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Home , Optic radiation, Visual cortex

Vision Basics Some Basic Anatomy Vision The Rods and Cones Ganglion Cells Center-on and Center-off Cells Parvocellular and MagnoCellular Streams

The Primary Visual Pathway The LGN: Lateral Geniculate Nucleus Layers and Topographic Mapping Visual Cells Layers and Topographic Mapping Vision Basics Some Basic Anatomy Vision

The Ventral & Dorsal Pathways The Ventral “What” Pathway Functions Dysfunction: Visual Form The Dorsal “Where/How” Pathway Functions Dysfunction: Optic Ataxia

Retinocalcarine Pathway Anatomic Elements Function Vision Basics Some Basic Anatomy Vision

Oculomotor Circuit for Control vs Eye and Head Movements Large, Small, & Minisaccades vs Visual Flutter Saccade Initiation, Fixation, & Inhibition of Return General Mechanisms of Saccade Initiation Priority Mapping

Mechanisms of Visual Stability Function Neuroanatomical System For FEF Vision Basics

Hi Kids! I’m Gato--the kitten with freakishly large --and I’m going to be your guide through the primary visual pathway! We’ll look at each major junction. Iris: The pigmented tissue that Aqueous humor: control light flowing into the eye. The clear water liquid filling the area between the cornea and lens.

Cornea: the glassy, transparent front covering of the eye providing 2/3s of Pupil: The the refraction. center hole in the iris. Let’s begin with the eye. Vitreous humor: The Sclera: The tough gelatinous, diaphanous white tissue that fluid filling the eye sack. begins where the cornea ends : The multi-layered organ covering Suspensory Ligament: the surface of the vitreous chamber from The fibrous tissue the ciliary muscles to the optic nerve. connecting the lens and the ciliary muscle. Optic Nerve: Lens: The optic element of from the eye composed of water- the soluble structural proteins. ganglion cells of And let’s not the forget about... eyes. Ciliary Muscle: The muscle supporting the lens. Fovea: The 3 by 5 mm retinal area located off-center towards the temple containing tightly packed cones. Pigment Layer 100 Rod and Cone Layer 50 Outer Limiting Membrane 20 Outer Nuclear 0 Membrane 350400 500 600 700 Outer Plexiform Layer Wavelength (nanometers)

Inner Nuclear Membrane The retina is a seven-layer Inner Plexiform structure containing four Layer types of light-sensitive Ganglion cells (rods & cones). Cell Layer Nerve Fiber Layer Pigment Layer 100 Rod and Cone Layer 50 Outer Limiting Membrane 20 Outer Nuclear 0 Membrane 350400 500 600 700 Outer Plexiform Layer Wavelength (nanometers)

Inner Nuclear Membrane The 120,000,000 rods and cones Inner Plexiform converge to 1,000,000 ganglion Layer cells (6 to 1 or 1 to 1 for cones) Ganglion Cell Layer in the retina. Nerve Fiber Layer Ganglion Cells

Visual processing starts before the ganglion cells leave the eyes. Already two types of cells exhibit a specific sort of feature extraction called center-surround antagonism. On-center cells exhibit an excitatory response to light in the center of their and an inhibitory response to light in the surrounding area of their receptive field.

On-center center-surround receptive fields - - - - + - - + + - - + - - - - Ganglion Cells

Off-center cells exhibit an inhibitory response to light in the center and an excitatory response to light in the surrounding area. Researchers have determined that the cells for a receptive field synapse onto both on-and off-center ganglion cells. This allows populations of ganglion cells to detect contrasts and rapid changes across the .

