Vicki J. Volbrecht

Isolation of short-wavelength-sensitive cone photoreceptors in 4–6-week-old human infants

Spectral sensitivity was measured for nine infants, 4-6 weeks of age, and three adults under conditions of chromatic adaptation chosen to reveal the presence of short-wavelength-sensitive cones. Monochromatic test stimuli (400-550 nm) were presented at 2 Hz superimposed on a broadband, yellow background. Following 4 min of adaptation to the background, test stimuli were presented while recording the steady-state, visually-evoked cortical potential (VECP). Response averages were obtained for several radiance levels at each test wavelength, and the amplitude of the fundamental frequency was extracted from the digitized response with a fast-Fourier transform. These data were used to construct response vs intensity functions for each wavelength. A fixed criterion response was chosen from the latter family of functions to generate individual spectral sensitivity curves. These VECP spectral sensitivity functions matched the psychophysically-determined functions of adults, measured by the method of adjustment and with the same stimulus configuration. Peak sensitivity for infants and adults under these conditions occurred at about 440 nm, and the main lobe of the curve (400-500 nm) was well fitted by the Vos-Walraven short-wavelength cone fundamental. The only major difference between the infant and adult data was in the relative sensitivity of the secondary mode of the curves (above 500 nm). These results demonstrate the presence of short-wavelength-sensitive cones and a functional pathway to the visual cortex by 4-6 weeks of age.

The effects of perceptual condition on proofreading for misspellings

In two experiments, subjects proofread text in which misspelled words were created by replacing a single letter with another one. The following perceptual conditions were included: clear-cut photocopies of typewritten text, clear-cut text presented on a CRT screen, ditto copies that varied in legibility, and text that included extraneous noise characters superimposed on some letters. In all conditions, subjects adopted a hierarchical feature test that gave first priority to resolving letter envelope and second priority to discriminating other letter features. When clear-cut text with no extraneous noise was proofread, subjects used a sophisticated-guessing decision rule that tolerated misspellings involving missing letter features (as when c replaced e) but not added ones (as when e replaced c). This asymmetrical rule was modified, however, when subjects were exposed to text that included extraneous noise that was confusable with the letter features. In those circumstances, subjects adopted a decision rule that tolerated misspellings involving missing features or any added features that resembled the noise.

Influences of Variation in Lenticular and Macular Pigmentation on Dichromatic Neutral Points

Protanopic, deuteranopic and tritanopic neutral points were computed by determining the wavelength of light that produced the same quantal-catch ratio in the photopigments as that produced by a broad-band light of specified color temperature (range: 2 800–6 600 K). The Vos-Walraven primaries were used as photopigment absorption spectra that were screened by varying densities of ocular (0.5–2,5 at 400 nm) and macular (0.0–1.0 at 460nm) pigmentation. The computations were carried out in 1 nm steps for the wavelength range of 380 to 720 nm. Most of the empirically determined mean, neutral-point loci in the literature were predicted from these computations to within 1–2 nm when average ocular and macular pigment densities were used. The neutral-point range associated with the extreme values of the prereceptoral screening pigments was up to 25 nm for protanopes and deuteranopes and up to 13 nm for tritanopes.

Temporal induction of blackness--I. Color appearance.

Three color-normal observers described the appearance of colors under conditions of temporal induction. One of four inducing fields (unique blue, unique green, unique yellow, unique white) was foveally viewed for 5 sec followed immediately by a 400 msec reference stimulus of the same size (0.75 deg) and spatial location as the inducing field. The reference stimulus was 3 or 5 td and appeared achromatic to the observer when viewed without the inducing field. After presentation of this temporal sequence, the observers described the reference stimulus by assigning percentages to the terms red, green, yellow, blue, white and black. The range of inducing field intensities was -0.4-3.2 log td. Both chromatic and achromatic induction occurred at low illuminance levels of the inducing fields. As the illuminance of the inducing field increased, the reference stimulus gradually became blacker and the chromatic components less prominent. The minimum illuminance level at which the reference appeared 100% black was the same for the four different inducing fields.

