Metacontrast with black and white stimuli: evidence for inhibition of on- and off-sustained activity by either on- or off-transient activity

The phase coherence of cortical oscillations predicts dynamic changes in perceived visibility

The phase synchronization of brain oscillations plays an important role in visual processing, perceptual awareness, and performance. Yet, the cortical mechanisms underlying modulatory effects of post-stimulus phase coherence and frequency-specific oscillations associated with different aspects of vision are still subject to debate. In this study, we aimed to identify the post-stimulus phase coherence of cortical oscillations associated with perceived visibility and contour discrimination. We analyzed electroencephalogram data from two masking experiments where target visibility was manipulated by the contrast ratio or polarity of the mask under various onset timing conditions (stimulus onset asynchronies, SOAs). The behavioral results indicated an SOA-dependent suppression of target visibility due to masking. The time-frequency analyses revealed significant modulations of phase coherence over occipital and parieto-occipital regions. We particularly identified modulations of phase coherence in the (i) 2-5 Hz frequency range, which may reflect feedforward-mediated contour detection and sustained visibility; and (ii) 10-25 Hz frequency range, which may be associated with suppressed visibility through inhibitory interactions between and within synchronized neural pathways. Taken together, our findings provide evidence that oscillatory phase alignments, not only in the pre-stimulus but also in the post-stimulus window, play a crucial role in shaping perceived visibility and dynamic vision.

Neural correlates of metacontrast masking across different contrast polarities

Metacontrast masking is a powerful illusion to investigate the dynamics of perceptual processing and to control conscious visual perception. However, the neural mechanisms underlying this fundamental investigative tool are still debated. In the present study, we examined metacontrast masking across different contrast polarities by employing a contour discrimination task combined with EEG (Electroencephalography). When the target and mask had the same contrast polarity, a typical U-shaped metacontrast function was observed. A change in mask polarity (i.e., opposite mask polarity) shifted this masking function to a monotonic increasing function such that the target visibility was strongly suppressed at stimulus onset asynchronies less than 50 ms. This transition in metacontrast function has been typically interpreted as an increase in intrachannel inhibition of the sustained activities functionally linked to object visibility and identity. Our EEG analyses revealed an early (160-300 ms) and a late (300-550 ms) spatiotemporal cluster associated with this effect of polarity. The early cluster was mainly over occipital and parieto-occipital scalp sites. On the other hand, the later modulations of the evoked activities were centered over parietal and centro-parietal sites. Since both of these clusters were beyond 160 ms, the EEG results point to late recurrent inhibitory mechanisms. Although the findings here do not directly preclude other proposed mechanisms for metacontrast, they highlight the involvement of recurrent intrachannel inhibition in metacontrast masking.

Metacontrast masking and stimulus contrast polarity

A recent report [Becker, M. W., & Anstis S. (2004). Metacontrast masking is specific to luminance polarity. Vision Research, 44, 2537-2543] of a failure to obtain metacontrast with target and mask stimuli of opposite contrast polarity is reexamined in an experiment that systematically varies not only stimulus contrast polarity but also target size and target-mask onset asynchrony (SOA). The results show that (a) although, as previously shown [Breitmeyer, B. G. (1978a). Metacontrast with black and white stimuli: Evidence of inhibition of on and off sustained activity by either on or off transient activity. Vision Research, 18, 1443-1448], metacontrast is weaker with stimuli of opposite contrast polarity, (b) substantial metacontrast can be obtained with targets and masks of opposite contrast polarity, especially (c) when the target is small. We conclude that Becker and Anstis's failure to obtain metacontrast with stimuli of opposite contrast polarity is due to their use of a single, relatively large, SOA value.

