Abstract
Our visual memory percepts of whether we have encountered specific objects or scenes before are hypothesized to manifest as decrements in neural responses in inferotemporal cortex (IT) with stimulus repetition. To evaluate this proposal, we recorded IT neural responses as two monkeys performed a single-exposure visual memory task designed to measure the rates of forgetting with time. We found that a weighted linear read-out of IT was a better predictor of the monkeys' forgetting rates and reaction time patterns than a strict instantiation of the repetition suppression hypothesis, expressed as a total spike count scheme. Behavioral predictions could be attributed to visual memory signals that were reflected as repetition suppression and were intermingled with visual selectivity, but only when combined across the most sensitive neurons.
Highlights
The everyday act of viewing the things around us leaves us with memories of the things that we have encountered
We found that while the spike count classifier (SCC) was a better predictor of behavior than the Fisher Linear Discriminant (FLD) for smaller sized populations, the FLD was a better predictor of behavior 332 overall (Fig 7a)
Returning to the two scenarios presented at the beginning of this section, these results suggest that better FLD as compared to SCC behavioral predictions could largely be attributed to the FLD preferentially weighting the neurons with the strongest visual memory signals, as opposed to the inability of the SCC to appropriately weight reliable, mixed sign modulation. 446 Together, these results suggest that largely accurate behavioral predictions could be attributed to ~50% of inferotemporal cortex (IT) units whose memory signals were reflected as repetition suppression, and within this top-ranked subpopulation, spike counts could largely be summed
Summary
The everyday act of viewing the things around us leaves us with memories of the things that we have encountered. Where and how are visual memories stored and where and how is the percept of visual memory signaled? 30 One candidate mechanism for signaling visual memory percepts is the adaptation-like response reduction that occurs in high-level visual brain areas with stimulus repetition, known as “repetition suppression” (Fahy et al, 1993; Li et al, 1993; Miller and Desimone, 1994; Riches et al., 1991; Xiang and Brown, 1998). If it were the case that visual recognition memories were reflected by changes in the total numbers of spikes or equivalently population response vector length, this could minimize interference when superimposing visual memories and visual identity representations within the same network (Fig 1).
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