Chapter 16 Lateralization, Language, & the Split Brain Dr. Mascolo

Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

Cerebral Lateralization of Function: Introduction

 Scientists long assumed that the brain functioned as whole -- like the lungs & kidneys -- that is, with no areas of specialized function.

 This assumption has been countered by evidence that certain areas of the brain perform specific functions -- the concept is termed brain lateralization -- meaning specialization -- because the primary example of specific brain function is the difference between left and right hemispheric abilities. Look for the Table in your text that compares and summarizes left & right hemispheric functions.

 Understanding the evidence for brain lateralization requires understanding 2 sets of brain structures: 1. The Cerebral Commissures connect the left & right hemispheres. We’re going to limit our focus to the Big Daddy of them all -- the Corpus Callosum. 2. The Optic Chiasm is where axons projecting from each nasal hemiretina decussate to the contralateral cerebral hemisphere (maybe a good time to review how contralaterality happens in the mammalian visual system -- Chapter 6).

 The evidence for brain lateralization came in the form of 19th century clinical case studies -- in particular, patients suffering aphasia (failure in language ability) and patients suffering apraxia (failure in voluntary responses to verbal instruction)

 Neurology includes lots of terminology with the prefix a (not). We studied a type of agnosia -- failure to know -- in Chapters 6 & 7. And there are lots of forms of agnosia; we focused on visual agnosia -- even more specifically, prosopagnosia.

 Like agnosia, there are lots of forms of aphasia. I’ve made a table of the 2 chief forms -- the first is a failure of language production, and the other is a failure of language comprehension (also called reception).

 These 2 forms of aphasia are not mutually exclusive -- that is, their symptoms overlap. After all, can you really say that after 4 years of college classes you can speak German just fine but when you traveled to Germany you couldn’t understand a word people said to you? Or that you could understand what people were saying but couldn’t say anything in response? Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

Nevertheless, here is a broad-brush summary of these 2 basic forms of aphasia:

Language Production-- Reception Aspect “Broca’s Aphasia”-- “Wernicke’s Aphasia”-- “Expressive Aphasia” “Receptive Aphasia” articulation halting, difficult normal

comprehension variably impaired severely impaired awareness of yes no deficits

syntactic closed-class words open-class words vulnerability (e.g., prepositions, (e.g., nouns, verbs) conjunctions)

 The bottom row -- syntactic vulnerability -- refers to the observation that each form of aphasia includes difficulties with different parts of speech.

 So a patient with Broca’s Aphasia -- struggling with prepositions & conjunctions -- would have great difficulty with this sentence:

“The man was seen carrying a gun by the police, and so he was arrested”

 Here’s where these 2 forms of aphasia overlap -- the Broca’s patient would stutter and stammer if asked to read this sentence aloud (articulation), and the patient would also struggle to understand who had the gun and who was arrested (comprehension)

 Another example -- the Broca’s patient would stutter and stammer if asked to read this phrase aloud:

“To be or not to be”

but this same patient would read this phrase much more easily:

“Two bee oar knot two bee”

 You may have noticed that these two sentences require the exact same articulation -- that is, each word in the second sentence is a homonym of the word in the same position in first sentence. The only difference is that the first phrase is filled with prepositions & conjunctions -- the Achilles’ heel of Broca’s patients, but the second phrase is filled with nouns! Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

 Meanwhile, Wernicke’s patients struggle with nouns and verbs -- the chief symptom is anomia -- yes, another a word --this refers to difficulty naming things -- i.e., word finding difficulty, e.g.:

I plugged in the -- uh -- you know -- the thing that -- cooks -- um -- bread -- you know -- so the butter melts -- for breakfast -- (you mean toaster?) Yeah -- I plugged in the toaster.

 So the assumption that the brain worked as a whole became increasingly untenable in the 1800s -- case studies suggested instead that specific areas of brain damage led to specific sets of symptoms --. Patients suffering apraxia revealed that the left hemisphere was almost always the dominant hemisphere, the right hemisphere earning the name minor hemisphere.

 Your text also describes certain tests of lateralization -- in particular, language abilities.  The culmination of these is based on modern technologies like Functional MRIs, but I can tell you that neurosurgeons still rely on good ol’ Sodium Amytal (Wada Test) -- anesthetizing one hemisphere at a time to confirm where the most valued ability of all human abilities is located -- of course I speak of language. Can you think of a case in which a patient might direct surgeons to protect a cognitive ability other than language?

 Your text notes that the lateralization of language interacts with handedness; nonetheless, it remains true that almost all right-handers (left hemisphere dominant) and the majority of left-handers (right hemisphere dominant) have their language abilities controlled by the left hemisphere.

 Your text addresses the long-held belief that males are more lateralized for language than are females. Overall, the evidence supporting this notion is mixed at best -- there are many contradictory findings, so this version of Vive la Difference may turn out to be a scientific version of an urban myth!

The Split Brain

 In the second half of the 20th century, clinical case studies supporting the concept of cerebral lateralization was extended to seizure disorder patients requiring a radical surgery -- a commissurotomy -- severing the brain’s primary commissure -- the Corpus Callosum.

