PATTERNS OF LOCAL CONNECTIVITY IN THE NEOCORTEX

Dual intracellular recording of nearby pairs of pyramidal cells in sli ces of rat visual cortex has shown that there are significant differen ces in functional connectivity between the superficial and deep layers (Mason et al. 1991; Nicoll and Blakemore 1993). For pairs of cells no farther than 300 mum apart, synaptic connections between layer 2/3 py ramidal neurons were individually weaker (median peak amplitude, A, of single-fiber excitatory postsynaptic potentials, EPSPs, = 0.4 mV) but more frequent (connection probability, p = 0.087) than those between layer 5 pyramidal neurons (mean A = 0.8 mV, p < 0.015). Taken in combi nation with plausible estimates of the density of pyramidal cells, the total numbers of synapses on them and the number of synapses formed o n their intracortical axons, the present analysis of the above data su ggests that roughly 70% of the excitatory synapses on any layer 2/3 py ramid, but fewer than 1% of those on a layer 5 pyramidal neuron, are d erived from neighboring pyramidal neurons in its near vicinity. Even a ssuming very extreme values for some parameters, chosen to erode this difference, the calculated proportion of ''local synapses'' for layer 5 pyramids was always markedly lower than f or layer 2/3 pyramidal neu rons. These results imply that local excitatory connections are much m ore likely to provide significant ''intracortical amplification'' of a fferent signals in layer 2/3 than in layer 5 of rat visual cortex.