The inferior longitudinal fasciculus: a reexamination in humans and monkeys.

The inferior longitudinal fasciculus is commonly considered to be a long association fiber bundle interconnecting the occipital and temporal lobes. Based on blunt dissections of human and monkey brains, we have found that the only long fiber bundle common to both the occipital and temporal lobes is the geniculostriate pathway (i.e., optic radiations), located within the external sagittal stratum. In addition, our autoradiographic experiments indicate that the pathway from the occipital to the temporal cortex in monkeys consists of a series of U fibers that course beneath the cortical mantle to connect adjacent regions in striate, prestriate, and inferior temporal cortex. We suggest that the occipital and temporal lobes in human beings are similarly connected by a series of U fibers and that the term inferior longitudinal fasciculus be replaced with the term occipitotemporal projection system. Different clinical syndromes attributed to lesions of the inferior longitudinal fasciculus, including visual agnosia, prosopagnosia, and impaired visual recent memory, are probably due to interruption of fibers at different points along this projection system.