The Effects of Luminance Boundaries on Color Perception

The luminance and red-green chromatic detection mechanisms respond to, respectively, the sum and difference of the long-wave (L) and middle-wave (M) zone contrast signals. The most-detectable stimulus is not a small patch of luminance drifting grating, as suggested by others, but rather a small, foveal red-green chromatic flash. Even at the smallest test size examined, 2.3\u27 diameter, the red-green mechanism i~s more sensitive than the luminance mechanism, which has profound implication for visual physiology. When a suprathreshold luminance flash (a pedestal) occurs coincidentally with a red-green chromatic flash, detection of color is facilitated ~2-fold, regardless of spot size, as shown by forced-choice results, and this constant facilitation contrasts with the much larger facilitation reported earlier for small flashes. The lack of chromatic masking by suprathreshold luminance pedestals supports the view of separable luminance and red-green detectors. Isolation of the red-green mechanism with large test flashes on different colored backgrounds showed that the red-green mechanism responds to an equally-weighted difference of L and M cone contrast on each background. Even for fields as low as 400 trolands, sensitivity is controlled by cone-selective adaptation (as well as second-site adaptation), which is surprising in view of recent physiological recordings suggesting that light adaptation in cones is insignificant below 2000 trolands. Motion mechanisms receiving L and M cone signals were studied with 1 cpd, flickering and drifting gratings. At low velocity, a spectrally-opponent (SPO) motion mechanism is more sensitive than the luminance (LUM) mechanism, which summates L and M signals. The SPO mechanism has equal L and M contrast weights at low velocity but is L-cone dominated at intermediate and high velocity, whereas the LUM mechanism shows the reverse pattern of weights. The SPO motion mechanism appears distinct frown a red-green hue mechanism, for the latter has balanced L and M inputs at all temporal frequencies. The two motion mechanisms can be distinguished by the relative phase shifts of the L and M inputs: large shifts are seen for the LUM mechanism at intermediate frequency (4-9 Hz), where SPO shows very little shifts

Texture variations suppress suprathreshold brightness and colour variations

Discriminating material changes from illumination changes is a key function of early vision. Luminance cues are ambiguous in this regard, but can be disambiguated by co-incident changes in colour and texture. Thus, colour and texture are likely to be given greater prominence than luminance for object segmentation, and better segmentation should in turn produce stronger grouping. We sought to measure the relative strengths of combined luminance, colour and texture contrast using a suprathreshhold, psychophysical grouping task. Stimuli comprised diagonal grids of circular patches bordered by a thin black line and contained combinations of luminance decrements with either violet, red, or texture increments. There were two tasks. In the Separate task the different cues were presented separately in a two-interval design, and participants indicated which interval contained the stronger orientation structure. In the Combined task the cues were combined to produce competing orientation structure in a single image. Participants had to indicate which orientation, and therefore which cue was dominant. Thus we established the relative grouping strength of each cue pair presented separately, and compared this to their relative grouping strength when combined. In this way we observed suprathreshold interactions between cues and were able to assess cue dominance at ecologically relevant signal levels. Participants required significantly more luminance and colour compared to texture contrast in the Combined compared to Separate conditions (contrast ratios differed by about 0.1 log units), showing that suprathreshold texture dominates colour and luminance when the different cues are presented in combination

The Effects of Luminance Boundaries on Color Perception

The luminance and red-green chromatic detection mechanisms respond to, respectively, the sum and difference of the long-wave (L) and middle-wave (M) zone contrast signals. The most-detectable stimulus is not a small patch of luminance drifting grating, as suggested by others, but rather a small, foveal red-green chromatic flash. Even at the smallest test size examined, 2.3\u27 diameter, the red-green mechanism i~s more sensitive than the luminance mechanism, which has profound implication for visual physiology. When a suprathreshold luminance flash (a pedestal) occurs coincidentally with a red-green chromatic flash, detection of color is facilitated ~2-fold, regardless of spot size, as shown by forced-choice results, and this constant facilitation contrasts with the much larger facilitation reported earlier for small flashes. The lack of chromatic masking by suprathreshold luminance pedestals supports the view of separable luminance and red-green detectors. Isolation of the red-green mechanism with large test flashes on different colored backgrounds showed that the red-green mechanism responds to an equally-weighted difference of L and M cone contrast on each background. Even for fields as low as 400 trolands, sensitivity is controlled by cone-selective adaptation (as well as second-site adaptation), which is surprising in view of recent physiological recordings suggesting that light adaptation in cones is insignificant below 2000 trolands. Motion mechanisms receiving L and M cone signals were studied with 1 cpd, flickering and drifting gratings. At low velocity, a spectrally-opponent (SPO) motion mechanism is more sensitive than the luminance (LUM) mechanism, which summates L and M signals. The SPO mechanism has equal L and M contrast weights at low velocity but is L-cone dominated at intermediate and high velocity, whereas the LUM mechanism shows the reverse pattern of weights. The SPO motion mechanism appears distinct frown a red-green hue mechanism, for the latter has balanced L and M inputs at all temporal frequencies. The two motion mechanisms can be distinguished by the relative phase shifts of the L and M inputs: large shifts are seen for the LUM mechanism at intermediate frequency (4-9 Hz), where SPO shows very little shifts

