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Biomedical subjects

C W Tyler

Publications and source records attributed to C W Tyler.

At least 19 recordsLinked to original sources
PubMed1992

Disparity tuning in mechanisms of human stereopsis.

The change in sensitivity across some stimulus dimension which follows adaptation to a particular stimulus can reveal a great deal about the tuning characteristics of underlying sensory/perceptual mechanisms. In this study, a psychophysical adaptation paradigm was employed to characterize the disparity tuning of perceptual mechanisms involved in stereopsis. The stimulus was a dynamic random-dot stereogram (DRDS) portraying a surface which varied in interocular correlation (IOC) and retinal disparity. Adaptation to a fully correlated DRDS surface produced an elevation in IOC threshold over a relatively narrow range of disparities, with maximum effect at the disparity of the adapting stimulus. The width of these disparity tuning functions varied from 5 arc min for adaptation at the horopter to 20 arc min for adaptation at 20 arc min disparity. Frequently, IOC sensitivity was enhanced for disparities on either side of the adapted disparity, suggesting that an opponent center-surround organization operates at an early level of disparity processing. A model of underlying channel structure consistent with these data is presented.

Adaptation, Ocular↗
PubMed1992

Failure of rivalry at low contrast: evidence of a suprathreshold binocular summation process.

Presentation of different images to the two eyes normally results in a time-varying alternation between the two images (binocular rivalry). However, we find that when orthogonal gratings are viewed dichoptically at low contrast, a stable summation between the two images is perceived in the form of a dichoptic plaid. The range of perception of the dichoptic plaid depends on spatial frequency, contrast and luminance of the gratings. This phenomenon differs from the "false fusion", a fleeting summation of different images perceived only under very brief presentation of the stimuli. The observations suggest that there exists a neural process that performs a summation of dissimilar images, and that is distinct from the competitive process of suppression and binocular rivalry.

Contrast Sensitivity↗
PubMed1992

Different spatial tunings for ON and OFF pathway stimulation.

We compared the spatial tuning of sensitivity to luminance increments and decrements in three types of localized stimulus presentation with a smooth spatiotemporal envelope. The first type consisted of spatiotemporal Gabor grating functions with either a positive or a negative bias in luminance. The spatial tuning showed a substantially narrower bandwidth and greater peak sensitivity for the positive Gabors. A similar description could be applied to the results for detection of spots with a 2-D difference-of-Gaussian profile, although the shape of the tuning function differed in several respects. We also used the biased Gabor modulation in a contrast self-masking paradigm, where the increment (or decrement) was presented against a steady background with the same spatial configuration, over a range of base contrasts. At medium spatial frequencies the biased masking functions were similar to those typically found for unbiased gratings, in showing a threshold facilitation (dipper) at low contrast, and sub-Weber masking behaviour at higher contrasts. At low spatial frequencies, however, a pronounced asymmetry appeared. Stimuli with a positive bias again showed typical masking behaviour, but a virtually flat masking function was obtained for negative bias stimuli. We conclude that stimuli without abrupt luminance transients reveal pronounced differences in the spatial tuning of responses to positive and negative stimuli, which probably reflect differences in the neural connectivity of the ON and OFF processing systems.

Contrast Sensitivity↗
PubMed1992

Psychophysical derivation of the impulse response through generation of ultrabrief responses: complex inverse estimation without minimum-phase assumptions.

A new technique is presented for the psychophysical measurement of the response phase that requires no assumptions of the relation between the phase and amplitude components of the frequency response. The amplitude attenuation is measured by a standard threshold paradigm and is then compensated for by increasing harmonic strength in proportion to the visual attenuation. Then the phase of each frequency component is adjusted to maximize the internal response amplitude. The amplitude and phase values may be converted by inverse Fourier transformation into the estimated visual impulse response in any stable set of stimulus conditions and are insensitive to a wide variety of response nonlinearities. A key aspect of the phase measurement technique is the generation of a stimulus to obtain the minimum time spread of the internal response, creating a briefer response than for any other stimulus.

Computer Simulation↗
PubMed1992

Analysis of visual modulation sensitivity. V. Faster visual response for G- than for R-cone pathway?

To determine the linear, unadapted responses of the cone pathways, we have measured the critical fusion frequency (CFF) for green (555-nm) and red (642-nm) flicker as a function of retinal illuminance. Both functions obeyed the Ferry-Porter law (CFF proportional to log illuminance) to high accuracy over a > or = 5-log-unit range. In both foveola and periphery the CFF/illuminance functions were significantly steeper for green light than for red light. The peripheral 555-nm function had an average slope 1.26 times the average slope of the 642-nm function. An additive model of flicker detection could not account for the observed differences in slope. A threshold independence model, in which detection is based on the most sensitive mechanism, accurately fits the data. Whichever model is assumed, the presence of different slopes for the two wavelength flicker conditions strongly implies that the R- and G-cone pathways have different temporal properties. The occurrence of steeper CFF/illuminance slopes in response to green light implies that the linear (near-CFF) response of the G-cone pathways in inherently faster than that of the R-cone pathways at both retinal loci. These differences in R- and G-cone-mediated temporal properties complicate the fundamental concept of luminance and invalidate it for precise application over the full illuminance range.

