Extreme capsule

The extreme capsule is a very thin region of white matter located between the claustrum medially and the insular cortex laterally. Its position can be understood from the medial-to-lateral arrangement:

Putamen → external capsule → claustrum → extreme capsule → insular cortex.

Association fibres

The extreme capsule contains short and long association fibres. The short fibres connect the insular cortex with the adjacent opercular cortex, while the long association pathways include the inferior fronto-occipital fasciculus and uncinate fasciculus, which pass through and converge within this region. Shekari et al. (2021) therefore suggests that, in humans, the extreme capsule is better regarded as a bottleneck region through which several association fibre pathways pass, rather than simply as a single, discrete fibre tract.

Functional and clinical correlates

The fibres passing through the extreme capsule (i.e. the inferior fronto-occipital fasciculus and the uncinate fasciculus) form part of the ventral language pathway and contribute particularly to semantic processing and language comprehension. Consequently, damage involving these ventral-pathway fibres in the extreme capsule region may be associated with impaired language comprehension and semantic processing.