Abstract
The categorical context in which an image is viewed influences how well it will be remembered. For example, an image of a dog presented in a sequence of other images of dogs is typically less memorable than the same image viewed in a random image sequence. To determine the neural correlates of these contextual influences on image memorability, we recorded from large populations of isolated neurons in monkey inferotemporal cortex (ITC) while one rhesus monkey performed a single-exposure visual memory task. The monkey viewed one image per trial and determined whether it was novel or repeated. Images were presented in one of two types of blocks: 1) categorical blocks in which 80% of the images came from a single category (expected images) and the other 20% came from random categories (oddballs) and 2) random blocks in which images came from several categories. Like humans, the monkey showed worse remembering for expected images as compared to oddball and random images. We also found that this pattern of behavior was at least qualitatively predicted by neural responses in ITC. To test the neural mechanisms that shaped these ITC neural responses, we explored two non-mutually exclusive hypotheses. First, that these effects result from sensory adaptation, characterized by context-dependent changes in the robustness of ITC visual representations. Second, that these effects result from changes in memory mechanisms, reflected by altered ITC memory signals. In the expected condition, we found minimal evidence for sensory adaptation (1.1% reduction in the firing rates to novel images), but strong reductions in memory signals (20% reduction in repetition suppression). These results suggest that the contextual modulations of image memorability are dominated by changes in memory mechanisms as opposed to sensory adaptation.