Metacontrast masking suggests interaction between visual pathways with different spatial and temporal properties

Acute exercise has specific effects on the formation process and pathway of visual perception in healthy young men

Visual perception is formed over time through the formation process and visual pathway. Exercise improves visual perception, but it is unclear whether exercise modulates nonspecifically or specifically the formation process and pathway of visual perception. Healthy young men performed the visual detection task in a backward masking paradigm before and during cycling exercise at a mild intensity or rest (control). The task presented gratings of a circular patch (target) and annulus (mask) arranged concentrically as a visual stimulus and asked if the presence and striped pattern (feature) of the target were detected. The relationship between the orientations of the gratings of the target and the mask included iso-orientation and orthogonal orientation to investigate the orientation selectivity of the masking effect. The masking effect was evaluated by perceptual suppressive index (PSI). Exercise improved feature detection (∆PSI; Exercise: -20.6%, Control: 1.7%) but not presence detection (∆PSI; Exercise: 8.9%, Control: 29.6%) compared to the control condition, and the improving effect resulted from the attenuation of the non-orientation-selective (∆PSI; Exercise: -29.0%, Control: 16.8%) but not orientation-selective masking effect (∆PSI; Exercise: -3.1%, Control: 11.7%). These results suggest that exercise affects the formation process of the perceptual feature of the target stimulus by suppressively modulating the neural networks responsible for the non-orientation-selective surround interaction in the subcortical visual pathways, whose effects are inherited by the cortical visual pathways necessary for perceptual image formation. In conclusion, our findings suggest that acute exercise improves visual perception transiently through the modulation of a specific formation process of visual processing.

Internal noise measures in coarse and fine motion direction discrimination tasks and the correlation with autism traits

Motion perception is essential for visual guidance of behavior and is known to be limited by both internal additive noise (i.e., a constant level of random fluctuations in neural activity independent of the stimulus) and motion pooling (global integration of local motion signals across space). People with autism spectrum disorder (ASD) display abnormalities in motion processing, which have been linked to both elevated noise and abnormal pooling. However, to date, the impact of a third limit-induced internal noise (internal noise that scales up with increases in external stimulus noise)-has not been investigated in motion perception of any group. Here, we describe an extension on the double-pass paradigm to quantify additive noise and induced noise in a motion paradigm. We also introduce a new way to experimentally estimate motion pooling. We measured the impact of induced noise on direction discrimination, which we ascribe to fluctuations in decision-related variables. Our results are suggestive of higher internal noise in individuals with high ASD traits only on coarse but not fine motion direction discrimination tasks. However, we report no significant correlations between autism traits and additive noise, induced noise, or motion pooling in either task. We conclude that, under some conditions, the internal noise may be higher in individuals with pronounced ASD traits and that the assessment of induced internal noise is a useful way of exploring decision-related limits on motion perception, irrespective of ASD traits.

Short-term monocular deprivation induces an interocular delay

Short term monocular deprivation modulates ocular dominance, such that the previously deprived eye's contribution to the binocular percept increases, supposedly as a result of changes in contrast-gain. Therefore, the processing time of the previously patched eye would be expected to speed up as a result of an increase in contrast gain. In order to test this hypothesis, this study examines the effects of short-term monocular deprivation on interocular synchronicity. The present study uses a paradigm based on the Pulfrich phenomenon. The stimulus used for testing consists of elements defining a cylinder rotating in depth, that allows measurement of any interocular delay. The interocular delay was measured at baseline before patching and at outcome, after one hour of monocular deprivation with an opaque or translucent patch. Contrary to expectations, short-term monocular deprivation induces an interocular delay, albeit not always significant, in the previously patched eye. The amplitude of this effect is larger with opaque patching compared to translucent patching. These results are the first report of a non-beneficial effect - i.e. a slowing down in the processing time of the previously patched-eye. They indicate that the plasticity effects of monocular deprivation are not exclusively mediated by contrast gain mechanisms and that light adaptation mechanisms might also be involved in the plasticity resulting from short-term monocular deprivation.

Distinct spatiotemporal mechanisms underlie extra-classical receptive field modulation in macaque V1 microcircuits

Complex scene perception depends upon the interaction between signals from the classical receptive field (CRF) and the extra-classical receptive field (eCRF) in primary visual cortex (V1) neurons. Although much is known about V1 eCRF properties, we do not yet know how the underlying mechanisms map onto the cortical microcircuit. We probed the spatio-temporal dynamics of eCRF modulation using a reverse correlation paradigm, and found three principal eCRF mechanisms: tuned-facilitation, untuned-suppression, and tuned-suppression. Each mechanism had a distinct timing and spatial profile. Laminar analysis showed that the timing, orientation-tuning, and strength of eCRF mechanisms had distinct signatures within magnocellular and parvocellular processing streams in the V1 microcircuit. The existence of multiple eCRF mechanisms provides new insights into how V1 responds to spatial context. Modeling revealed that the differences in timing and scale of these mechanisms predicted distinct patterns of net modulation, reconciling many previous disparate physiological and psychophysical findings.

