What is hemispheric specialization? - Chapter 4

How did the investigation into hemispheric specialization start?

For centuries, the effects of unilateral brain damage have revealed major functional differences between the two hemispheres. The most dramatic and most studied has been the effect of the left-hemisphere damage on language functions. 

The major lobes (frontal, parietal, occipital and temporal) appear, at last superficially, to be symmetrical, and each half of the cerebral cortex of the human brain has approximately the same size and surface area. The two hemispheres are offset, however. The right protrudes in front, and the left protrudes in back. Anatomists of the 19th century found that the Sylvian fissure - the large sulcus that defines the superior border of the temporal lobe - has a more prominent upward curl in the right hemisphere than it does in the left hemisphere, where it is flat. Geschwind measured the temporal lobe and came to the conclusion that the planum temporale, around Wernicke's area, was larger in the left hemisphere. 

The asymmetry of the planum temporale is one of the few examples in which an anatomical index is correlated with a well-defined functional asymmetry. By studying the cellular basis of hemispheric specialization, we seek to understand whether differences in neural circuits between the hemispheres might underlie functional asymmetries in tasks such as language. A promising approach is to look for specializations in cortical circuitry within homotopic areas - such as differences in the cortical microcircuitry between the two hemispheres both anterior and posterior. 

Additional structural differences have been documented in both anterior and posterior language cortex. These asymmetries include cell sizes differences between the hemispheres. Cortical areas have a basic underlying organization, documenting cortical locations involved in certain functions should be distinguished in terms of form and variety, between the neural structures common to all regions and the structures critical for a region to carry out particular cognitive functions. 

What is the anatomy of communication?

The corpus callosum and the commissures

The left and right cerebral hemispheres are connected by the largest white matter structure in the brain (the corpus callosum) and the two much smaller fiber tracts (the anterior and posterior commissures). The corpus callosum is divided on macroscopic level into the genu, the body and the splenium. The splenium is the most posterior portion of the corpus callosum. When the posterior half of the callosum is sectioned in humans, the transfer of visual, tactile and auditory sensory information is severely disrupted. By using the diffusion tensor imaging (DTI) technique researchers have traced the white fiber tracts from one hemisphere across the corpus callosum to the other hemisphere. The corpus callosum can be partitioned into vertical segments carrying homotopic connections - those that go into the corresponding region in the other hemisphere - and heterotopic connections - those that travel to a different region in the other hemisphere. 

The callosal fibers also connect heterotopic areas. These are regions with different locations in the two hemispheres. These projections generally mirror the ones found within a hemisphere. The anterior commissure is a much smaller band of fibers connecting the two hemispheres, including the two amygdalae. The posterior commisssure is even smaller and also carries some interhemispheric fibers. 

What is the function of the corpus callosum?

The corpus callosum is the primary highway between the two cerebral hemispheres, and people want to know how and what exactly is being transported between the two hemispheres. In young developing humans and animals callosal projections are diffuse and more evenly distributed across the cortical surface. Cats and monkeys, for instance, lose about 70% of their callosal axons during development. But the reason axon loss doesn't lead to cell loss in both hemispheres is that a signal cell body can send out more than one axon terminal. 

The difference in some corpus callosum sizes may also be attributed to differences in brain size. For instance, the size of the corpus callosum is bigger in men than women. 

Can you split the brain? Cortical disconnection

Myers and Sperry did some experiments with animals to assess whether the corpus callosum was crucial for unified cortical function. They first trained the cats to choose a 'plus' stimulus versus a 'circle' stimulus. Then they made the discovery that when the corpus callosum and anterior commissure were sectioned, such visual discriminations learned by one hemisphere did not transfer to the other hemisphere. 

Corpus callosotomy, or split-brain surgery, is used to treat intractable epilepsy when other forms of treatment, such as medication, fail to function. The first time this was executed, it was done by a professor in NY, named Van Wagenen. To everyone's relief, the surgery was a great success. The patient appeared and felt completely normal. The main method of testing the perceptual and cognitive functions of each hemisphere has changed little over the past 50 years. The ability to communicate solely to one hemisphere is based on the anatomy of the optic nerve. When you look at an object in front of you, information from the right side of you visual field hits the left side of the retina (both eyes), and information from the left side of your visual field hits the right side of the retina. 

There are a number of methodological issues that arise in evaluations of the performance of split-brain patients:

1. Bear in mind that these patients were not neurologically normal before their callosotomy. Therefore it is unreasonable to ask whether they provide an appropriate barometer of noral hemispheric function after the operation. 

2. It is also important to consider whether the transcortical connections were completely sectioned, or whether some fibers remained intact. 

3. Experiments must be meticulously designed to eliminate the possibility of cross-cuing, which occurs when one hemisphere initiates a behavior that the other hemisphere detects externally, giving it a cue about the answer to a test. 

When the corpus callosum is fully sectioned, little or no perceptual or cognitive interaction can occur between the two hemispheres. Surgeons therefor sometimes perform the split-brain procedure in stages, first the anterior or posterior part of the corpus callosum. The remaining fibers are sectioned in a second operation only if the seizures continue to persist. 

When the posterior half of the callosum is sectioned, transfer of visual, tactile, and auditory sensory information is severely disrupted, but the remaining intact anterior region of the callosum is still able to transfer higher-order information. 

What is the evidence of lateralized brain functions from split-brain patients?