Estimating Middle-Wavelength and Long-Wavelength Cone Sensitivity with Large, Long-Duration Targets and Small, Brief Targets

Incremental threshold versus field-intensity curves (tvi) and spectral sensitivity functions were measured for 500 nm and 667 nm test flashes, either under the standard two-colour threshold conditions of Stiles (target: 1.0 deg, 200 ms)—which are known to favour detection by the chromatically-opponent pathways—or with a smaller (0.1 deg), shorter-duration (10 ms) target—chosen to favour detection by the non-opponent (achromatic) pathway. The data reveal differences between the two conditions: for the small, brief target, the tvi curves were shallower and less dependent upon wavelength, and the spectal sensitivity functions were narrower than for the standard target.

Temporal induction of blackness—II. Spectral efficiency and tests of additivity

The perception of blackness was investigated by measuring spectral efficiency and field additivity under conditions of temporal induction. For both purposes, observers foveally viewed a 0.75 deg, inducing stimulus for 5 sec followed immediately (or 200 msec later) by a broadband (5500 K) reference stimulus of the same size and spatial location. For spectral efficiency measurements, the inducing field was a monochromatic light between 400 and 700 nm (10 nm steps), while the additivity studies involved various wavelength mixtures. The psychophysical task was to increase the radiance of the inducing field until the reference stimulus just turned completely black. For two observers the spectral efficiency function of temporally-induced blackness more closely resembled their heterochromatic flicker photometry (HFP) function than their direct brightness-matching function. Their brightness functions were characterized by an inflection at about 580 nm which is generally ascribed to subtractive cone interactions, but their blackness-induction and HFP functions did not show this inflection. The brightness function of the third observer did not show an inflection at 580 nm, thereby making it difficult to differentiate between her three spectral efficiency functions. Overall, the subtle differences between the various spectral efficiency functions made it difficult to determine whether blackness induction was more similar to HFP or brightness matching. The results from the additivity tests of blackness induction, HFP and direct brightness matching removed this ambiguity from the spectral efficiency findings. Blackness induction and HFP were shown to be additive, whereas the results from brightness matching showed clear additivity failures of the cancellation type. These data support the view that the perception of blackness is mediated by neural mechanisms that additively combine the inputs of middle- and long-wave photoreceptors.

Small field tritanopia in the peripheral retina.

If stimuli are made sufficiently small, color-normal individuals report a loss in hue perception, in particular a decrease in the perception of green, in both the fovea and peripheral retina. This effect is referred to as small field tritanopia. It is not clear, however, how rod input may alter the dynamics of small field tritanopia in the peripheral retina. This paper looks at peripheral hue-naming data obtained for small stimuli at mesopic and photopic retinal illuminances under conditions that minimize (bleach) and maximize (no bleach) rod contribution. The data show that attenuation in the perception of green occurs with larger stimuli in the no-bleach condition than in the bleach condition. As retinal illuminance increases, the stimulus size that elicits small field tritanopia decreases, but the stimulus size is still larger under the no-bleach condition. Small field tritanopia in both the bleach and no-bleach conditions may be related to short-wavelength-sensitive (S) cone activity and its potential role in the mediation of the perception of green. The differences in stimulus size for small field tritanopia may be explained by rod input into the magnocellular and koniocellular pathways, which compromises the strength of the chromatic signals and creates a differential loss in the perception of green as compared to the other elemental hues.

Perception of electronic display colours as a function of retinal illuminance

Abstract Colour-naming data were objected from four colour-normal observers to determine the nature of the changes in colour appearance that can occur with changes in illuminance for spectral and nonspectral stimuli comparable to those used in electronic displays. Test stimuli were presented as one degree, foveal flashes of one second duration in Maxwellian view. The retinal illuminance of the nine test stimuli varied from −1.0 to 4.0 log trolands (0.01-1 147fL). Shifts in perceived colour were observed with changes in stimulus intensity. At low illuminance levels, the stimuli appeared desaturated, with black and white responses dominating chromatic responses. At higher retinal illuminances, the stimuli were more saturated, although perceived colour was not constant with changing light levels. In general, blue and yellow responses increased at a faster rate relative to red and green responses with increasing illuminance. The data have important implications for the use of colour on electronic displays.