Shedding light on learning and memory: optical interrogation of the synaptic circuitry

Abstract

In the quest for the physical substrate of learning and memory, a consensus gradually emerges that memory traces are stored in specific neuronal populations and the synaptic circuits that connect them. In this review, we discuss recent progresses in understanding the reorganization of synaptic circuits and neuronal assemblies associated with learning and memory, with an emphasis on optical techniques for in vivo interrogations. We also highlight some open questions on the missing link between synaptic modifications and neuronal coding, and how stable memory persists despite synaptic and neuronal fluctuations.

Figure 1

Learning strengthens specific neuronal connections via synaptic modifications. (a) A schematic drawing of spines on the apical dendritic arbor of a pyramidal neuron. (b) During learning some existing synapses (magenta) are strengthened and new synapses (blue) are formed, leading to stronger connections between the input axons (not shown) and the pyramidal neuron. (c) Selective weakening of the strengthened and newly formed synapses (green) erases the acquired memory.

Figure 2

Memory coding at synaptic and neuronal levels. Two different memories may recruit two partially overlapping sets of synapses on the same neuron (a) and/or two partially overlapping sets of neurons (b).