Parvalbumin (PV) basket cells are widespread local interneurons that inhibit principal neurons and each other through perisomatic boutons. They enhance network function and regulate local ensemble activities, particularly in the γ range. Organized network activity is critically important for long-term memory consolidation during a late time window 11-15 h after acquisition. Here, we discuss the role of learning-related plasticity in PV neurons for long-term memory consolidation. The plasticity can lead to enhanced (high-PV) or reduced (low-PV) expression of PV/GAD67. High-PV plasticity is induced upon definite reinforced learning in early-born PV basket cells, whereas low-PV plasticity is induced upon provisional reinforced learning in late-born PV basket cells. The plasticity is first detectable 6 h after acquisition, at the end of a time window for memory specification through experience, and is critically important 11-15 h after acquisition for enhanced network activity and longterm memory consolidation. High-and low-PV plasticity appear to regulate activity in distinct local networks of principal neurons and PV basket cells. These findings suggest how flexibility and stability in learning and memory might be implemented through parallel circuits and networks.

The structure and function of the vertebrate retina have been extensively studied across species with an isolated, ex vivo preparation. Retinal function in vivo, however, remains elusive, especially in awake animals. Here we performed single-unit extracellular recordings in the optic tract of head-fixed mice to compare the output of awake, anesthetized, and ex vivo retinas. While the visual response properties were overall similar, we found that awake retinal output had 1) faster kinetics with less variability in the response latencies across different cell types; and 2) higher firing activity, by ~20 Hz on average, for both baseline and visually evoked responses. Notably, unlike the other conditions, many awake ON cells did not increase firing in response to light increments due to high baseline activity near saturation. Instead, they encoded light intensity fluctuations primarily by decreasing firing upon light decrements. In either condition, the visual message remains the same: the more spikes, the higher light intensity. The awake response patterns, however, violate efficient coding principles, predicting that sensory systems should favor firing patterns minimizing energy consumption. Our findings suggest that the retina employs dense coding in vivo, rather than sparse efficient coding as suggested from previous ex vivo studies.

No abstract