Visual discrimination, categorical identification, and categorical rating in brief displays of curved lines: Implications for discrete encoding processes.

Abstract

Visual discrimination, categorical identification, and categorical rating measurements were made on sets of curved-line stimuli drawn from a theoretically uniform continuum with curvature parameter s. In Experiment 1, discriminability of pairs of curved lines separated by a constant distance on the .s scale was measured at successive points along the scale. Curved lines were presented four at a time in a 100-msec display, which was followed by a random-dot mask. Discrimination performance was found to vary nonsmoothly with s. In Experiment 2, a categorical identification task was performed in which subjects labeled the curved-line stimuli of Experiment 1 straight, just curved, and more than just curved. From these data, a theoretical discrimination performance was computed that was closely congruent to the discrimination performance of Experiment 1. In Experiment 3, three different categorical rating scajes with two, three, and four intervals were tested and each was shown to be less effective than the categorical identification scale for predicting discrimination performance. Mean ratings were, however, highly linear with s, suggesting that the curved-line continuum was psychometrically uniform. Experiment 4 provided further evidence for the uniformity of the curved-line continuum by measuring conventional acuity for curvature. Two rather than four curved lines were presented in each display; duration was increased to 2 sec; and the poststimulus mask was omitted. Acuity was found to vary linearly with s. It was concluded that under conditions in which attention is distributed over a number of elements in the field and in which viewing and effective visual processing time are restricted, performance in discriminating curved-line stimuli may be determined by relatively coarse, discrete visual processes.

Categorical perception of familiar objects

“…To illustrate our point here, consider morphing between a spindle and a barrel. Somewhere along this morph sequence, because of the change in curvature between concave and convex, a cylinder may be perceived (Foster, 1983). This cylinder may constitute a distinct perceptual category for the observer.…”

Characterization of discrete and continuous modes of visual pattern discrimination

“…When visual attention is distributed over several subpatterns (typically four or more) in a display and display duration is short, visual discrimination seems to be determined by coarse, discrete encoding processes. Such performance has been reported in the discrimination of simple 3-dot figures when their deviation from collinearity is varied systematically (Foster, 1979(Foster, , 1982 and in the discrimination of curved lines as curvature is varied systematically (Foster, 1983). Discrete modes of performance have also been observed in low-level perception tasks involving grouping effects with multiple arrays of geometric elements forming textures (see, for example, Olson and Attneave, 1970;Julesz, 1971Julesz, , 1980Beck, 1972;Beck andAmbler, 1972, 1973;Fox, 1978).…”

“…It may be noted, however, that the result ec(S ) > ed(S ) for all s is not inconsistent with experimental findings, suggesting that the level of discrimination performance involving continuous attributes is greater than that involving discrete attributes (Foster, 1982(Foster, , 1983. Nevertheless, it is possible to obtain a criterion to characterize discrete and continuous discrimination performances using the qualitative features of de(s) and d'c(S) resulting from definitions (5) …”

“…Discrete modes of performance have also been observed in low-level perception tasks involving grouping effects with multiple arrays of geometric elements forming textures (see, for example, Olson and Attneave, 1970;Julesz, 1971Julesz, , 1980Beck, 1972;Beck andAmbler, 1972, 1973;Fox, 1978). In contrast, when the number of subpatterns in the visual field is reduced and prolonged viewing is permitted, * On leave from Istituto di Fisiologia e Chimica Biologica, Universitfi di Torino, Italy visual discrimination appears to be determined by smooth, continuous processes (Foster, 1982(Foster, , 1983. This situation is characteristic of conventional acuity and hyperacuity performance (Ludvigh, 1953 ;Westheimer and McKee, 1977;Westheimer, 1979Westheimer, , 1981.…”

“…A lower bound for the value of As may be provided by the limit on visual acuity (Foster, 1982) and on discrimination in displays of curved lines (Foster 1983). The design of these experiments was such that conditions (C1) and (C2) were satisfied and (C3) was not seriously violated.…”