Lightness Similarity

Evidence from my own laboratory relevant to theoretical accounts of the development of object perception has focused on when and how humans become capable of grouping parts of a stimulus together to form a coherent whole. A study by Quinn, Burke, and Rush (1993) hints at answers to both questions. The stimuli are shown in the left half of Figure 5.1. As reported by Wertheimer (1923/1958), adult participants group together the elements of such stimuli on the basis of lightness similarity and represent the top pattern shown in Panel (a) as a set of columns, and the bottom pattern depicted in Panel (b) as a set of rows. Quinn et al. reasoned that if young infants, 3 months of age, organize the patterns shown in Panels (a) and (b) of Figure 5.1 into columns and rows based on lightness similarity, then they should respond differentially to the vertical- and horizontal-grating test stimuli shown in the right half of the figure. On the assumption that there is no spontaneous preference between the vertical and horizontal stripes, infants familiarized with columns should generalize to verticals and prefer horizontals, whereas infants familiarized with rows should generalize to horizontals and prefer verticals. This is precisely the result that was obtained—a finding indicating that young infants were able to use lightness similarity to represent the row- and column-like organization of the individual light and dark squares.

One could take issue with the grouping via lightness similarity interpretation proposed by Quinn et al. (1993) by claiming that the apparent perceptual grouping is actually a by-product of immature

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Figure 5.1 Familiarization and test stimuli used to test adherence to the lightness similarity Gestalt organizational principle in Quinn, Burke, and Rush (1993). The rationale is that if infants can organize familiar stimulus (a) into columns, then the vertical-column test stimulus should be perceived as familiar, and the horizontal-row test stimulus should be preferred. Similarly, if infants can organize the familiar stimulus (b) into rows, then the horizontal-row test stimulus should be perceived as familiar, and the vertical-column test stimulus should be preferred.

Figure 5.1 Familiarization and test stimuli used to test adherence to the lightness similarity Gestalt organizational principle in Quinn, Burke, and Rush (1993). The rationale is that if infants can organize familiar stimulus (a) into columns, then the vertical-column test stimulus should be perceived as familiar, and the horizontal-row test stimulus should be preferred. Similarly, if infants can organize the familiar stimulus (b) into rows, then the horizontal-row test stimulus should be perceived as familiar, and the vertical-column test stimulus should be preferred.

peripheral filtering by the infant's visual system (Banks & Ginsburg, 1985; Banks & Salapatek, 1981). That is, one could argue that immature resolution acuity and contrast sensitivity rendered the dark squares as indistinguishable from other dark squares and the light squares as indistinguishable from other light squares. The representation of familiar stimuli (a) and (b) resulting from such "low-pass" filtering would best be described as "two dark vertical (or horizontal) bars on a light background." Performance of the 3-month-olds could thus be explained by simple generalization from representations of this nature to the vertical (or horizontal bars) used as test stimuli, without invoking any grouping mechanism (cf. Ginsburg, 1986).

Quinn et al. (1993) attempted to choose between the Gestalt grouping and low-pass filtering explanations of the findings with both computer simulation and experimental evidence. First, the images of the familiar stimuli shown in Figure 5.1, and additional stimuli in which either the dark or light elements were changed from square to diamond, were fed into a low-pass spatial frequency filter that removed spatial frequencies above 4 cycles/degree, the cutoff spatial frequency for 3-month-olds as estimated by preferential looking techniques (e.g., Atkinson, Braddick, & Moar, 1977; Banks & Salapatek, 1978). Figure 5.2 shows the resulting patterns. It can be seen that the stimuli have lost some sharpness, but the individual elements comprising the patterns are clearly discernable. Figure 5.2 thus suggests that 3-month-olds have enough resolution acuity and contrast sensitivity to perceive the individual elements of the patterns, and that the initial preference results cannot be explained on the basis of peripheral immaturities in the young infant's visual system.

This suggestion was subsequently confirmed in a discrimination experiment utilizing the familiarization/novelty-preference methodology. Infants were familiarized with stimuli like those shown in the left half of Figure 5.1 and then presented with novel stimuli in which only the dark or light elements were changed from square to diamond. Preference for the novel stimuli was above chance in both cases, indicating that infants were able to process shape information for both the light and dark elements. The combined findings from the simulation and discrimination experiment thus provide strong evidence in favor of a Gestalt interpretation of the original preference results: infants had perceived the individual elements of the patterns and grouped these elements into alternating light and dark columns or rows on the basis of lightness similarity.

Figure 5.2 Patterns with spatial frequencies above 4 cycles/degree removed. While the individual elements of the patterns have lost some "sharpness," their shape (square or diamond) remains clearly discernable. (From Quinn, Burke, & Rush, 1993.)

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