Biology subjects

Hacker, C. M.

Publications and source records attributed to Hacker, C. M..

3 recordsLinked to original sources

bioRxiv2025

Resolving a paradox about how vision is transformed into familiarity

How is the act of seeing an image transformed into the memory that it has been seen? To investigate, we leveraged the systematic variation with which some images are better remembered than others, image memorability, to compare neural responses in inferotemporal cortex (ITC) and the hippocampus (HC) as macaque monkeys performed a single-exposure visual familiarity task. We found evidence for a two-stage algorithmic transformation from visual representations to familiarity, including a previously undescribed computational transformation of familiarity in the medial temporal lobe. At the first stage, more memorable images elicited more vigorous ITC firing-rate responses and stronger ITC familiarity signals (reflected as repetition suppression). This led to a counterintuitive intermingling of familiarity and memorability signals in ITC, but a representation that could be read out by a linear decoder. At the next stage, the medial temporal lobe selectively extracted ITC familiarity signals to produce a more isolated familiarity representation with minimal memorability modulation, reflected downstream in HC. These results shed light on how seeing is transformed into familiarity, and they establish the existence of a previously undescribed medial temporal lobe computation.

neuroscience↗

bioRxiv2025

Sharpened visual memory representations are reflected in inferotemporal cortex

Humans and other primates can robustly report whether theyve seen specific images before, even when those images are extremely similar to ones theyve previously seen. Multiple lines of evidence suggest that pattern separation computations in the hippocampus (HC) contribute to this behavior by shaping the fidelity of visual memory. However, unclear is whether HC uniquely determines memory fidelity or whether computations in other brain areas also contribute. To investigate, we recorded neural signals from inferotemporal cortex (ITC) and HC of two rhesus monkeys as they performed a memory task in which they judged whether images were novel or exactly repeated in the presence of visually similar lure images with a range of visual similarities. We found behavioral evidence for sharpening, reflected as memory performance that was nonlinearly transformed relative to a benchmark defined by visual representations in ITC. As expected, we found that behavioral sharpening aligned with visual memory representations in HC. Surprisingly, and unaccounted for by HC pattern separation proposals, we also found neural correlates of behavioral sharpening reflected in ITC. These results, coupled with further analysis of the data, suggest that ITC contributes to shaping the fidelity of visual memory in the transformation from visual processing to memory storage and signaling. SignificanceVisual recognition memories are stored with remarkable visual fidelity, allowing humans and other primates to distinguish images they have encountered from visually similar images they have not. This fidelity has long been attributed to computations in the hippocampus that sharpen visual representations before memory storage ("pattern separation"). Unclear is how this proposal aligns with other evidence that visual memories are stored within high-level visual cortex itself, before signals reach the hippocampus. Here we demonstrate that, like the hippocampus, inferotemporal cortex also reflects sharpened visual memory representations, suggesting that visual cortex contributes to shaping the visual fidelity of visual memory.

neuroscience↗

bioRxiv2025

The representation of mood in primate anterior insular cortex

Understanding how the brain reflects and shapes mood requires resolving the disconnect between behavioral measures of mood that can only be made in humans (typically based on subjective reports of happiness) and detailed measures of brain activity only available in animals. To achieve this, we developed a mood model to predict behavioral fluctuations in human subjective happiness as individuals experienced wins and losses during a gambling task. Next, we investigated how this operationalization of mood was reflected behaviorally and in the brains of two monkeys engaged in the same gambling task. We found a remarkable alignment between human mood model signatures, the impact of estimated mood on monkey choice, and the persistent responses of units in monkey anterior insular cortex -- including a matched timescale of integration across events. In comparison, the same signatures were only weakly reflected in lateral prefrontal cortex, suggesting that insular mood representations do not trivially follow from a signal broadcast to all higher brain areas. These results are consistent with a model in which the brain transforms experiences into mood by integrating events through a recurrently connected network of excitatory and inhibitory pools of neurons. These are among the first detailed insights into the nature of putative mood representations in the primate brain.

neuroscience↗