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Localisation and lateralisation of cognitive function

An important use of fMRI in patients with epilepsy is to delineate areas of brain that are
responsible for specific functions, such as the primary sensory and motor cortex, and to
identify their anatomical relation to areas of planned resection66,67. In patients with cerebral
lesions, the localisation of cognitive activation may differ from the pattern in normal subjects.
These data may be helpful in the planning of neocortical resections of epileptic foci, in order
to minimise the risk of causing a fixed deficit.

Lateralisation of language function may also be accomplished using fMRI68. There was a
strong correlation between language lateralisation measured with the carotid amytal test, and
using fMRI with a single-word semantic decision task69 and other fMRI language studies
have generally concurred with carotid amytal testing70. The high proportion (33%) of left-
TLE patients showing bilateral or right hemispheric language-related lateralisation with fMRI
implies plasticity of language representation in patients with intractable TLE71.

fMRI results do not always accord with carotid amytal data72. A combination of language
tasks may be more reliable than a single task73,74. Artefacts and technical difficulties may
adversely affect both methods and false lateralisations may occur75. Further, identification of
the areas of brain involved in language is not the same as determining if someone can speak
when half of the brain is anaesthetised.

As well as predicting the lateralisation of language function, fMRI may localise cerebral areas
involved in language76-78. For example, in a fMRI study of healthy right-handed subjects,
tasks of reading comprehension activated the superior temporal gyri, and verbal fluency and
verb generation tasks activated the left inferior and middle frontal gyri and left insula79.
Generally, verbal fluency usually gives stronger and wider activations than verb generation80.
These data may assist in planning surgical resections in the language-dominant hemisphere.
There are, however, important caveats. Absence of activation on one language task does not
guarantee that that part of the brain is inert. Conversely, an area that is activated may have
only a peripheral and non-essential role in verbal communication.

Left temporal lobe epilepsy is associated with increased likelihood of expressive language
activation in the right frontal lobe, and atypical dominance is more likely with early onset of
epilepsy81. In patients with lesions close to Broca’s area expressive language function may
be shown in perilesional cortex82. There is considerable heterogeneity of this effect between
individuals, related to the underlying pathology83, which needs to be taken into account when
planning surgical treatment close to language areas. There has been concern that language
lateralisation with fMRI may be less reliable in the presence of structural lesions84, and
caution is required with clinical interpretation.

Up to 40% of individuals will have significant language deficits, particularly a decline in
naming ability after a left anterior temporal lobe resection. Preoperative language fMRI
predicted significant language decline, with greater activation in the left hemisphere,
particularly the temporal lobe, being associated with greater risk of post-operative
impairment85.

Following temporal lobe resection there is evidence of both intra- and interhemispheric
reorganisation of language functions86. After left temporal lobe resection, reading sentences
activated the right inferior frontal and right temporal lobes, in addition to the remaining left
temporal lobe87.