Biomedical subjects

B R Stephens

Publications and source records attributed to B R Stephens.

12 recordsLinked to original sources

PubMed2001

Asymmetries in contrast polarity processing in young human infants.

Luminance increments and decrements of equal magnitude are processed asymmetrically in the adult visual system. At detection threshold, decrements are slightly easier to detect than increments. At suprathreshold contrast levels decrements appear to have more contrast than increments when both differ from the background by the same absolute amount. Two experiments are reported with 3.5-month-old human infants examining the processing of luminance increments and decrements. Using two different methods to measure the relative salience of positive and negative polarity high contrast bars, we found consistent evidence that dark bars appeared more salient to infants than light bars when both differed from the background by the same absolute amount. The asymmetry may be explained by noting that when luminance increments and decrements have the same Weber contrast, the decrements will have greater Michelson contrast. Perceived contrast in adults follows Michelson contrast more closely than Weber contrast, and a similar metric may characterize the relations between negative and positive contrasts in young human infants.

Contrast Sensitivity↗

PubMed2000

Contrast discrimination under temporally varying contrast conditions.

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was tested using a paradigm that alternated test and masking gratings at 8 Hz. Masking contrasts were lower than, equal to, or higher than the test contrasts. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contrast curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them diagonally by the ratio of their masking contrasts. It is shown that this behavior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 msec, and contrast discrimination is a function of this time-averaged contrast.

Contrast Sensitivity↗

PubMed1998

Contrast gain control in psychophysical contrast discrimination.

Contrast masking may in part reflect the operation of contrast gain control mechanisms in the visual system. Four experiments were conducted in order to examine contrast discrimination under conditions in which the base contrast was interrupted with various maskers. Interrupting the base contrast grating at 8 Hz--with a uniform field, with a randomly phase-shifted version of the same grating, or with a higher contrast version of the same grating--interfered with contrast discrimination. These effects occurred only when the background contrast was above its own threshold. These results may be explained by assuming that the networks responsible for contrast gain control operate by temporally averaging contrast over a period greater than 62.5 msec. When this time-averaged contrast is different from the base contrast, contrast discrimination suffers.

Contrast Sensitivity↗

PubMed1988

Effect of reinforcement on infants' performance in a preferential looking acuity task.

Two experiments examined the effect of reinforcement on infants' performance in a preferential looking acuity task. In experiment 1, performance of 3- and 5-month-old infants was assessed under three conditions. In one condition, reinforcement was contingent upon performance. In another, no reinforcement was provided. In the last condition, reinforcement was provided, but it was not contingent upon performance. Similar performance was observed in all three conditions. In experiment 2, 7-month-olds were tested in the first and second conditions. No differences in performance were observed between these two groups. Thus, the reinforcement used in preferential looking measurements does not appear to improve performance significantly for 3- to 7-month-old infants. Several interpretations of these results are presented. The most plausible is that performance under nonreinforced conditions is already nearly optimal, so the addition of reinforcement has little impact. General implications for estimates of infant visual sensitivity in the laboratory and clinic are discussed.

Humans↗

PubMed1988

Effect of feedback on estimates of visual sensitivity.

Estimates of visual sensitivity are often difficult to interpret because nonsensory factors may influence performance on a psychophysical task. We examined for 19 naive students, ages 10 to 40 yr., the effect of feedback on estimates of contrast sensitivity and found no evidence that feedback improves performance when compared to no feedback conditions.

Adolescent↗

PubMed1988

A critical test of infant pattern preference models.

Models of infant visual preferences with predictions based on the physical attributes of visual patterns were evaluated using pairs of schematic faces and abstract patterns that were identical except for contrast reversals. Preferences at 6 weeks were entirely consistent with the predictions of these models. However, at 12 weeks the preferences for facelike images were in clear violation of the predictions of these models. These results represent the first unambiguous demonstration of a face preference in young human infants. The results also allow rejection of all current stimulus-based models of visual preference and suggest that a fundamental change in the determinants of visual preference occurs between 6 and 12 weeks postnatally.

Attention↗

PubMed1987

Contrast discrimination in human infants.

The ability to detect differences in spatial contrast is crucial to object recognition and identification. This ability is generally examined by measuring the contrast discrimination function. This function represents, for a variety of conditions, the smallest contrast difference required to discriminate otherwise identical patterns. We examined human infants' ability to discriminate patterns on the basis of differences in spatial contrast. The forced-choice preferential looking procedure was used to estimate contrast increment thresholds at a number of background contrasts. The Weber fractions of 6- and 12-week-old infants were about 1 log unit higher than adult values for background contrasts ranging from 0.14 to 0.55. Furthermore, the slopes of infants' discrimination functions were much shallower than those of adults. These age differences in contrast discrimination imply certain changes in the neural mechanisms that underlie contrast encoding. They also aid our understanding of the anomalies observed in early pattern vision.

Attention↗

PubMed1985

The development of basic mechanisms of pattern vision: spatial frequency channels.

