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Reinterpreting behavioral receptive fields: lightness induction alters visually completed shape.

Keane BP, Lu H, Papathomas TV, Silverstein SM, Kellman PJ - PLoS ONE (2013)

Bottom Line: This pattern arose when pixels immediately adjacent to the discriminated boundaries were excluded from the analysis (Experiment 2) and also when the noise was restricted to the key regions so that the noise never overlapped with the physically visible edges (Experiment 3).Moreover, behavioral receptive fields derived in CI studies do not correspond to contours per se but to filled-in surface regions contained by those contours.The relevance of lightness to two-dimensional shape completion supplies a new constraint for models of object perception.

View Article: PubMed Central - PubMed

Affiliation: Center for Cognitive Science, Rutgers University, New Brunswick, Piscataway, New Jersey, USA. Brian.Keane@gmail.com

ABSTRACT

Background: A classification image (CI) technique has shown that static luminance noise near visually completed contours affects the discrimination of fat and thin Kanizsa shapes. These influential noise regions were proposed to reveal "behavioral receptive fields" of completed contours-the same regions to which early cortical cells respond in neurophysiological studies of contour completion. Here, we hypothesized that 1) influential noise regions correspond to the surfaces that distinguish fat and thin shapes (hereafter, key regions); and 2) key region noise biases a "fat" response to the extent that its contrast polarity (lighter or darker than background) matches the shape's filled-in surface color.

Results: To test our hypothesis, we had observers discriminate fat and thin noise-embedded rectangles that were defined by either illusory or luminance-defined contours (Experiment 1). Surrounding elements ("inducers") caused the shapes to appear either lighter or darker than the background-a process sometimes referred to as lightness induction. For both illusory and luminance-defined rectangles, key region noise biased a fat response to the extent that its contrast polarity (light or dark) matched the induced surface color. When lightness induction was minimized, luminance noise had no consistent influence on shape discrimination. This pattern arose when pixels immediately adjacent to the discriminated boundaries were excluded from the analysis (Experiment 2) and also when the noise was restricted to the key regions so that the noise never overlapped with the physically visible edges (Experiment 3). The lightness effects did not occur in the absence of enclosing boundaries (Experiment 4).

Conclusions: Under noisy conditions, lightness induction alters visually completed shape. Moreover, behavioral receptive fields derived in CI studies do not correspond to contours per se but to filled-in surface regions contained by those contours. The relevance of lightness to two-dimensional shape completion supplies a new constraint for models of object perception.

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Stimuli and results for Experiment 3 and Experiment 4.(A) Shapes appeared with slender noise bars (identified by red arrows) that were centered within the key regions of the illusory condition. The noise bars were embedded in a fine-grained pattern shown schematically by gray rectangles. The pattern was constant for all trials and appeared under the translating ovals. The top and bottom parts of the fragmented stimuli were trapezoidal and appeared disconnected. (B) A CI was computed with the noise bar values for each subject and ROIs consisted of just the noise regions. Mean ROI values are shown for each polarity condition for the illusory shape discrimination of Experiment 3 and the fragmented discrimination of Experiment 4. Errors are +/− SEM.
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pone-0062505-g007: Stimuli and results for Experiment 3 and Experiment 4.(A) Shapes appeared with slender noise bars (identified by red arrows) that were centered within the key regions of the illusory condition. The noise bars were embedded in a fine-grained pattern shown schematically by gray rectangles. The pattern was constant for all trials and appeared under the translating ovals. The top and bottom parts of the fragmented stimuli were trapezoidal and appeared disconnected. (B) A CI was computed with the noise bar values for each subject and ROIs consisted of just the noise regions. Mean ROI values are shown for each polarity condition for the illusory shape discrimination of Experiment 3 and the fragmented discrimination of Experiment 4. Errors are +/− SEM.

Mentions: A concern may be that the key regions were used only on trials where the noise destroyed the physically visible contours. In these trials, the participants might have been unable to see any shape and would thus be forced to respond only on the basis of surface cues. To better test whether the key regions change shape perception or whether they serve as a shape substitute, we ran a third experiment in which the global noise field was replaced with two slender noise bars centered within the two key regions (see Figure 7A). On each trial, the bars were freshly drawn with a single randomly chosen Weber contrast value. The bars appeared at a fixed location–either immediately outside a thin shape or just within a fat shape. The bars were embedded in a non-varying patterned background so that they would appear as surface features rather than separate objects. Upon computing CIs and performing an ROI analysis on the noise bar regions, inducer polarity once again was found to alter shape perception via lightness induction, F(2,28) = 11.5, p = .0002. The ROI value was highest when inducers were dark, lowest when inducers were light, and intermediate otherwise (Figure 7B). The effect was strikingly consistent: For all fifteen observers, the ROI value in the dark polarity condition was higher than that in the light, t(14) = 5.36, p = .0001. The outcome is especially noteworthy in that participants were repeatedly asked to ignore the noise bars throughout each experimental session. The capacity of lightness to alter shape perception apparently cannot be easily abolished through observer strategy, at least not when the inducers are noise-less and high-contrast.