Spatial Masking Does Not Reveal Mechanisms Selective to Combined Luminance and Red-Green Color

Detection thresholds plotted in the L and M cone-contrast plane have shown that there are two primary detection mechanisms, a red–green hue mechanism and a light–dark luminance mechanism. However, previous masking results suggest there may be additional mechanisms, responsive to combined features like bright and red or dark and green. We measured detection thresholds for a 1.2 c deg−1 sine-wave grating in the presence of a spatially matched mask grating which was either stationary, dynamically jittered or flickered. The stimuli could be set to any direction in the L,M plane. The appearance of selectivity for combined hue and luminance arose only in conditions where adding the test to the mask modified the spatial phase offset between the luminance and red–green stimulus components. Sensitivity was very high for detecting this spatial phase offset. When this extra cue was eliminated, masking contours in the L,M plane could be largely described by the classical red–green and luminance mechanisms

Motion Detection on Flashed, Stationary Pedestal Gratings: Evidence for an Opponent-Motion Mechanism

Contrast thresholds were measured for discriminating left vs right motion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion. This test was presented on a 1 c/deg stationary grating (pedestal) of twice-threshold, flashed for the duration of the test motion. Lu and Sperling [(1995). Vision Research, 35, 2697–2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the ‘feature wobble’ which it induces). On the contrary, we observe that the stationary pedestal has large effects on motion detection at 7 and 15 Hz, and smaller effects at 0.9–3.7 Hz, evidenced by a spatial phase dependency between the stationary pedestal and moving test. At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedestal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test motion reverses with the pedestal in the opposite phase. Phase dependency was also explored using a very brief (∼ 1 msec) static pedestal presented with the moving test. The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left and right moving components which interact with the moving test. The pedestal effects can be explained by the visual system\u27s much higher sensitivity to the difference of the contrast of right vs left moving components than to either component alone

Second-Site Adaptation in the Red-Green Detection Pathway: Only Elicited by Low-Spatial-Frequency Test Stimuli

he red–green (RG) detection mechanism was revealed by measuring threshold detection contours in the L and M cone contrast plane for sine-wave test gratings of 0.8–6 c deg−1 on bright adapting fields of yellow or red. The slope of the RG detection contours was unity, indicating that the L and M contrast signals contribute equally (with opposite signs) on both the yellow and the red fields; this reflects first-site, cone-selective adaptation. Second-site adaptation, which may reflect saturation at a color-opponent site, was evidenced by the RG detection contours being further out from the origin of the cone contrast plane on the red field than on the yellow field. Second-site adaptation was strong (3-fold) for low spatial frequency test gratings but greatly diminished by 6 c deg−1. The disappearance of second-site adaptation with increasing spatial frequency can be explained by spatial frequency channels. The most sensitive detectors may comprise a low spatial frequency channel which is susceptible to masking by the chromatic, spatial DC component of the red field. The 6 c deg−1 patterns may be detected by a less sensitive, higher frequency channel which is less affected by the uniform red field. The RG spatial frequency channels likely arise in the cortex, implicating a partially central site for the second-site effect

Colour Adaptation Modifies the Long-Wave Versus Middle-Wave Cone Weights and Temporal Phases in Human Luminance (But Not Red-Green) Mechanism

