Adam Reeves

Simultaneous color constancy

Observers matched patches (simulated Munsell papers) in two simultaneously presented computer-controlled displays, a standard array presented under 6500-K illumination and a test array under 4000 or 10,000 K. Adaptation to the test illuminants was limited. The adjusted patch was surrounded by a single color (annulus display) or by many colors (Mondrian display). Observers either matched hue and saturation or made surface-color (paper) matches in which the subject was asked to make the test patch look as if it were cut from the same piece of paper as the standard patch. For two of the three subjects, the paper matches were approximately color constant. The hue-saturation matches showed little color constancy. Moreover, the illumination difference between the two displays was always visible. Our data show that simultaneous mechanisms alone (e.g., simultaneous color contrast) alter hues and saturations too little to produce hue constancy.

Focused auditory attention and frequency selectivity

The probe-signal method (Greenberg & Larkin, 1968) was used to determine the percentage of trials in which unpracticed subjects detected (two-interval, forced-choice) a soft, expected sound as compared with an unexpected sound. Pure tones at or near an expected frequency were detected in about 90% of the trials. Tones more than one-half critical band away were detected near chance (50%). Complex sounds (a band of noise or a multitone complex) were detected better if they were inside the same critical band as the expected signal than if they were outside the band. A signal that differed spectrally from the expected sound was not detected even though it had the same low pitch, based on a common fundamental frequency. The results may mean that under some conditions focused attention alters sensitivity in the auditory system.

Perceived lightness, but not brightness, of achromatic surfaces depends on perceived depth information

Three experiments were conducted in an attempt to replicate and clarify Gilchrist’s (1977, 1980) experiments on the effects of depth information on judgments of achromatic surface color. Gilchrist found that coplanarity, and not retinal adjacency, was the dominant factor in determining achromatic color matches. Because such matches can be made on the basis of either brightness or lightness, we obtained judgments of both qualities. Stereopsis was added to enhance the perceived depth effect of Gilchrist’s display, which was otherwise simulated closely on a high-resolution CRT. The results for lightness followed the same pattern as those of Gilchrist, but were smaller in magnitude. This discrepancy may reflect reduced extraneous lighting effects in our displays. Our results therefore agree with related studies in suggesting that lightness matches are based on relationships among coplanar surfaces. Brightness matches, however, were not influenced by perceived depth.

Visual Imagery Selectively Reduces Vernier Acuity

Mental imagery interferes with perception. This, an example of the ‘Perky effect’, was studied for vernier acuity. Mean accuracy for reporting the offset of vertical line targets declined from 80% to 65% when subjects were requested to imagine vertical lines near fixation. Images of horizontal lines or of a grey mist in the fixation region lowered accuracy to a similar extent. However, accuracy was barely affected when the image was requested 1.5 deg or more from the target. The Perky effect remained strong for at least 4 s after an instruction to ‘clear’ the image away. The results were not due to imagery-induced changes in fixation, pupil diameter, or accommodation, or (at least primarily) to central attentional or decisional factors. Rather, imagery produces a local, pattern-insensitive, and relatively long-lasting reduction in visual sensitivity. The sensitivity loss may be mimicked by a 0.24 log unit reduction in target energy.

Effect of luminance on suprathreshold contrast perception

Perceived contrast was measured under natural viewing conditions with the use of contrast-matching and magnitude-estimation paradigms and found to be independent of luminance over a range of luminances from 37.5 down to 8 cd/m2. However, this contrast constancy broke down when the dimmer target was below 8 cd/m2. The perceived contrast of the dimmer target then fell below that expected from contrast constancy. The extended range of contrast constancy previously reported [J. Physiol. 252, 627 (1975); Vision Res. 16, 1419 (1976)] has been thought to imply neural mechanisms with unlimited constancy, but these researchers permitted differential adaptation to the brighter and dimmer targets, which were seen haploscopically (by different eyes). As our natural-viewing procedure ensured that both bright and dim targets were presented to retinal areas in a roughly constant state of adaptation, our failure to find extended contrast constancy implies an important limitation on the neural processing of contrast.

Development of an image/threshold database for designing and testing human vision models

Models that predict human performance on narrow classes of visual stimuli abound in the vision science literature. However, the vision and the applied imaging communities need robust general-purpose, rather than narrow, computational human visual system models to evaluate image fidelity and quality and ultimately improve imaging algorithms. Psychophysical measure of image imaging algorithms. Psychophysical measures of image quality are too costly and time consuming to gather to evaluate the impact each algorithm modification might have on image quality.

