Cellular basis for the response to second-order motion cues in Y retinal ganglion cells

We perceive motion when presented with spatiotemporal changes in contrast ( second-order cue). This requires linear signals to be rectified and then su mmed in temporal order to compute direction. Although both operations have been attributed to cortex, rectification might occur in retina, prior to th e ganglion cell. Here we show that the Y ganglion cell does indeed respond to spatiotemporal contrast modulations of a second-order motion stimulus. R esponses in an OFF ganglion cell are caused by an EPSP/IPSP sequence evoked from within the dendritic field; in ON cells inhibition is indirect. Inhib itory effects, which are blocked by tetrodotoxin, clamp the response near r esting potential thus preventing saturation. Apparently the computation for second-order motion can be initiated by Y cells and completed by cortical cells that sum outputs of multiple Y cells in a directionally selective man ner.