Damage

Damage to the cortex, either physical or secondary to infection, leads to significant problems. This is due to the central role the cortex has in mediating functions such as seeing, or hearing. However, the study of damage to the cortex has revealed much concerning the function each area of the brain has. The following are examples of disorders that occur due to impairment of the associated cortex.

Damage to Frontal lobe

The motor cortex and associated areas are situated at the posterior of the frontal lobe, and damage to such regions of cortex can lead to problems with control of movement. Damage to the primary motor cortex results in paralysis of voluntary movement. Strokes in this region can lead to contralateral hemiplegia, which is the inability to move the side of the body controlled by the damaged hemisphere. Damage to the premotor cortex, or Brodmann’s area 6, results in loss of learned activity.

The prefrontal cortex also has many roles in controlling the human body, and damage here can lead to personality changes, loss of judgement and loss of inhibitions. Phineas Gage was a railroad worker who survived an accident which resulted in an iron rod destroying the majority of his frontal lobe, subsequently profoundly altering his personality. Damage here therefore leads to noticeable changes of attitude and behaviour.

 Studies by Hoesen et al. (2000) have shown a high incidence of neurofibrillary tangles, associated with Alzheimers disease, in layers III and V of the orbitalfrontal complex. They suggest this pathology of the cortex may contribute to many ‘non memory-related behaviours’ associated with Alzheimers disease.

Damage to Parietal lobe

Major areas of cortex, within the parietal lobe, include regions dedicated to sensation and spatial coordination. Damage to the primary somatosensory cortex and associated cortex results in the inability to recognise objects by touch.

Damage to other areas of the parietal lobe is dependent on the side of the hemisphere. Lesions to the left parietal lobe can lead to Gertsmann’s Syndrome, which involves right-left confusion, problems with writing and mathematics. Lesions to the right parietal lobe can result in neglect of controlateral body parts, which affects washing and dressing.

Damage to Temporal lobe

The Perisylvian region surrounds the Sylvian Fissure that separates the temporal lobe from the rest of the brain. Damage here can result in aphasia. This is the inability to comprehend language or difficulty in speaking, and usually occurs due to damage to parts of the auditory cortex, and some parts of motor association cortex.

Broca’s aphasia is characterised by non-fluency of speech, and impaired language repetition, caused by damage to Broca’s area.

Lesions in Wernicke’s area cause Wernicke’s aphasia, which is characterised by fluent speech and paraphasia, i.e. words are substituted for unintended sounds and syllables. There is also impairment to comprehension of language.

Global aphasia is a result of damage to the entire cortex around the Sylvian fissure, and as a consequence, is characterised by non-fluent speech, impaired comprehension and poor repetition of language.

Damage to Occipital lobe

The visual cortex is found in the occipital lobe, therefore damage here has implications within vision. Lesions to the primary visual cortex results in blindspots in the visual field, however if the lesion is relatively large then the result can be total blindness.

Damage to the secondary visual cortex could lead to visual agnosia. This condition dissociates the meaning from the object i.e. the vision is not impaired but the individual cannot identify or understand the function of objects. Lesions to the ventral occipital lobe, however, results in cerebral achromatopsia, i.e. problems with discriminating colour.

Did You Know…

We lose, on average, 31 million neocortical neurons a year

Figure 1. Phineas Gage skull drawing. Image courtesy of Wikimedia Commons in the public domain