Hemispheric Lateralization of Function

Hello and welcome to the debate about

the different functions of the cerebral hemispheres.

Key Terms are:

Cerebral lateralization, hemispheric lateralization, cerebral cortex, left brain, right brain, left hemisphere, right hemisphere, functional localization, language, spatial function, attention, memory, plasticity, development, and psychopathology.


Chapter 1, Phrenology (1790-1820, interest persists today for historical reasons):

The debate about the different functions of our left and right hemispheres begin with the scientific and public interest in the possibility that different brain regions have different functions.  This thesis was popularized by Franz Gall and Johann Spurzheim beginning in the 1790s and peaking in the 1820s with the general notion of both functional localization and phrenology (the study of skull features as an indirect measure of brain features).  Functional localization is correct, different parts of the brain do have different function even while it takes a distributed network of neurons to support a function.  Phrenology is wrong, skull features do not change as a function of our skill development and development of the underlying brain.  We cannot examine a healthy person’s skull and make valid predictions about personality and mental function.

Chapter 2, Neurology (1860):

Functional localization gets formal research support in the 1860s when the French physician Paul Broca discovers that damage to the left inferior frontal lobe can cause immense disruption or loss of the ability to generate speech.  His studies showed that multiple patients with damage to “Broca’s area” (Broadmann areas 44 and 45) all had great disruption of language production, but not language comprehension.
A second language area is discovered in the 1870s by the German physician Carl Wernicke.  Wernicke’s area allows for speech comprehension.  People with damage to this part of the superior temporal gyrus (Broadmann area 22) have great disruptions to language comprehension and while they can produce fluent speech sounds, those sounds are gibberish.  Thus we have the functional localization of language production and comprehension demonstrated by people with damage to two different and specific regions of the left cerebral cortex.  Similar damage to the right hemispheres does not cause these problems, but the right hemisphere does process language information such as prosody (rhythm, sound of language) and the right hemisphere language processing seems to allow us to understand context, jokes, and implications.  Language functions are perhaps the most well known examples of both cerebral localization of function and hemispheric functional asymmetry.  Image courtesy of (https://ankiweb.net/shared/info/295487534)


To enter the debate on exactly how the brain’s hemispheres have different structures and different functions we need to learn about the following issues:

1) Neuroanatomy – What are the major parts of the brain, what is a cortex, what is gray matter and white matter, and what are the major functions that we believe can be located in specific regions.

2) Structural Asymmetry – Which parts of the hemispheres are different in location, size, and structure?

3)  Functional Asymmetry – Given the structural differences, what are all of the functional differences that are supported by valid research?  Here we must avoid or compare the popularized over-generalizations about hemispheric differences to the actual research

4)  Methods of brain study – It is not possible to interpret the experiments without knowing how different methods such as functional imaging, EEG, and neurology/neuropsychology compare to each other.  Different observational and experimental methods yield different data and conclusions.  What are the primary methods?  The first ones were neuropsychological arguments discovered by the physicians Paul Broca and Carl Wernicke – study brain damage to see what functions are disrupted.  This is just the beginning.

5)  Nuance – be aware that there is a large amount of published research on these topics and we need to read the reviews in order to piece together a contemporary understanding.  This changes with better methods, experiments, and time, so everybody will need to update their knowledge every few years.

Resources – Here are some beginning resources to help you explore.

Hemispheric Asymmetry, by pag101
Power Point Presentation