Steven K. Shevell

The Subjective Dates of Natural Events in Very-Long-Term Memory

This research explores the problem of how people determine the time of public events, such as the attempted assassination of Ronald Reagan or the Three-Mile Island accident. According to what here is called the accessibility principle, the subjective dates of these events depend in part on the amount that can be recalled about them: The more known, the more recent the event will seem. Experiments 1 and 2 demonstrate this effect when subjects estimate explicit dates for important news stories of the 1970s and 1980s. The same effect appears in Experiment 3 for subjects who rate the recency of less known events drawn from a single week. Accessibility also contributes to the amount of time needed to compare the subjective date of an event (e.g., the Jonestown suicides) to an explicitly presented date (e.g., November 1979), as shown in Experiment 4. The accessibility principle for time estimation can be conceived as one of a related group of retrieval-based inferences that plays a part in judgments of frequency and probability and judgments about the falsity of a putative fact.

FOVEAL CONE DETECTION STATISTICS IN COLOR-NORMALS AND DICHROMATS

We measured for six male observers, the psychometric functions for the detection of two simultaneously presented points of light. The test stimuli were two 1 min point sources separated by 17 min arc, and pulsed for 0.5 msec. The stimuli varied in wavelength from 500 to 620 nm. The psychometric functions were fit with a model that assumes ideal detection and the following properties: (1) Poisson-distributed quantal absorptions; (2) binomial sampling of foveal long-wavelength-sensitive (LWS) and middle-wavelength-sensitive (MWS) cones; (3) independent responses of the LWS and MWS cones; and (4) the Smith-Pokorny fundamentals for cone spectral sensitivities. Based on chi 2 fits to the psychometric functions for detecting neither, one or both of the two-point stimuli presented, estimates were derived for the minimum quantal catch by a single cone for detection (C), the number of effective cones illuminated by a point stimulus at threshold (N) and the proportion of central foveal cones of the LWS type (PL). Three observers were color-normal, two were protanopes and one was a deuteranope. A second deuteranope was included in the design but his data were too unreliable for an unambiguous solution. The estimated quantal requirement C was consistently near 5(4-6), and the effective number of illuminated cones always was 1 or 2. The plausible range of PL (98% confidence interval) for the color-normal observers was 0.48-0.68 (observer YY), 0.76-0.90 (observer MW) and 0.77-0.95 (observer DF). The best fitting PL solution for these observers were 0.61, 0.82 and 0.88, respectively. These were comparable to the values obtained from flicker photometric data. The best PL value for each of the protanopes was 0.00 and for the deuteranope the best PL value was 0.98.

Stereo disparity improves color constancy

Binocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception.

Large shifts in color appearance from patterned chromatic backgrounds

The perceived color of a light varies with the background on which it is seen. In the present study, patterned backgrounds composed of two different chromaticities caused larger shifts in perceived color than did a uniform background at either chromaticity within the pattern. Cortical receptive-field organization, but not optical factors or known retinal neurons, can account for the color shifts from patterned backgrounds.

Color perception under chromatic adaptation: equilibrium yellow and long-wavelength adaptation.

Observers viewed a thin (0.8-1.3) annulus composed of a mixture of 540 and 660 nm monochromatic lights (denoted delta G and delta R, respectively). The annular mixture was superimposed upon a larger (2.7) 660 nm circular background field. The observer adjusted the radiance of either delta G or delta R so that the annulus appeared a perfect (i.e. neither reddish nor greenish) yellow. In the first experiment, the background and annulus both were presented steadily. The results showed that the background, varied over a range from 10 to 1000 td. always contributed less to the color appearance of the annular test area than would be expected from the simple admixture of lights. The second experiment examined the effect of briefly removing the background-field quanta during the period when the annulus was judged. After several minutes of adapting to the background, the background was momentarily extinguished for 1 sec once every 6 sec; the observer adjusted the radiance of delta R so that during the 1 sec period the continuously presented annular mixture appeared equilibrium yellow. With steady backgrounds, the delta G to delta R luminance ratio decreased with test annulus luminance; for judgments made while the background momentarily was extinguished, the luminance ratio generally increased with annulus luminance. All of the empirical observations can be accounted for quantitatively by a two-process theory of chromatic adaptation; in two processes are (1) gain changes and (2) a restoring signal that tends to drive back toward equilibrium the opponent response resulting from the adapting light. Results from a third experiment, in which the background-off interval was reduced from 1 sec to 500, 200 or 150 msec. also are consistent with this model.

