Assumptions concerning orthogonality in threshold-scaled versus cone-contrast colour spaces.
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Color Perception↗
Biomedical subjects
M J Sankeralli
Publications and source records attributed to M J Sankeralli.
Evidence for mild blue-yellow colour vision deficits immediately following fluorescein angiography.
AIMS: We have investigated the short term effects of fluorescein angiography on the blue-yellow, red-green, and luminance contrast sensitivity of patients with early age-related macular degeneration (ARMD). METHODS: Nine ARMD patients with no exudative complications and a visual acuity of 20/60 or better in the tested eye were selected. Cardinal colour directions for the isolation of the red-green, blue-yellow and achromatic (luminance) visual mechanisms were determined for each patient. Contrast sensitivity was measured in each cardinal colour direction immediately before and 20 min after standard 20-flash fluorescein angiography. RESULTS: A significant, albeit mild, reduction for blue-yellow contrast sensitivity following angiography was observed (ANOVA, alpha = 0.05). Fluorescein angiographic exposure had no significant effect on red-green or luminance contrast sensitivity. CONCLUSION: Our results show that fluorescein angiography causes at least a short term deficit selective to blue-yellow contrast sensitivity in our patient group.
Aged↗
Evidence for the stochastic independence of the blue-yellow, red-green and luminance detection mechanisms revealed by subthreshold summation.
We investigated the manner in which the outputs of the three postreceptoral mechanisms (red-green, blue-yellow and luminance) combine to determine contrast threshold. We used a subthreshold summation paradigm to test whether the combination of the postreceptoral mechanism outputs could be described by a probability summation model which assumes stochastic independence of the mechanisms, and determined the best fitting summation exponent. Stimuli were Gaussian enveloped 1 c/d sinusoidal gratings represented in a 3D cardinal space transformed from cone contrast axes, and normalized to detection threshold. The use of this space avoids the presence of elongated threshold contours, allowing a reliable model fit to include the less sensitive blue-yellow and luminance mechanisms. Our results were well fitted by the probability summation model and hence support the underlying stochastic independence of the three postreceptoral mechanisms.
Chi-Square Distribution↗
Ratio model for suprathreshold hue-increment detection.
We use psychophysical techniques to investigate the neural mechanisms subserving suprathreshold chromatic discrimination in human vision. We address two questions: (1) How are the postreceptoral detection mechanism responses combined to form suprathreshold chromatic discriminators? and (2) How do these discriminators contribute to color perception? We use a pedestal paradigm in which the subject is required to distinguish between a pedestal stimulus and the same pedestal added to a chromatic increment (the test). Our stimuli are represented in a cardinal space, in which the axes express the responses of the three postreceptoral detection mechanisms normalized relative to their respective detection thresholds. In the main experiment the test (a hue increment) was fixed in the direction orthogonal to the pedestal in our cardinal space. We found that, for high pedestal contrasts, the test threshold varied proportionally with the pedestal contrast. This result suggests the presence of a hue-increment detector dependent on the ratio of the outputs from the red-green and blue-yellow postreceptoral detection mechanisms. The exception to this was for pedestals and tests fixed along the cardinal axes. In that case detection was enhanced by direct input from the postreceptoral mechanism capable of detecting the test in isolation. Our results also indicate that discrimination in the red-green/luminance and blue-yellow/luminance planes exhibits a behavior similar to discrimination within the isoluminant plane. In the final experiment we observed that thresholds for hue-increment identification (e.g., selecting the bluer of two stimuli) are also governed by a ratio relationship. This finding suggests that our ratio-based mechanisms play an important role in color-difference perception.
Adult↗
Postreceptoral chromatic detection mechanisms revealed by noise masking in three-dimensional cone contrast space.
We used a noise masking technique to test the hypothesis that detection is subserved by only two chromatic postreceptoral mechanisms (red-green and blue-yellow) and one achromatic (luminance) mechanism. The task was to detect a 1-c/deg Gaussian enveloped grating presented in a mask of static, spatially low-passed binary or Gaussian distributed noise. In the main experiment, the direction of the test stimulus (termed the signal) was constant in cone contrast space, and the direction of the noise was sampled in equally spaced directions within a plane (the noise plane) in the space. The signal was chosen to coincide with one of the three cardinal directions of three postulated mechanisms. The noise plane was selected to span two of the cardinal directions, including that chosen as the signal direction. As the noise direction was sampled around the noise plane, the signal detection threshold was found to vary in accordance with a linear cosine model, which predicted noise directions yielding maximum and minimum masking of the signal. In the direction of minimum masking (termed a null direction), the noise was found to have no masking effect on the signal. Moreover, the null was not orthogonal to the signal direction but lay instead in one of the cardinal directions. Our findings suggest that detection is mediated by only three mechanisms. In a further experiment we found little or no cross masking between each pair of cardinal directions up to the limit of our noise mask contrasts. This further supports the presence of no more than three independent postreceptoral mechanisms.
Artifacts↗
Color and luminance vision in human amblyopia: shifts in isoluminance, contrast sensitivity losses, and positional deficits.
The deficits for contrast detection and positional accuracy were compared for chromatic and luminance mechanisms within a group of strabismic and anisometropic amblyopes. We found that the isoluminant point was shifted towards red in the amblyopic compared to the fellow normal eye. This was not accounted for by eccentric fixation by the amblyopic eye. Contrast sensitivity deficits were similar for luminance and color stimuli in normal and amblyopic visual systems. In the majority of our amblyopic subjects, however, the deficits in positional acuity were greater for the chromatic than the luminance stimuli.
Adolescent↗
Bipolar or rectified chromatic detection mechanisms?
It is widely accepted that human color vision is based on two types of cone-opponent mechanism, one differencing L and M cone types (loosely termed "red-green"), and the other differencing S with the L and M cones (loosely termed "blue-yellow"). The traditional view of the early processing of human color vision suggests that each of these cone-opponent mechanisms respond in a bipolar fashion to signal two opponent colors (red vs. green, blue vs. yellow). An alternative possibility is that each cone-opponent response, as well as the luminance response, is rectified, so producing separable signals for each pole (red, green, blue, yellow, light, and dark). In this study, we use psychophysical noise masking to determine whether the rectified model applies to detection by the postreceptoral mechanisms. We measured the contrast-detection thresholds of six test stimuli (red, green, blue, yellow, light, and dark), corresponding to the two poles of each of the three postreceptoral mechanisms. For each test, we determined whether noise presented to the cross pole had the same masking effect as noise presented to the same pole (e.g. comparing masking of luminance increments by luminance decrement noise (cross pole) and luminance increment noise (same pole)). To avoid stimulus cancellation, the test and mask were presented asynchronously in a "sandwich" arrangement (mask-test-mask). For the six test stimuli, we observed that noise masks presented to the cross pole did not raise the detection thresholds of the test, whereas noise presented to the same pole produced a substantial masking. This result suggests that each color signal (red, green, blue, and yellow) and luminance signal, (light and dark) is subserved by a separable mechanism. We suggest that the cone-opponent and luminance mechanisms have similar physiological bases, since a functional separation of the processing of cone increments and cone decrements could underlie both the separation of the luminance system into ON and OFF pathways as well as the splitting of the cone-opponent mechanisms into separable color poles.
Color Perception↗