Conversion of object identity to object-general semantic value in the primate temporal cortex

Faulty remembrance of objects past The primate brain analyzes visual input along the ventral processing stream to extract the identity of an object. The final stage of this stream, the perirhinal cortex, plays a crucial role in object recognition. Tamura et al. systematically biased the judgments of monkeys in an old-new object recognition task by using either optogenetic or electrical stimulation. The monkeys judged an encountered object as familiar when the stimulation site was in a hotspot where memory neurons were clustered. However, at the hotspot's fringe region, where neurons lost selective responses to the learned objects, electrical microstimulation led the monkeys to mistakenly judge an object as never seen before. Science, this issue p. 687 Optogenetic activation of perirhinal memory neurons in primates drives a subjective feeling of having previously seen an object. At the final stage of the ventral visual stream, perirhinal neurons encode the identity of memorized objects through learning. However, it remains elusive whether and how object percepts alone, or concomitantly a nonphysical attribute of the objects (“learned”), are decoded from perirhinal activities. By combining monkey psychophysics with optogenetic and electrical stimulations, we found a focal spot of memory neurons where both stimulations led monkeys to preferentially judge presented objects as “already seen.” In an adjacent fringe area, where neurons did not exhibit selective responses to the learned objects, electrical stimulation induced the opposite behavioral bias toward “never seen before,” whereas optogenetic stimulation still induced bias toward “already seen.” These results suggest that mnemonic judgment of objects emerges via the decoding of their nonphysical attributes encoded by perirhinal neurons.