Figure 1. The Feedforward Model of Orientation Selectivity in Primary Visual Cortex.

(A) The feedforward model as originally proposed by Hubel and Wiesel (1962). Four relay cells from the LGN (top right), whose receptive fields are shown to the left, synapse onto a V1 simple cell (bottom right). The simple cell derives its preferred orientation from the axis of alignment of these relay cell receptive fields and others like them that are not shown.

(B) The response of the feedforward model to drifting gratings in the preferred orientation (top) and the orthogonal orientation (bottom). LGN neurons with spatially offset receptive fields respond synchronously for the preferred orientation and asynchronously for the orthogonal orientation (middle panels). The average feedforward input increases in response to both stimuli, but only the preferred orientation response is sufficient to cross threshold (dotted lines) and evoke action potentials (right panels).

(C) The spatial relationship between the receptive fields of 23 recorded LGN relay cells (circles) and the receptive field of their postsynaptic simple cells (ovals). Each simple cell receptive field, along with its presynaptic LGN cell receptive fields, have been scaled and shifted to superimpose on an idealized receptive field. The image is adapted from Reid and Alonso (1995). Not shown is a tendency for LGN cells overlapping the center of a simple cell subregion to make stronger connections than those overlapping the periphery of the subregions.

(D) The receptive fields of two different sets of LGN relay cells that terminate in one column of V1 (circles), superimposed on the receptive field of a V1 simple cell recorded in layer 4 of the same column (square). The image is adapted from Chapman et al. (1991).