Temporal asymmetry in motion masking: a shortening of the temporal impulse response function

Morgan and Chubb observed a striking temporal asymmetry in motion masking (Vis. Res. 39 (1999) 4217). Motion was produced with a two-frame sequence of gratings presented in spatial quadrature phase; the second grating (100 ms) was presented immediately after the first grating (100 ms), with no temporal overlap. The contrast threshold for detecting the direction of motion of the stimulus pair was facilitated when the first grating was of low-contrast and the second grating was of high-contrast, but strong masking occurred when the order was reversed, so the high-contrast grating came first. We replicated this result, but showed that the masking mostly disappeared when the two gratings temporally overlapped only slightly. The high sensitivity to the precise temporal pattern of the stimulus can be explained by a small temporal 'shortening' of the temporal impulse response function (IRF) as stimulus contrast is increased. The IRF is biphasic with a negative inhibitory lobe. When the first grating has high-contrast, its flash response (owing to the shortening of the IRF) may be in a fairly strong negative phase by the time that the positive response to the second, lower-contrast grating has reached appreciable strength--this reduces the magnitude of the motion signal generated by the two flashes and can account for the masking. A shortening of the IRF with increased contrast (a nonlinearity) is supported by psychophysical studies in humans and by recordings of magnocellular retinal ganglion cells in macaque, and the present results bolster this concept.     

Change in detection threshold caused by peripheral gratings: Dependence on contrast and separation

The detection threshold for a sinusoidal grating in the presence of peripheral gratings was determined as a function of peripheral-grating contrast and separation between the two gratings, with phase relation as a parameter. The result showed that the peripheral gratings, in the range of low contrast, yielded a facilitatory or an inhibitory effect dependent on the phase relation, but in the range of high contrast, yielded an inhibitory effect irrespective of the phase relation. This suggests that two separate mechanisms may underlie the grating induction effect of the detection threshold.     

Two expressions of "surround suppression" in V1 that arise independent of cortical mechanisms of suppression

The preferred stimulus size of a V1 neuron decreases with increases in stimulus contrast. It has been supposed that stimulus contrast is the primary determinant of such spatial summation in V1 cells, though the extent to which it depends on other stimulus attributes such as orientation and spatial frequency remains untested. We investigated this by recording from single cells in V1 of anaesthetized cats and monkeys, measuring size-tuning curves for high-contrast drifting gratings of optimal spatial configuration, and comparing these curves with those obtained at lower contrast or at sub-optimal orientations or spatial frequencies. For drifting gratings of optimal spatial configuration, lower contrasts produced less surround suppression resulting in increases in preferred size. High contrast gratings of sub-optimal spatial configuration produced more surround suppression than optimal low-contrast gratings, and as much or more surround suppression than optimal high-contrast gratings. For sub-optimal spatial frequencies, preferred size was similar to that for the optimal high-contrast stimulus, whereas for sub-optimal orientations, preferred size was smaller than that for the optimal high-contrast stimulus. These results indicate that, while contrast is an important determinant of spatial summation in V1, it is not the only determinant. Simulation of these experiments on a cortical receptive field modeled as a Gabor revealed that the small preferred sizes observed for non-preferred stimuli could result simply from linear filtering by the classical receptive field. Further simulations show that surround suppression in retinal ganglion cells and LGN cells can be propagated to neurons in V1, though certain properties of the surround seen in cortex indicate that it is not solely inherited from earlier stages of processing.     

Superposition of edges and gratings: Interaction between central and peripheral regions

The test pattern was a bipartite field with a sharp edge (step) masked by suprathreshold sinusoidal gratings of variable spatial frequency and phase. With grating contrast increasing an increase of the step threshold contrast was found for cosine phase gratings. In addition to the above effect sine phase gratings lead to a decrease or increase of the step threshold according to the polarity of the grating. It was possible to show that the inhibitory component can be attributed to more peripheral regions of the grating, while the excitatory or inhibitory component is due to a region close to the onset of the edge. Experiments with spatial frequency adaptation showed that the inhibitory influence is selectively impaired. Dichoptic presentation of step and gratings showed an interocular transfer of the inhibitory component, while an interocular transfer of the excitatory or inhibitory component is only found for identical dichoptic stimuli. The results are discussed from the point of view of masking by structure and its interpretation by interchannel inhibition.     

Contrast adaptation in striate cortex of macaque

We have characterized the contrast-response relationships for simple and complex cells in striate cortex of macaque monkey, before and during adaptation to high-contrast sinusoidal gratings of the optimal spatial-frequency and orientation. Adaptation brings about systematic changes in the steepness of contrast-response curves and in the effective contrast of stimuli. Adaptation reduces the detectability of low-contrast gratings by almost a factor of three, but by extending the operating range of most cells it appears to improve the discriminability of high-contrast stimuli that previously gave rise to responses of saturating amplitude.     

