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S K Shevell

Publications and source records attributed to S K Shevell.

17 recordsLinked to original sources
PubMed1992

Two separate neural mechanisms of brightness induction.

A particular rate of quantal absorption by photoreceptors may result in a dim or an intense percept, depending on light stimulating other parts of the retina. The brightness of an object in a natural scene, therefore, depends on the amount of light reflected from the object in comparison to light from other parts of the scene. We show this phenomenon is mediated by two separate neural mechanisms at distinct levels of the visual system. The first mechanism depends on retinal image contrast between adjacent regions. The second mechanism depends on the binocularly fused "cyclopian" representation and is influenced by more remote, noncontiguous areas of the visual field.

Adaptation, Ocular↗
PubMed1992

Color perception within a chromatic context: changes in red/green equilibria caused by noncontiguous light.

We measured changes in the color appearance of one light caused by another light presented in a well-separated region. Observers viewed a 1 degrees test field superimposed on a 3 degrees, 540 or 660 nm adapting field (32 or 320 td). The change in appearance due to noncontiguous light was determined by surrounding the 3 degrees adapting field with a continguous 3 degrees i.d., 5 degrees o.d. ring of either 32 or 320 td. The ring was 540, 660 nm or achromatic (tungsten-halogen "white"). The test was an admixture of 549 and 660 nm light, and varied from 6 to 1000 td. The observer adjusted the ratio of 549 to 660 nm test light so the test appeared neither reddish nor greenish. A 540 or 660 nm ring had a chromatic inducing effect on the small test that mimicked a simple surround contiguous with the test. Results with an achromatic ring were more complex: an isolated achromatic ring (no adapting field present) had virtually no effect on the color appearance of the test, but the same achromatic ring surrounding a chromatic adapting field shifted the test toward the color appearance of the adapting light (e.g. introducing a "white" ring surrounding a "green" adapting field shifted the test toward greenness). A thin pencil-width band of "white" light superimposed on a larger 5 degrees adapting field had an effect similar to a "white" 3-5 degrees ring. These results demonstrate (1) strong effects of the remote noncontiguous lights and (2) that the change in color appearance they cause is not a simple function of only the light in the noncontinguous region. The change depends on other lights in view. The visual processes revealed in these experiments are considered in terms of inferred illumination and surface reflectances of objects in natural scenes.

Adaptation, Ocular↗
PubMed1992

Redness from short-wavelength-sensitive cones does not induce greenness.

According to opponent-colors theory, a reddish surround induces greenness in a central test field. Color-appearance measurements verify this with a long-wavelength reddish surround (660 nm) but not with a short-wavelength reddish surround (440 nm). Surprisingly, a short-wavelength reddish surround shifts the appearance of a test toward redness. Four possible explanations are: (1) stray light from the short-wavelength reddish surround falls in the test area; (2) receptoral sensitivity changes overwhelm induced greenness from the surround; (3) a neural process of assimilation, rather than contrast, to the surrounding light; and (4) short-wavelength-sensitive (S) cones do not contribute to induced redness/greenness. Chromatic cancellation experiments confirm the fourth explanation. There was no change in induced redness/greenness when quantal absorption by only S cones in the surround was varied by 30-fold (using tritanopic metamers), even though varying stimulation of S cones strongly affected the color appearance of the surround. The redness induced by a short-wavelength surround is accounted for by opponent chromatic induction mediated by only middle- and long-wavelength-sensitive cones.

Color Perception↗
PubMed1992

Ocular toxicity of prednisone in pediatric patients with inflammatory bowel disease.

We performed ocular examinations on 58 corticosteroid-treated pediatric patients with inflammatory bowel disease (IBD) and on 58 age-matched controls. Posterior subcapsular cataracts (PSC) were detected in 12 of the 58 treated patients (20.7%) and in none of the controls. The difference in mean intraocular pressure (IOP) between the treated patients (15.89 +/- 4.11 mm Hg) and control subjects (13.63 +/- 2.35 mm Hg) was significant statistically (P < 0.001). Twenty-one patients (36.2%) were characterized as "IOP responders" (IOP > or = 20 mm Hg, change in IOP > or = 6 mm Hg between visits, or a difference in IOP > or = 6 mm Hg between the two eyes). Formation of PSC was not correlated significantly (P > 0.05) with the total dose of prednisone, duration of treatment, average daily dose, or number of days on high doses (> or = 25 mg). Raised IOP was correlated (P = 0.005) only with average daily dose (12.4 +/- 10.9 mg/day; range, 0-47 mg/day) 30 days before examination. When the dose of corticosteroid was reduced to < 10 mg/day, 2 patients manifested regression of PSC, and 12 IOP responders showed a decrease in IOP to within 2 SD of the mean control IOP. Only 3 of the 58 treated patients (5.2%) manifested both PSC and raised IOP. A significant inverse correlation (P = 0.02) was established between IOP at first examination and formation of PSC. We propose that the mechanisms for steroid-induced lens opacities and raised IOP do not share the same genetic basis. Because 52% of these children developed either PSC or raised IOP with prednisone therapy, we advocate careful ophthalmologic monitoring of pediatric patients receiving corticosteroids for IBD or any other condition.

