Incoming +visual stimuli are converted into neural signals by the retina and transmitted to the primary visual cortex. Often these visual stimuli are preserved in temporary memory traces so that the information is available for use for a few seconds after the stimulus is gone. The primate visual system has evolved two distinct pathways for serving this kind of visual memory. One is a dorsal pathway for spatial memory (the "where" pathway), and the other is a ventral pathway for object memory (the "what" pathway). The dorsal pathway carries visual information specialized for spatial location from the visual areas through the parietal cortex and to the dorsal lateral prefrontal cortex; in the monkey brain, this latter region is just anterior to the frontal eye field. The ventral visual memory pathway carries parallel information, specialized for object recognition, from the visual cortex through temporal regions and on to the middle and inferior frontal cortex.

These pathways have been well characterized in the central nervous systems of nonhuman primates. Now, the implementation of functional magnetic resonance imaging (fMRI) to measure regional changes in blood oxygenation has allowed the evaluation of these visual memory pathways in the human brain.

Illustrated above are the results of a human fMRI experiment that compared the visual memory task of remembering the location of a face presented on a screen with the task of remembering the identity of that face. The prefrontal cortex showed sustained activity during the memory delay period of both tasks after the stimulus was removed from view. The amount of activation in different regions within the prefrontal cortex differed depending on the type of information held in memory. A region in the superior frontal sulcus showed more activity during spatial memory (activated regions outlined in red in the figure above), and a region in the inferior frontal cortex showed more activity during face memory (activated regions outlined in green). The region involved in spatial memory in the superior frontal sulcus is located just anterior to the frontal eye field, located in the precentral sulcus. Both the spatial memory region and the frontal eye field are located more superior and posterior in the human than are the analogous areas in the nonhuman primate brain. The area serving face memory in the inferior frontal cortex is more inferior in the human than the analogous area in the nonhuman primate.

We speculate that the development of cortical regions in humans to serve language (in the parietotemporal regions) and human executive functions (in the frontal cortex) have pushed these two pathways further apart in the human brain than in the nonhuman primate brain, more superiorly for spatial memory and more inferiorly for object memory. On the basis of their distinct localizations, these two components of visual memory are differentially vulnerable to localized lesions.

Image is courtesy of Dr. Courtney.