Metacontrast masking and the cortical representation of surface color: dynamical aspects of edge integration and contrast gain control

This paper reviews recent theoretical and experimental work supporting the idea that brightness is computed in a series of neural stages involving edge integration and contrast gain control. It is proposed here that metacontrast and paracontrast masking occur as byproducts of the dynamical properties of these neural mechanisms. The brightness computation model assumes, more specifically, that early visual neurons in the retina, and cortical areas V1 and V2, encode local edge signals whose magnitudes are proportional to the logarithms of the luminance ratios at luminance edges within the retinal image. These local edge signals give rise to secondary neural lightness and darkness spatial induction signals, which are summed at a later stage of cortical processing to produce a neural representation of surface color, or achromatic color, in the case of the chromatically neutral stimuli considered here. Prior to the spatial summation of these edge-based induction signals, the weights assigned to local edge contrast are adjusted by cortical gain mechanisms involving both lateral interactions between neural edge detectors and top-down attentional control. We have previously constructed and computer-simulated a neural model of achromatic color perception based on these principles and have shown that our model gives a good quantitative account of the results of several brightness matching experiments. Adding to this model the realistic dynamical assumptions that 1) the neurons that encode local contrast exhibit transient firing rate enhancement at the onset of an edge, and 2) that the effects of contrast gain control take time to spread between edges, results in a dynamic model of brightness computation that predicts the existence Broca-Sulzer transient brightness enhancement of the target, Type B metacontrast masking, and a form of paracontrast masking in which the target brightness is enhanced when the mask precedes the target in time.

Luminance processing in object substitution masking

We probed how processing of luminance increments and decrements interacts with attention dependent substitution masking. Results showed that a target was identified better when surrounded by an opposite polarity mask as compared to the same polarity mask. Opposite polarity mask decreased an effect of distracters, indicating influence on the time of directing attention to a target. The opposite polarity mask decreased masking when delayed for longer than 100 ms. Stimuli with the same polarity but different contrast showed increased masking with high contrast mask. Luminance processing, particularly polarity processing, probably enables faster formation of distinct object representation, interacting with attentional selection processes in object substitution masking.

Metacontrast masking is specific to luminance polarity

A 1 degrees -spot was flashed up on a screen, followed by a snugly fitting annular mask. We measured the amount of masking as a function of stimulus luminance. The surround was always mid-gray, the masking ring was either black or white, and the luminance of the spot target ranged from 0% to 100% of white in 4% steps. Observers reported the apparent lightness of the masked spot by adjusting a matching spot.

RESULTS

A black annular mask made all spots that were darker than the gray surround appear to be transparent, that is, of the same luminance as the surround (complete masking). The black ring had virtually no masking effect on spots that were lighter than the surround. Conversely, a white ring made all spots that were lighter than the gray surround look apparently the same luminance as the surround (complete masking), but had virtually no masking effect on spots that were darker than the surround. In summary, a black ring masked spatial decrements but not increments, whilst a white ring masked spatial increments but not decrements. Thus masking occurred only when the spot and the ring had the same luminance polarity. This same-polarity masking still occurred when the target spot was larger than the 'donut hole' of the masking ring, so that the target and ring partly overlapped. This ruled out simple edge-cancellation theories. Instead, masking disrupts the filling-in process that normally propagates inward from the edges of a spot [Vision Res. 31 (7-8) (1991) 1221]. We conclude that metacontrast masking occurs within, but not between, separate visual ON and OFF pathways.

Is there an association between functional vision and learning to read?

