C.F. Stromeyer

Visual interactions with luminance and chromatic stimuli

The visibility of a 1 degree, 200-msec flash on a large yellow field was measured as a function of the intensity of a coincident pedestal flash (a flash that was the same in both temporal intervals of a two-alternative forced-choice trial). The various flashes were incremental (+Lum) or decremental (-Lum) yellow luminance flashes or green (+Chr) or red (-Chr) isoluminant chromatic flashes. With uncrossed conditions (Lum tests on Lum pedestals or Chr tests on Chr pedestals), we obtained the conventional dipper function, that is, the function of threshold test intensity was highly asymmetric about zero pedestal intensity, and strong pedestals induced strong masking. Crossed conditions produced neither effect: for example, with Chr tests on Lum pedestals, there was no dipper function: the function of threshold test intensity was symmetric about zero pedestal intensity, and strong pedestals produced no masking. Instead, the suprathreshold luminance pedestals facilitated chromatic detection by as much as 2-3X and also linearized the chromatic psychometric function, further enhancing sensitivity to weak chromatic stimuli. (Chromatic sensitivity on the suprathreshold luminance pedestal was approximately 25X higher than luminance sensitivity on the uniform field.) A pedestal consisting of a thin luminance ring that surrounded the chromatic test produced facilitation equal to that of the uniform-luminance pedestal: the pedestal may thus act to demarcate the test spatially and promote chromatic comparison with the surround. Removing the uniform yellow surround eliminated this crossed facilitation but did not eliminate the uncrossed facilitation (the dipper function), suggesting that different mechanisms mediate the crossed and uncrossed facilitations.

Second-site adaptation in the red-green chromatic pathways

On different chromatic adapting fields, thresholds were measured with a 1.2 deg flash consisting of simultaneous incremental and decremental red and green components that stimulate the M and L cones in any desired ratio. Thresholds were plotted in normalized coordinates in which the quantal change in the M and L cones due to the flash was divided by the quantal catch due to the field. Detection contours for a wide range of test flashes provide evidence for luminance and chromatic mechanisms that respectively respond to the sum and difference of the M and L cone signals. Field color has little influence on the luminance mechanism but strongly affects chromatic detection, with sensitivity being maximal on yellow fields and declining slightly on green fields and declining strongly on red fields. Similar effects were obtained for long (200 msec) and very brief flashes, although the shape of the contours differed considerably. The results provide evidence for a second adaptation site within the red-green chromatic pathways, similar to the second-site in the S cone pathways. Chromatic fields (green and red) polarize the site and reduce sensitivity to chromatic flashes.

Contribution of human short‐wave cones to luminance and motion detection.

1. Human short‐wave S cone signals are important for colour vision and here we examine whether the S cone signals also contribute to motion and luminance. 2. Detection was measured with moving patterns that selectively stimulated S cones‐violet sine‐wave gratings of 1 cycle deg‐1 on an intense yellowish field. For rates up to 12 Hz, detection was governed by non‐directional mechanisms, possibly of a chromatic nature, as shown by three findings: moving gratings had to be suprathreshold for their direction to be identified; the threshold ratio of counterphase flickering versus moving gratings was low; and direction‐selective adaptation was essentially absent. 3. Evidence for less sensitive, directional mechanisms includes the following: at high velocity, the direction of movement of the violet gratings can be identified just slightly above the detection threshold; directional adaptation was strong with a suprathreshold test pattern; velocity was seen veridically for clearly suprathreshold patterns; and a counterphase flickering test, added in spatial‐temporal quadrature phase to a similar suprathreshold mask, had identical detection and direction‐identification thresholds. 4. Interactions of long‐wave L cone and S cone signals in direction‐selective mechanisms were measured with an orange counterphase grating and a violet counterphase test, both flickering at the same rate and presented in spatial quadrature phase on the yellowish adapting field. Direction identification thresholds, measured as a function of the temporal phase of two gratings, demonstrated both that the S cone signal lags considerably behind the L cone signal (an effect that strongly varies with S cone light adaptation), and more strikingly, the S cone signal summates with a negative sign and thus is effectively inverted in direction‐selective mechanisms. 5. Quantitatively similar temporal phase functions were obtained with uniform violet and orange flicker when a luminance discrimination criterion was used: thus the S cone signal summates negatively with the L cone signal for both discrimination of luminance flicker and the direction of motion. 6. The temporal phase functions accurately predicted threshold summation for identifying the direction of motion of a pair of violet and orange gratings moving with the same velocity but with different spatial phase offsets. Once the relative temporal phase lag of the S cones was compensated for, there was linear threshold summation for the violet and orange patterns when presented in effective (physiological) spatial antiphase, and clear cancellation when presented in phase. This and related experiments show a linear summation of S, M and L cone signals for direction detection, with the S cones having a negative sign.(ABSTRACT TRUNCATED AT 400 WORDS)

Low spatial-frequency channels in human vision: Adaptation and masking

Previous work showed that adapting to low spatial frequency gratings (below 1.5 cycles/degree) may cause maximal spatial adaptation at a significantly higher spatial frequency. It has been suggested that there are no adaptable spatial-frequency channels tuned to below 1.5 c/deg. Contrary to this view, we found that adaptation and masking with low spatial frequencies (0.12-1.0 c/deg) produced maximal threshold elevations when the test patterns were the same spatial frequency as the adapting or masking pattern. These results were obtained using test patterns that turned on and off gradually or sharply. The results suggest that there are form mechanisms optimally sensitive to very low spatial frequencies. Adaptation was selective to position (phase) and orientation at low spatial frequencies; masking was observed to be selective to orientation at a spatial frequency as low as 0.2 c/deg. A clear dichotomy between transient, motion channels and sustained, form channels at low spatial and temporal frequencies may represent an unrealistic simplification. There may exist directionally-selective motion mechanisms sensitive to very slow motion, and these may play a role in the discrimination of form. The discussion considers the bandwidths of the low spatial frequency mechanisms.

