Localisation of Function

Localisation of Function in the Brain

Localisation of function is the principle that specific areas of the brain are responsible for specific psychological functions — that the brain is organised as a collection of specialised regions, each contributing to distinct aspects of perception, movement, language, memory, and cognition. This principle, established largely through post-mortem studies in the nineteenth century and confirmed and refined by twentieth and twenty-first century neuroimaging, is central to the biological approach in psychology.

The Cerebral Cortex and Its Lobes

The cerebral cortex — the highly folded outer layer of the brain — is divided into four lobes in each hemisphere, each associated with particular functional domains:

• Frontal lobe: executive function (planning, reasoning, impulse control), voluntary motor control (primary motor cortex), and speech production (Broca's area in the left hemisphere).
• Parietal lobe: processing of somatosensory information — touch, pain, temperature, proprioception — via the primary somatosensory cortex.
• Temporal lobe: auditory processing, language comprehension (Wernicke's area, left hemisphere), and declarative memory (medial temporal structures including the hippocampus).
• Occipital lobe: visual processing — primary visual cortex receives raw visual input; surrounding areas process colour, form, motion, and object recognition.

Motor and Somatosensory Areas

The primary motor cortex runs along the precentral gyrus (at the back of the frontal lobe) and controls voluntary movement. Different body regions are represented in different parts of the motor cortex — with larger areas devoted to body parts requiring fine motor control (hands, face, lips). This topographic mapping is represented in the motor homunculus. The adjacent primary somatosensory cortex in the parietal lobe receives and processes sensory input from the body, similarly organised topographically.

Language Areas

Broca's area (left posterior inferior frontal gyrus) is responsible for the production of speech and language. Damage to Broca's area produces Broca's aphasia — characterised by slow, effortful, telegraphic speech with preserved comprehension. Wernicke's area (left posterior superior temporal gyrus) is responsible for language comprehension. Damage produces Wernicke's aphasia — fluent but nonsensical speech with severely impaired comprehension. The two areas are connected by the arcuate fasciculus; damage to this pathway produces conduction aphasia — preserved production and comprehension but impaired repetition.

Visual Processing

The primary visual cortex (V1) in the occipital lobe receives input from the retina via the lateral geniculate nucleus of the thalamus. Visual processing then proceeds via two pathways: the dorsal stream ('where' or 'how' pathway — processes spatial location and guides action) and the ventral stream ('what' pathway — processes object identity and recognition). Damage to specific areas within the visual system produces characteristic deficits: damage to V1 produces cortical blindness in the contralateral visual field; damage to specific ventral stream areas produces prosopagnosia (inability to recognise faces) or achromatopsia (inability to perceive colour).

Evaluation of Localisation

Localisation is supported by converging evidence from lesion studies, neuroimaging (fMRI activation studies), direct cortical stimulation, and transcranial magnetic stimulation (TMS). However, it is increasingly recognised that most complex psychological functions are distributed across networks of interconnected regions rather than residing in a single 'centre'. Language, for example, involves not only Broca's and Wernicke's areas but a wide bilateral network of frontal, temporal, and parietal regions. Modern neuroscience therefore emphasises localisation within networks rather than strict one-function-one-region mapping.