Feedback signal from medial temporal lobe mediates visual associative mnemonic codes of inferotemporal neurons

Functional roles of the cortical backward signal in long-term memory formation were studied in monkeys performing a visual pair-association task. Before learning of the task, the anterior commissure of the monkeys was transected, disconnecting the anterior temporal cortex of each hemisphere. After training with 12 pairs of pictures, we injected a grid of ibotenic acid unilaterally into the entorhinal and perirhinal cortex that provide massive backward projections ipsilaterally to the inferotemporal cortex. According to the histological examination, the lesions covered medial and lateral banks of the rhinal sulcus completely and most of the entorhinal and perirhinal cortex. After the injection, the monkeys fixated the cue stimulus normally, relearned the preoperatively learned set (set-A) and learned a new set (set-B) of paired associates. Then single units were recorded from the same area as that for the prelesion control. We found that (i) in spite of the lesion, the sampled neurons responded strongly and selectively to both the set-A and set-B patterns, and that (ii) the paired associates elicited significantly correlated responses in the control neurons but not in the cells tested after the lesion either for set-A or set-B stimuli. We conclude that the ability of inferotemporal neurons to represent association between picture pairs was lost after disruption of backward neural signals from the limbic cortex to the inferotemporal neurons, while the ability of the neurons to respond to a particular visual stimulus was left intact.