Relationship between acuity for gratings and for tumbling-E letters in peripheral vision

Earlier studies have reported that grating resolution is sampling-limited i n peripheral vision but that letter acuity is generally poorer than grating acuity. These results suggest that peripheral resolution of objects with r ich Fourier spectra may be limited by some factor other than neural samplin g. To examine this suggestion we formulated and tested the hypothesis that letter acuity in the periphery is sampling-limited, just as it is for exten ded and truncated gratings. We tested this hypothesis with improved methodo logy to avoid the confounding factors of target similarity, alphabet size, individual variation, peripheral refractive error, and stimulus size. Acuit y was measured for an orientation-discrimination task (horizontal versus ve rtical) for a three-bar resolution target and for a block-E letter in which all strokes have the same length. We confirmed previous reports in the lit erature that acuity for these targets is worse than for extended sinusoidal gratings. To account for these results quantitatively, we used difference- spectrum analysis to identify those frequency components of the targets tha t might form a basis for performing the visual discrimination task. We find that discrimination performance for the three-bar targets and the block-E letters can be accounted for by a sampling-limited model, provided that the limited number of cycles that are present in the characteristic frequency of the stimulus is taken into account. Quantitative differences in acuity f or discriminating other letter pairs (e.g., right versus left letters E or characters with short central strokes) could not be attributed to undersamp ling of either the characteristic frequency or the frequency of maximum ene rgy in the difference spectrum. These results suggest additional tests of t he sampling theory of visual resolution, which are the subject of a compani on paper [J. Opt. Sec. Am. A. 16, 2334-2342 (1999)]. (C) 1999 Optical Socie ty of America [S0740-3232(99)00710-3].