Contrast amplification in global texture orientation discrimination

We show that adding a low-contrast texture stimulus that is far below its own detection threshold to an ambiguously oriented high-contrast texture can produce an easily perceived global orientation. When such a low-contrast (e.g., 0.1%) test texture and a high-contrast (e.g., 2%) amplifier texture are interleaved, the effective strength for global orientation detection closely approximates the product of the two contrasts. Therefore, adding two ambiguous textures, an amplifier texture at 5x its threshold contrast for global orientation discrimination and a test texture at 1/5x its threshold contrast, produces threshold global orientation discrimination, that is, 5x amplification of the below-threshold test texture. The observed 5x amplification factors are larger than facilitation effects reported in other pattern tasks. Amplification is 11x when orientation discrimination thresholds are compared to absolute detection thresholds. For second-order textures, maximum contrast amplification is about 2.5x. A contrast gain control model is presented that accounts for 90% of the variance in observed d' for texture patterns of differing geometries, exposure durations, and component contrasts. In the model, very low-contrast orientations are represented by power functions of their contrasts, with an exponent greater than two. As the contrast of an amplifier texture increases beyond about 4%, feed-forward gain control exerted by the amplifier ultimately nullifies the amplification effect and produces masking.     

Systematic misestimation in a vernier task arising from contrast mismatch

Luminance signals mediated by the magnocellular (MC) pathway play an important role in vernier tasks. MC ganglion cells show a phase advance in their responses to sinusoidal stimuli with increasing contrast due to contrast gain control mechanisms. If the phase information in MC ganglion cell responses were utilized by central mechanisms in vernier tasks, one might expect systematic errors caused by the phase advance. This systematic error may contribute to the contrast paradox phenomenon, where vernier performance deteriorates, rather than improves, when only one of the target pair increases in contrast. Vernier psychometric functions for a pair of gratings of mismatched contrast were measured to seek such misestimation. In associated electrophysiological experiments, MC and parvocellular (PC) ganglion cells' responses to similar stimuli were measured to provide a physiological reference. The psychophysical experiments show that a high-contrast grating is perceived as phase advanced in the drift direction compared to a low-contrast grating, especially at a high drift rate (8 Hz). The size of the phase advance was comparable to that seen in MC cells under similar stimulus conditions. These results are consistent with the MC pathway supporting vernier performance with achromatic gratings. The shifts in vernier psychometric functions were negligible for pairs of chromatic gratings under the conditions tested here, consistent with the lack of phase advance both in responses of PC ganglion cells and in frequency-doubled chromatic responses of MC ganglion cells.     

Interactions among spatial frequency and orientation channels adapted concurrently

Interactions between size and orientation-specific mechanisms in the human visual system were investigated using a sequential adaptation technique. Subjects adapted to a vertical, 4 c/deg high-contrast (0.7) sinewave grating that was interleaved at a rate of 0.5 Hz with another adapting grating differing either in (1) spatial frequency or (2) orientation. Before and after adaptation contrast thresholds were measured for a vertical 4 c/deg sinewave test grating. The resultant elevation in contrast threshold was plotted as a function of the (1) spatial frequency or (2) orientation differences between the first and second adapting gratings. Maximum threshold elevation was found when both adapting gratings shared the same spatial frequency and orientation. Minimum elevations were found when the second grating's spatial frequency or orientation differed by approx. 1.5 octaves or 45 deg, respectively. Beyond these values threshold elevations reapproached the baseline value measured in a control condition, where the 4.0 c/deg adapting grating was interleaved with a blank. The minimum threshold elevations were 0.2-0.3 log units below the baseline level. The results suggest the existence of inhibitory interactions between neural mechanisms tuned to the size and orientation of retinal images.     

Two-dimensional optokinetic nystagmus induced by moving plaids and texture boundaries. Evidence for multiple visual pathways.

Horizontal and vertical components of optokinetic nystagmus (OKN) were measured using the magnetic search coil technique in normal human adults during presentation of simple and complex moving patterns. Simple patterns were gratings moving horizontally and obliquely. Complex moving patterns consisted of plaids formed by superimposed oblique motion of two sets of gratings or of illusory contours formed by offset discontinuities in gratings. Slow-phase OKN gains (eye velocity divided by stimulus velocity) induced by high-contrast type I and type II plaids were comparable with those generated by one-dimensional moving gratings. The axis of OKN for high-contrast plaids was along the resultant direction determined by the intersection-of-constraints rule and not along any component. With low-contrast presentations, OKN induced by type I patterns remained in the resultant direction, but the OKN direction induced by type II patterns was biased toward the components' directions. The OKN generated by texture boundaries embedded in real pattern motion was measured for motion of illusory contours having systematically varying directions. The gain of OKN induced by real motion was independent of the direction of illusory contour motion, but the gain to illusory contour motion decreased with increasing contour angles. All these results suggest that input signals for driving the optokinetic system come from visual areas extracting higher order two-dimensional motion information.     

