Specification: Localisation of function in the brain: motor, somatosensory, visual, auditory and language centres; Broca’s and Wernicke’s areas.

Motor area

The motor area is located in the frontal lobe and is responsible for voluntary movements by sending signals to the muscles in the body. Hitzig and Fritsch (1870) first discovered that different muscles are coordinated by different areas of the motor cortex by electrically stimulating the motor area of dogs. This resulted in muscular contractions in different areas of the body depending on where the probe was inserted. The regions of the motor area are arranged in a logical order, for example, the region that controls finger movement is located next to the region that controls the hand and arm and so on.

Somatosensory area

The somatosensory area is located in the parietal lobe and receives incoming sensory information from the skin to produce sensations related to pressure, pain, temperature, etc. Different parts of the somatosensory area receive messages from different locations of the body. Robertson (1995) found that this area of the brain is highly adaptable, with Braille readers having larger areas in the somatosensory area for their fingertips compared to normal sighted participants.

Visual area

At the back of the brain, in the occipital lobe is the visual area, which receives and processes visual information. Information from the right-hand side visual field is processed in the left hemisphere, and information from the left-hand side visual field is processed in the right hemisphere. The visual area contains different parts that process different types of information including colour, shape or movement.

Auditory area

The auditory area is located in the temporal lobe and is responsible for analysing and processing acoustic information. Information from the left ear goes primarily to the right hemisphere and information from the right ear goes primarily to the left hemisphere. The auditory area contains different parts, and the primary auditory area is involved in processing simple features of sound, including volume, tempo and pitch.

Broca’s area

The Broca’s area is named after Paul Broca, who discovered this region while treating a patient named Leborgne, who was more commonly referred to as ‘Tan’. Tan could understand spoken language but was unable to produce any coherent words, and could only say ‘Tan’.

After Tan’s death, Broca conducted a post-mortem examination on Tan’s brain and discovered that he had a lesion in the left frontal lobe. This led Broca to conclude that this area was responsible for speech production. People with damage to this area experience Broca’s aphasia, which results in slow and inarticulate speech.

Extension: Due to the significance of this finding, Dronkers et al. (2007) decided to conduct an MRI scan on Tan’s brain, to try to confirm Broca’s original work. Although there was a lesion found in Broca’s area, they also found evidence to suggest that other areas may have also contributed to the failure in speech production. Therefore it is likely that the Broca’s area is not solely responsible for speech production, as other areas may also play a role.

Wernicke’s area

At a similar time, Carl Wernicke discovered another area of the brain that was involved in understanding language. Wernicke found that patients with lesions to Wernicke’s area were still able to speak, but were unable to comprehend language.

Wernicke’s area is found in the left temporal lobe, and it is thought to be involved in language processing/comprehension. People with damage to this area struggle to comprehend language, often producing sentences that are fluent, but meaningless (Wernicke’s aphasia).

Wernicke concluded that language involves a separate motor and sensory region. The motor region is located in Broca’s area, and the sensory region is located in Wernicke’s area.

Extension: However, research by Saygin et al. (2003) found that some patients displayed symptoms of Wernicke’s aphasia without any damage to this area. This suggests that language comprehension is much more complex than originally thought. Further evidence has also been found which suggests some left-handed people process language in the right hemisphere.

Evaluation of localisation of function

The claim that functions are localised to certain areas of the brain has been criticised. Lashley proposed the equipotentiality theory, which suggests that the basic motor and sensory functions are localised, but that higher mental functions are not. He claimed that intact areas of the cortex could take over responsibility for specific cognitive functions following brain injury. This therefore casts doubt on theories about the localisation of functions, suggesting that functions are not localised to just one region, as other regions can take over specific functions following brain injury.

There is a wealth of case studies on patients with damage to Broca’s and Wernicke’s areas that have demonstrated their functions. For example, Broca’s aphasia is an impaired ability to produce language; in most cases, this is caused by brain damage in Broca’s area. Wernicke’s aphasia is an impairment of language perception, demonstrating the important role played by this brain region in the comprehension of language. However, although there is evidence from case studies to support the function of the Broca’s area and Wernicke’s area, more recent research has provided contradictory evidence. Dronkers et al. (2007) conducted an MRI scan on Tan’s brain, to try to confirm Broca’s findings. Although there was a lesion found in Broca’s area, they also found evidence to suggest other areas may have contributed to the failure in speech production. These results suggest that the Broca’s area may not be the only region responsible for speech production and the deficits found in patients with Broca’s aphasia could be the result of damage to other neighbouring regions.

Furthermore, psychologists suggest that it is more important to investigate how the brain areas communicate with each other, rather than focusing on specific brain regions. Wernicke claimed that although the different areas of the brain are independent, they must interact with each other in order to function. An example to demonstrate this is a man who lost his ability to read, following damage to the connection between the visual cortex and the Wernicke’s area, which was reported by Dejerine. This suggests that interactions between different areas produce complex behaviours such as language. Therefore, damage to the connection between any two points can result in impairments that resemble damage to the localised brain region associated with that specific function. This reduces the credibility of the localisation theory. o Also, critics argue that theories of localisation are biologically reductionist in nature and try to reduce very complex human behaviours and cognitive processes to one specific brain region. Such critics suggest that a more thorough understanding of the brain is required to truly understand complex cognitive processes like language.

Finally, some psychologists argue that the idea of localisation fails to take into account individual differences. Herasty (1997) found that women have proportionally larger Broca’s and Wernicke’s areas than men, which can perhaps explain the greater ease of language use amongst women. This, however, suggests a level of beta bias in the theory: the differences between men and woman are ignored, and variations in the pattern of activation and the size of areas observed during various language activities are not considered. 

Possible exam questions

1. Describe and evaluate what research has shown about localisation of function in the brain. (16 marks)