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Biomedical subjects

C W Tyler

Publications and source records attributed to C W Tyler.

At least 73 records · Page 4Linked to original sources
PubMed1988

High visual contrast sensitivity in the young human infant.

The visual evoked potential (VEP) was used to estimate photopic contrast sensitivity of 10-week-old infants over a wide range of spatial frequencies including the acuity limit. Adult and infant VEP contrast sensitivity was compared for sinusoidal luminance gratings reversed in contrast at 6 Hz. Space-average luminance was 220 cd/m2. Grating contrast was swept from well below the measured thresholds to well above them in 10 sec trials. Contrast thresholds were defined as the zero voltage intercept of the initial rising portion of the VEP amplitude versus contrast function. The VEP contrast sensitivity of 10-week-old infants was close to that of the adults for spatial frequencies below about 1 cycle (c)/deg.

Adult↗
PubMed1987

Postmenopausal smoking, estrogen replacement therapy, and the risk of endometrial cancer.

Previous studies of the association between cigarette smoking and endometrial cancer have yielded inconsistent results. There is some evidence that this discrepancy may be explained by differences in menopausal status between the groups of women studied. We addressed the issue of the effects of postmenopausal smoking on endometrial cancer risk using data from the Cancer and Steroid Hormone Study, a multicenter, population-based, case-control study of gynecologic cancers in the United States. We found that smoking after natural menopause is associated with a 70% reduced risk of endometrial cancer among estrogen users and a 50% reduced risk among nonusers of estrogen. These findings are consistent with previously proposed biologic effects of smoking on estrogen metabolism, which may have important clinical implications.

Adult↗
PubMed1987

Analysis of visual modulation sensitivity. III. Meridional variations in peripheral flicker sensitivity.

Critical flicker frequency (CFF) and modulation sensitivity were measured throughout the visual field as a function of meridian, eccentricity, and luminance. The stimulus conditions were designed to excite preferentially the long-wavelength-sensitive (R) cones in equal numbers at each retinal location. Temporal-frequency responses under these conditions can be partitioned into two components by following the analysis of Kelly [J. Opt. Soc. Am. 51, 422 (1961)]: a high-frequency linear component unaffected by the mean luminance level and a nonlinear adaptive component that conforms to Weber's law of proportionality to luminance. The results suggest that the time constant of the linear component varies to some extent with meridian at a constant eccentricity, in addition to the substantial increase with eccentricity along all meridians. Retinal variations in sensitivity of the nonlinear, adaptive aspect of the visual response were small when equal numbers of cones were stimulated, and the variations did not account for the changes in CFF with either eccentricity or meridian.

Humans↗
PubMed1986

Different encoding mechanisms for phase and contrast.

Phase sensitivity was assessed over a large range of exposure durations by means of a 2AFC staircase procedure where the observer had to detect the relative position of sinusoidal gratings relative to a superimposed thin, dark line. Phase discrimination thresholds decreased as a function of exposure duration although the contrast of the stimuli was weighted for equal detectability at all durations. Phase sensitivity improved markedly with contrast, as opposed to the degradation with contrast seen in contrast discrimination paradigms. The contrast and time functions of phase sensitivity both support the hypothesis that phase is processed separately from contrast by a pathway with different temporal and contrast characteristics. We propose a model where phase sensitivity depends on a luminance subtraction process with a time constant of about 130 msec.

Discrimination, Psychological↗
PubMed1986

Electrophysiological assessment of contrast sensitivity in human infants.

Contrast sensitivity functions (CSF's) were measured for a group of 6-month-old infants using the visual evoked potential (VEP). Sine-wave luminance gratings were counterphase modulated at 12 contrast reversals per s and simultaneously swept in contrast. Each contrast sweep lasted 10 s, spanning a range of 0.5 to 40% contrast in 19 equal logarithmic steps. The amplitude and phase of the response at the second harmonic were determined by a discrete Fourier transform. Contrast thresholds were estimated from a linear extrapolation to zero-amplitude of the VEP amplitude vs. log-contrast function. Contrast sensitivity was found to be nearly adult-like at 1 c/deg, but was not yet mature at higher spatial frequencies.

Child Development↗
PubMed1986

Comparative study of electrophysiological and psychophysical measurement of the contrast sensitivity function in humans.

We measured the human contrast sensitivity function (CSF) both electrophysiologically with the steady-state visual evoked potential (VEP) and psychophysically using a method of ascending limits. VEP contrast thresholds were determined using a rapid recording technique in which the contrast of a counterphase modulated sine wave grating was swept logarithmically from 0.5 to 40% over a period of 10 s. For this pattern reversal stimulus the amplitude and phase of the second harmonic response as a function of contrast were measured using a discrete Fourier transform (DFT). Psychophysical thresholds were determined on the same trials used to record the VEP. Near threshold the VEP amplitude vs. contrast function was approximately linear and VEP contrast thresholds were estimated by a linear extrapolation to zero amplitude. The contrast thresholds obtained by the two methods correlated at 0.914 for 5 observers, with a mean discrepancy of only 12%. At higher contrasts, the VEP amplitude vs. contrast function often became nonmonotonic, sometimes showing two amplitude peaks.