Visual adaptation to patterns containing two-dimensional spatial structure

Adaptation experiments have been carried out using gratings periodic in two dimensions. The threshold elevation effect was measured using such patterns both as test gratings and as adaptation gratings. It is shown that the threshold elevation effect is selective for the length of bars in the gratings, the maximum effect occurring when the bar length in the adaptation grating is the same as that in the test grating. This selectivity holds for bar lengths less than about three times the bar width, whereas for longer bars, the threshold elevation effect is independent of the bar length. It is suggested that there are two populations of mechanism; one population which is length selective and responds to bars with length to width ratio less than three, and another population which is not length selective and responds to bars with length to width ratio greater than three. The orientation selectivity is calculated for the latter population of mechanism and shown to be in agreement with experimental data on the orientation selectivity of the threshold elevation effect for linear gratings.     

The spatial tuning of "motion streak" mechanisms revealed by masking and adaptation

We previously reported that fast-moving dot arrays cause orientation-tuned masking of static gratings (D. Apthorp, J. Cass, & D. Alais, 2010), which we attribute to "motion streaks." Using similar "streaky" dot motion, we describe spatial frequency tuning of grating threshold elevations caused by masking (Experiment 1) and adaptation (Experiment 2) to motion. To compare the streaks with psychophysical tunings, we Fourier analyzed time-averaged translating dots, which were bandpass (peaking at ～2.3 c/deg). Masking, however, was strongest at lower test frequencies (≤1 c/deg) and largely isotropic over orientation, although a small orientation-tuned effect occurred at ～1.2 c/deg. Results were broadly similar across monoptic and dichoptic conditions. Adaptation to fast motion produced spatially bandpass threshold elevations for parallel test gratings, peaking slightly lower than the peak Fourier frequency, with little elevation below 1 c/deg (unlike the low-pass elevation resulting from masking). Slow adaptation produced little elevation for parallel gratings. For orthogonal test gratings, fast motion adaptation produced low-pass threshold elevations and slow motion produced bandpass elevations, suggesting that separable mechanisms process fast (streaky) and slow motion. The different threshold elevation patterns over spatial frequency for masking and adaptation suggest that the adaptation effects are mainly within-channel suppression, whereas the masking effects may be mainly due to between-channel suppression.     

Word acuity threshold as a function of contrast and retinal eccentricity.

BACKGROUND: We determined word acuity thresholds as a function of contrast and retinal eccentricity to determine the rate of threshold alteration in the normal retinal periphery. METHODS: Subjects identified words presented foveally (0 degrees eccentricity) or above the point of fixation at retinal eccentricities of 0.5, 1, 2, 3, 6, and 8 degrees for three contrast levels of 10, 45, and 85%. A descending method of limits was used to determine thresholds for random four-letter words flashed for 90 ms at the different retinal eccentricities. RESULTS: For high-contrast letters, word acuity displayed threshold elevation in the periphery similar to previous reports and similar threshold elevation to those reported for vernier acuity. Lower contrast levels displayed different threshold change as a function of eccentricity, approaching levels reported for grating acuity. When comparing the relative elevation of word acuity thresholds for the different contrast levels (85 vs. 45% and 85 vs. 10%), both comparisons showed that the most rapid decline in word acuity threshold occurs within 2 degrees of the fovea. CONCLUSIONS: The peripheral retina displays a reduction in word acuity threshold that is dependent on letter contrast and shows a change similar to those reported for higher cortical functions such as vernier thresholds. The greatest word threshold elevation occurs within the central 2 degrees of the fovea.     

