Neuronal firing rate responses to complete faces.

<p>The firing rate responses of an 8×8 sample of fourth (output) layer cells are shown before training (left) and after training (right) with the full set of 1600 complete faces. Each of the 8×8 sub-plots represents an individual cell in the output layer, and shows its firing-rate profile to all 1600 faces presented during testing. For each subplot, the horizontal axis denotes the position of the test face within the identity space and the vertical axis denotes the position within the expression space. The responses of the neuron are represented on a grey scale where black indicates high firing. After training, some individual neurons learned to respond to local portions of their preferred space, either identity or expression, irrespective of the position in the alternative space. This results in the appearance of vertical or horizontal ‘bars’ of activation in the subplots. If a neuron responds selectively to a local region of the identity space, then this results in a vertical bar in the subplot. In contrast, if a neuron responds to a local region within the expression space, then this results in a horizontal bar. Furthermore, due to the effects of the SOM architecture, cells with similar response profiles form close together. This causes spatial clustering of cells that respond preferentially to either facial identity or facial expression.