Electrophysiology↗
PubMed1986

New look at Bloch's law for contrast.

It has been commonly reported that the temporal integration of grating contrast proceeds more slowly as spatial frequency is increased. Such results have been based on the critical duration for sensitivity to contrast pulses varying in duration, but the analyses have not assumed full integration at short durations and have neglected the effects of probability summation over time. To take such effects into account, we discuss a class of analytical models based on nonlinear temporal integration. On the assumption that the temporal impulse response of the visual system determines contrast integration over time, we develop both a high-threshold model and a signal-detection approach involving multiple and independent nonlinear signal detectors with a time-limited integration span. The redefined critical durations predicted by the models and verified by the data are about 35 msec and vary by no more than 10 msec across spatial frequency. This variation is entirely attributable to a change in the strength of inhibition with spatial frequency, and the analysis implies that the excitatory component is constant at all spatial frequencies, contrary to previous accounts.

Humans↗
PubMed1986

Phase discrimination of compound gratings: generalized autocorrelation analysis.

The Julesz paradigm involving statistical constraints for the study of texture discrimination is extended to continuous-contrast repetitive patterns by using nth-order autocorrelation functions rather than Julesz's nth-order statistics. Second-order autocorrelations specify the power spectrum of the pattern, and the higher-order autocorrelations specify different levels of phase relationships in the pattern. A method is presented for generating patterns with statistical constraints of any order, in one and two dimensions. We show that, without scrutiny by foveal attention, discrimination of continuous textures fails at about the level of fourth-order constraints. An explanation for this failure based on the bandwidth of spatial-frequency-tuned mechanisms is excluded. The autocorrelation approach therefore may provide a general metric for the description of phase discrimination of repetitive textures.

Biometry↗
PubMed1986

Invariance of the slope of the psychometric function with spatial summation.

The high-threshold probability summation model for improved detection of near-threshold gratings with increased spatial extent of the patterns assumes that the detection psychometric function is a Weibull function. This model predicts that the slope of the psychometric function should not change as the number of mechanisms stimulated increases, although other models predict that the slope should vary. We confirm for a two-alternative, forced-choice paradigm that the slope parameter does not vary systematically with spatial frequency or with number of periods of the grating, although there are reliable differences in average slope between observers.

Female↗
PubMed1985

Endometrial cancer: how does cigarette smoking influence the risk of women under age 55 years having this tumor?

This analysis of the Cancer and Steroid Hormone Study, a multicenter, population-based case control investigation of hormone use by women of reproductive age and endometrial, breast, and ovarian cancer shows that cigarette smoking is not associated with either an increased or a decreased risk of endometrial cancer. This study included 437 women with endometrial cancer and 3200 control subjects, all of whom were between the ages of 20 and 54 years at the time of interview. The absence of any alteration of the risk of endometrial cancer and smoking was found consistently no matter which variable was used as a measure of smoking--ever or never smoked cigarettes, former or current smoking, light or heavy smoking, or age smoking began.

Adult↗
PubMed1985

Infant VEP acuity measurements: analysis of individual differences and measurement error.

The reliability of visual evoked potential (VEP) measurements of acuity was determined by estimating acuity for sinewave luminance gratings which were counterphase modulated at either 12 or 20 reversals/sec (rps). Gratings were swept in spatial frequency beyond the acuity limit and acuity was estimated from an extrapolation based on the last peak in the VEP amplitude versus spatial frequency function. Twenty-five infants ranging in age from 17 to 25 weeks were studied. Individual 10 sec sweeps resulted in records with a criterion response in 65-75% of trials. The reliability of acuities obtained from individual 10 sec sweeps was +/- 0.54 octaves at 95% confidence across 12 and 20 rps recording conditions (RMS error of +/- 0.27 octaves). The best acuity attained by each infant on either a single sweep, or on their vector average, was reliable to +/- 0.38 octaves at 95% confidence (RMS error of +/- 0.19 octaves) compared to a range of individual acuities of about 2 octaves. Much of the variability of sweep VEP acuity in cross-sectional samples of infants is therefore attributable to reliable individual differences rather than to measurement error. In testing individual infants our analysis indicates that choice of temporal frequency accounts for only 14% of the variation in acuity estimates within subjects.

Electroencephalography↗
PubMed1985

Electrophysiological evidence for the existence of coarse and fine disparity mechanisms in human.

Visual sensitivity to stereoscopic disparity changes was measured both psychophysically and by means of evoked potentials. The binocular disparity of a dynamic random-dot stereogram portraying a single flat plane alternated between two values symmetrical about the plane of fixation. The threshold for disparity alternation of the stereoscopic plane was determined at alternation rates between 4 and 12 depth reversals per second (rps). Evoked potential and forced-choice psychophysical estimates of stereoscopic threshold at each reversal frequency agreed, with a mean discrepancy of only +/- 0.1 log units. Evoked potential amplitude was a linear function of log disparity up to about 15 arc min peak to peak disparity. For larger disparities, the evoked potential amplitude versus log disparity function was found to be nonmonotonic with a dip occurring at approximately 26 arc min disparity. Responses to fine disparities of less than 20 arc min lay close to one temporal phase while those evoked by coarse disparities greater than 40 arc min lay near a different phase. The data suggest that disparity processing mechanisms either undergo dynamic changes as disparity increases or that processing shifts between at least two independent mechanisms.

