green upper loop), seen from the midline section of the brain. The two cortical half-domes, with a thalamic traffic hub on hippocampus is colored purple and amygdala orange. They are each side, create an extraordinary biological structure. actually embedded inside of the temporal lobe. Source : Heimer and Think of the thalamus as a relay station: almost all Van Hoesen, 2006. input stops off at the thalamus on the way to cortex; almost all output also stops off at the thalamus, going out to the muscles and glands. Fibers emanating from cortical cells spread in the cingulate is somehow involved in detecting or every direction, flowing horizontally to neighboring resolving such conflicting signals. cells, hanging in great bundles on their way to dis- The lower arc of the limbic lobe is originally a part tant regions of cortex, and converging downward on of the smell brain, the rhinal cortex, and is therefore the great traffic hub, the thalamus, of each half of the called the perirhinal cortex, ( ‘ peri- ’ means ‘ around ’ and cortex. In addition, hundreds of millions of axons flow ‘ rhinal ’ means ‘ nose ’ ). Recall that we stated earlier that crosswise, from one hemisphere to the other, creating not all cortex has six layers; only the giant mammalian white axon bridges called commissures ( Figure 5.38 ). cortex does, which is why it is called ‘ neocortex ’ (new The largest crosswise fiber bridge is called the corpus cortex, because it only emerged 200 million years ago). callosum, or ‘ calloused body’ . When the brain is sliced Older regions of cortex are also found in reptiles, like straight through the midline, you can see the corpus salamanders, for example, such as the limbic cortex. callosum as a curved white bow shape. The white This region has five cortical layers and is sometimes color, again, comes from the myelin surrounding the referred to as ‘ paleocortex ’ . It is often associated with cortical axons that form the great bridge connecting emotion and memory and, in the case of the upper arc the two hemispheres. of the limbic region, with decision-making and the res- Finally , cortical sensory and motor pathways olution of competing impulses. In addition, the limbic make up the incoming and outgoing highways of the cortex flows continuously into the hippocampus and brain ( Figure 5.39 ). All of these pathways flow from amygdala, which are hidden inside the temporal lobe, the bottom of the brain. The sensory and motor path- and therefore invisible from the medial perspective. ways can be divided into two sets. One set of path- Recent research shows very close interaction between ways emerge through small holes in the cranium, these ancient regions of cortex and episodic memory, the upper skull, and are therefore called the cranial i.e. memory for conscious experiences. This is the nerves. These include the optic nerve from the back ancient reptilian brain, which is, however, still a vital of the eyes, the auditory, olfactory, and taste nerves, center of activity in humans and other mammals. as well as the feelings of touch and pain from the face and oral cavity; on the motor side, our facial expres- sions, vocal apparatus, and mouth, tongue, and so 3.4 The massive interconnectivity of the on are also controlled by cranial nerves. The second cortex and thalamus set of pathways flows into the spinal cord, and controls all our bodily functions, both voluntary – like While the lobes may be thought of as the continents movements of the torso, arms and legs – and vegeta- of the brain, their processes are nonetheless intricately tive (autonomic), like blood pressure and sweating.

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SUPEROLATERAL SURFACE OF HEMISPHERE MEDIAL SURFACE OF HEMISPHERE

Nuclei of the midline Mediodorsal nucleus

Anterior nuclear group Interthalamic adhesion Reticular nucleus

Pulvinar Ventral anterior nucleus

Centromedian nucleus Ventral lateral nucleus

Medial geniculate body

Dorsal lateral nucleus Lateral geniculate body Ventral posterolateral nucleus Lateral posterior nucleus Ventral posteromedial nucleus Intralaminar nuclei

FIGURE 5.37 Cortex and thalamus: a single unified system. A schematic drawing showing a color-coded mapping of connections from the thalamus to cortical regions. Source : Standring, 2005.

On the input side, sensory nerves from the body give speak of the ‘thalam-o-cortical ’ connections. Signal us all the information, both conscious and uncon- flow from cortex to thalamus is called corticothalamic, scious, that we receive from the body. While these and, believe it or not, neuronal traffic can even be pathways are complex in detail, the overview is cortico-thalamo-cortical. It’s a little less complicated straightforward. if you think about it as the traffic flow in a city, or It is conventional to put an ‘ -o- ’ between the names even as the World Wide Web, connecting millions of of brain regions that are connected, so that we can computers by way of major hubs and pathways.)

