At first glance, the left and right cerebral hemispheres of the human brain look to be almost perfectly symmetrical. However, there are in fact anatomical and functional asymmetries.
Functional asymmetry
Functional asymmetry is not simply a case of one half of the brain being solely responsible for a given function. Rather, there tends to be a predominance of a particular function in either the left (LH) or right (RH) hemispheres. Language is said to be “the most clearly lateralised higher function” (Martin, 2006, p.160) - Broca’s area and Wernicke’s area in the LH are known to be involved in language processing. Lesions to either of these areas in the LH are associated with aphasia – a deficit in the communication (Broca’s aphasia) or comprehension (Wernicke’s aphasia) of language. The LH is also associated with mathematical ability and analysis as well as complex and ipsilateral movement (Sterling & Elliot, 2008).
The RH is understood to be dominant in visuospatial ability, a function said to be more lateralized (in the RH) in men than with women. This may explain why men appear to be better at mental rotation of objects. The RH is associated with pattern recognition, perceptual tasks and creativity. Patients with RH damage are shown to have impairments in spatial orientation, mental rotation and face recognition.
Anatomical asymmetry
Looking from above, the right frontal lobe is wider than the left frontal region and extends further forwards by several millimetres whilst the left occipital lobe extends further back than the right occipital lobe. The RH is larger and heavier than the LH. The planum temporal (which encompasses Wernike’s area) is larger on the left. Two Heschl’s gyri (primary auditory cortex) are found in the RH as opposed to just one found in the LH. The Sylvian fissure (the dividing line between the frontal and temporal lobes) is larger in the LH.
The corpus callosum
As briefly outlined above, there is clear evidence of relatively greater localisation of function in either the LH or RH. However, the two hemispheres communicate with each other through the corpus callosum – the largest bundle of nerve fibres in the nervous system (Colman, 2006). This communication between the hemispheres is clearly illustrated by looking at visual input to the brain. The right side of the visual field projects to the visual cortex of the LH – the reverse in found of the left visual field. In spilt brain patients (where the corpus callosum is severed, typically to address epilepsy) the two hemispheres are in effect isolated and unable to communicate.
Split brain experiments
In a series of experiments with split brain patients in the 1960s, Roger Sperry and colleagues were able to demonstrate task specialisation in each hemisphere. Patients were asked to fixate on a spot in the middle of a screen. Words flashed on the right side of the screen (received by the LH) were correctly reported verbally by the patients. However, words flashed on the left side of the screen (received by the RH) could not be verbalised by the patient. In fact, patients reported that they hadn’t seen anything at all. Despite ‘not having seen anything’, they were able to correctly identify and pick up the corresponding item with their left hand when asked to choose from a selection.
References
Martin, G. N. (2006). Hemispheric localization and lateralization of function. In Human Neuropsychology (2nd ed.). Essex: Pearson Education Ltd
Stirling, J. & Elliot, R. (2008). Lateralisation. In Introducing Neuropsychology (2nd ed.). Hove: Psychology Press
No comments:
Post a Comment