Adolescent↗
PubMed1991

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.

Color Perception↗
PubMed1990

Normal endothelial cell density range in childhood.

Specular microscopy of the in vivo corneal endothelium of 214 clinically normal eyes in children ranging from 5 to 14 years of age showed a regular mosaic of hexagonal cells. The cell population density of individuals presented some variation, as it doses in older subjects. Quantitative analysis permitted us to determine the normal range of the endothelial cell count at each age. The mean (+/- SD) value ranged from 3591 +/- 399 cells per square millimeter at age 5 years to 2697 +/- 246 cells per square millimeter for the oldest subjects. Our data show a rapid decrease in cell density up to age 10 years. We estimate from our data a decrease in cell density of 13% between ages 5 and 7 years and an additional decrease of 12% by age 10 years.

Adolescent↗
PubMed1989

On neural signals that mediate induced blackness.

A small patch of achromatic light viewed within a large achromatic surround appears gray or black when the radiance of the surround is well above that of the patch. No single light can match the appearance of the patch because no light in isolation appears blackish; blackness is induced by a second stimulus. The present experiments examine the locus of the mechanisms mediating induction of blackness. They test whether induced blackness can be completely explained by interactions among signals from only one eye (retinal lateral inhibition, for example, though no explicit process is assumed here). If so, a surround affects the appearance of a patch only by modifying a signal that represents the patch at a monocular neural level. This signal may depend on retinal mechanisms and on purely monocular central processes. The alternative hypothesis is that induced blackness depends, at least in part, on a central binocular process driven by more information from each eye than can be carried by a neural signal representing only the patch. Measurements with fused binocular stimuli support the alternative hypothesis, implying retinal mechanisms alone are an incomplete explanation of induced blackness.

Adaptation, Ocular↗
PubMed1989

Foveal cone thresholds.

The method of constant stimuli was used to estimate the psychometric functions for detection of one or two flashes when two light pulses were presented. The test stimulus consisted of two simultaneous 0.5 msec, 1' pulses separated by 17'. Observers reported seeing 0, 1 or 2 flashes. A computer-controlled direct-view apparatus allowed sampling of slightly different foveal locations on each trial. The data were analyzed assuming a binomial probability for sampling of L and M cones and Poisson distributed quantal fluctuation. Under these assumptions, the measurements imply that detection requires a minimum of 5-7 quanta absorbed per cone, and that the effective number of cones illuminated by the 1', 0.5 msec pulse is two. The estimated L/M cone ratio was 1.6 for one observer and 4.0 for the other; each observer's ratio was in general agreement with the value estimated independently by heterochromatic flicker photometry.

Adult↗
PubMed1988

Color perception under chromatic adaptation: red/green equilibria with adapted short-wavelength-sensitive cones.

Chromatic adaptation can dramatically alter the color appearance of a light. The specific effect of adapting short-wavelength-sensitive (SWS) cones is examined by using two adapting wavelengths that lie on a tritanopic confusion line. The change in color appearance caused by signals from adapted SWS cones is isolated by restricting the wavelengths of the test light to 550 nm or longer. Thus the test negligibly stimulates SWS cones, so their sensitivity does not affect the test's appearance. The results show that adapted SWS cones contribute redness to the appearance of a superimposed test light, while not affecting sensitivity of MWS and LWS cones. Quantitatively, the redness from SWS cones illuminated by a large adapting field approaches physical admixture of test and adapting lights. This is very different from an adapting field that stimulates only MWS and LWS cones which, due to a postreceptoral process, contributes much less redness to a small superimposed test than expected from admixture. The difference between the adapted SWS-cone and the adapted MWS/LWS-cone contributions to the color of a small test explains a surprising result: a bluish-green (491 nm) adapting field contributes redness to a superimposed test light.

