John Uhlarik

The Relational Determination of Length Illusions and Length Aftereffects

When two lines were presented simultaneously, in an illusion paradigm, the judged length of one of the lines (the focal line) assimilated toward the length of the contextual line. When the lines were presented sequentially, in an aftereffect paradigm, the apparent length of the focal line was displaced away from the length of a previously presented contextual line (i.e. contrast). The largest effects obtained at inverse contextual-to-focal length ratios: 1·67:1 for contextual lines longer than the focal line and 0·60:1 for contextual lines shorter than the focal line. Overestimation of focal length was larger than underestimation for both illusions and aftereffects despite the fact that identical stimuli produced effects in opposite directions in the two paradigms. Results were discussed in terms of a neural length-coding model and were related to visual size constancy.

Contextual Influences on Judgments of Linear Extent.

A series of experiments was performed to test the predictions of adaptation level, efferent readiness, and framing explanations of illusions of extent. The framing notion, that the ratio of total figure length to shaft length (i.e., the framing ratio) determines the magnitude of illusions, was supported for the Müller-Lyer, Baldwin, and divided line illusions. Peak overestimation of shaft length obtained when the framing ratio was 3:2. Variation of proximal figure size, achieved by altering either viewing distance or distal figure size, was found to be directly related to the magnitude of the illusions. A model was proposed to account for the effects of both the framing ratio and proximal shaft length on judgments of focal length.

Color aftereffects contingent on perceptual organization

The present experiment demonstrated that (a) cognitive organizational factors influence an orientation-contingent color (McCollugh) aftereffect, and (b) adaptation to colored gratings affected the perceptual organization of a reversible figure. Following adaptation to colored gratings, color aftereffects were reported in only one of two organizations of a reversible figure, and the colors conformed to the subjective contours of the test pattern rather than exclusively to the regions defined by horizontal and vertical pattern elements. In addition, an organization of the reversible figure that segregated vertical and horizontal pattern elements was dominant subsequent to adaptation.

Length contrast in the Müller-Lyer figure: Functional equivalence of temporal and spatial separation

The distortion of perceived line length produced by the Müller-Lyer figures reverses fromassimilation to contrast under conditions of temporal separation of contextual fins from judged shaft (Experiment 1) and spatial separation of contextual and judged elements (Experiment 2). In both experiments, the outgoing fins produced underestimation of judged shaft length whereas ingoing fins produced overestimation. These findings were predicted by the “pool and store” model of perceptual distortions (Girgus & Coren, 1982) and point to the need for a unified model of assimilation and contrast of perceived size. Extensions and refinements of the “pool and store” model are proposed.

Assimilation and contrast of perceived length depend on temporal factors

The parallel lines configuration produces assimilation of linear extent when the contextual and focal lines are presented simultaneously and contrast when the contextual line is presented prior to the focal judgment. The interpretation of the shift from assimilation to contrast is confounded by the covariation of mode of presentation (simultaneous or sequential) of contextual and focal stimuli and the duration of contextual exposure in previous research. This confound was removed by varying contextual exposure duration within the sequential mode of presentation (Experiment 1) and by combining the simultaneous and sequential modes of presentation (Experiment 2). The results indicated that the shift from assimilation to contrast is determined by the mode of presentation of contextual and focal stimuli. The data are discussed as they relate to Coren and Girgus’s (1978) “pool and store” model of assimilation and contrast.

Size scaling in two-dimensional pictorial arrays

The study examined effects of instructional sets (objective, phenomenal, projective, and retinal) on the judged sizes of blocks placed at various “distances” in a pictorial array. Magnitude estimations of size were consistent with previous studies of size constancy in three-dimensional arrays. Chronometric analyses indicated that reaction time increased with distal size, but was not affected by perceived distance. The results suggested that size was scaled relative to a perceptual unit. Instructions affected the nature of the scale unit (proximal vs. distal), but not the scaling process itself.

Fragmentation and identifiability of repeatedly presented brief visual stimuli

Ss were given repeated brief presentations of geometric forms and drew what they thought they saw. The stimulus patterns were presented tachistoscopically for as many repetitions as necessary for correct identification. The nature and sequential properties of pattern fragments reported prior to correct identification were examined. The first fragments to be reported were generally straight line elements of the stimulus patterns. With repeated presentations, more features were gradually reported until the complete figure was correctly identified. The pattern of construction appeared similar to patterns of fragmentation found in research on stabilized retinal images, prolonged afterimages, and conditions of steady fixation. Implications of these findings for theories of pattern information processing were discussed.

A device for presenting targets and recording positioning responses in one dimension

Apparatus for recording manual target pointing localizations is described. The device, utilizing a position transducer and digital voltmeter, is capable of determining the position of either manual limb when localizing visual and/or auditory targets. The pointing localizations involve naturalistic limb movements made in a relatively unrestricted manner. The apparatus has the advantages of rapid accurate measurement, reliability, ease of reading, and the potential for automatic recording. When used for research involving adaptation to sensory rearrangement, there is the further advantage that the dependent measures are obtained in the same context as during exposure to the rearrangement.

The role of some spatial parameters of gratings on the McCollough effect

Ss were alternately adapted to vertical and horizontal gratings that consisted of black bars and colored slits. The slits of one grating were green and of the other, magenta. The widths of the black bars and the colored slits were varied independently during adaptation and testing. This design separates the relative influence of bar width, slit width, and spatial frequency on an orientation specific color aftereffect known as the McCollough effect. Black bar width had the major influence on the strength of the aftereffect, suggesting that the neurophysiological mechanism underlying the McCullough effect might consist of orientation specific units that are sensitive to both the widths of black bars and the chromatic characteristics of their surrounds.

Contextual influences on a spatial-frequency-contingent color aftereffect

Spatial-frequency-contingent color aftereffects were induced in eight observers in order to examine the role of apparent spatial frequency in the aftereffects. Test stimuli consisted of achromatic gratings presented either with or without the context of a receding pictorial corridor array. A neutral-point procedure was used to assess the strength of the color aftereffects. The results indicated that both retinal and apparent spatial frequency affected the color aftereffects on those test gratings presented in the context of the corridor array. However, the color aftereffects on the test gratings presented without context were affected only by variations in retinal spatial frequency. It is concluded that the stimulus for the color aftereffects is not simply the retinal representation of the test stimulus. Therefore, any description of contingent color aftereffects based on selective chromatic adaptation of retinotopically organized cortical feature detectors is not complete.