Hemispheric Specialization

Goals

Today's lecture considers the differences between the cerebral hemispheres. My goal for today's lecture is to cover the following topics:

• Historical concepts of cerebral dominance
• Asymmetries in brain anatomy
• Visual fields and dichotic listening studies
• Unilateral brain damage
• Split-brain and disconnection syndrome
• Control of facial emotional expressions

Topic Slide

Roger Sperry (1913-1994) won the Nobel Prize in 1981 for his work on split-brain patients. Born in Hartford (mother was Florence Bushnell), he won a scholarship to Oberlin and afterwards received his Ph.D. at the University of Chicago. He worked as a post-doc with Karl Lashley at Harvard and later became a professor a Cal Tech.

Michael Gazzaniga (b. 1939) received his Ph.D. at Cal Tech from Sperry, and has been the leader in research concerning hemispheric differences. Gazzaniga was instrumental in establishing the field of Cognitive Neuroscience, and was the founder of the Cognitive Neuroscience Society.

Reading

Hemispheric specialization is not covered in PN6. Please read the linked review and watch the embedded video.

• Michael S. Gazzaniga, Cerebral specialization and interhemispheric communication: Does the corpus callosum enable the human condition? Brain, 2000.
• The video embedded below is from a Scientific American documentary on the work of Michael Gazzaniga.

Historical concepts

Why do we have two cerebral hemispheres, and what, if anything, differentiates them on a functional basis. This question has been debated for centuries, and I presented several perspectives from antiquity to the the 20th century.

This quote from the Greek physician Diocles (a contemporary of Hippocrates) in the 4th century BC summarizes a rather modern view:

Accordingly, there are two brains in the head, one which gives understanding, and another provides sense perception. That is to say, the one which is lying on the right side is the one that perceives; with the left one, however, we understand.

However, a commonly held view was that the hemispheres were duplicates – for example, neither Flourens nor Gall thought the hemispheres were differentiated.

A minority view began to emerge in the 19th century that the hemispheres were differentiated. This influenced psychiatry and also the popular imagination (Dr. Jekyll and Mr. Hyde). Cesare Lombroso’s remarkably racist theory of born criminals argued that symmetry between the hemispheres reflected a more primitive brain, and thus asymmetric brains represented a higher form of development.

As we discussed in my prior lecture, Broca’s and other’s work established the left hemisphere as primary for language processing – in that left lesions caused different forms of aphasia.

In the 20th century, the left hemisphere was seen as ‘dominant’ primarily because of its language capabilities, while the right hemisphere was usually described as the ‘minor’ or ‘non-dominant’ hemisphere.

Karl Lashley thought the corpus callosum primarily provided structural support for the two hemispheres – a surprisingly backwards view for the 20th century.

Brain anatomy

The two cerebral hemispheres look similar, but careful measurement has revealed differences. I provided several examples, as follows:

• Anatomical studies beginning in the 19th century showed that part of the temporal lobe (planum temporale) is larger in the left than right hemisphere. This was presumed to reflect language specialization.
• MRI measures of asymmetry by Toga show asymmetries in anterior and posterior language areas.
• Cerebral Petalias – the brain appears as though torque has been applied. The left hemisphere is enlarged posteriorly while the right frontal also shows a leftward shift.
• Hominid endocasts are studied by physical anthropologists to look for evidence in skulls that the brain was becoming asymmetric. Cerebral Petalias are seen as far back as Australopithecus (3.5 mya), but only in hominids. This suggests an anatomical asymmetry that might have formed the substrate for language development.

Experimental studies

Investigators have taken advantage of the fact that the left visual field projects to the right occipital cortex, and the right VF projects to the left occipital cortex to study:

• RVF – LH advantage for words (right visual field – left hemisphere)
• LVF – RH advantage for faces
• LVF – RH advantage for gaze directions
  • The gaze direction that appeared in the LVF-RH biased what the subject reported.

Hemispheric differences also appear for global/local processing. Global refers to the large structure (e.g., house), while Local refers to the detail (e.g., windows, doors). Studies have suggested that the LVF-RH shows an advantage for global processing, while RVF-LH shows and advantage for local processing.

• In many studies, Global is a letter that is composed of many instances of a same or different smaller letters, while Local are those smaller letters
• Another way to think about Global/Local differences is the spatial frequency of the object. For example, a blurred face (low frequency) is Global while a high frequency or line-drawn face (high frequency) is Local.
• I showed an amazing dissociation between global and local in patients with left or right brain lesions. Patients with left lesions draw the Global while patients with right lesions draw the Local.