When you want to understand the neural bases of language, it is useful to distinguish between grammatical and lexical functions. Grammar is the rule-based system that humans have for ordering words to facilitate communication. The lexicon is the dictionary of the mind, where words are associated with specific meanings. The grammar-lexicon distinction is more apparent when you are learning a new language. You often learn stock phrases that you speak as a unit rather than struggling with the grammar. 

Language and speech are rarely present in both hemispheres; they are either in one or the other. The left hemisphere normally comprehends all aspect of language, the right hemisphere does have linguistic capabilities, although they are uncommon. Both hemispheres also show a word superiority effect. This means that people are better able to identify letters in the context of a real word, than in the context of a pseudoword. In sum, there appear to be two lexicons, one in each hemisphere. 

What is visuospatial processing?

Early testing made it clear that the two hemispheres have different visuo-spatial capabilities. The right hemisphere is specialized for efficiently detecting upright faces and discriminating among similar faces. The left hemisphere is not good at distinguishing among similar faces, but is able to distinguish among dissimilar ones when it can tag the feature differences with words. Both hemispheres can generate spontaneous facial expressions, but you need your left hemisphere to produce voluntary facial expressions. When a split-brain patient gives it left hemisphere the command to smile, the lower-right side of the face responds first, while the left side responds about 180 ms later. Why does it respond at all? Most likely, the signal is rerouted through secondary ipsilateral pathways that connect to both facial nuclei, which then eventually send the signal over to the left-side facial muscles. So, the left hemisphere can trigger voluntary facial expressions, but both hemispheres can trigger involuntary expressions. 

What is the interaction of attention and perception?

After cortical disconnection, perceptual information is not shared between the two cerebral hemispheres. We noted earlier that split-brain patients cannot integrate visual information between the two visual fields. The same is true for certain types of somatosensory information presented to each hand. Thus, when holding an object in the left hand, a split-brain patient is unable to find an identical object with the right hand. Experiments showed that spatial attention can be directed with ease to either visual fields, and this raised the question of whether each separate cognitive system in the split-brain patient, if instructed to do so, could independently and simultaneously direct attention to a part of its own visual field. Some forms of attention are integrated at the subcortical level, and other forms act independently in the separated hemispheres. Split-brain patients can use either hemisphere to direct attention to positions in either the left or right visual field. 

The interpreter

A hallmark of human intelligence is that it is our ability to make causal interpretations about the world around us. For instance, when you walk outside and see a gray sky and a wet ground you probably automatically assume that it has rained. Even though you did not witness the rain and also nobody told you it had rained. A large part of the right hemisphere's impoverishment can be attributed to the finding that causal inferences and interpretations appear to be a specialized ability of the left hemisphere. The left hemisphere appears to have a specialized ability to make causal inferences and form hypotheses. This unique specialization of the left hemisphere is also called interpreter.

A typical observation occurs when the speaking left hemisphere offers some kind of rationalization to explain actions that were initiated by the right hemisphere but were spurred on by a motivation unknown to the left hemisphere. For example, when a split-brain patient was getting a command to stand up, only available to the right hemisphere, the patient stood up. When asked the patient why he was getting up the left hemisphere immediately came up with a plausible explanation: ''I felt like getting a coke''. If the corpus callosum would be intact, the patient would have responded that he stood up because that was the instruction he had received. When predicting which of the two events will occur, the left hemisphere uses a frequency-matching strategy, where-as the right hemisphere uses a maximizing strategy. The left hemisphere is also better at making causal inferences, but the right one is better at judgments of causal perception. 

What is the evidence of lateral brain functions, comparing the normal and malfunctioning brain?

Researchers have also designed experiments to test the differential processing of the two hemispheres in people with intact brains. Studies of auditory perception similarly attempt to isolate the input to one hemisphere. As in vision work - the stimuli can be presented monaurally - that is, restricted to one ear. An alternative methodology for isolating the input is the dichotic listening task. In this task two competing messages are presented simultaneously, one to each ear, and the participant tries to report both messages. But there are some limitations to this kind of studies:

• The effects are small and inconsistent, perhaps because healthy people have two functioning hemispheres connected by an intact corpus callosum that transfers information quite rapidly. 

• There is an bias in the scientific review process towards publishing papers that find significant differences over papers that report no differences. It is much more exciting to report asymmetries in the way we remember lateralized pictures of faces than to report that effects are similar.

• Interpretation is problematic. What can be inferred from an observed asymmetry in performance with lateralized stimuli? 

How do you map functional and anatomical connectivity?

Researchers can also use fMRI techniques to explore hemispheric differences in healthy individuals. On measuring the functional connectivity of brain regions within the same hemisphere and between the two hemispheres, they found that the left and right hemispheres had different patterns of functional connectivity. Neurologically healthy participants exhibit a right-ear advantage when performing the dichotic listening task. When listening to songs, however, while there is a right-ear advantage for the song's words, there is a left-ear advantage for the melodies of the songs. 

What is the evolutionary basis of hemispheric specialization?

In this chapter we have reviewed general principles of hemispheric specializations in humans. Because of the central role of language in hemispheric specialization, laterality research has focused primarily on humans. But the evolutionary pressures that underlie hemispheric specialization would also be potentially advantageous to other species. Humans show handedness, favoring either the left or right hand, dogs and cats show pawedness. But males and females show opposite preferences. Males favor their left paws and females favor their right paws.