Off-center center-surround receptive fields + + + ++ - + ++ -- - - - + + + + + + + Ganglion Cells

In addition to on- and off-center cells. Ganglion cells also fall into two classes, each containing on- and off-center cells. P cells (parvocellular i.e., small) are small, have small receptive fields, exhibit selective sensitivity to specific wavelengths, and exhibit a sustained response to stimulation. P cells function to analyze form and . They exhibit medium-conduction-velocity when they leave the retina to form the . Ganglion Cells

M cells (Magnocellular, i.e., large) exhibit a burst of firing when light hits their receptive fields, but show a decrease in firing as time elapses. M cells are larger, have larger receptive fields, and exhibit the above- mentioned transient response to light in their receptive fields. M cells are fast-conducting. M cells function to analyze the gross features and motions of the objects in their receptive fields. M and P cells form separate and parallel processing pathways.

Optic Tracts: Primary : The post-chasm The last and largest stop lateralized for feature extraction. optic nerve. Lateral Geniculate Bodies: The second major structure Optic Nerves: in the pathway involved in Retinal ganglion feature extraction. cell axons and support cells combine to form the optic nerve. Optic chasm: ganglion cells crossover so that cells with Once the axons of receptive fields in the left the ganglion cells half of each retina go to the leave the eyes, left hemisphere while the the main cells with receptive fields in processing stream the right half of each retina looks as follows. go to the right hemisphere. Vision Basics Our Friend The Within the Thalamus there one finds several visually important structures. These include; The Pulvinar Nucleus, The Medialdorsal Nucleus, and the next stop on the primary visual pathway--The Lateral Geniculate Nucleus Vision Basics Our Friend The Thalamus

Interthalamic Adhesion

Here’s a different angle for you. Lateral Nuclei Pulvinar Medial Nuclei Lateral Geniculate Anterior Nuclei Medial Geniculate Body Nucleus Ganglion cells synapse in the Receptive LGN. Cells in the LGN have a Field Locations center-surround configuration AB and can be classified as P or M CD cells. LGN cells also form a topographic retinal map (fovea disproportionally represented), A B C and fall into six distinct layers, D each having approximately the same receptive field location on the retina.

Layer 6 Receptive Each layer receives input Field Locations from one eye. 2,3, and 5 AB receive input from the CD ipsilateral (same side) eye, 1,4, and 6 receive input from the contralateral (opposite A B side) eye. The M pathway C goes to layers 1&2, while the D P pathway goes to 3,4,5,& 6.

Layer 6 Visual Cortex Cortical Cells

- + - - Simple cortical cells have + - excitatory and inhibitory fields + - arranged side by side. The + ++ - - effect of this arrangement is + to make them show a + + - - preference for stationary bars + of a particular orientation. + - - + - - - While simple cortical cells respond weakly to spots, and strongly to stationary bars. Complex cortical cells respond only to bars that move in a particular direction and at a particular orientation. Hypercomplex Cortical Cells (or end-stopped cells): Hypercomplex cortical cells respond only to moving lines of a particular length or to moving corners or angles. Judging from the glazed- over looks in people’s eyes, it must be movie time!!! Orientation Columns I II III Cortical IV layers V VI

Orientation Columns: The different cell types of the visual cortex are organized into orientation columns. Cells within an share the same rough receptive field position and orientation. Each column has cells of each of the types discussed. Ocular I dominance columns II III Cortical IV layers V VI

Orientation columns are grouped together into ocular dominance columns. Each contains cells for all orientations that show preferential response to the same receptive field. Hypercolumn I II III Cortical IV layers V VI

Ocular dominance columns are then organized into hypercolumns consisting of an ocular dominance column for each eye corresponding to the same receptive field on the retina and containing cells from each orientation. Cortical I Pegs II III Cortical IV layers V VI

Cortical Pegs are regions where one finds cells specific for . Pegs, also known as blobs, do not appear in layer IV. Don’t be fooled by Wallis’ crampy diagrams, the actual structure of the hypercolumns in the cortex are more like a pin-wheel of cells with concentric receptive feilds as seen in this imaged spatial frequency map. Figure 5. Map of iso-orientation preference contours (black lines), ocular dominance boundaries (white lines), and spatial frequency preferences of cells in the cat V1 (redrawn from Issa et al. 2000, with permission). Red regions correspond to low spatial-frequency preference, violet to high. Bressloff, P. C., and Cowan, J. D. (2003) Spherical model of orientation and spatial frequency tuning in a cortical hypercolumn. Phil. Trans. Roy. Soc. B , 358:1643-1667. Other Topographic Maps

