Rhea T. Eskew

Chromatic masking in the (ΔL/L, ΔM/M) plane of cone-contrast space reveals only two detection mechanisms

Abstract The post-receptoral mechanisms that mediate detection of stimuli in the (ΔL/L, ΔM/M) plane of color space were characterized using noise masking. Chromatic masking noises of different chromaticities and spatial configurations were used, and threshold contours for the detection of Gaussian and Gabor tests were measured. The results do not show masking that is narrowly-selective for the chromaticity of the noise. On the contrary, our findings suggest that detection of these tests is mediated only by an opponent chromatic mechanism (a red-green mechanism) and a non-opponent luminance mechanism. These results are not consistent with the hypothesis of multiple chromatic mechanisms mediating detection in this color plane [1] .

ON and OFF S-cone pathways have different long-wave cone inputs

Three experiments compared thresholds for S-cone increments and decrements under steady and transient adaptation conditions, to investigate whether stimuli of both polarities are detected by the same cone-opponent psychophysical mechanism. The results could not be accounted for by a standard model of the S-cone detection pathway [Polden & Mollon (1980) Proceedings of the Royal Society of London, B, 210, 235-272]. In particular, a transient tritanopia detection paradigm that measured threshold elevation following the offset of long-wavelength fields produced different field sensitivities for S-cone increment and decrement tests. The decrement field sensitivity function was shifted to shorter wavelengths relative to the increment function. L-cone opponency is apparently stronger for S-cone increments than for decrements. The most plausible substrates of the two different psychophysical detection mechanisms are the ON and OFF channels. The results suggest that S-ON (bistratified) and S-OFF ganglion cells receive different relative amounts of L- and M-cone input.

Visual noise disrupts conceptual integration in reading

The Effortfulness Hypothesis suggests that sensory impairment (either simulated or age-related) may decrease capacity for semantic integration in language comprehension. We directly tested this hypothesis by measuring resource allocation to different levels of processing during reading (i.e., word vs. semantic analysis). College students read three sets of passages word-by-word, one at each of three levels of dynamic visual noise. There was a reliable interaction between processing level and noise, such that visual noise increased resources allocated to word-level processing, at the cost of attention paid to semantic analysis. Recall of the most important ideas also decreased with increasing visual noise. Results suggest that sensory challenge can impair higher-level cognitive functions in learning from text, supporting the Effortfulness Hypothesis.

Rapid sensitivity changes on flickering backgrounds: tests of models of light adaptation

To investigate mechanisms underlying sensitivity changes that are capable of following rapid variations in intensity of the background field, we measured the threshold radiance needed to detect a 2-ms probe flash presented at various phases relative to a sinusoidally flickering background. The temporal frequency, mean luminance, and modulation of the background were systematically varied. The sensitivity change consisted of two components: a phase-insensitive increase in threshold that occurs at all the phases of the background field (a change in the dc level of the threshold), and a phase-dependent variation in threshold. Both components can reliably be measured at temporal frequencies up to approximately 50 Hz. On a 30-Hz background, the threshold varied with phase over roughly 0.5 log unit within a half-cycle (17 ms). For background flicker rates of 20-40 Hz the probe threshold increased with increasing instantaneous background radiance, following a typical threshold-versus-radiance template, and approaching Weber-law behavior during the peak of the background flicker. This pattern of threshold elevation was measured at mean background illuminances from 580 to 9100 Td (trolands), with the dimmer backgrounds being slightly less effective in producing threshold elevations. The measured increase in the dc level commenced as soon as the modulation of the background flicker began, and the amount of threshold elevation followed the envelope of the background flicker, ruling out modulation gain control explanations for the change in sensitivity on flickering backgrounds. The threshold elevations measured on a 30-Hz, 25% modulation background were lower than those measured on a 30-Hz, 100% modulation background at all phases. The measured changes in threshold with changes in background modulation rule out all adaptation models consisting of a multiplicative and a subtractive adaptation processes followed by a single, late, static nonlinearity.

Theory of chromatic noise masking applied to testing linearity of S-cone detection mechanisms

A method for testing the linearity of cone combination of chromatic detection mechanisms is applied to S-cone detection. This approach uses the concept of mechanism noise, the noise as seen by a postreceptoral neural mechanism, to represent the effects of superposing chromatic noise components in elevating thresholds and leads to a parameter-free prediction for a linear mechanism. The method also provides a test for the presence of multiple linear detectors and off-axis looking. No evidence for multiple linear mechanisms was found when using either S-cone increment or decrement tests. The results for both S-cone test polarities demonstrate that these mechanisms combine their cone inputs nonlinearly.

Noise masking of S-cone increments and decrements.

S-cone increment and decrement detection thresholds were measured in the presence of bipolar, dynamic noise masks. Noise chromaticities were the L-, M-, and S-cone directions, as well as L-M, L+M, and achromatic (L+M+S) directions. Noise contrast power was varied to measure threshold Energy versus Noise (EvN) functions. S+ and S- thresholds were similarly, and weakly, raised by achromatic noise. However, S+ thresholds were much more elevated by S, L+M, L-M, L- and M-cone noises than were S- thresholds, even though the noises consisted of two symmetric chromatic polarities of equal contrast power. A linear cone combination model accounts for the overall pattern of masking of a single test polarity well. L and M cones have opposite signs in their effects upon raising S+ and S- thresholds. The results strongly indicate that the psychophysical mechanisms responsible for S+ and S- detection, presumably based on S-ON and S-OFF pathways, are distinct, unipolar mechanisms, and that they have different spatiotemporal sampling characteristics, or contrast gains, or both.