Investigators have also taken advantage of the fact that the right ear projects primarily (but not exclusively) to the left hemisphere, while the left ear projects primarily (but not exclusively) to the right hemisphere. These studies have revealed:

• Right ear advantage for words
• Left ear advantage for melodies

Sodium amytal test

Although not ‘damage’ per se, the amytal test can be used to reversibly silence one hemisphere. Remember what we learned about blood flow to the brain. The two carotid arteries (one per hemisphere) perfuse most of the cerebral hemispheres. Injecting sodium amytal into the left carotid artery puts most of the left hemisphere to sleep, while inject the right carotid puts most of the right hemisphere to sleep. You can test the awake hemisphere to see if it can speak. In this way, language laterality can be definitively determined. You can also test the awake hemisphere for memory. In this way you can determine if the hippocampus and medial temporal lobe structures in the awake hemisphere are functionally intact (i.e., can support memory).

Split-brain patients

Callosal sectioning in epilepsy

In some cases of severe epilepsy, the corpus callosum is surgically severed to prevent seizures from spreading from one hemisphere to the other. Nowadays, most patients undergoing this operation have only a portion of their callosum split (i.e., ‘partial’ split-brains). However, there are a number of patients who had complete callosal sections. These patients have been studied extensively by Michael Gazzaniga and his students. I showed two video clips in which Gazzaniga discussed (and demonstrated) his split-brain research. One was produced in the 1960s and can be found here. The second was more recent and involved an interview of Gazzaniga by the actor Alan Alda. That can be found here.

I showed extensive examples (from a YouTube video) of a television show in which Alan Alda interviewed Michael Gazzaniga. They observed a high-functioning complete split-brain subject on a variety of tasks. The link to this video is listed above, so students can watch it outside of class.

When the split-brain patient was shown different words to the left and right hemisphere, the patient would speak the word presented to the RVF-LH, but if asked to draw, he would draw a picture of the object projected to the LVF-RH with his left hand (controlled by the RH).

Similarly, when the split-brain patient was shown different words to the left and right hemisphere, the patient would speak the word presented to the RVF-LH, but retrieve the unseen object named in the LVF-RH from behind a screen with his left hand.

The split-brain patient can draw an associated picture to a picture shown only to the LVF-RH. For example, if a picture of a horse is shown briefly to the LVF-RH, and the RVF-LH is blank, the patient will report not seeing anything. However, if asked to draw what goes with the picture, the left hand will draw a related object (e.g., a saddle)

If shown the RVF-LH is shown an Arcimboldo picture of a face composed of meats and vegetables, the patient will report seeing vegetables or meats. If the same picture is shown to the LVF-RH, the patient will report seeing a face.

The right hemisphere can read words and draw pictures, but it cannot put sequences of words together. Language understanding is poor.

In a decision-making task in which stimuli were presented at skewed probabilities (e.g., 80/20), the right hemisphere was shown to maximize – i.e., responds with the same response on every trial to maximize winnings. The left hemisphere was observed to try to match frequencies (e.g., respond 80% of time with one hand, 20% of time with other hand). This strategy does NOT maximize winnings.

The left hand can complete a simple pattern-matching puzzle. The right hand cannot do this. When both hands try to solve the puzzle, the left hand can appear to fight with the right hand to solve the puzzle.

In an exemplar learning task, the RH responds best to the exemplars on which it is trained, and NOT to the template from which the exemplars were generated. The LH responds quickly to the template (or prototype), even though it had never seen it before.

The 'Interpreter'

Gazzaniga reports (and we observed in the video) that patients sometimes verbally make up a story about why they made a particular response if that response was determined by the non-speaking RH. That is, the LH doesn’t know why a response was made, so it confabulates a story.

Gazzaniga proposes that the left hemisphere creates a narrative for our actions. When the hemispheres are disconnected, this narrative can be shown to be uninformed by reality.

I raised the possibility that the Interpreter is the basis for anosognosia – i.e., the name given to situations where patients lack self-awareness of their deficits and sometimes confabulate when asked to explain their behavior.

Control of emotional facial expression

I briefly presented evidence that the control of emotional facial expressions showed a differential pattern of subcortical and cortical control. Voluntary expressions, such as a social/fake smile, involve cortical control, involve the corpus callosum, and appear to be initiated in the left hemisphere. Spontaneous facial expressions, such as a genuine smile in response to a seeing a loved one, appear to be generated in subcortical regions (such as the basal ganglia) and involve subcortical pathways that do not involve the corpus callosum. These ‘older’ pathways seem also to be involved in facial expressions made by non-human primates.

I showed two individuals – one subject had right hemisphere cortical damage. This subject could spontaneously smile. However, when attempting a voluntary smile, the smile appeared only on the right side of his face. That is, the left hemisphere initiated the smile on the contralateral side of the face, but the damaged right hemisphere could not complete the smile on the left side of the face.

The second individual had Parkinson’s disease, which we have discussed as involved the basal ganglia. This patient could produce a voluntary smile on command. However, he did not generate spontaneous smiles.

Videos

Prerecorded lectures for fall 2020

The video embedded below was recorded live in fall, 2019.