Responses to stimulation of retinal regions remote from the classical receptive field were recorded from optic tract fibres in lightly anaesthetized cats. 2. X- and Y-cells gave reliably different responses to the sudden reversal of the phase of a high contrast grating that fell on the retina more than 15 deg from the centre of the receptive field. 3. The mechanism that generates these responses ('shift effect' or 'periphery effect') in Y-cells is insensitive to the spatial phase of the stimulating grating. It can resolve gratings of higher spatial frequency than can be resolved by the classical receptive field mechanisms of Y-cells but its temporal resolution is poorer. 4. Signals that contribute to peripherally evoked responses are accumulated over a region that extends to at least 35 deg from the centre of the receptive field. Although this region is not uniformly sensitive, regions in the periphery of the visual field are as effective as regions around the area centralis in eliciting the responses, and do not require coarser gratings. 5. In some Y-cells the response to peripheral stimulation was amplified by increasing (on-centre units) or decreasing (off-centre units) the steady illumination of the centre of the receptive field. This confirms Krüger & Fisher (1973), but the effect is only found in a proportion of cells. 6. The mechanism that generates peripherally evoked responses is tentatively identified with the 'rectifying subunits' postulated by Hochstein & Shapley (1976b) to account for the spatial non-linearity in the receptive fields of Y-cells. Transient (bistratified) amacrine cells are known to have many of the properties attributed to these mechanisms (Chan & Naka, 1976).