Depth Perception↗
PubMed1985

Effects of contrast, orientation and binocularity in the pattern evoked potential.

Monocular and binocular visual evoked potentials were studied as a function of modulation depth (contrast) of a counterphase sinusoidal grating stimulus. A range of spatial and temporal frequencies of stimulation were used. The contrast functions showed many different forms, and were in some cases nonmonotonic. The binocular response usually had a steeper slope than the monocular response. Extrapolating the slope to the zero intercept gave electrophysiological thresholds which did not correspond with psychophysical threshold at any spatial or temporal frequency used. The binocular response was reduced to the monocular level when the orientation difference between the two eyes exceeded about 20 degrees. The data show that the pattern evoked potential is highly specific to many stimulus variables.

Depth Perception↗
PubMed1985

Spatial frequency sweep VEP: visual acuity during the first year of life.

The grating acuity of 197 infants from 1 week to 53 weeks of age was measured using the visual evoked potential (VEP) in response to counterphase grating stimulation. The gratings were presented as a 10 sec spatial frequency sweep which spanned the acuity limit. The amplitude and phase of the second harmonic response were extracted by discrete Fourier analysis. The VEP amplitude versus spatial frequency function showed narrow spatial frequency tuning with amplitude peaks at one or more spatial frequencies. The phase of the response at medium to high spatial frequencies was generally constant at a spatial frequency peak, followed by a progressive phase lag with increasing spatial frequency. Grating acuity was estimated by linear extrapolation to zero microvolts of the highest spatial frequency peak in the VEP amplitude versus spatial frequency function. This visual acuity estimate increased from a mean of 4.5 c/deg during the first month to about 20 c/deg at 8-13 months of age. The VEP acuities at 1 month are a factor of three to five higher than previously reported for pattern reversal or pattern appearance stimuli. By 8 months VEP grating resolution was not reliably different from adult levels in the same apparatus.

Adult↗
PubMed1985

Analysis of visual modulation sensitivity. II. Peripheral retina and the role of photoreceptor dimensions.

Temporal-frequency characteristics were measured as a function of retinal location, with test-field size scaled to provide equivalent sensitivity at each eccentricity. The results showed that the temporal-frequency limits increased uniformly by about a factor of 2 between the fovea and 45 degrees eccentricity, corresponding to a decrease in the response-time constant from 70 to 35 msec. These data were compared with the change in inner- and outer-segment diameters of the cones across the same retinal range. The data conform to the hypothesis that visual time constant varies inversely with cone outer-segment diameter. A second hypothesis that visual sensitivity increases in proportion to come outer-segment length in the central fovea was also supported.

Flicker Fusion↗
PubMed1984

Temporal frequency limits for stereoscopic apparent motion processes.

Temporal processing limits were determined for two types of stereoscopic percept associated with square wave disparity alternation: apparent depth motion and depth pulsation. The stimuli were dynamic random dot stereograms containing no monocular cues for either target motion or disparity change. The percept of a single noise plane undergoing apparent depth motion coincided with the extent of a large peak in the low frequency portion of the evoked potential amplitude spectrum. The limit for apparent depth motion was approximately 6 Hz. Above this frequency two pulsating depth planes were seen simultaneously. Depth pulsations wer visible up to 14 Hz and an evoked potential occurred in synchrony with each disparity change (up to 28 depth reversals/sec). Above 14 Hz two transparent planes were perceived without depth pulsation and no stereoscopic evoked potential could be recorded. The results indicate a higher temporal resolution for stereoscopic position change than has been reported in previous studies of apparent depth motion.

Depth Perception↗
PubMed1984

Perceived velocity of moving chromatic gratings.

Equiluminous red-green sine-wave gratings were drifted at a uniform rate in the bottom half of a 10-deg field. In the top half of the display was a sinusoidal-luminance grating of the same spatial frequency and 95% contrast that drifted in the opposite direction. Observers, while fixating a point in the display center, adjusted the speed of this upper comparison grating so that it appeared to match the velocity of the chromatic grating below. At low spatial frequencies, equiluminous gratings were appreciably slowed and sometimes stopped even though the individual bars of the grating could be easily resolved. The amount of slowing was proportionally greatest for gratings with slow drift rates. Blue-yellow sine-wave gratings showed similar effects. When luminance contrast was held constant, increasing chrominance modulation caused further decreases in apparent velocity, ruling out the possibility that the slowing was simply due to decreased luminance contrast. Perceived velocity appears to be a weighted average of luminance and chrominance velocity information.

Color Perception↗