Distributed Hierarchical Neural Systems for Visual Memory in Human Cortex

Summary

The visual cortex in human and nonhuman primates consists of multiple areas that are hierarchically organized into processing pathways. A ventral pathway in the occipitotemporal cortex is critical for the perception of object identity, and a dorsal pathway in the occipitoparietal cortex is critical for the perception of the spatial relations among objects, the perception of movement, and the direction of movements toward objects. The ventral object vision and dorsal spatial vision pathways have projections into different parts of the prefrontal cortex. Functional brain imaging studies of visual working memory for faces and spatial locations reveal activity throughout the ventral object vision and dorsal spatial vision pathways, respectively. Different prefrontal areas are associated with working memory for faces and for locations. Face working memory is selectively associated with regions in the inferior and mid-frontal cortex. Location working memory is selectively associated with a more dorsal and posterior region in the superior frontal sulcus. These prefrontal regions demonstrate sustained activity during working memory delays demonstrating the role these areas play in maintenance of an active representation of a visual working memory. Functional brain imaging studies of long-term episodic memory also demonstrate a mnemonic role for prefrontal areas. Encoding new long-term memories for faces was associated with activity in the right hippocampus and in the left prefrontal and inferior temporal cortex. Recognition of memorized faces, on the other hand, was not associated with hippocampal activity but was associated with increased activity in the right prefrontal and parietal cortex. This research shows that widely distributed neural systems are associated with working and episodic visual memory. Mnemonic functions involve the concerted activity of the multiple regions in the posterior extrastriate and prefrontal cortices.