Spatial phase dependence and the role of motion detection in monocular and dichoptic forward masking

Contrast thresholds for briefly flashed gratings were measured by the QUEST procedure, under conditions of forward masking by gratings of the same spatial frequency (usually 1 c/deg). Low-contrast masks reduced threshold at short onset asynchronies (0 to 50 msec), while higher contrasts raised threshold over a broader temporal range (0 to 100-140 msec). Both effects depended on the spatial phase relation, but in different ways. Threshold reduction at 0- +/- 90 deg phases probably arises from spatio-temporal filtering by direction-selective mechanisms. This conclusion was supported by computer simulation of a motion detector model. The direction-selective stage of motion analysis may be entirely monocular, since facilitation at 90 deg was abolished by dichoptic presentation. Threshold elevation was phase-dependent at short SOA's (20-50 msec), with a minimum at +/- 90 deg, but was not phase-dependent at longer SOA's (70-140 msec). In-phase masking (0 deg) was about equally strong monocularly and dichoptically, but dichoptic threshold elevation showed no phase-dependence at any SOA. Threshold elevation at longer SOA's, and with dichoptic presentation, may reflect a purely suppressive binocular masking effect, unselective for spatial phase, and its basis may be the same as contrast adaptation. At short SOA's, monocular and binocular masking data apparently reflect a mixture of this phase-independent suppression and phase-selective facilitation.     

Apparent contrast of a sinusoidal grating in the simultaneous presence of peripheral gratings

Apparent contrast of a vertical sinusoidal grating in the simultaneous presence of peripheral gratings was measured as a function of peripheral contrast, with test contrast, and relative phase and position of the two gratings as parameters. When the peripheral gratings were horizontally adjacent to the test grating, irrespective of the phase relation, the apparent contrast was raised in the range of peripheral contrast below the test contrast, but depressed in the range of peripheral contrast above the test contrast. When the peripheral gratings were vertically adjacent to the test grating, a similar tendency as mentioned above was observed under the in-phase condition. Under the opposite-phase condition, the apparent contrast was raised monotonically with an increase in peripheral contrast. These effects can be explained in terms of three processes of brightness induction, spatial summation and interaction between the spatial-frequency selective mechanisms.     

Associations between subjective and objective visual function in patients with unilateral macular holes

Forty-six patients with uniocular macular holes and unaffected, fellow eyes were studied to evaluate inter- and intraocular associations between various objective tests of visual function and perceived visual ability. The affected eye had significant associations between visual acuity (VA) and the fovea threshold test, but for the fellow eye only VA and low-contrast VA 10% were associated. The reduction in visual acuity under low-contrast conditions relative to high-contrast did not differ between the affected eye and the healthy eye. Subjective visual ability seems to depend more on the visual acuity of the affected eye than the healthy eye.     

Suppressive effect of sustained low-contrast adaptation followed by transient high-contrast on peripheral target detection

We observed that presenting a low-contrast Gabor patch (2 cpd, 5 degrees eccentricity, contrast=4%) for 8 s and then flashing a 20-30 ms high-contrast patch over it could elicit the perceptual disappearance of a subsequent low-contrast stimulus, whereas neither low-contrast adaptation nor high-contrast flash alone had any considerable effect (p<0.00001). In other experiments we found (a) suppressive components are phase-insensitive, (b) the effect transfers between eyes, (c) suppression is selective for orientation, and (d) the induction by the transient high-contrast Gabor patch could be transferred to another previously adapted location up to a few degrees. Results indicate synergy between contrast and adaptation through a non-linear interaction between rapid gain adjustment to transient change and adaptation to sustained spatial patterns. Findings are compatible with non-local mechanisms presumably at the cortical level.     

Position-based motion perception for color and texture stimuli: effects of contrast and speed

Motion can be perceived either through low-level, motion-energy detection or through tracking the change in position of features. Previously we have shown that, while luminance-based motion likely is detected with velocity-sensitive motion-energy units, patterns defined by texture or binocular disparity ('second-order' stimuli) were tracked by a position-sensitive mechanism (Seiffert & Cavanagh (1998) Vision Research, 38, 3569-3582). Here, we use the same technique, measuring motion amplitude thresholds of oscillating gratings over a range of temporal frequencies and find that the motion of low-contrast equiluminant red/green gratings is also detected with position tracking. In addition, we find that as contrast or speed increases these results change: high-contrast or high-speed equiluminant color or texture-based motion is detected by velocity-sensitive mechanisms. These results help resolve the dispute over the processes which detect the motion of non-luminance based stimuli. Both systems are available, but their relative efficiency changes as a function of contrast and speed. A position-tracking process is more sensitive at low contrasts and low speeds whereas a motion-energy system is more sensitive at high contrasts and high speeds.     

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.     

Threshold elevation following adaptation to coloured gratings

In two experiments, luminance thresholds were measured for horizontal or vertical gratings of various wavelengths, following adaptation to gratings which were of the same or different orientation, and of the same or different wavelength. In the first experiment, in which the adaptation period was relatively brief, there was a small orientation-specific threshold elevation effect but only minimal evidence that this was wavelength specific. In the second experiment, which involved prolonged adaptation, there was a large orientation-specific threshold elevation but no evidence for wavelength specificity.