The mature visual system possesses mechanisms that analyze visual inputs into bands of spatial frequency. This analysis appears to be important to several visual capabilities. We have investigated the development of these spatial-frequency channels in young infants. Experiment 1 used a masking paradigm to test 6-week-olds, 12-week-olds, and adults. The detectability of sine wave gratings of different spatial frequencies was measured in the presence and the absence of a narrowband noise masker. The 12-week data showed that at least two spatial-frequency channels with adultlike specificity are present at 12 weeks. The 6-week data did not reveal the presence of narrowband spatial-frequency channels. Experiment 2 used a different paradigm to investigate the same issue. The detectability of gratings composed of two sine wave components was measured in 6-week-olds and adults. The results were entirely consistent with those of experiment 1. The 12-week and adult data indicated the presence of narrowband spatial-frequency channels. The 6-week data did not. The results of these experiments suggest that the manner in which pattern information is processed changes fundamentally between 6 and 12 weeks of age.

Adult↗

PubMed1985

The development of contrast constancy.

The mature visual system possesses mechanisms that enable invariant perception of the contrast of an object and its features as the object undergoes changes in distance. This phenomenon, which has been called contrast constancy, obtains at suprathreshold contrasts only. Some models of contrast constancy assume the presence of narrowband spatial-frequency channels. An implication of M.S. Banks, B.R. Stephens, and E.E. Hartmann (1985, Journal of Experimental Child Psychology, 40, 501-527) is that contrast constancy should not be observed at 6 weeks but may be observed at 12 weeks. We examined this implication by investigating the development of contrast constancy in 6- and 12-week-old infants. Two sine wave gratings, differing in spatial frequency by a factor of 3, were presented side-by-side. The contrast of one grating was varied in order to estimate the contrast at which preference for the two gratings was equal. The equal preference points for 6-week-olds were predictable from their contrast thresholds. The 12-week-olds' equal preference points for low-contrast stimuli were predictable from their contrast thresholds, but those for intermediate and high-contrast stimuli were not. Thus, if one accepts the assumption that equal preference in infants is analogous to apparent contrast matches in adults, these data imply that contrast constancy is observed at 12 weeks but not 6 weeks. The perceptual consequences of this developmental transition are discussed.

Adult↗

PubMed1982

The contrast sensitivity of human infants to gratings differing in duty cycle.

Human infants' contrast thresholds for rectangular wave gratings differing in duty cycle were measured. The forced-choice preferential looking technique was used to estimate thresholds in fifteen 8- to 10-week old infants. The results indicated that contrast threshold varied systematically with duty cycle. Our findings were consistent with the predictions of a linear, multiple channel model and one version of a linear, single channel model but were inconsistent with the predictions of a contour density and another version of a single channel model.

Form Perception↗

PubMed1982

A failure to observe negative preference in infant acuity testing.

Held et al. (1979) Vision Res. 19, 1377-1379, reported that infants' psychometric functions in preferential looking experiments exhibit a region of below chance performance or "negative preference". They argued from this that previous preferential looking experiments may have systematically underestimated acuity because they ignored this negative preference. We present data from an experiment designed to reveal negative preference in the version of the preferential looking paradigm used in our laboratory. The results do not exhibit negative preference apart from random variations about 50%. Gwiazda et al. (1980) Am. J. Optom. physiol. Opt. 57, 420-427; 428-432, developed a psychophysical procedure for infant testing that is designed to yield accurate threshold estimates in a short period of time. We present the results of computer simulations of more conventional psychophysical procedures. The simulations demonstrate that an up-down staircase procedure is more efficient than the Gwiazda et al. procedure.

Computers↗

PubMed1982

Sample preparation in determination of lead in garden vegetables by flame atomic absorption spectrophotometry.

Dry and wet ashing methods have been used in the analysis of garden vegetables for Pb. The reliability of wet ashing has been verified by the method of standard additions. Comparison of dry and wet ashing showed good agreement for a variety of garden vegetables. Sample size was more strictly limited for the wet-ashed samples, which led to lower sensitivity. Vegetable samples are commonly analyzed for a number of trace elements, which introduces additional constraints on sample preparation, notably because of Cd loss on dry ashing. Pretreatment with HNO3/H2SO4 ash aid eliminated Cd loss. Reliability of dry ashing with pretreatment was shown with NBS SRM Orchard Leaves, Pine Needles, Spinach, and Tomato Leaves. The analysis was insensitive to ashing temperature in the range 480-625 degrees C. A practical detection limit for the method is about 2 ppm Pb, dry weight basis (DWB). Care must be exercised to avoid contamination of the sample with lead at this level by improper handling. Segregation and acid washing of glassware and protection of the sample from contact with any object not demonstrably clean was necessary. No evidence was found of Pb contamination at this level from tap water washing of fresh vegetables, forced-air oven drying, or grinding with mortar and pestle. No special clean room facilities or laboratory air purification measures were used. Sensitivity was increased 3-fold by extraction with dithizone in CHCl3 followed by back-extraction into dilute HCl. Detection limits were not improved, however, because of variation in the extraction results. The instrumental method for assessing effective correction for back-ground absorbance showed adequate compensation, although comparison of direct and extractive determinations showed a small but significant difference between the methods of about 1 ppm Pb (DWB).

Hot Temperature↗