1. The human luminance (LUM) mechanism detects rapid flicker and motion, responding to a linear sum of contrast signals, L\u27 and M\u27, from the long‐wave (L) and middle‐wave (M) cones. The red‐green mechanism detects hue variations, responding to a linear difference of L\u27 and M\u27 contrast signals.2. The two detection mechanisms were isolated to assess how chromatic adaptation affects summation of L\u27 and M\u27 signals in each mechanism. On coloured background (from blue to red), we measured, as a function of temporal frequency, both the relative temporal phase of the L\u27 and M\u27 signals producing optimal summation and the relative L\u27 and M\u27 contrast weights of the signals (at the optimal phase for summation).3. Within the red‐green mechanism at 6 Hz, the phase shift between the L\u27 and M\u27 signals was negligible on each coloured field, and the L\u27 and M\u27 contrast weights were equal and of opposite sign.4. Relative phase shifts between the L\u27 and M\u27 signals in the LUM mechanism were markedly affected by adapting field colour. For stimuli of 1 cycle deg‐1 and 9 Hz, the temporal phase shift was zero on a green‐yellow field (approximately 570 nm). On an orange field, the L\u27 signal lagged M\u27 by as much as 70 deg phase while on a green field M\u27 lagged L\u27 by as much as 70 deg. The asymmetric phase shift about yellow adaptation reveals a spectrally opponent process which controls the phase shift. The phase shift occurs at an early site, for colour adaptation of the other eye had no effect, and the phase shift measured monocularly was identical for flicker and motion, thus occurring before the motion signal is extracted (this requires an extra delay).5. The L\u27 versus M\u27 phase shift in the LUM mechanism was generally greatest at intermediate temporal frequencies (4‐12 Hz) and was small at high frequencies (20‐25 Hz). The phase shift was greatest at low spatial frequencies and strongly reduced at high spatial frequencies (5 cycle deg‐1), indicating that the receptive field surround of neurones is important for the phase shift.6. These temporal phase shifts were confirmed by measuring motion contrast thresholds for drifting L cone and M cone gratings summed in different spatial phases. Owing to the large phase shifts on green or orange fields, the L and M components were detected about equally well by the LUM mechanism (at 1 cycle deg‐1 and 9 Hz) when summed spatially in phase or in antiphase. Antiphase summation is typically thought to produce an equiluminant red‐green grating. 7. At low spatial frequency, the relative L\u27 and M\u27 contrast weights in the LUM mechanism (assessed at the optimal phase for summation) changed strongly with field colour and temporal frequency. 8. The phase shifts and changing contrast weights were modelled with phasic retinal ganglion cells, with chromatic adaptation strongly modifying the receptive field surround. The cells summate L\u27 and M\u27 in their centre, while the surround L\u27 and M\u27 signals are both antagonistic to the centre for approximately 570 nm yellow adaptation. Green or orange adaptation is assumed to modify the L and M surround inputs, causing them to be opponent with respect to each other, but with reversed polarity on the green versus orange field (to explain the chromatic reversal of the phase shift). Large changes in the relative L\u27 and M\u27 weights on green versus orange fields indicate the clear presence of the spectrally opponent surround even at 20 Hz. The spectrally opponent surround appears sluggish, with a long delay (approximately 20 ms) relative to the centre

Separable Red-Green and Luminance Detectors for Small Flashes

Detection contours were measured in L and M cone contrast coordinates for foveal flashes of 200 msec duration and 2.3, 5, 10 and 15 min arc diameter on a bright yellow field. The test flash consisted of simultaneous incremental and decremental red and green lights in various amplitude ratios. At all sizes, the most sensitive detection mechanism was not a luminance mechanism, but rather a red-green mechanism that responds to the linear difference of equally weighted L and M cone contrasts, and signals red or green sensations at the detection threshold. Both temporal and spatial integration were greater for red-green detection than luminance detection. A coincident, subthreshold, yellow flash (a luminance pedestal) did not affect the threshold of the red-green mechanism. Such a pedestal is a sum of equal L and M cone contrast—it represents a vector parallel to the red-green detection contour and thus is expected not to stimulate directly the red-green mechanism. When suprathreshold, the coincident pedestal facilitated chromatic detection by ~2 × at all tested sizes; intense pedestals did not mask chromatic detection. This insensitivity to intense luminance pedestals further indicates that the red-green mechanism has fixed spectral tuning with balanced opponent L and M contrast inputs. This view of fixed spectral weights contrasts with the “variable tuning hypothesis”, which postulates that the weights change with spatial-temporal variations in the test stimulus. The red-green mechanism, when facilitated by the suprathreshold luminance pedestal, can account for the color discrimination of small, slightly suprathreshold, incremental monochromatic flashes, so it is not necessary to postulate an array of detectors with variable spectral tuning for small flashes