Reinterpreting behavioral receptive fields: lightness induction alters visually completed shape.

Keane BP, Lu H, Papathomas TV, Silverstein SM, Kellman PJ - PLoS ONE (2013)

Stimuli and results for Experiment 3 and Experiment 4.(A) Shapes appeared with slender noise bars (identified by red arrows) that were centered within the key regions of the illusory condition. The noise bars were embedded in a fine-grained pattern shown schematically by gray rectangles. The pattern was constant for all trials and appeared under the translating ovals. The top and bottom parts of the fragmented stimuli were trapezoidal and appeared disconnected. (B) A CI was computed with the noise bar values for each subject and ROIs consisted of just the noise regions. Mean ROI values are shown for each polarity condition for the illusory shape discrimination of Experiment 3 and the fragmented discrimination of Experiment 4. Errors are +/− SEM.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3672097&req=5

pone-0062505-g007: Stimuli and results for Experiment 3 and Experiment 4.(A) Shapes appeared with slender noise bars (identified by red arrows) that were centered within the key regions of the illusory condition. The noise bars were embedded in a fine-grained pattern shown schematically by gray rectangles. The pattern was constant for all trials and appeared under the translating ovals. The top and bottom parts of the fragmented stimuli were trapezoidal and appeared disconnected. (B) A CI was computed with the noise bar values for each subject and ROIs consisted of just the noise regions. Mean ROI values are shown for each polarity condition for the illusory shape discrimination of Experiment 3 and the fragmented discrimination of Experiment 4. Errors are +/− SEM.
Mentions: A concern may be that the key regions were used only on trials where the noise destroyed the physically visible contours. In these trials, the participants might have been unable to see any shape and would thus be forced to respond only on the basis of surface cues. To better test whether the key regions change shape perception or whether they serve as a shape substitute, we ran a third experiment in which the global noise field was replaced with two slender noise bars centered within the two key regions (see Figure 7A). On each trial, the bars were freshly drawn with a single randomly chosen Weber contrast value. The bars appeared at a fixed location–either immediately outside a thin shape or just within a fat shape. The bars were embedded in a non-varying patterned background so that they would appear as surface features rather than separate objects. Upon computing CIs and performing an ROI analysis on the noise bar regions, inducer polarity once again was found to alter shape perception via lightness induction, F(2,28) = 11.5, p = .0002. The ROI value was highest when inducers were dark, lowest when inducers were light, and intermediate otherwise (Figure 7B). The effect was strikingly consistent: For all fifteen observers, the ROI value in the dark polarity condition was higher than that in the light, t(14) = 5.36, p = .0001. The outcome is especially noteworthy in that participants were repeatedly asked to ignore the noise bars throughout each experimental session. The capacity of lightness to alter shape perception apparently cannot be easily abolished through observer strategy, at least not when the inducers are noise-less and high-contrast.

Bottom Line: This pattern arose when pixels immediately adjacent to the discriminated boundaries were excluded from the analysis (Experiment 2) and also when the noise was restricted to the key regions so that the noise never overlapped with the physically visible edges (Experiment 3).Moreover, behavioral receptive fields derived in CI studies do not correspond to contours per se but to filled-in surface regions contained by those contours.The relevance of lightness to two-dimensional shape completion supplies a new constraint for models of object perception.

View Article: PubMed Central - PubMed

Affiliation: Center for Cognitive Science, Rutgers University, New Brunswick, Piscataway, New Jersey, USA. Brian.Keane@gmail.com

ABSTRACT

Background: A classification image (CI) technique has shown that static luminance noise near visually completed contours affects the discrimination of fat and thin Kanizsa shapes. These influential noise regions were proposed to reveal "behavioral receptive fields" of completed contours-the same regions to which early cortical cells respond in neurophysiological studies of contour completion. Here, we hypothesized that 1) influential noise regions correspond to the surfaces that distinguish fat and thin shapes (hereafter, key regions); and 2) key region noise biases a "fat" response to the extent that its contrast polarity (lighter or darker than background) matches the shape's filled-in surface color.

Results: To test our hypothesis, we had observers discriminate fat and thin noise-embedded rectangles that were defined by either illusory or luminance-defined contours (Experiment 1). Surrounding elements ("inducers") caused the shapes to appear either lighter or darker than the background-a process sometimes referred to as lightness induction. For both illusory and luminance-defined rectangles, key region noise biased a fat response to the extent that its contrast polarity (light or dark) matched the induced surface color. When lightness induction was minimized, luminance noise had no consistent influence on shape discrimination. This pattern arose when pixels immediately adjacent to the discriminated boundaries were excluded from the analysis (Experiment 2) and also when the noise was restricted to the key regions so that the noise never overlapped with the physically visible edges (Experiment 3). The lightness effects did not occur in the absence of enclosing boundaries (Experiment 4).

Conclusions: Under noisy conditions, lightness induction alters visually completed shape. Moreover, behavioral receptive fields derived in CI studies do not correspond to contours per se but to filled-in surface regions contained by those contours. The relevance of lightness to two-dimensional shape completion supplies a new constraint for models of object perception.

Show MeSH
Related in: MedlinePlus