A model of selective masking in chromatic detection

Narrowly tuned, selective noise masking of chromatic detection has been taken as evidence for the existence of a large number of color mechanisms (i.e., higher order color mechanisms). Here we replicate earlier observations of selective masking of tests in the (L,M) plane of cone space when the noise is placed near the corners of the detection contour. We used unipolar Gaussian blob tests with three different noise color directions, and we show that there are substantial asymmetries in the detection contours-asymmetries that would have been missed with bipolar tests such as Gabor patches. We develop a new chromatic detection model, which is based on probability summation of linear cone combinations, and incorporates a linear contrast energy versus noise power relationship that predicts how the sensitivity of these mechanisms changes with noise contrast and chromaticity. With only six unipolar color mechanisms (the same number as the cardinal model), the new model accounts for the threshold contours across the different noise conditions, including the asymmetries and the selective effects of the noises. The key for producing selective noise masking in the (L,M) plane is having more than two mechanisms with opposed L- and M-cone inputs, in which case selective masking can be produced without large numbers of color mechanisms.

Asymmetric high-contrast masking in S cone increment and decrement pathways

Abstract Physiological, anatomical, and psychophysical evidence points to important differences between visual processing of short‐wave cone increments and decrement (S+ and S−) stimuli. The present study uses the pedestal discrimination paradigm to investigate potential differences, using S+ and S− tests presented on (L)ong‐wave, (M)edium‐wave, S, L+M, L−M, and achromatic pedestals, of both contrast polarities. Results show that high contrast ‘purplish’ (S+ or −(L+M)) pedestals produce substantially more masking of both S+ and S− tests than ‘yellowish’ (S− or +(L+M)) pedestals do. The other pedestals produce no masking. These findings suggest greater nonlinearity – either a static nonlinearity or contrast gain control – in the mechanisms responsible for the ‘purplish’ polarity, likely the S ON pathway.

Selective noise masking of L and M cone stimuli: unipolar tests reveal theoretically significant asymmetries.

In an important study, Hansen & Gegenfurtner (2013) showed that chromatic masking noise placed near the corners of detection contours in the ΔL/L, ΔM/M plane of cone contrast color space can produce selective masking. Rather than being indicative of the existence of large numbers of mechanisms, as they suggested, we (Eskew & Shepard, 2013) demonstrated that this result could be accounted for by a model with a very limited number of mechanisms. Both studies used bipolar test stimuli that contain equal and opposite chromatic contrasts. The symmetry of such stimuli enforces symmetry on the threshold contour and the model fit that can obscure the presence of asymmetric mechanisms-ones that do not form pairs with equal and opposite cone weights. Unipolar mechanisms like this can act as if there were more mechanisms than actually exist, since different mechanisms can detect the two poles of bipolar stimuli independently (Eskew, 2009). Here, for the first time, we report that detection contours for unipolar stimuli in the LM plane may be asymmetric when masking noise is added near the corners of the detection contour. Thresholds for stimuli comprised of nearly equal L and M increments are, for almost all observers, higher than the thresholds for the corresponding stimuli comprised of nearly equal L and M decrements. There are also differences in the cone contrast weights for the R and G mechanisms (responsible for the long flanks of the detection contour), differences that predict selective masking, as we previously reported, without requiring large numbers of mechanisms. A plausible physiological reason for these asymmetries will be discussed. A very limited number (4 to 6) of unipolar, asymmetric mechanisms may be able to account for all the bipolar and unipolar test detection contours. Meeting abstract presented at VSS 2015.

Harmonics added to a flickering light can upset the balance between ON and OFF pathways to produce illusory colors

Significance By varying the temporal waveforms of complex flickering stimuli, we can produce alterations in their mean color that can be predicted by a physiologically based model of visual processing. The model highlights the perceptual effects of a well-known feature of most visual pathways, namely the early separation of visual signals into increments and decrements. The role of this separation in improving the efficiency and sensitivity of the visual system has been discussed before, but its effect on perception has been neglected. The application of a model incorporating half-wave rectification offers an exciting psychophysical method for investigating the inner workings of the human visual system. The neural signals generated by the light-sensitive photoreceptors in the human eye are substantially processed and recoded in the retina before being transmitted to the brain via the optic nerve. A key aspect of this recoding is the splitting of the signals within the two major cone-driven visual pathways into distinct ON and OFF branches that transmit information about increases and decreases in the neural signal around its mean level. While this separation is clearly important physiologically, its effect on perception is unclear. We have developed a model of the ON and OFF pathways in early color processing. Using this model as a guide, we can produce imbalances in the ON and OFF pathways by changing the shapes of time-varying stimulus waveforms and thus make reliable and predictable alterations to the perceived average color of the stimulus—although the physical mean of the waveforms does not change. The key components in the model are the early half-wave rectifying synapses that split retinal photoreceptor outputs into the ON and OFF pathways and later sigmoidal nonlinearities in each pathway. The ability to systematically vary the waveforms to change a perceptual quality by changing the balance of signals between the ON and OFF visual pathways provides a powerful psychophysical tool for disentangling and investigating the neural workings of human vision.