Color Constancy: Phenomenal or Projective?

Naive observers viewed a sequence of colored Mondrian patterns, simulated on a color monitor. Each pattern was presented twice in succession, first under one daylight illuminant with a correlated color temperature of either 16,000 or 4,000 K and then under the other, to test for color constancy. The observers compared the central square of the pattern across illuminants, either rating it for sameness of material appearance or sameness of hue and saturation or judging an objective property—that is, whether its change of color originated from a change in material or only from a change in illumination. Average color constancy indices were high for material appearance ratings and binary judgments of origin and low for hue-saturation ratings. Individuals’ performance varied, but judgments of material and of hue and saturation remained demarcated. Observers seem able to separate phenomenal percepts from their ontological projections of mental appearance onto physical phenomena; thus, even when a chromatic change alters perceived hue and saturation, observers can reliably infer the cause, the constancy of the underlying surface spectral reflectance.

Rotating objects to determine orientation, not identity: Evidence from a backward-masking/dual-task procedure

The effects of picture-plane rotations on times taken to name familiar objects (RTs) may reflect a process of mental rotation to stored viewpoint-specific representations: therotate-to-recognize hypothesis. Alternatively, mental rotation might be used after stored object representations are activated by a misoriented stimulus in order to verify a weak or distorted shape percept: thedouble-checking hypothesis. We tested these two accounts of rotation effects in object recognition by having subjects verify the orientations (to within 90°) and basic-level names of 14-msec, backward-masked depictions of common objects. The stimulus-mask interval (SOA) varied from 14 to 41 msec, permitting interpolation of the SOA required for 75% accuracy (SOAc). Whereas the SOAc to verify orientation increased with rotation up to 180°, the SOAc to verify identity was briefer and asymptoted at ∼60°. We therefore reject the rotate-to-recognize hypothesis, which implies that SOAc should increase steadily with rotation in both tasks. Instead, we suggest that upright and near-upright stimuli are matched by a fast direct process and that misoriented stimuli are matched at a featural level by a slightly slower view-independent process. We also suggest that rotation effects on RTs reflect apostrecognition stage of orientation verification: therotate-to-orient hypothesis, a version of double-checking that also explains the well-known reduction in orientation effects on RTs when naming repeated objects.

The Effect of Photon Noise on the Detection of White Flashes

Thresholds for detecting brief, white, foveal test flashes drop abruptly within 0.2 sec of the offset of a white adapting field. The magnitude of the abrupt drop is proportional to the square root of field intensity (square root of I) correct for bleaching and dark light. Thresholds are then stable out to 1.6 sec for 200 msec tests, or recover only slightly for 20 msec tests. These results exclude some simple deterministic models in which Weber-like gain controls in the luminance pathway are assumed to recover exponentially in the dark, but can be explained parsimoniously if turning off the field abolishes photon-driven noise, improving the S/N ratio while leaving visual responsivity virtually unaltered. This theory was first put forward by Krauskopf and Reeves [(1980) Vision Research, 20, 193-196] for S-cone thresholds; it implies that the Weber law for increment thresholds is not due to a single gain control, but rather expresses the product of two distinct square root of I factors, adjustment of responsivity and photon-driven noise. Removal of the noise, not recovery of gain, permits thresholds to fall in early dark adaptation.

Flicker brightness enhancement and visual nonlinearity

The purpose of this study was to investigate the nonlinear mechanism underlying brightness enhancement, in which a flickering stimulus appears brighter than a steady stimulus of equal mean luminance. The flickering and matching stimuli were temporally alternated. Both were cosine windowed to minimize the potential effects of temporal transients. Subjects adjusted the amplitude of the matching stimulus to match it in brightness to the flickering stimulus. The temporal frequency, modulation, and waveform of the flickering stimulus were varied. With sinusoidal flicker, brightness enhancement increased with increasing modulation at all frequencies, peaking at about 16 Hz at full modulation. The results were modeled by a broad temporal filter followed by a single accelerating nonlinearity. The derived temporal sensitivity of the early filter inferred from brightness enhancement decreased more slowly at high frequencies than the filter(s) inferred from flicker modulation thresholds. With low frequency sawtooth flicker, brightness enhancement was phase-dependent at low, but not at high modulations, suggesting that multiple neural mechanisms may also be involved in addition to an early nonlinearity.