Trichromatic color vision with only two spectrally distinct photopigments

Protanomaly is a common, X-linked abnormality of color vision. Like people with normal color vision, protanomalous observers are trichromatic, but their ability to discriminate colors in the red–green part of the spectrum is reduced because the photopigments that mediate discrimination in this range are abnormally similar. Whereas normal subjects have pigments whose wavelengths of peak sensitivity differ by about 30 nm, the peak wavelengths for protanomalous observers are thought to differ by only a few nanometers. We found, however, that although this difference occurred in some protanomalous subjects, others had pigments whose peak wavelengths were identical. Genetic and psychophysical results from the latter class indicated that limited red–green discrimination can be achieved with pigments that have the same peak wavelength sensitivity and that differ only in optical density. A single amino acid substitution was correlated with trichromacy in these subjects, suggesting that differences in pigment sequence may regulate the optical density of the cone.

Variation in color matching and discrimination among deuteranomalous trichromats: theoretical implications of small differences in photopigments

Individual differences in abnormal color vision are well known. A fundamental unresolved problem is the great variation in color vision even among those classified as having the same color-vision defect. Several physiological hypotheses have been proposed to account for this variation but little consideration has been given to how (and how much) color matching and discrimination are affected by the posited physiological mechanisms. Advances in molecular genetics have renewed interest in this problem, which is at the foundation of the relation between genotype and phenotype. We report here theoretical Rayleigh ranges (chromatic discrimination) and quantal matches for deuteranomalous trichromats with photopigments in the red/green range that vary in their separation and optical density. The results show there is relatively little loss of discrimination with pigments of normal optical density separated by as little as 2-3 nm. With pigments separated by 4 nm or less, however, optical density can strongly influence discrimination when varied independently in the two types of cone. Moderately lower (or higher) optical density in only one cone-type affects discrimination by altering the shape of the cones relative spectral sensitivity function. The lack of correlation between Rayleigh-match midpoint and range, which is reported in the literature, may be accounted for by independent variation in pigment separation and optical density.

Developmental changes in face processing: Results from multidimensional scaling

Multidimensional scaling procedures were used to investigate developmental changes in the ability to process previously unfamiliar faces. Eighty male subjects, aged 7, 9, 12, or adult, rated the similarity of pairs of faces. The faces were presented to subjects in either the upright or the inverted orientation. Multidimensional scaling analyses suggest that subjects of all ages use similar information in judging the similarity of faces. However, for upright faces, individual subjects under age 10 seem to use fewer features at a time. The results argue against a qualitative shift in face processing at age 10, and suggest that the improvement in face recognition ability noted at this age is due at least in part to an increased ability to consider more features simultaneously.

A year's memories : The calendar effect in autobiographical recall

When asked to recall autobiographical events from the past year, students tend to recall more incidents from the beginning and the end of school terms than from other periods. We investigated thiscalendar effect in Experiment 1 by comparing free recall at schools with different academic calendars. The event distributions tracked the individual calendars, helping to eliminate the possibility that the calendar effect is due to seasonal, nonschool factors, such as holidays. In Experiments 2–4, we checked explanations based on the ideas that events at term boundaries are more important or distinctive than others, that events are incorrectly dated too near the boundaries, and that boundaries serve as implicit cues for recall. These experiments revealed no evidence that importance or errors in dating could explain the effect. Manipulating cues, however, did change the size of the effect, implicating retrieval from very long-term memory as the effect’s source. We suggest that when people have to search episodic memory, they consider their own calendar rhythms (such as a student’s academic schedule) and let the temporal structure of their personal context guide their search.

Saturation in human cones

Abstract Increment threshold intensities were measured for a foveally presented 20msec test flash which fell on a flashed 4.1° white background field. The test field was varied in color (red or green) and size (1° or 0.57°). In one condition (green 0.57° test) the onset asynchrony of the background and test fields was also varied. The increment threshold measurements directly demonstrate saturation of theπ4 and π5 color mechanisms and, based on the cone signal equation given by Alpern, Rushton and Torii (1970c), provide estimates of the semi-saturation constant σ. For a given color mechanism, a single value of σ was found to be adequate for all test flash sizes and asynchrony intervals tested; however, these values of σ are about 100 times smaller than those found by Alpern et al. using the contrast-flash technique. The Alpern et al. cone signal equation is essentially a simple gain control model. It is shown that when light intensities are correctly expressed in terms of quantum catch, the gain control model can account explicitly for both the observed cone mechanism saturation in the absence of pigment bleaching and the absence of saturation in the presence of bleaching by steady backgrounds.