Color Perception↗
PubMed1991

The diagnosis of ovarian cancer by pathologists: how often do diagnoses by contributing pathologists agree with a panel of gynecologic pathologists?

The Cancer and Steroid Hormone Study, a multicenter, population-based, case-control study of ovarian, breast, and endometrial cancer in women 20 to 54 years of age, permitted the diagnoses of contributing pathologists to be compared with those of a panel of three gynecologic pathologists. A diagnosis of ovarian cancer was made by contributing pathologists on 477 subjects. Agreement between the two groups of pathologists was 97% for primary epithelial ovarian cancer and 89% for primary nonepithelial ovarian malignancies. Agreement on diagnosis of major cellular subtypes of ovarian malignancy ranged between 73% for endometrioid cancer and 100% for clear cell carcinomas. We conclude that the diagnosis of pathologic features of primary ovarian cancer is highly predictable. Nonetheless, diagnosis by histologic type varies sufficiently that a review process should be considered for clinical or investigative decisions involving specific histologic diagnoses of ovarian cancer.

Adult↗
PubMed1991

Temporal summation in dark-adapted 10-week old infants.

The effect of stimulus duration on the b-wave and psychophysical responses of dark-adapted 10-week-old infants and adult control subjects is reported. Both infant and adult b-wave sensitivities vary with stimulus duration, show summation for brief duration stimuli, critical durations estimated at 88-155 msec, and little variation in sensitivity for longer durations. There are however, substantial differences between the infant and adult psychophysical temporal summation functions. The infant function is described by a straight line, slope about -0.5, across all flash durations while adults show summation at durations less than 100 msec and critical durations of 136 to 151 msec. Adult, but not infant, b-wave integration times and b-wave rise and fall times show duration-dependent changes. Thus, both ERG and psychophysical measures demonstrate immaturities in the rod mediated function of the infant retina.

Adult↗
PubMed1991

Purely chromatic perception of motion in depth: two eyes as sensitive as one.

Motion hyperacuity (phase) thresholds were measured for both lateral and stereoscopic oscillatory motion in both luminance and equiluminant red/green gratings of 2 cycles per degree. Thresholds for lateral chromatic motion did not exhibit the inhibitory fall-off at low temporal frequencies that was found for luminance motion. Phase thresholds for purely chromatic motion were substantially higher than those for luminance gratings, in proportion to the ratio of cone signal modulation, but they could be predicted from the corresponding contrast sensitivities for both types of stimulus. Stereomovement thresholds in luminance gratings showed the stereomovement suppression effect relative to monocular motion sensitivity previously reported for line stimuli, but purely chromatic gratings did not. Together with the lack of an inhibitory fall-off, these results imply that chromatic and luminance motion are processed by different neural pathways, and that the chrominance pathway is capable of supporting a strong percept of stereoscopic motion from purely chromatic gratings.

Attention↗
PubMed1991

Analysis of normal flicker sensitivity and its variability in the visuogram test.

Flicker sensitivity was measured in groups of younger and older adult observers to assess its mean values and the test-retest variability both between sessions and in a single session. Test-retest sensitivities differed by less than 15% but tended to decrease slightly in a session and increase slightly between sessions. Optical blur had little effect on the measured sensitivities, implying that they were not mediated by the edge information in the display. There were no significant differences between eyes other than those attributable to testing order. Within eyes, inherent variations in human flicker sensitivity accounted for about half of the variance, and within-session variability accounted for most of the remainder.

Adolescent↗
PubMed1990

Development of contrast sensitivity in the human infant.

Contrast sensitivity and grating acuity were measured using the sweep VEP method in a group of 48 infants from 2 to 40 weeks of age and in a group of 10 adults. Sinusoidal gratings were reversed in contrast at 12 alternations per sec at a space-average luminance of 220 cd/m2. During 10 sec trials, either the contrast or the spatial frequency was increased in a series of 19 steps. Thresholds were estimated by extrapolation of the VEP response functions to zero amplitude. The contrast threshold at low spatial frequencies developed rapidly from 7% contrast at 2-3 weeks to an asymptote of 0.5% at 9 weeks. For adults, maximum sensitivity at low spatial frequencies was 0.32-0.22%. The sweep VEP estimate of grating acuity showed a gradual increase in spatial frequency with age, starting at 5 c/deg during the first month and reaching 16.3 c/deg at 8 months. The mean adult acuity was 31.9 c/deg. There appeared to be two phases in the development of contrast sensitivity and acuity. Between 4 and 9 weeks overall contrast sensitivity increased by a factor of 4-5 at all spatial frequencies. Beyond 9 weeks, contrast sensitivity at low spatial frequencies remained constant, while sensitivity increased systematically at higher spatial frequencies.

