Local lateral connectivity of inhibitory clutch cells in layer 4 of cat visual cortex (area 17).

To characterise spatially a major component of the anatomical basis of local lateral inhibition in layer 4 of cat visual cortex (area 17), we analysed the lateral distribution of neuronal somata postsynaptic to electrophysiologically characterised GABAergic clutch (basket) cell axons (CC1 and CC2). We report two main results. First, the clutch cell axons appear to show isotropic lateral connectivity near their cell body (less than 50 microm radius), but beyond this core region they show anisotropic lateral connectivity, preferring particular angular sectors around their cell body. Second, we estimated the probability of lateral connection for each axon arbor as a function of radial distance from the parent soma. We found that this radial function has a brief rising phase, to a peak at 30-45 microm, and a longer, exponential decaying phase, with a space constant of around 50 microm. The shape of the radial connection probability function suggests that most lateral inhibitory connections of clutch cells are formed with neurons in nearest-neighbour cortical columns. Taken together, the results suggest that these individual layer-4 clutch cell axons may inhibit all (isotropic) nearest-neighbour cortical columns with a relatively high probability of connection, but outside this core region may provide a type of anisotropic lateral inhibition of cortical columns with a radially decreasing probability of connection.