Experiments on the effect of synaptic disruption on polychronous group formation: detailed methods and results

Izhikevich (2006a) claims that groups of strongly connected neurons known as polychronous neural groups (PNGs) could provide the basis for memory in the brain. Polychronous groups exist as spatio-temporal patterns of connection lengths and weights; when activated by a matching input pattern, they are able to produce a causal chain of firing events that is not synchronous but is nevertheless precisely timed and consistently reproducible. Izhikevich et al. (2004) have previously shown that synaptic disruption produces a dramatic decrease in PNG counts, leading them to the conclusion that PNG formation depends on activity-dependent changes in synaptic plasticity. However, despite the disruption some polychronous groups remain in the network, suggesting that polychronous groups can sometimes be formed by chance arrangements of synaptic weights in the network. In the following report we reproduce and extend the work of Izhikevich et al. (2004) by delving further into the nature of PNG formation: firstly, we examine the effects of synaptic disruption on the groups that remain; we then examine the considerable inter-network variation in PNG counts over the time-course of network maturation and categorize the resulting profiles into discrete classes of behavior. The first experiment found a highly significant decrease in PNG count following synaptic disruption (paired t(19) = 9.0; p < 0.001), reproducing the previous report (Izhikevich et al., 2004). However, we also found a highly significant decrease in both size (paired t(19) = 10.7; p < 0.001) and temporal length (paired t(19) = 10.8; p < 0.001) in the remaining polychronous groups. A further experiment employing sampling at multiple time-points found two previously unreported phenomena: firstly, there is a consistent small peak in the PNG counts immediately following initialization; secondly, comparison of the temporal profiles of multiple networks over the course of maturation shows two broad classes of network behavior: either cyclic, or an initial burst of productivity followed by a slow decline. These results are discussed in the context of a model of PNG formation involving an activitydependent interaction between supported and adapted groups which is at the heart of PNG formation.