Spike timing-dependent plasticity: Hebb’s postulate revisited

Sixty years agoDonaldHebbpostulated that perceptualmemory of sensory experience may be stored in the neural circuit by strengthening the synapses among a specific group of cells (‘‘cell assembly’’), through the action of reverberatory activities createdby sensory experience within the cell assembly. In this lecture, I will summarizeourworkover thepast10years inaddressing thevalidity of the Hebb’s postulate. First, I will describe our discovery of spike timing-dependent plasticity (STDP) in the developing Xenopus retinotectal system, indicating that the timing of preand postsynaptic activities plays an important role in the synaptic modification by correlated activities. I will then address the idea of synapsespecificity associatedwith activity-induced LTP and LTD. I will show that LTP/LTDmay undergo extensive spread both in vitro and in vivo from the site of induction to other unstimulated synaptic sites within the neural circuit, in a manner that suggests specific longrange cytoplasmic signaling. Finally, Iwill present our recent studies of neuronal ensemble activities in the visual system. We found that following the termination of repetitive conditioning visual stimuli, specific cell assembly in the zebrafish optic tectum undergoes regular reverberatory activities at the conditioned time interval (in the absence of sensory stimuli), suggesting circuit reverberatory activities may serve as a mechanism for short-term memory of the time interval of the visual stimuli. Inspired initially by Hebb’s postulate, these studies have revealed rather complex rules of activity-dependent synaptic and circuit modifications, the implications of which for perception andmemory remain to be elucidated.