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Short fibers remarkably elegant shapes, like loops, horns, and egg- Superior longitudinal fasciculus Corpus callosum like ovals. Cingulum The satellite regions are especially important in cognitive neuroscience. The thalamus, often called the gateway to cortex, was described above; the two thalami reside at the very center of the brain, on both sides of the midline (so you can’t actually see them in the medial view). The thalami also connect differing cortical regions, so there are important cortico- thalamo-cortical circuits that have been shown to play Uncinate fasciculus a role in attentional processing and other higher order Inferior longitudinal fasciculus cognition functions. The thalami are nestled above the Perpendicular fasciculus brainstem and just below cortex, a perfect location to serve their role as the relay station for the brain. FIGURE 5.38 Schematic drawing of the connectivity of the brain, showing major fiber patterns. Source : Standring, 2005. The hippocampal complex (see Figure 5.17 ) is critical to remembering conscious experiences, and appears as two small sausages embedded in each temporal cortex. However, it is now known that areas adjacent to the ‘ sausage ’ of hippocampus are also needed for episodic (experiential) memory. For that reason we will talk about the entire hippocampal complex, rather than just the hippocampus alone. At the very front tip of each hippocampus is the amygdala, Latin for ‘almond ’ (see Figure 5.18). It has a small spherical nut-like shape, profoundly important for emotions like fear and anger, as well as learn- ing processes that involve those emotions. Finally, the basal ganglia (see Figure 5.19 ) are complex disk-and- loop structures just outside of each thalamus, and the cerebellum (or little brain) rides on the back of the entire upper brainstem and thalami. The basal ganglia have been implicated in action planning and unconscious cognitive operations. New evidence, however, has linked the basal ganglia to higher order cognitive functions, such as decoding the grammar, or syntax, of language. The cerebellum is seated on the rear of the lower brainstem. It is itself a very large structure. In many FIGURE 5.39 White bundles of myelinated axons run in all mammals, the cerebellum has as many neurons as the directions through the cortical domes. Source : Mario Lazar, with cortex itself, though they have shorter axons. Most kind permission. cerebellar neurons are connected locally, in small clusters. Historically, the cerebellum was thought to be mainly involved in controlling fine motor movements, like the finger movements of a typist or musician. It 3.5 The satellites of the subcortex is now also known to be necessary for cognitive functions as well. Indeed, functional imaging shows the Because the human cortex is so large, it covers impor- cerebellum to ‘ light up ’ in almost any cognitive task. tant subcortical organs, which act as satellites to the The reason for this is not completely understood. cortex, constantly interacting with it. These subcortical Finally, a number of tiny nuclei of the brainstem structures don’t look like the popular idea of a brain and basal forebrain send cell fibers widely through at all – they are giant clusters of neurons often called the upper brain. These neuromodulating nuclei are ‘ ganglia ’ or ‘ nuclei ’ . Subcortical organs often have sometimes informally called ‘ spritzers ’ , because they