Adult↗
PubMed1990

A stereoscopic view of visual processing streams.

Recent anatomical and physiological studies of the visual pathway suggest the existence of at least three parallel processing streams in the lateral geniculate/primary cortex structure--a magno/interblob stream for motion and transient information; a parvo/interblob stream for high spatial frequency, static information; and a parvo/blob stream for chromatic and low spatial frequency information. How does this functional typology relate to the processing for stereoscopic depth? Human stereopsis may be viewed as consisting of three distinct types of disparity processing: coarse, local stereopsis suitable for stereomovement processing by the magno/interblob stream; fine, global stereopsis suitable for the processing of complex random-dot stereograms by the parvo/interblob stream; and simple, protostereopsis for processing size differences between the two eyes by the parvo/blob stream. Extensive psychophysical evidence supports the identification of these three disparity processes with the three processing streams.

Color Perception↗
PubMed1990

Analysis of visual modulation sensitivity. IV. Validity of the Ferry-Porter law.

The maximum flicker frequency was determined over a 5+6-log-unit range of retinal illuminance for a stimulus configuration designed to isolate the linear response from long-wavelength (R) cones. For a particular retinal location, the data conformed to the Ferry-Porter law and departed significantly from the predictions of the diffusion equation. The slope of the function was an invariant characteristic and was unaffected by stimulus intensity or area, modulation waveform, or modulation amplitude. However, the slope varied substantially with retinal locus, increasing by more than a factor of 2 between the foveola and 35 degrees eccentricity. This increase shows that the time constant of the linear, unadapted visual response decreases with increasing eccentricity. The difference between foveola and periphery remained at high spatial frequencies, implying that it was not attributable to lateral inhibitory effects.

Algorithms↗
PubMed1989

Measurement of spatial contrast sensitivity with the swept contrast VEP.

Contrast response functions (CRFs) for the VEP were obtained with a Discrete Fourier Transform (DFT) technique employing swept contrast gratings. VEP CRFs in infants were found to have a form similar to those observed in adults, being linear functions of log contrast over a range of near-threshold contrasts. CRFs with low and high contrast lobes were present in infants, as they are in adults. Contrast thresholds were estimated by extrapolation of the CRF to zero microvolts. The effects of additive EEG noise and of the DFT data window on the shape of the measured CRF are considered. For large signals, the measured CRF is nearly independent of the additive noise, but at small signal values additive noise introduces a small bias towards larger amplitudes. The VEP signal-plus-noise distribution was modeled as a family of Rice distributions in order to evaluate the effects of bias on the estimates of threshold. The amount of bias depends inversely upon the slope of the CRF. The amount of bias introduced by a smoothing window also depends upon slope of the CRF as well as the sweep rate. The combined effects of additive noise and window bias were such that the total bias was nearly independent of CRF slope. Sweep VEP contrast thresholds were shown empirically to be unaffected by changes in the range of contrast swept.

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↗
PubMed1979

Local versus general anesthesia: which is safer for performing suction curettage abortions?

The relative safety of suction curettage abortions performed with either local or general anesthesia has not been clearly established. To compare the safety of these two anesthetic techniques, we studied 36,430 women who received local anesthetics and 17,725 who received general anesthetics for this operation in the United States from 1971 through 1975. The aggregated major complication rates for the two groups were similar, but there were significant differences between local and general anesthesia for specific complications and treatments. Local anesthesia was associated with higher rates of febrile and convulsive morbidity; however, general anesthesia was associated with higher rates of hemorrhage, cervical injury, and uterine perforation. Both anesthetic techniques appear to be safe, with similar degrees of overall safety, although each is associated with a different spectrum of complications.

Abdomen↗
PubMed1979

Epidemiology of hepatocellular adenoma. The role of oral contraceptive use.

A case-control study of hepatocellular adenoma (HCA), a serious though nonmalignant liver tumor, was conducted by the Center for Disease Control and the Armed Forces Institute of Pathology (AFIP). Interviews with 79 women with HCA and with 220 age- and neighborhood-matched controls were completed. Limited information was obtained on nine additional patients who had died. Women with HCA and hemorrhage have a greater risk of morbidity and death than those with other symptoms. Increasing duration of OC use increases the risk of HCA. Use of OCs with high hormonal potency and age over 30 years may further increase a woman's risk of HCA. Long-term users of OCs have an estimated annual incidence of HCA of 3 to 4 per 100,000.

Adolescent↗