The primate visual system is functionally and anatomically divided into two processing streams. One processes the “what” of the visual field, e.g. the color of an object. The other processes the “where” of the visual field, such as movement of an object and distance of landmarks from each other and from the self. How exactly these two streams meet, and where exactly their information is bound, is unclear. It has been hypothesized that this segregation continues all the way through the entorhinal cortex, in which the medial entorhinal cortex (MEC) computes spatial representations, while the lateral entorhinal cortex (LEC) is responsible for object representation. There is, however, evidence that this may not be the case.
It is well established that the primary role of the MEC is spatial information processing. The MEC receives projections from high order visual areas of the “where” stream, such as the postrhinal cortex, an area involved in the processing of visual scenes. The MEC’s spatially responsive neurons, such as grid cells, are highly selective in the information they convey. This selectively is relatively stable over time. The LEC may show spatially tuned activity in an open arena, but these representations are nowhere near as stable as those of the MEC. Further, LEC lesions do not impair water maze performance, nor on performance of tasks that rely on path integration. Lesions to the MEC do impair performance on these taks. These two areas send and receive anatomically segregated (for the most part) projections to and from CA1 and the subiculum. However, their projections to the dentate gyrus and CA3 converge on single cells.
So is it in these hippocampal regions where space and the objects in it are bound?
Actually, evidence has shown that spatial modulation of LEC activity can occur in the presence of objects. Though the upstream perirhinal cortex is free of object-dependent spatial modulation, LEC lesions produce deficits in knowing whether an object has been moved to a different location. Though performing this task was similarly impaired by MEC lesions, object novelty was only impaired by LEC lesions.
The author thus concludes that, while the MEC does not seem to represent any object-related information, the LEC represents spatial relationships in the presence of objects. It is then hypothesized that, rather than a spatial vs. object MEC-LEC dichotomy, the MEC processes space and movement in it through internally driven signals, while the LEC mainly processes external sensory input to compute object-place representations. Indeed, as mentioned MEC lesions produce deficits on path integration tasks, while LEC do not.
So where exactly are “what” and “where” bound? I don’t think the answer to this seems very simple. First, the degree and nature of crosstalk between what/where pathways, but also specifically between the LEC and MEC and perirhinal and postrhinal cortex, do not seem to be clear. Further, that LEC and MEC projections are bound in the DG and CA3 while separate in CA1 which then receives CA3 input anyway, seems to complicate this. It may seem that since the LEC is involved in both object and (to a degree) spatial computations while the upstream perirhinal cortex only represents obects, the LEC may be the first area in the brain to bind the two streams of information. However, it was noted previously, that while the perirhinal cortex specializes in objects and plays no role in scenic/spatial information, the postrhinal cortex, a specialist in visual scene processing, does in fact receive some object-related information from its perirhinal neighbor.
It’s not quite clear to me how the internal-external MEC-LEC dichotomy would hold up if the MEC receives postrhinal projections. The postrhinal cortex, it has been argued, may very well be a high-order visual processing area, selective for visual scenes.