Background: Controversy exists about the role of visual parameters and vision in learning to read. This study aims to determine whether ocular parameters or performance on a dynamic test of visual function differs for children of differing reading ability. Methods: Two hundred and eighty-four children (mean age 9.9 +/- 1.8 years) received a vision screening emphasising binocular anomalies associated with discomfort at near (distance and near visual acuity, distance vision challenged with binocular +1 D lenses, near heterophoria, near point of convergence, stereopsis and accommodative facility). Non-verbal mentation age and reading accuracy were assessed. One hundred and six children performed a computerised task of motion coherence detection. Children were classified as normal readers (n = 195), children with dyslexia (n = 49) or learning disabled children (n = 40) based on their mentation age and their reading age. Results: There were no statistically significant differences or correlations between visual parameters and reading performance. Over thirty per cent of the children had accommodative facilities below or equal to six cycles per minute. Children with learning disabilities performed worst on the motion coherence task but this was statistically significant only when compared to the performance of dyslexics. Discussion: The lack of association between ophthalmic parameters and poor reading ability supports the view of the Committee on Children with Disabilities. However, 39 per cent of the children might be expected to experience difficulty 'reading to learn', as suggested by the American Academy of Optometry, as they showed anomalies associated with visual discomfort with prolonged reading. The motion coherence test did not differentiate dyslexics from normal readers and was worst in children with learning disability. Accommodative facility testing remained the most useful predictor of potential visual discomfort.

Vernier acuity with opposite-contrast stimuli

Vernier acuity has usually been tested with stimuli of the same contrast polarity (SC). This traditional vernier acuity was compared to that obtained with stimuli of opposite-contrast (OC) in which one target was brighter than the background and the other was darker. For both bar and dot targets vernier acuity with OC stimuli was about half as good as with SC stimuli. There were large individual differences in the size of the disadvantage with OC stimuli, although thresholds remained within the hyperacuity range. There were also individually-differing biases to see a dark vernier stimulus on one or the other side of a bright stimulus. Differences between OC and SC vernier acuities persisted over a wide range of interstimulus spacings, widths, and contrasts. At extremes of these spatial manipulations acuities became similar, but only because SC acuities were degraded to the level of OC acuities. Subjects showed little improvement in OC vernier acuity, even after 50,000 trials. It is concluded that finest judgements of spatial position arise in a level of the visual system at which light and dark stimuli are treated independently.

Response latency of brisk-sustained (X) and brisk-transient (Y) cells in the cat retina

1. Several methods for evaluating light-evoked response latency and its variability in brisk-sustained (X) and brisk-transient (Y) retinal ganglion cells were tested. The most accurate procedure proved to be that described by Levick (1973), in which the time of the occurrence of the fourth impulse after stimulus onset is taken as an estimate of the latency.2. The shortest response latencies are obtained when the stimuli are the same size as the receptive field centre. At medium and high response amplitudes (> 150 impulses/sec) the response of brisk-transient (Y) cells to these optimal stimuli is 10-15 msec faster than that of adjacent brisk-sustained (X) cells.3. The response latency of brisk-sustained (X) cells for stimuli larger than the receptive field centre increases, whereas that of brisk-transient (Y) cells remains constant. Brisk-sustained (X) cells respond faster than do brisk-transient (Y) cells to stimuli smaller than the receptive field centre.4. No systematic difference exists between brisk-sustained (X) and brisk-transient (Y) cells in regard to the temporal variability of the response. The standard deviation of the latency for stimuli of optimal size decreases from 2.0-8.0 msec at medium stimulus contrast to 0.6-2.0 msec at high stimulus contrast.5. The response of OFF-centre cells to the disappearance of a light spot is always slower than that of an ON-centre cell of the same class to the onset of this stimulus. However, when OFF-centre cells are stimulated with dark spots, their response latency does not differ from that of ON-centre cells of the same class.6. No simple relationship exists between the response latency and the response amplitude. At medium and high discharge rates, most brisk-transient (Y) cells respond faster than an adjacent brisk-sustained (X) cell with equal response. At the same response amplitude, the latencies become shorter as the background illumination is raised. The same discharge rate can be obtained with stimuli of sub-optimal and supra-optimal size, but the latency for the larger stimulus is shorter than that for the smaller one. Latency, therefore, is an additional parameter characterizing the light-evoked response.

A test for contrast-polarity selectivity in the tilt aftereffect

The tilt aftereffect (TAE) was studied with adapting and test stimuli consisting of black or white bars (experiment 1), and of luminance edges (experiment 2). Both experiments failed to demonstrate selectivity of the TAE to the polarity of luminance contrast.