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spray neurochemicals from their axon terminals so between the different senses. In fact, anatomists who that those chemicals are widely dispersed. Spritzers have studied the reticular formation have pointed to may contain only a few thousand neurons, but they its resemblance to Aristotle’s concept. Scheibel and are crucial to a healthy brain. Major disorders like Scheibel (1965) point out that ‘ Anatomical studies of Parkinson’s disease, characterized by disabling motor Kohnstamm and Quensel, which suggested pooling tremor, result from defects of such neuromodulators. of a number of afferent and efferent systems upon the They also control the daily sleep-waking cycle. reticular core, led them to propose this area as a “cen- We end this section with a description of the retic- trum receptorium 2 ” or “ sensorium commune ” – a ular formation, located at a central point in the brain common sensory pool for the neuraxis ’ . ( Figure 5.40 ). This is a particularly intriguing area of Moreover , these authors note that ‘ . . . the reticu- the brain in terms of its role in human conscious expe- lar core mediates specific delimitation of the focus of rience. The reticular formation is called ‘ reticular ’ (i.e. consciousness with concordant suppression of those network-like) because the neuronal axons in this sys- sensory inputs that have been temporarily relegated tem are usually very short, suggesting a great amount to a sensory role’ (p. 579). Along similar lines, Gastaut of interaction between adjacent neurons. Further, it (1958) describes the brainstem reticular formation as receives input from all sensory and motor systems, an area of ‘ convergence . . . where signals are concen- as well as from other major structures in the brain. trated before being redistributed in a divergent way to Through its connections with the thalamus, it can the cortex ’ . Thus, different sensory contents can sup- send information to, and receive it from, all areas of press each other, as we would indeed expect of input the cortex. to a global workspace. This suggests that different What does this suggest about the role of the reticu- specialized processors can compete for access to the lar formation in conscious experience? There is neuro- ERTAS. physiological evidence that specialist systems in the How does this ‘ blackboard ’ concept actually work brain can cooperate and compete for access to a cen- in terms of neural processes and how are messages tral integrative ‘ blackboard ’ . There is reason to think broadcast? In one possible scenario, one sensory that the extended reticular-thalamic system (ERTAS) cor- projection area of the cortex provides input to the responds to this ‘ blackboard ’ . ERTAS. If this input prevails over competing inputs, This is not a new notion; Aristotle’s ‘ common it becomes a global message which is widely distrib- sense’ was supposed to be a domain of integration uted to other areas of the brain, including the rest of the cortex. Thus, one selected input to the ERTAS is amplified and broadcast at the expense of others. Thus, in this way, the ERTAS underlies the ‘ global broadcasting ’ function of consciousness, while a selected perceptual ‘ processor ’ in the cortex supplies the particular contents of consciousness which are to be broadcast. What is the role of the ERTAS in conscious thought? It may be the case that any cortical activity must trig- T ger ERTAS ‘ support ’ in a circulating flow of information, before it can be broadcast globally and become conscious (e.g. Scheibel and Scheibel, 1965; Shevrin and Dickman, 1980). Dixon (1971) has also argued that ARAS a circulating flow of information between the reticular formation and the sensory areas of the cortex is required before sensory input becomes conscious. The possible role of the ERTAS in conscious experience is an intriguing one! It makes intuitive sense that there must be some kind of broadcast system in the brain that allows for all modes of sensory processing – FIGURE 5.40 The ascending reticular activating system sight, hearing, touch – to combine with conscious (ARAS) is found in the brainstem and thalamus, and sends projections throughout cortex. The ARAS is thought to be required for the thought and experience in order to focus on some normal conscious waking state. Source : Filley, 2002. inputs and suppress others. Clearly, the ERTAS does

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not work in isolation in these types of brain functions. compared to non-human primates. While there are sep- The thalami and regions in the prefrontal cortex are arable regions and parts of the brain, such as the two likely closely intertwined with ERTAS-like pro cesses. hemispheres and the four major lobes, nonetheless, the Nevertheless, ERTAS seems to play a key role in brain is highly interconnected with an extensive fiber human conscious experience. pathway system that connects the hemispheres, the lobes, and provides circuits to subcortical regions. Some important questions about human brain 4.0 SUMMARY structure and function remain a puzzle to us. Why do we see so much evidence of duality in the brain, with It is a difficult task indeed to attempt to describe a two hemispheres, two thalami, for example, when we dynamic and complex biological structure such as have one mind? What role do the mirror image regions the brain in a few short pages. Our aim in this chap- of the brain play in human cognition? While some ter was to provide you with the basic structures and ‘ newer ’ regions of the brain, such as the prefrontal cor- regions of the brain and their function in human cogni- tex and the inferior parietal lobe, seem to be the site tion. Some important points to remember are that the for higher order associative cognition, there are also brain has developed and changed through time and some ancient regions, such as the reticular formation, so some areas of the brain are ‘older ’ than others. The that seem to play a key role in consciousness. New and cortex or neocortex represents recent brain develop- ancient, the many regions of the brain come together to ments in the human, and the frontal and parietal lobes form a dynamic and intricate biological structure that have expanded their neural territory tremendously as holds many more puzzles for scientists to unravel.

5.0 CHAPTER REVIEW 4 Briefly describe the role of the thalami in brain processing. 5.1 Study questions 5 How are the hemispheres linked? Are there any differences in how they function? 1 Why is cortex sometimes referred to as 6 What is the reticular formation and what role ‘ neocortex ’ ? may it play in conscious thought? 2 What are the four major lobes of the brain and what are some of their key functions in human 5.2 Drawing exercises cognition? 3 Where is the medial temporal lobe located? Show the locations and names of the major brain What are its key structures? landmarks using Figure 5.41 on page 156.

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FIGURE 5.41 Building the brain figure for the Drawing Exercise.

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