Adaptation, Ocular↗
PubMed1988

Light spread and scatter from some common adapting stimuli: computations based on the point-source light profile.

A point source of light is not so imaged on the photoreceptor mosaic. The light is distributed over the retina by diffraction, imperfections in the optics of the eye, and scatter. The spatial distribution of light is specified quantitatively by the point-source light profile, which can be used in convolution to determine light spread from more complex visual stimuli. We use the Vos, Walraven and van Meeteren [Vision Res. 16, 215-219 (1976)] light profile to determine spread light in specific regions within the spared area of an illuminated surround, and within a thin spared ring in an otherwise uniformly illuminated circular field. Surrounds of different sizes and rings of various widths and diameters are evaluated.

Adaptation, Ocular↗
PubMed1987

Answering autobiographical questions: the impact of memory and inference on surveys.

Survey questions often probe respondents for quantitative facts about events in their past: "During the last 2 weeks, on days when you drank liquor, about how many drinks did you have?" "During the past 12 months, how many visits did you make to a dentist?" "When did you last work at a full-time job?" are all examples from national surveys. Although questions like these make an implicit demand to remember and enumerate specific autobiographical episodes, respondents frequently have trouble complying because of limits on their ability to recall. In these situations, respondents resort to inferences that use partial information from memory to construct a numeric answer. Results from cognitive psychology can be useful in understanding and investigating these phenomena. In particular, cognitive research can help in identifying situations that inhibit or facilitate recall and can reveal inferences that affect the accuracy of respondents' answers.

Cognition↗
PubMed1987

A central binocular mechanism affects chromatic adaptation.

Two experiments explored the role of central binocular mechanisms in color perception. The first experiment examined the effect of adapting to simultaneous, binocularly fused fields. Each eye adapted to a slowly flickering (0.5 Hz) long-wavelength light. The two eyes were adapted either inphase (both eyes stimulated at the same moment) or out-of-phase (only one eye stimulated at any given moment). Both adapting procedures shifted equilibrium yellow toward longer wavelengths, but a significantly greater shift was found when adapting light stimulated both eyes simultaneously. This reveals that a central binocular mechanism affects chromatic adaptation. The second experiment tested whether the binocular mechanism could shift equilibrium yellow measurements made with both eyes (identical, binocularly fused fields presented to each eye) outside of the range of measurements established by left-eye monocular viewing and right-eye monocular viewing. Differences were found between monocular left-eye and monocular right-eye color appearance under conditions of moderate chromatic adaptation, but binocularly fused measurements fell within the range established by the monocular results. This is consistent with the view that central mechanisms serve to keep the two eyes in balance, rather than systematically alter color appearance from colors perceived under monocular viewing.

Adaptation, Ocular↗
PubMed1986

On neural signals that mediate brightness.

The brightness of any single light depends on other lights in view. These experiments examine how adapting light affects the brightness of a small incremental patch. The original purpose was to determine the quantitative effect of adaptation on the eye's brightness signal for the increment. However, this was found to be a fruitless enterprise because the results showed there exists no single signal from the eye that encodes brightness of the increment. This means that binocular brightness cannot be explained by any model that combines a signal from the left eye and a signal from the right eye. Instead, at least two independent neural signals from each eye are transmitted to a central locus where multi-attribute information about the complete left-eye stimulus configuration is combined with multi-attribute information about the right-eye stimulus configuration.

Adaptation, Ocular↗
PubMed1985

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.

Adult↗
PubMed1985

Color perception with binocularly fused adapting fields of different wavelengths.

By presenting to one eye a small test superimposed on a background field, and to the other eye only a similar background field of a different wavelength (arranged so that the fused percept is the test centered upon a single fused background), the color appearance of the perceived background can be changed while keeping constant the light stimulating the test eye. Measurements demonstrate that the contralateral field clearly influences the color of the test, even when the left- and right-eye backgrounds are very different in wavelength and illuminance. This change in color appearance cannot be explained (a) by simple contribution of a color signal from the contralateral eye, (b) by the perceived color of the fused background, or (c) by combining the effects of contralateral adaptation (alone) and monocular adaptation (alone). Instead, the central-mechanism response depends on the particular pair of wavelengths that are fused. The results suggest chromatic coding of neural signals arriving at the central locus.

Adaptation, Ocular↗