Patterns of Local Connectivity in the Neocortex

Abstract

Dual intracellular recording of nearby pairs of pyramidal cells in slices of rat visual cortex has shown that there are significant differences 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 μm apart, synaptic connections between layer 2/3 pyramidal 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 combination with plausible estimates of the density of pyramidal cells, the total numbers of synapses on them and the number of synapses formed on their intracortical axons, the present analysis of the above data suggests that roughly 70% of the excitatory synapses on any layer 2/3 pyramid, but fewer than 1% of those on a layer 5 pyramidal neuron, are derived from neighboring pyramidal neurons in its near vicinity. Even assuming 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 for layer 2/3 pyramidal neurons. These results imply that local excitatory connections are much more likely to provide significant "intracortical amplification" of afferent signals in layer 2/3 than in layer 5 of rat visual cortex.

A quantitative analysis of the local connectivity between pyramidal neurons in layers 2/3 of the rat visual cortex

“…Thus, 70% of the synapses should be local. This is just within the order of magnitude that has been reported in other studies for local cortical connections (Gruner et al 1974;Nicoll and Blakemore 1993). In other words, there is indeed evidence that the approach of this study estimates real synaptic connectivity to a good approximation.…”

Broca's Area and the Discrimination of Frequency Transitions: A Functional MRI Study

“…In addition, the activation peaks identified in the present study are within a few millimeters to those found in neuroimaging studies for verb generation (Warburton et al, 1996), semantic decision (Müller et al, submitted), and syntactic comprehension (Caplan et al, 2000). Even though currently available neuroimaging techniques do not provide sufficient spatial resolution to determine whether shared neuronal assemblies are involved in linguistic and nonlinguistic processing, the spatial vicinity and the known density of local cortical connectivity (Nicoll & Blakemore, 1993) make it likely that such findings reflect participation of overlapping neural tissue of association cortex in multiple functional domains subserved by multiple sensorimotor modalities.…”

Interocular suppression in the visual cortex of strabismic cats

“…Given current knowledge of the microcircuitry of the primary visual cortex (see Douglas et al, 1989;Douglas and Martin, 1991;Nicoll and Blakemore, 1993) it seems likely that intracortical inhibitory connections, mainly within cortical layers 2/3, form the substrate for interocular suppression in strabismus, which is then fed down to infragranular layers. However, there is also the possibility that suppressive effects observed in area 17 are partly due to feedback inhibition from extrastriate visual cortical areas, especially when the latencies of onset of suppression are unusually long (see Fig.…”

“…Since the majority of excitatory synapses on neurons in area 17 derive from closely neighboring cells rather than from thalamic afferents (Kisvarday et al, 1986;Douglas and Martin, 1991) the responsiveness of cortical neurons is likely to depend crucially on "amplification" performed by local excitatory circuitry within the same OD column (Douglas et al, 1989;Nicoll and Blakemore, 1993). The intrinsic excitatory circuitry mediating this local amplification may indeed be unaffected by squint (Lowe1 and Singer, 1992;KGnig et al, 1993).…”