Peripheral chromatic sensitivity for flashes: A post-peceptoral red-green asymmetry

Thresholds of luminance and red-green chromatic flashes (200 msec) were measured on a yellow adapting field in the fovea and periphery (up to 12 degrees eccentricity for 1 degree flashes and 21 degrees eccentricity for 2 degrees flashes). Chromatic sensitivity (in cone contrast coordinates) is about 7 times higher than luminance sensitivity in the fovea but falls faster with eccentricity, so that luminance and chromatic sensitivities are similar at eccentricities of 20 degrees or less. At eccentricities greater than about 14 degrees, there is a clear asymmetry wherein green chromatic flashes are considerably less detectable than red ones. By measuring complete detection contours for many ratios of incremental and decremental red and green flashes, we isolated the red and green chromatic detection mechanisms, and demonstrated that the red-green asymmetry is not a property of the L- or M-cone response per se, but rather is a property of the post-receptoral, chromatic mechanisms. The peripheral luminance and chromatic mechanisms could be further separated with a suprathreshold luminance flash (a pedestal), since an intense pedestal masks coincident luminance test flashes but facilitates the chromatic flashes. The luminance pedestal approximately linearizes the chromatic detection function (the psychometric function).

On inhibition between spatial frequency channels: Adaptation to complex gratings

Abstract Previous studies have shown that adapting to a complex grating may produce little rise in the contrast threshold of a test grating whose spatial frequency matches one of the higher harmonics of the adapting pattern. The present study shows that adaptation may be strong if the adapting component of a complex grating appears visible as a separate grating. The adapting component could be made to appear as a separate grating by sufficiently separating the spatial frequencies of the components, or by drifting the components relative to each other, or by substituting a jittering, noisy component for the stationary component. Under these conditions, the visible component produced strong adaptation. Inhibition between mechanisms tuned to different spatial frequencies does not readily account for our data. An alternate model with lateral inhibition between spatially adjacent mechanisms is in agreement with known results.

Detection uncertainty and the facilitation of chromatic detection by luminance contours

A suprathreshold luminance flash (1 degree, 200 msec) on a large uniform yellow field facilitates detection of a coincident (1 degree, 200 msec) red or green equiluminant flash and approximately linearizes the psychometric function for detecting the chromatic flash. The facilitation is produced by the suprathreshold contour created by the luminance flash. We tested whether the contour facilitates detection by reducing spatiotemporal uncertainty in detecting the chromatic flash. Uncertainty increase false alarms, and this effect can be factored out by correcting yes-no psychometric functions for guessing. Uncertainty also alters the shape of the receiver operating characteristic. Measurements of yes-no psychometric functions and receiver operating characteristics do not support the uncertainty reduction hypothesis.

Response saturation of short-wavelength cone pathways controlled by color-opponent mechanisms.

Abstract Response saturation of the short-wavelength cone pathways was measured with foveal, violet test flashes on steady or flashed violet fields. These violet stimuli were presented on an intense steady yellow “auxiliary” field that suppressed the Π4 and Π5 mechanisms. At given violet field intensities the increment threshold curve became very steep, demonstrating response saturation of the short-wavelength pathways. This response saturation could be greatly reduced or eliminated by flashing the yellow auxiliary field, rather than having it steady, or by adding a flashed yellow field to the steady yellow auxiliary field. The response saturation was strongly promoted by presenting a negative yellow field flash (i.e. a decrement of the steady yellow field) with the positive violet field flash. The results show that the response saturation is controlled, not by independently acting short-wavelength cones, but by cancellative mechanisms that receive signals of opposite sign from the short-wavelength cones and the middle- and long-wavelength cones.

Visibility of red and green spatial patterns upon spectrally mixed adapting fields

Abstract Light-adaptation of mechanisms that detect red and green spatial patterns was studied by measuring increment thresholds on spectrally mixed adapting fields. Adding uniform green light to a bright red adapting field increased the detectability of red test patterns of long duration and low spatial frequency. In contrast, adding uniform red light to similar green patterns on a green field had little or no effect. Spectrally mixed adapting fields acted approximately additively in controlling sensitivity to fine red or green spatial patterns. The results demonstrate that the spatial and also temporal properties of test stimuli in part determine sensitivity to chromatic adapting fields. The results are related to Stiles π-mechanisms and possible adaptive mechanisms are discussed.

Visibility of chromatic flicker upon spectrally mixed adapting fields

Abstract Adding steady green light to a low-frequency flickering red field or adding steady red light to a low-frequency green field reduced the flicker threshold. The red and green adapting lights act in a cancellative manner, lowering the threshold. This is contrary to a model of independent Π-mechanisms acting like fundamental color mechanisms. When the flicker frequency was increased to 12 Hz, red and green adapting fields acted approximately additively in raising the threshold. At this frequency the spectral field sensitivity of the middle-wavelength π4 and long-wavelength π5 mechanisms was measured with the two color increment threshold method of Stiles. The spectral functions were then used to estimate the flicker sensitivities (Weber fractions) of π4 and π5 at 12 Hz, which were found to be similar.