 Patients who underwent this surgery were dubbed “split-brain patients” -- the surgeon’s tool is literally lowered down the longitudinal fissure between the two cerebral hemispheres and severs the thick band of axons that make up the corpus callosum.

But First: Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

Your text presents the work of Myers and Sperry (yes, the same Sperry who severed & rotated the optic nerves of frogs). They devised a visual discrimination task with 2 choices. An untrained cat will achieve a 50% success rate just by guessing (i.e., pure chance). With training, the cat will improve up that and eventually reach a100% success rate.

So now the experiment:

 Cat is trained with a patch covering one eye – shouldn’t matter, the unpatched eye sends information to both hemispheres, and anyway the Corpus Callosum shares information between hemispheres. And in fact, the cat with an eye patch shows the learning curve described above – the cat’s success rate climbs from 50% to 100%.

 Then the patch is moved to the other eye, and the cat is retrained. Again, it shouldn’t matter – the cat shouldn’t have to start over – both hemispheres learned the task, so the initial training should transfer to the retraining, and the cat’s success rate should continue at 100%-- right? Thing is, this whole training-retraining regimen was carried out on 4 groups of cats (you knew that if Sperry were involved there had to be some surgery involved): 1. Eye patch + severed Optic Chiasm & severed Corpus Callosum 2. Eye patch + severed Optic Chiasm 3. Eye patch + severed Corpus Callosum 4. Eye patch only (no surgery)

 Your text reveals the end of the story – I’ll just tell you that 3 of the groups remained at the 100% success rate – the last group had to start over.

Armed with these experimental results, the severing of the corpus callosum (but not the optic chiasm) was applied to humans with disabling epilepsy (Vogel & Bogen) -- these were the split-brain patients referred to above.

 These clinical patients were studied experimentally to determine the neuropsychological consequences of the surgery (Gazzaniga & Sperry). Your text shows the basic methodology used to isolate each hemisphere and determine its performance.

 The essential results confirmed that the 2 hemispheres now functioned independently. The Chimeric Figures Test is just one example of this -- with surprising results.

 Deprived of internal (neural) communication with each other, researchers discovered that split-brain hemispheres developed external strategies (e.g., cross cuing and the help hand phenomenon).

 The basic methodology employed by Gazzaniga and Sperry was enormously successful, but studies involving visual perception were limited because a visual stimulus could only be displayed for a tenth of a second in order to guarantee that Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

each hemisphere received only the contralateral half of the stimulus. This limitation was lifted by the development of the Z lens

Differences Between the Left & Right Hemispheres

Examples of Cerebral Lateralization of Function

 I have already referred to the Table in your text that compares and summarizes left & right hemispheric functions. -- it summarizes a huge body of research, but the author of your text cautions against oversimplified, superficial interpretation of such results.

 I would like to add to this concern -- this research is a prime example of how psychology can introduce a societal fad. In the 1990s -- my first decade of teaching -- both students and educators were quick to explain academic performance in terms of right-brain and left-brain dominance. So. it was common to hear statements like “I hate essay exams - I’m just not left-brained!” or “He’s extremely right-brained, so he’ll excel in math.”

 Research results aside, statements like these are also logically flawed. They are examples of circular reasoning -- the premise that supposedly supports the conclusion is either just a restatement of the conclusion itself or is equally unproven.

 So for example, the first statement could be expanded into this conversation:

“I hate essay exams - I’m just not left-brained!” “How do you know you are not left-brained?” “I’ve fail every essay exam I’ve ever taken!”

 This student may as well cut out the middleman and claim, “I hate essay exams because I always fail them!”

 Nevertheless, at the moment it seems safe to conclude that the right hemisphere is superior to the left hemisphere in emotional and musical abilities.

 Your text even describes an example of how the left hemisphere’s (metaphorical) “interpreter” – can be a detriment to performance on certain tests and so abdicates superiority to the right hemisphere -- which earns its victory by performing pretty much the same as rats and pigeons! Chapter 16 Lateralization, Language, & the Split Brain – Dr. Mascolo

Some additional material:

Anatomical Asymmetries of the Brain

 Your text explains that research into hemispheric asymmetries have focused on language areas. I’m focusing on your text diagram depicting 2 areas located in the Temporal Lobes: Heschl’s Gyrus (the primary auditory cortex) and the Planum Temporale (Wernicke’s Area). Though measuring these areas is complicated, the general finding is that the combined area of these two structures is larger in the left hemisphere than it is in the right hemisphere.

 In addition, the Planum temporale / Heschl’s Gyrus ratio is larger in the left hemisphere than it is in the right hemisphere.

 Complications aside, all these findings are consistent with the idea that the left hemisphere is dominant in most language abilities.

Evolutionary Perspective of Cerebral Lateralization and Language

Your text persents a lot of compllicated ideas in a very short section of this chapter. Three theoretical perspectives are introduced to explain how lateralization and language may make sense in Evolutionary theory, and only one of these perspectives focuses on language abilities!

Complications aside, the case of W.L. certainly seems to strngthen the argument that the language areas covered in this chapter – in this case Broca’s area – contribute to language abilities, not something more general.