Sensitivity loss in odd-symmetric mechanisms and phase anomalies in peripheral vision.

The ability to detect, discriminate and identify spatial stimuli is much poorer in the peripheral than in the central visual field. Some deficits are eliminated by scaling stimulus size. For example, grating detectibility is roughly constant across the visual field when spatial frequency and target extent are scaled appropriately. Other deficits persist despite scaling. For instance, some readily detectable patterns are more difficult to identify peripherally than in the fovea. This deficit is caused, at least partially, by a reduced ability to encode spatial phase (or relative position). To specify the properties of foveal and peripheral phase-encoding mechanisms, we measured discrimination thresholds for compound gratings at several eccentricities. Our observations are consistent with a two-channel model of phase encoding based on even- and odd-symmetric mechanisms (see Fig. 1), but the sensitivity of the odd-symmetric mechanisms decreases dramatically with eccentricity. Thus, the loss of sensitivity in one type of mechanism may underlie the reduced ability to encode spatial phase peripherally.