Sensory Cortex Dorsal & Ventral Pathways

The visual cortex projects along two pathways. One pathway (nicknamed the “What” system) runs ventrally to region TE in the inferior . The what pathway processes color, fine texture, shape, & fine depth information. The what pathway What contains projections primarily from (ventral) pathway to parvocellular layers of the LGN, & Temporal has many cells sensitive to particular Cortex types of shapes and colors and large numbers of cells with receptors in the fovea (functional studies). Dorsal & Ventral Pathways

Mishkin et al. (1983) published a classic study identifying the dorsal and ventral pathways and determining their respective functions. After lesioning cells (chemically or surgically) in the temporal lobe area, monkeys become severely impaired in their ability to What discriminate among shapes and fine (ventral) patterns, but are relatively unimpaired pathway to Temporal in their ability to discriminate locations. Cortex For instance, lesioned monkeys have difficulty with object recognition tasks, but not landmark discrimination tasks.

Mishkin, M., Ungerleider, L., & Macko, K. “Object vision and spatial vision: Two cortical pathways.” in Trends in Neurosciences, Vol 6(10), Oct 1983. pp. 414-417. Dorsal & Ventral Pathways

Where/how (dorsal) pathway to the Parietal Cortex The other pathway (nicknamed the “where” system) runs dorsally to area PG in the . The where pathway contains projections primarily from magnocellular layers of the LGN. Cells in this pathway are What not particularly sensitive to color or (ventral) shape and rarely include the fovea in pathway to Temporal their receptive fields. They are Cortex sensitive to direction of motion and to an object’s location. Dorsal & Ventral Pathways

Where/how (dorsal) pathway to the Parietal Cortex After lesioning cells (chemically or surgically) in this area, monkeys become severely impaired in their ability to discriminate locations, but retain a relatively unimpaired ability to discriminate patterns and colors. For What instance, they perform relatively normally (ventral) on object recognition tasks, but perform pathway to poorly on landmark discrimination tasks. Temporal Cortex In both pathways, similar results are observable when strokes cause corresponding damage in humans. Parietal Lobe Magnocellular Magnocellular Ganglion Cells LGN Cells V1 in the Occipital Cortex Parvocellular Parvocellular Ganglion Cells LGN Cells Temporal We can trace the Magno and Lobe Parvocellular pathways through the primary visual pathway and into the ventral and dorsal pathways as follows. follows Vision Basics Retinotectal Pathway (SC) Pulvinar Nucleus (P) Associative Cortices

Cells of the pulvinar nucleus exhibit similar complex visual responses as

Temporal those in the cortex. Researchers Nasal believe that the pulvinar nucleus Temporal Optic plays a significant role in creating a stable visual image across bodily movements. The pulvinar nucleus Pulvinar Nucleus Lateral Geniculate recieves afferents from both the Nucleus Superior optic nerve and the LGN and Colliculus projects to the motion sensitive visual areas in the dorsal stream.

Primary Visual Cortex http://thebrain.mcgill.ca/flash/a/a_02/a_02_cr/a_02_cr_vis/a_02_cr_vis.html Vision Basics Motion Saccades vs Eye and Head Movements Large, Small, & Minisaccades vs Visual Flutter

Saccade Initiation, Fixation, & Inhibition of Return

http://4colorvision.com/reading/saccades.htm Oculomotor Circuit for Saccade Control

