Alan Gilchrist - Academia.edu (original) (raw)

Papers by Alan Gilchrist

Perception, 1997

Relative luminance is fundamental to lightness perception, but can be used to predict specific li... more Relative luminance is fundamental to lightness perception, but can be used to predict specific lightness values only when coupled with an anchoring rule. Empirical results indicate that, in simple displays, lightness is anchored by the highest luminance (white) rather than by the average luminance (middle gray). This implies that increasing the luminance range of a stimulus causes grayness induction: the lower luminance values become darker gray while the highest luminance remains unchanged, as many so-called brightness induction experiments have shown. Yet sometimes increasing the luminance range of a stimulus causes luminosity induction: the highest luminance becomes increasingly self-luminous while the lowest luminances remain unchanged. Whether grayness induction or luminosity induction results from an increase in stimulus contrast depends on relative area. A few simple, yet hitherto unrecognised, rules that describe how anchoring by highest luminance combines with anchoring by ...

Journal of Vision, 2009

Surface lightness is widely thought to depend on the relative luminance coming from neighboring s... more Surface lightness is widely thought to depend on the relative luminance coming from neighboring surfaces. But relative luminance can produce only relative lightness values. Specific lightness values can be derived only with an anchoring rule that specifies how relative luminance values in the retinal image are mapped onto the lightness scale. We explored the anchoring rules governing very simple images consisting of two adjacent surfaces that fill the entire visual field. These were painted onto the interior of a large hemisphere that surrounded the observer's head. Lighter and darker radial sectors of the same two shades of gray were painted onto nine such hemispheres, but with different relative areas. The region of highest luminance was always seen as white. The lightness of the darker sector depended on relative area, appearing lighter as the darker sector became larger, but this effect was stronger when the darker sector was larger than the lighter, a pattern of results shown to be consistent with over a dozen prior studies of relative area and lightness.

Journal of Vision, 2010

We used a novel probe disk technique to test for the existence of functional frames of reference ... more We used a novel probe disk technique to test for the existence of functional frames of reference for lightness perception in complex images. Thirteen identical gray disks were electronically pasted into the photograph Trastevere, which shows two large regions of sunlight and shadow. Observers matched the lightness of each disk with a Munsell scale. The data revealed a framework effect. That is, lightness differences within either the sunlight or shadow region were small relative to the pronounced step function at the framework boundary. Additional experiments testing the perceived embeddedness of the disks showed that the framework effect was increased when disk size and shape were altered to conform to the perspective shown in the photograph and when the disks were blurred slightly to conform to the graininess of the photograph. The effect was further increased when the photograph was viewed through a pinhole and when the disks were presented one by one. The effect was reduced when paper disks of equal luminance and visual angle were pasted onto the glass front of the CRT screen. When the sunlight framework was covered with black paper, the remained disks within the shadow region appeared white, as predicted by the anchoring theory (A. Gilchrist, 2006).

Perception

... Perception 29 ECVP Abstract Supplement. Lightness determination for an object under two illum... more ... Perception 29 ECVP Abstract Supplement. Lightness determination for an object under two illuminations. S Zdravkovic, A L Gilchrist. ...

Perception, 2002

The concept of articulation was first introduced by Katz [1935 The World of Colour (London: Kegan... more The concept of articulation was first introduced by Katz [1935 The World of Colour (London: Kegan Paul, Trench, Trubner & Co)] to refer to the degree of complexity within a field. Katz, who created the basic research methods for studying lightness constancy, found that the greater the degree of articulation within a field of illumination, the greater the degree of constancy. Even though this concept has been largely forgotten, there is much empirical evidence for Katz's principle, and the effects on lightness are very strong. However, when articulation is increased within a framework that does not coincide with a region of illumination, constancy is weakened. Kardos (1934 Zeitschrift für Psychologie Ergänzungband 23) advanced the concept of co-determination, according to which the lightness of a surface is determined relative to more than one field of illumination. Gilchrist et al (1999 Psychological Review106 795–834) argue that the fields concept should be replaced by the more...

Journal of Experimental Psychology: Human Perception and Performance, 2012

This research was funded by the NSF grant: BCS 0236701. We would like to thank Adam Reeves for ve... more This research was funded by the NSF grant: BCS 0236701. We would like to thank Adam Reeves for very insightful comments that have greatly improved the paper. Also, we thank the two anonymous reviewers for their suggestions.

Perception & Psychophysics, 2004

The lightness hangover illusion is an unusually robust, long-lasting, prior-experience-based ligh... more The lightness hangover illusion is an unusually robust, long-lasting, prior-experience-based lightness effect. The effect occurs in the Mondrian world, a miniature chamber with interior walls covered with dark gray to black patches. The lightest patch in this scene, physically dark gray, looks white. When real whites and light grays are added to the scene, all the patches darken, but at an unusually slow rate. For several seconds, the white patches look self-luminous and the other patches continue to look very light. The luminosity fades and the other patches darken only after 2 min. We tested three possible explanations for this illusion: retinal adaptation, lightness persistence, and anchor persistence. The results clearly support anchor persistence, which is caused by the presence of steady patches, surfaces that retain their luminance values across scenes. The data also show that the size of the illusion varies directly with the number of these steady patches.

Perception & Psychophysics, 1995

Psychological Review, 1999

Perception Ecvp Abstract, Aug 25, 2014

Perception, 2015

Guest editorial Perception and the social psychology of 'The Dress' By now most people, especiall... more Guest editorial Perception and the social psychology of 'The Dress' By now most people, especially in the field of perception, will be familiar with an image that has provoked intense discussion and controversy. It is a photograph of a dress, shown in figure 2. Some observers see the dress as white and gold while others see it as blue and black. Disputes over the color of this dress have produced anger and have left some people wondering whether their vision is defective. I believe the difference in perceived color is associated with a difference in how the highly cropped image is perceptually parsed. But in this editorial I would like to discuss the social psychology of this phenomenon. Why would such apparently trivial disputes about color produce such strong emotional responses? The answer, I believe, can be found in the classic work of Solomon Asch (1956) on group pressure. In a series of trials, eight subjects sitting at a table are asked to judge which of three lines, A, B, or C, is equal in length to a standard line (see figure 1). Of course there is only one real subject. The other seven are all confederates of Asch. On many trials, the confederates give the obviously correct answer. But on certain trials they all give the same wrong answer. The key question on these trials is, what will the real subject do? Subjects yielded to the group pressure roughly a third of the time.

Nature Precedings

Seeing black, white and gray surfaces, called lightness perception, might seem simple because whi... more Seeing black, white and gray surfaces, called lightness perception, might seem simple because white surfaces reflect 90% of the light they receive while black surfaces reflect only 3%, and the human retina is composed of light sensitive cells. The problem is that, because illumination varies from time to time and from place to place, any amount of light can be reflected from any shade of gray. Thus the amount of light reflected by an object, called luminance, says nothing about its lightness. Experts agree that the lightness of a surface can be computed only by using the surrounding context, but they disagree about how the context is used. We have tested an image in which two major classes of theory, contrast theories and frame-of-reference theories, make very different predictions regarding what gray shades will be seen by human observers. We show that when frame-of-reference is varied while contrast is held constant, lightness varies strongly. But when contrast is varied but frame-of-reference is held constant, little or no variation is seen. These results suggest that efforts to discover the exact algorithm by which the human visual system segments the image received by the retina into frames of reference should be given high priority. The challenge confronting the human visual system in assigning black, white, and gray shades to visible surfaces is illustrated in Figure 1. Three identical disks have been pasted into this photograph.