Abstract
The coordinated propagation of activity across cortical layers enables simultaneous local computation and inter-areal interactions. A pattern of upward propagation from deeper to more superficial layers, which has been repeatedly demonstrated in spontaneous activity, would allow these functions to occur in parallel. But it remains unclear whether upward propagation also occurs for stimulus evoked activity, and how it relates to activity in other cortical areas. Here we used a new method to analyze relative delays between spikes obtained from simultaneous laminar recordings in primary sensory cortex (S1) of both hemispheres. The results identified systematic spike delays across cortical layers that showed a general upward propagation of activity in evoked and spontaneous activity. Systematic spike delays were also observed between hemispheres. After spikes in one S1 the delays in the other S1 were shortest at infragranular layers and increased in the upward direction. Model comparisons furthermore showed that upward propagation was better explained as a step-wise progression over cortical layers than as a traveling wave. The results are in line with the notion that upward propagation functionally integrates activity into local processing at superficial layers, while efficiently allowing for simultaneous inter-areal interactions.
Highlights
The structural organization of cerebral cortex consists of six cortical layers that can be distinguished based on cell-types, circuits and functional selectivity of activity[1,2,3,4]
Peak latencies of population spike likelihoods in iS1 occurred at longer latencies, and they showed considerable variation these suggest that spiking activity first peaked in L6 and L4, followed by activity in supragranular layers (Fig. 2B, supplementary Table S1 for average peak latencies and 95% confidence intervals)
The shortest interhemispheric delays occurred for infragranular layers and they increased with cortical layer, again consistent with a general upward propagation of spiking activity
Summary
The structural organization of cerebral cortex consists of six cortical layers that can be distinguished based on cell-types, circuits and functional selectivity of activity[1,2,3,4]. The circuitry of interlaminar structural connections is highly complex but a simplified, canonical circuit of excitatory axonal projections and activations has been proposed[2,5]. In this canonical pattern layer IV (L4) is the main recipient of thalamic sensory inputs that relays stimulus-evoked activity to supragranular layers (L2/3), which in turn relay information to infragranular layers (L5, L6). The results showed a laminar pattern of systematic delays that confirm a predominant upward propagation of activity both in stimulus-evoked and in spontaneous activity. Further analyses showed that upward propagation was better characterized as a step-wise propagation across cortical layers than as a wavelike propagation across tissue
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