Pedestal Contrast Research Articles

Analysis of the effect of pattern adaptation on pattern pedestal effects: A two-process model

Pattern contrast thresholds for vertical Gabor patterns were measured on pattern pedestals that were vertical or horizontal. Contrast of the pedestal was varied to measure the function relating target contrast threshold to pedestal contrast (TvC function). TvC functions were measured without an adaptor and after adaptation to vertical, horizontal and plaid patterns. For a pedestal with the same orientation as the target, the vertical and plaid adapters increased thresholds at low pedestal contrasts, but not high. For the pedestal orthogonal to the target, the same two adaptors increased thresholds over the whole range of pedestal contrasts. These asymmetric effects are described by a model of adaptation and masking derived from a model of masking (Foley,1994a) by allowing two parameters to vary with the adapt state; one of them is an additive parameter in the denominator of the response function, which can be interpreted as adaptor-produced divisive inhibition that persists after adaptor offset; the other is the sensitivity to pedestal-produced divisive inhibition, which is changed by adaptation for the pedestal orthogonal to the target. Other models do not account for both effects.

Contrast discrimination function: spatial cuing effects.

The effects of spatial cuing were measured for discrimination between an increment and a decrement on a target's pedestal contrast. Discrimination thresholds measured in the absence of a spatial cue were always higher than corresponding thresholds measured in the presence of a spatial cue, except when pedestal contrast was near zero. Uncued discrimination thresholds rose monotonically with pedestal contrast; cued discrimination thresholds formed a dipper function of pedestal contrast. A spatial-uncertainty model incorporating a nonlinear transducer produced similar results.

The Effect of Pedestal Contrast on Choice Reaction Times to Contrast Increments

Two-alternative forced-choice reaction times (RTs) were measured and psychometric functions constructed for ten contrast increments at seven pedestal contrasts ranging from 0% to 4.8%. Two sine gratings at 2 cycles deg−1 differing only in contrast were presented on a computer screen, and the subject's task was to indicate as quickly as possible whether the stimulus with higher contrast appeared to the left or to the right of a fixation point. There were 100 trials per stimulus pair from which the percentage of correct responses and the median correct RTs were calculated. As the contrast increment increased, the percentage of correct responses increased and RTs decreased reaching a minimum with large increments. However, RTs continued to decrease even when performance reached 100% correct. Contrast increment thresholds calculated at 82% correct level formed a classical dipper-shaped function when plotted as a function of pedestal contrast. Response variability, reflected in standard errors of increment thresholds, was greater at high pedestal contrasts. When RTs corresponding to threshold increments were interpolated and plotted against pedestal contrast, the functions were also dipper-shaped. Discriminative RTs were on average faster at low and slower at high pedestal contrasts compared to detection. These findings show that equalising the percentage of correct responses did not equalise RTs, and that processing time increased with pedestal contrast. The increase of both increment thresholds and RTs with pedestal contrast may be due to an increase in signal-dependent noise which increases response variability and slows down the decision process.

Suprathreshold Contrast Increment Detection is not Affected by Grating Edge-Type

We have shown that the spatial frequency components introduced by sharply truncating the edge of a sinusoidal grating do not affect detection thresholds (Kukkonen et al, 1996 Perception25 Supplement, 117). This could be due to the fact that contrast energy at these spatial frequencies is much lower than at the nominal frequency. Any effect upon detection attributable to the broadening of the Fourier spectrum might therefore exert itself most strongly at suprathreshold rather than threshold contrast levels. To establish whether these extra components interfere with performance at higher contrasts, we measured contrast increment thresholds of circular-aperture sinusoidal gratings with a pedestal contrast of 0.5. Gratings were either sharply truncated or Gaussian-edged having a half-contrast diameter equal to that of its corresponding sharp-edged stimulus. Gratings of 0.125 and 0.5 cycle deg−1 were used, each tested with 1 – 8 cycles. There was no significant difference between contrast increment thresholds for the sharp-edged and Gaussian-edged gratings, which were spectrally narrower. The frequency components introduced into the Fourier spectrum by the abrupt edge of the truncated gratings appear to have no effect upon subject performance even in suprathreshold contrast increment detection.

Bloch's Law for Contrast Increment Detection: The Effects of Pedestal Contrast and Light Level

The duration over which contrast detection improves (Bloch's regime) decreases with increasing light level and is often thought to reflect the temporal characteristics of the visual system. There is also some evidence to suggest that the temporal characteristics of the visual system might also change with increasing contrast level (M A Georgeson, 1987 Vision Research27 765 – 780). Here we compare temporal summation for stimuli presented on a blank field or on a high contrast background. On each trial a test grating was presented for X ms with the use of a spatial-alternate forced-choice procedure. The test grating (2 cycles deg−1) was presented superimposed on a similar pedestal grating which was also present for 500 ms prior to and after the test grating. Pedestal contrasts of 0% and 32% were tested at mean luminance levels of 150 cd m−2 and 1.5 cd m−2. The results show that both increasing light level and increasing contrast level resulted in smaller temporal summation times. In the current conditions both these effects approximately halve the summation time such that for a stimulus of low light level and of low pedestal contrast the summation time was ∼60 ms; low light, high contrast ∼30 ms; high light, low contrast ∼30 ms; and high light, high contrast ∼15 ms. The results imply that the temporal response of the visual system quickens with increasing contrast.