Supplementary Lateral Interparietal Eye Feilds (SEF) Area (LIP) Frontal Eye The cortical-basal- Fields (FEF) ganglia-thalamic-cortical skeletomotor circuit is one of five known cortical-basal-ganglia- cortical circuits. These circuits all work to modulate cortical activity through inhibition and Superior Colliculus encompass skeletomotor, oculomotor, cognitive, Pars Reticulata Mesencephalic and Pontine and emotional functions. Reticular Formations

Diagram of the oculomotor circuit from modified from Goldberg 2000 p.793 Oculomotor Circuit for Saccade Control

Supplementary Lateral Interparietal Eye Feilds (SEF) Area (LIP) (FEF) Input to both the FEF and the LIP appears to come from the superior temporal (STS) and the dorsal and Caudate ventral prestriate areas. Nucleus Superior Colliculus

Substantia Nigra Pars Reticulata Mesencephalic and Pontine Reticular Formations Oculomotor Circuit for Saccade Control The FEF also has an Supplementary Lateral Interparietal indirect pathway to the Eye Feilds (SEF) Area (LIP) superior colliculus Frontal Eye Fields (FEF) through the caudate nucleus and the substantia nigra pars reticulata. The superior colliculus has input from the optic nerve (approximately 10% of Caudate the fibers). The superior Nucleus Superior colliculus projects to the Colliculus mesencephalic and Substantia Nigra pontine reticular Pars Reticulata Mesencephalic and Pontine formations. Reticular Formations Oculomotor Circuit for Saccade Control

Supplementary Lateral Interparietal Eye Feilds (SEF) Area (LIP) Frontal Eye Fields (FEF) The mesencephalic reticular formation controls vertical saccadic action, while the pontine reticular formation controls horizontal eye Caudate movements. Nucleus Superior Colliculus

Substantia Nigra Pars Reticulata Mesencephalic and Pontine Reticular Formations Oculomotor Circuit for Saccade Control

Supplementary Lateral Interparietal Area (LIP) Eye Feilds (SEF) The Frontal Eye Fields (FEF) component of the oculomotor circuit works as an inhibitory gate on the superior colliculus. Excitatory from the FEF to the caudate nucleus Caudate causes it to inhibit Nucleus Superior substantia nigra pars Colliculus reticulata which in turn Substantia Nigra disinhibits the superior Pars Reticulata Mesencephalic and Pontine colliculus. Reticular Formations Oculomotor Circuit for Saccade Control

Supplementary Lateral Interparietal Eye Feilds (SEF) Area (LIP) Frontal Eye Fields (FEF) The superior colliculus has both fixation and saccade cells within a retiotopic motor map that vie with one another to send signals to mesencephalic and Caudate Nucleus pontine reticular Superior Colliculus formations.

Substantia Nigra Pars Reticulata Mesencephalic and Pontine Reticular Formations Vision Basics Motion Saccade Initiation, Fixation, & Inhibition of Return Priority Mapping (i) in this network encode visual information in a featureless manner (ii) lesions involving these structures produce deficits in attentional selection. (iii) electrically stimulating these regions facilitates the selection of objects with attention. (iv) these structures receive information from the ventral visual pathway which provides the input necessary for summing the relative salience of an object. (Fecteau & Munoz 2006, p.383) Vision Basics Motion

Visual fMRI responses in human superior colliculus show a temporal-nasal asymmetry that is absent in lateral geniculate and visual cortex. Sylvester, Richard; Josephs, Oliver; Driver, Jon; Journal of Neurophysiology, Vol 97(2), Feb 2007. pp. 1495-1502. Presents evidence that the phylogenetically older retinotectal pathway contributes to reflex orienting of visual attention in normal humans. Vision Basics Mechanisms of Visual Stability Function Neuroanatomical System For FEF

Pathway for collary discharge to modify visual representation. (a) Lateral view of the monkey showing the connection from the SC in the midbrain to the MDN (Mediodorsal) of the thalamus and then to the FEF that is believed to convey corollary discharge from the SC to the FEF. Munoz 2006, Sommer, M.A. & Wurtz 2007