On the Anatomical Basis of Field Size ,

Neurons in layer 4C in macaque striate cortex show a diier-ential change in receptive eld size and achromatic contrast sensitivity with depth, and exhibit orientation selective responses in the upper 4C sublayer. Using a computational model we rst demonstrate that the observed change in receptive eld size and contrast sensitivity can arise from a diierential convergence of aaerents from the P and M subdivisions of the lateral geniculate nucleus onto layer 4C spiny stellate cells-if one postulates that the two anatomically identiied M1 and M2 sub-populations of the M aaerents diierentially project to diierent depth in the 4C subdivision. Number ratios and response properties of both M subpopulations are predicted and may now be tested experimentally. We then show that realistic orientation selective responses in upper 4C can emerge intracortically as a result of local lateral interactions, which are anisotropic, between spiny stellate cells and inhibitory interneurons. The model assumes that orientation bias and tuning are generated by the same cortical circuits and predicts a receptive eld dynamics with an initial non orientation speciic response.