Contrast discrimination at high contrasts reveals the influence of local light adaptation on contrast processing

Previous measurements of contrast discrimination threshold, Δ C, as a function of pedestal contrast, C, for sine-wave gratings have shown a power law relationship between Δ C and C at suprathreshold levels of C. However, these studies have rarely used contrasts greater than 50%. Whittle (1986), using incremental and decremental patches, found that Δ C increased with C only up to about 50%. At higher contrasts it decreased. Since a periodic stimulus can be considered to be composed of increments and decrements, we thought we might find such an inverse U-shaped function for gratings if we used contrasts up to 100%. We tested this for both sine-wave and square-wave stimuli at spatial frequencies from 0.0625 to 8.0 c/deg. We found that for frequencies up to 0.5 c/deg, Δ C in nearly all cases ‘dipped down’ after about C = 50% contrast. At 4.0 and 8.0 c/deg, however, no dip-down occurred. Additional experiments showed that the dip-down was unlikely to be due to cortical long-term adaptation and most likely an effect of localized light adaptation to the dark bars. We argue that the absence of dip-down at high spatial frequencies was mainly due to the attenuation of contrast by the optics of the eye. As for the results of Whittle (1986), a Weber's Law in W = ( L max − L min) L min describes the inverse U-shaped contrast discrimination function well. Two other contrast expressions also linearize the data on log-log plots. We show how some familiar notions about the physiological operation of localized light adaptation can easily account for the form of the contrast discrimination function. Finally we estimate the number of discriminable steps in contrast from detection threshold to maximum contrast for the various spatial frequencies tested.

Contrast detection facilitation by spatially separated targets and inducers

We measured contrast detection thresholds for a small (3.6 × 3.6 arc min) square target in the presence and absence of spatially identical pedestal stimuli, and of a spatially non-overlapping inducing line (3.6 × 23 arc min). Results for the pedestal stimuli replicated the classical “dipper function”, thresholds being reduced by near-threshold pedestals and increased at higher pedestal contrasts. An inducer without a pedestal also decreased detection thresholds. When the inducer and pedestal were combined, their effects were additive. Thus the inducer facilitated target detection when the pedestal was absent but raised detection thresholds when the pedestal contrast was sufficient by itself to lower threshold. Inducers of opposite polarity to the target did not consistently decrease target thresholds, even when they were clearly visible, arguing against spatial uncertainty as the explanation of the inducer effect. The inducer effect was independent of the length of the inducer except with small (< 3.6 arc min) stimuli, and was abolished by increasing target-inducer separation beyond about 10 arc min.

Contrast adaptation and contrast gain control

Electrophysiological measurements have shown that contrast adaptation can increase the contrast gain of cortical cells of the cat and the monkey. This implies that adaptation could enhance the contrast discrimination sensitivity. Psychophysical contrast discrimination experiments were performed with and without contrast adaptation. The stimuli were spatially separated stationary Gabor patterns. The pedestal contrast was varied from 6 to 75 per cent. The spatial frequency was 1.5, 5.0 or 20 cpd. After adaptation the contrast detection thresholds are elevated and the subjective contrast is lowered. The contrast discrimination thresholds remain unchanged.

Contrast discrimination in peripheral vision.

Contrast discrimination provides a psychophysical method for studying contrast coding in vision. Our purpose was to compare properties of contrast discrimination in central and peripheral vision. We used forced-choice procedures to measure contrast-increment thresholds as a function of pedestal contrast. Our stimuli were 2-cycle/deg Gaussian-windowed sine-wave grating patches. They were centered at retinal loci ranging from 10 degrees nasal to 20 degrees temporal on the horizontal meridian. At each eccentricity, curves relating increment threshold to pedestal contrast had the same shape. When increment thresholds and pedestal contrasts were both normalized by the contrast thresholds at the retinal eccentricity in question, the curves became superimposed and fell along the same dipper-shaped contrast-discrimination function. We conclude that, after scaling by the local contrast sensitivity, properties of contrast discrimination are qualitatively and quantitatively similar from 0 degree to 20 degrees on the retina. These findings suggest that mechanisms of contrast coding are similar in central and peripheral vision.

On the relation between summation and facilitation

Summation refers to the small reduction of threshold contrast as a signal is extended in space or time, or when multiple signals are presented simultaneously. Facilitation refers to the large reduction in the threshold contrast increment when it is superimposed on a near-threshold pedestal contrast. Separate hypotheses relate each of these effects to distinct measures of the log-log steepness of the psychometric function. This note shows that for 2-alternative forced-choice (2afc) proportion correct, the Weibull function P( c)= 1-(1− γ)exp[-( c/α) β] and the d′ power law d′( c) = ( c/c 0.76) b are practically indistinguishable. Furthermore, their steepness parameters are proportional to each other, β = 1.247 b. This implies a simple relation between 2afc summation and facilitation which can be tested without measuring the psychometric function.