Transient cells can be neurometrically sustained: the positional accuracy of retinal signals to moving targets

The spatial accuracy inherent in retinal ganglion cell responses to moving targets was investigated by measuring trial-to-trial variability in response locus. When moving bars were used as stimuli, analysis of impulse trains showed that parafoveal cells of the magnocellular (MC) pathway provided a consistently accurate spatial signal over a range of target velocities up to ~8 deg/sec. Parvocellular (PC) pathway cells delivered less accurate signals even at low velocities, and their signals became even less accurate at higher target speeds. Human vernier performance in parafovea resembled the physiological MC-cell result, which suggests this feature of MC-cell behavior is functionally utilized. A similar result held with moving gratings; the highest signal-to-noise ratio for MC-cells occurred at low temporal frequencies. Psychophysical vernier thresholds to grating targets resembled phase variability of MC-cell responses as a function of temporal frequency. The analyses of physiological data utilized both the number of impulses a cell generates and their timing; MC-cells' responses may have low peak rates to slow moving stimuli compared to fast stimuli, but a spatially precise signal may be derived because many impulses are evoked at lower speeds. The results show that transient neurons can yield precise information about slowly moving stimuli, provided appropriate central mechanisms for extracting this information are present. Such central mechanisms would require either a long integration time or a suitable spatiotemporal filter that integrates over the ganglion array. Because accurate vernier performance can be achieved with brief presentations, the latter alternative is indicated.