This chapter gives an overview of the functions of two neighboring cortical areas involved in the hippocampal system: the perirhinal (PER) and postrhinal (POR) cortices. These areas are largely thought of as being important for object memory and visuospatial memory, respectively.
It has been demonstrated that the PER processes object-related sensory information. Damage to the PER results in impairments of object memory. That is, without a PER, organisms can’t tell whether or not they’ve seen a particular object before. It is thought that the PER does this by receiving object information from the ventral visual stream to piece together a high-order representation of objects. Whether this area is involved specifically in object memory, or whether it is also involved in the perception of objects is debated. PER lesions result processing deficits for complex, but not simple visual objects, and can even impair processing of complex auditory sequences, but not simple ones. These studies reduced dependency on memory as much as possible to emphasize a role for the PER in perception.
The PER is also home to three cell types that respond to objects. Familiarity cells respond most when an object is unfamiliar, but reduce firing when it has been viewed many times. Recency cells fire in response to objects that have not been viewed recently (how long ago “recently” is I’m not sure of), regardless of whether or not they have been learning and reduce firing for recently viewed objects. And finally, novelty cells, very similar to familiarity cells fire rapidly in response to unfamilar objects, though they initially fire rapidly to familiar objects, but quickly attenuate their firing. This system thus signals both novelty and temporal information about objects.
The POR is hypothesized to be involved in visuospatial processing and responds to the presentation of visual scenes. While the PER can also be modulated by audio and even odor senssation, the POR responds strictly to visual input. While the PER processes information solely about objects, the POR receives projections from the PER, thus binding object information to spatial information. Both of these areas seem to project to area CA1 of the hippocampus. Though it wasn’t mentioned in this chapter, I’m assuming that these projections may play an important role in the development of object-place cells in CA1, i.e., cells that only respond to a specific object in a specific location.
Interestingly, the authors dicuss how in studying the function of the POR, it came to be shown that contextual memory processing was segregated from spatial navigation. Lesions to both the PER and POR impaired memory of fearful contexts, while sparing performance on the Morris Water Maze. Thus, if the PER is involved in any object perception, spatial navigation can rely on path integration mechanisms that combine information about the internal organismal state with the lower-level visual information processed upstream from the PER.
Finally, I was hoping that the authors would allude to the discussed “recency” cells as having a role in the formation of time cells (Chapter 11 in this book) since I’ve always been curious about them. However, it was noted that the discoverers of time cells have a different hypothesis for how the temporal information of memories is processed. So, pretty stoked for Chapter 11 to come around.