Global Spontaneous Activity and Local Structured (learned) Delay Activity in Cortex

Running head: Spontaneous and selective activity 2 SUMMARY AND CONCLUSIONS 1. We investigate the conditions under which cortical activity alone makes spontaneous activity self-reproducing and stable against uctuations of spike rates. Invoking simple assumptions about properties of integrate-and-re neurons it is shown that the stochastic background activity, of 1-5 spikes/second, cannot be stabilized when all neurons are excitatory. 2. On the other hand, spontaneous activity becomes self-stabilizing in presence of local inhibition: given reasonable values of the parameters of the network spontaneous activity reproduces itself and small uctuations in the rate are suppressed. a. If the integration time constants of excitatory and inhibitory neurons at the soma are equal, local excitatory and inhibitory inputs to a neuron must balance to provide local stablility. b. If inhibition integrates faster its synaptic inputs, spontaneous activity is stable even when local recurrent excitation predominates. 3. In a network sustaining spontaneous rates of 1-5 spikes/second, we study the eeect of learning in a local module, expressed in synaptic modiications in speciic populations of synapses. We nd: a. Initially no stimulus speciic delay activity manifests itself. Instead, following the presentation and removal of any stimulus there is, in the local module, a delay activity in which all neurons selective (responding visually) to any of the stimuli presented for learning have rates which gradually increase with the amplitude of synaptic potentiation. b. When the average LTP increases beyond a critical value, speciic local at-tractors appear abruptly against the background of the global uniform spontaneous attractor. This happens with either gradual or discrete stochastic LTP. 4. The above ndings predict that in the process of learning unfamiliar stimuli, there is a stage in which all neurons selective to any of the learned stimuli enhance their spontaneous activity relative to the rest. Then, abruptly, selective delay activity appears. Both facts could be observed in single unit recordings in delayed match to sample experiments. 5. Beyond this critical learning strength the local module has two types of collective activity. It either participates in the global spontaneous activity, or it maintains a stimulus selective elevated activity distribution following the removal of the stimulus. The particular mode of behavior depends on the stimulus: if it is unfamiliar, the activity is spontaneous; if similar to a learned stimulus, the delay activity is selective. These new attractors (delay activities) reeect the synaptic structure developed during learning. In each of 3 them a small …