Function Of Stimulus Contrast Research Articles

Orientation Discrimination at Isoluminance

Colour and form are important attributes of the objects in our visual environment. We tested the hypothesis that colour and orientation are processed independently in the visual system. Orientation perception for stimuli defined by luminance is characterised by a decrease in thresholds with increasing stimulus contrast, and by a strong oblique effect: stimuli along the horizontal or vertical axes are discriminated more easily. We determined orientation discrimination thresholds for stationary, slow (1 Hz) or fast (8 Hz) moving sine-wave gratings (1 cycle deg−1) defined by luminance or isoluminant (red — green) contrast. In a 4AFC paradigm, three of four stimulus patches were identical and the fourth differed in orientation, contrast, or in both. When we measured orientation discrimination thresholds as a function of stimulus contrast, thresholds decreased for all stimuli with increasing contrast. At all temporal frequencies the functions relating orientation thresholds to stimulus contrast had similar shapes for luminance and isoluminant gratings. On a cone-contrast metric, thresholds for stationary and slowly moving stimuli were consistently lower for isoluminant compared to luminance stimuli. For fast-moving stimuli orientation thresholds were similar for both kinds of gratings. For both types of stimuli a marked oblique effect was observed. To characterise processing of contrast and orientation completely, we measured simultaneous thresholds for contrast and orientation. The shapes of the resulting two-dimensional threshold contours were similar for luminance and isoluminant gratings, indicating similar rules for combining differences in contrast and orientation. We conclude that processing of isoluminant and luminance stimuli undergoes the same neural processing at least for the low spatial frequencies used here.

Illusory brightness step in the chevreul illusion

It is well known that a staircase luminance profile is not seen veridically, but appears as the scallopy-like Chevreul illusion. We have shown that adding thin lines (either light or dark) to the centre of each step creates an illusory brightness change at the point of the line. The regions between the added lines and the edges seem to be uniform, with a clear change in brightness at the point where the line was added. The conditions under which the illusion occurred were measured systematically, both by contrast matching and by annulment. One model that can readily account for the illusion is the local-energy model of feature detection (Morrone & Burr, 1988 Proceedings of the Royal Society of London B, 235, 221–245). Adding the bar to the step creates a peak in local energy at all scales. At the higher scales, the phase of the energy is near zero, the signal for a line; but at the lower scales the phase is near π/2, the signal for an edge. We propose that the edge signal of the lower scales causes the brightness illusion and that this brightness difference is structured by the feature defined sharply by the higher scales (even though that feature is not an edge). As well as predicting the existence of the illusion, simulations with the energy model predicted quantitatively the apparent contrast of the illusion as a function of stimulus contrast, bar-position and high-pass filter frequency.

Effect of pattern adaptation on spatial frequency discrimination.

The effect of pattern adaptation on spatial frequency discrimination was examined in two experiments. In the first experiment delta f/f was measured (with and without adaptation) as a function of stimulus contrast; in the second experiment the contrast required to discriminate a fixed delta f/f was measured (with and without adaptation) as a function of the value of delta f/f. Maximum precision, as measured by the asymptotic value that delta f/f approaches at medium and high contrasts, was not altered by adaptation. Rather, the effect of adaptation was to translate the functions relating delta f/f and contrast along the log-contrast axis, i.e., to increase by a constant factor the contrast required to achieve a given precision of discrimination. This factor agreed closely with the factor by which adaptation raised the contrast threshold for detection of the test stimuli. The largest effects were observed when the reference and the adapting stimuli had the same spatial frequency (5 cycles/deg). Smaller or negligible effects were observed when reference and adapting frequencies differed by an octave.

Development of visual inhibitory interactions in kittens

This study was designed to monitor the development of inhibitory interactions elicited in the cat visual system by oriented visual stimuli. Steady-state visual-evoked potentials (VEPs) were recorded from the scalp of 11 behaving and alert kittens while they viewed contrast-reversed sinusoidal gratings. In adult cats, the form of VEP contrast-response curves (the amplitude of second harmonic modulation as a function of stimulus contrast) was modified by superimposing a mask grating on the test. Parallel masks displaced the curves to a higher contrast region (probably via contrast gain-control mechanisms), increasing contrast threshold without affecting the slope of the curve. Orthogonal gratings, on the other hand, decrease the slope of the curve without affecting threshold (so called cross-orientation inhibition: Morrone et al., 1981). These effects are similar to those previously reported in human VEPs (Morrone & Burr, 1986; Burr & Morrone, 1987) and single cortical cat cells (Morrone et al., 1982). For young kittens of 20 days, the orthogonal mask had no effect whatsoever on the response curves, and the effect of the parallel mask was much less than for adult cats. At about 40 days, the orthogonal mask began to attenuate responses multiplicatively, and by 50 days the amount of multiplicative attenuation had reached adult levels. The effect of the parallel mask (as indicated by the increase in threshold elevation) increased gradually from 20-50 days. The results are consistent with the existence of at least two types of inhibition in cat visual neurones that develop at different rates.

Neural and psychophysical correlates of induced interocular transmission disparities.

We investigated the influences of luminance and temporal asymmetries on the pattern visually evoked potential (pVEP) as a function of stimulus contrast. Monocular and binocular baseline steady-state (ss) and transient (t) pVEP's were recorded on 10 visually normal young adults using a reversing checkerboard pattern ranging in contrast from 4 to 65%. Neutral density (ND) filters were then placed before the right eye in 0.5 ND increments (maximum 3.5 ND), as monocular and binocular pVEP's were recorded. Visual acuity, brightness sense, and the Pulfrich effect were then measured to similar luminance and temporal asymmetries. Neural and psychophysical data were compared. Monocular visual acuity and pVEP results appeared to be luminance-dependent. Brightness sense, the Pulfrich effect, and binocular pVEP amplitudes appeared to be temporarlly dependent. The binocular pVEP amplitude seemed to be less sensitive to temporal asymmetries for lower contrast targets.