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W H Swanson

Publications and source records attributed to W H Swanson.

28 records · Page 2Linked to original sources
PubMed1989

Response pooling between chromatic and luminance systems.

Two experiments were designed to examine interactions of the responses of the chromatic and luminance systems to suprathreshold stimuli. We measured simple reaction times (RT) to eight photometrically matched (1 cd/m2) wavelengths between 448 and 658 nm. These chromatic test stimuli were incrementally presented on either a spatially coextensive 1.2 or a larger 2 degrees steady white background. Sectors of the outer annulus region (between 1.2 and 2 degrees) could be removed to allow systematic variation of the extent of the spatial contour between test and background fields. When the white background was spatially coincident with the test field, RTs showed trichromatic saturation-like wavelength dependence, with a maximum RT at 572 nm. As white sectors were added to the annulus (introducing spatial transients between test and background fields), RTs became less and less wavelength-dependent, and were nearly wavelength-independent when a full annulus was used. The data were analysed in terms of a two-system RT model for processing chromaticity and luminance.

Color Perception↗
PubMed1988

Comparison of contrast responses across spatial mechanisms.

We have analysed several recent studies of suprathreshold contrast perception in terms of the normalization of CTFs originally suggested by Georgeson and Sullivan (1975). When suprathreshold responses for different spatial frequencies are compared it is necessary to consider the effects of normalization across CTFs of different spatial mechanisms.

Amblyopia↗
PubMed1988

Effects of chromatic adaptation on phase-dependent sensitivity to heterochromatic flicker.

Temporal modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies from 6 to 20 Hz. The difference between 180 degrees and the phase of least sensitivity was computed as the measured phase shift. A 2 degree test field was superimposed upon 8 degrees chromatic adapting fields with luminances from 100 to 3000 Td and chromaticities of 500, 600, and 650 nm. For each adapting field, the 564-nm source was set to 175 Td, and the 625-nm source was matched to it with heterochromatic flicker photometry (giving an effective mean luminance of 350 Td). The 650-nm adapting fields produced large changes in photometric setting but only small changes in the measured phase shift. The 600- and 500-nm adapting fields produced smaller changes in photometric setting but larger changes in the measured phase shift. In general, increased adapting luminance resulted in an increase in the measured phase shift for 600-nm adaptation and a decrease in the measured phase shift for 500-nm adaptation.

Adaptation, Physiological↗
PubMed1987

Temporal modulation sensitivity and pulse-detection thresholds for chromatic and luminance perturbations.

We studied temporal processing of chromatic and luminance perturbations of a 600-nm field, measuring both modulation sensitivity (sinusoidal frequencies from 0.25 to 40 Hz) and pulse-detection thresholds (pulse durations from 5 to 2560 msec) for mean luminances of 0.9 to 900 Td and field sizes of 0.5 degrees to 8 degrees. Chromatic stimuli were produced by antiphase modulation of lights matched by heterochromatic flicker photometry. Both mean luminance and field size affected sensitivity, and the magnitude of field-size effects increased with mean luminance. We derived both luminance and chromatic impulse response functions for each set of experimental conditions, using the modulation-sensitivity data. At high mean luminances and large field sizes the chromatic impulse response functions are complex, suggesting contributions from both chromatic and luminance mechanisms. Pulse-detection data were fitted by a peak detector model based on these impulse response functions.

Color↗
PubMed1987

Effects of temporal frequency on phase-dependent sensitivity to heterochromatic flicker.

Modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies ranging from 1 to 40 Hz. Measurements were made at a retinal illuminance of 900 Td, with a 2 degree field metameric to approximately 600 nm. For most frequencies, the phase of least sensitivity was other than 180 degrees, indicating the existence of a luminance response to antiphase modulation. Additional data sets examined the effects of field size (0.5 degrees-8 degrees) and mean chromaticity (583-610 nm). Variation in field size, chromaticity, and photometric setting had little effect on the phase of least sensitivity. The data were fitted by using vector summation between luminance and chromatic responses; the modulation-sensitivity functions that resulted from this analysis agreed with those of earlier studies.

Color Perception↗
PubMed1985

Eccentricity dependence of contrast matching and oblique masking.

Recently a model of spatial vision was derived by fitting foveal masking data. This model is composed of a small number of medium bandwidth mechanisms, each composed of a spatial filter followed by a compressive nonlinearity. The present study incorporates eccentricity variation into this model by scaling the peak frequency (but not the shape) of the spatial filters with eccentricity. Predictions of this scaled model were compared with both oblique masking data and contrast matching data gathered at peripheral locations and were found to produce good fits. Therefore, the mode of analysis that was used for foveal masking data is also applicable peripherally once the scaling factor is applied.

Fixation, Ocular↗
PubMed1984

Contrast matching data predicted from contrast increment thresholds.

Predictions for contrast matches were generated using a model with all parameters fixed, and gave fits to contrast matching data gathered using spatially localized 0.79 octave bandwidth patterns. The model has four mechanisms, each composed of a medium-bandwidth spatial filter followed by a contrast transfer function (a nonlinear function relating mechanism response to physical contrast). Parameters for the contrast transfer functions were fixed by fitting contrast increment threshold data. The success of the predictions shows that a small number of medium-bandwidth mechanisms can account for contrast matching results. Spatial pooling was shown to become insignificant at high contrasts. Relative spatial phase was found to be important in contrast matches using sums of frequencies one octave apart. This model shows that four mechanisms are sufficient to predict contrast matches, but does not rule out the possibility of additional mechanisms or of small changes in the mechanism bandwidths.

Filtration↗
PubMedDate not supplied

Color vision screening of young children.

Early detection of congenital color vision defects is desirable, but school screening studies have been stymied by lack of a suitable test. We evaluated a new color vision test, the APT-5, for use by volunteer screeners in schools and preschools. The screeners tested 1794 children, ages 3 to 13 years, and found the APT-5 easy to use with young children ages 5 years and up. Children who failed the screening were recruited for diagnostic color vision testing; for the children ages 5 to 13 years, 56% of those who failed the screening were successfully recruited. Data analysis indicated that the false-positive rate in this age group was 1% to 2%, and that for boys in this age group the positive predictive value was 71% to 81%. Retest data indicated that most false-positives were not due to the test itself, but to other factors in the school screening situation. Two thirds of all children scored as abnormal by anomaloscopy were simple deuteranomalous, indicating that the APT-5 effectively identified even mild color defects. The results of this trial indicate that the APT-5 is suitable for school color vision screening of children ages 5 years and up.

Adolescent↗
PubMedDate not supplied

The importance of controlled illumination in color vision testing in a pediatric ophthalmology clinical practice.

The AO-HRR pseudoisochromatic plates are commonly used for color vision testing in pediatric ophthalmology; however, the recommended procedure for obtaining proper illumination (a completely darkened room and standard illuminant) is typically not followed. To evaluate the role of the illuminant in clinical testing with the AO-HRR, 132 children, ages 3 to 16 years, were tested with and without the recommended illuminant (MacBeth Easel Lamp) and with the self-illuminated APT-5 Color Vision Tester. Twenty-two failed the AO-HRR with the recommended illuminant; 28 failed the AO-HRR without the illuminant. Only 13 failed the APT-5. The Cochran Q test for three related samples showed that the differences among the three groups were significant (Q = 17.1, P < .001). Diagnostic evaluation following clinical screening indicated that the differences among the tests were primarily due to false alarms, which were greatest without the recommended illuminant and least with the APT-5. These results demonstrate the importance of controlled illumination in color vision testing, either by using the recommended illumination with the AO-HRR or a self-illuminated test such as the APT-5.

Adolescent↗