Masked blindsight in normal observers: Measuring subjective and objective responses to two features of each stimulus

Recent visual masking studies that have measured visual awareness with graded subjective scales have often failed the show any evidence for unconscious visual processing in normal observers in a paradigm similar to that used in studies on blindsight patients. Without any reported awareness of the target, normal observers typically cannot discriminate target's features better than chance. The present study examined processing of color and orientation by measuring graded awareness and forced-choice discriminations for both features in each trial. When no awareness for either feature was reported, discrimination of each feature succeed better than expected by chance, even when the other feature was incorrectly discriminated in the same trial. However, the characteristics of the mask determined whether or not masked blindsight was observed. We conclude that when the processing channels are free from intra-channel interference, unbound or weakly bound features can guide behaviour without any reported awareness in normal observers.

Lateral effects in pattern vision

There is a large literature on lateral effects in pattern vision but no consensus about them or comprehensive model of them. This paper reviews the literature with a focus on the effects of parallel context in the central fovea. It describes seven experiments that measure detection and discrimination thresholds in annular and Gabor-pattern contexts at different separations. It presents a model of these effects, which is an elaboration of Foley's (1994) model. The model describes the results well, and it shows that lateral context affects the response to the target by both multiplicative excitation and additive inhibition. Both lateral effects extend for several wavelengths beyond the target. They vary in relative strength, producing near suppression and far enhancement of the response to the target. The model describes the detection and discrimination results well, and it also describes the results of experiments on lateral effects on perceived contrast. The model is consistent with the physiology of V1 cells.

Interference in lateral masking stimuli - The effects of relative phase, position, orientation, and spatial frequency

Lateral masking has been defined as the perception of a visual target stimulus being impaired when other stimuli are present in its adjacent surroundings. In such cases it has generally been assumed that the target stimulus presented along with a masking stimulus has the same stimulus power as when presented alone and that the reduced visibility reflects interactions in the visual system. It has, however, become clear that there may be interference between such stimuli [5]. Such interference, which takes place in the stimuli and is independent of the visual system, has the potential to reduce the stimulus power of target stimuli. The present report asks, employing 2-Dimensional Gabor functions as stimuli, how interference effects may depend on (1) relative spatial phase, (2) separation between target and masking stimuli, (3) difference in orientation, and (4) difference in spatial frequency between masking and target stimuli. Interference was estimated numerically based on the sums of the amplitudes in the Fourier spectra and the norms of these spectra. Clear evidence for interference was demonstrated with both measures. All the four parameters have the ability to influence the amount of interference. These findings, therefore, emphasize that one cannot count on a target stimulus presented along with masking stimuli to have the same stimulus power as when it is presented by itself.

Extrafoveally applied flashing light affects contrast thresholds of achromatic and S-cone isolating, but not L-M cone modulated stimuli

Flashing light stimulation is often used to investigate the visual system. However, the magnitude of the effect of this stimulus on the various subcortical pathways is not well investigated. The signals of conscious vision are conveyed by the magnocellular, parvocellular and koniocellular pathways. Parvocellular and koniocellular pathways (or more precisely, the L-M opponent and S-cone isolating channels) can be accessed by isoluminant red-green (L-M) and S-cone isolating stimuli, respectively. The main goal of the present study was to explore how costimulation with strong white extrafoveal light flashes alters the perception of stimuli specific to these pathways. Eleven healthy volunteers with negative neurological and ophthalmological history were enrolled for the study. Isoluminance of L-M and S-cone isolating sine-wave gratings was set individually, using the minimum motion procedure. The contrast thresholds for these stimuli as well as for achromatic gratings were determined by an adaptive staircase procedure where subjects had to indicate the orientation (horizontal, oblique or vertical) of the gratings. Thresholds were then determined again while a strong white peripheral light flash was presented 50 ms before each trial. Peripheral light flashes significantly (p < 0.05) increased the contrast thresholds of the achromatic and S-cone isolating stimuli. The threshold elevation was especially marked in case of the achromatic stimuli. However, the contrast threshold for the L-M stimuli was not significantly influenced by the light flashes. We conclude that extrafoveally applied light flashes influence predominantly the perception of achromatic stimuli.

The effects of orientation and attention during surround suppression of small image features: A 7 Tesla fMRI study

Although V1 responses are driven primarily by elements within a neuron's receptive field, which subtends about 1° visual angle in parafoveal regions, previous work has shown that localized fMRI responses to visual elements reflect not only local feature encoding but also long-range pattern attributes. However, separating the response to an image feature from the response to the surrounding stimulus and studying the interactions between these two responses demands both spatial precision and signal independence, which may be challenging to attain with fMRI. The present study used 7 Tesla fMRI with 1.2-mm resolution to measure the interactions between small sinusoidal grating patches (targets) at 3° eccentricity and surrounds of various sizes and orientations to test the conditions under which localized, context-dependent fMRI responses could be predicted from either psychophysical or electrophysiological data. Targets were presented at 8%, 16%, and 32% contrast while manipulating (a) spatial extent of parallel (strongly suppressive) or orthogonal (weakly suppressive) surrounds, (b) locus of attention, (c) stimulus onset asynchrony between target and surround, and (d) blocked versus event-related design. In all experiments, the V1 fMRI signal was lower when target stimuli were flanked by parallel versus orthogonal context. Attention amplified fMRI responses to all stimuli but did not show a selective effect on central target responses or a measurable effect on orientation-dependent surround suppression. Suppression of the V1 fMRI response by parallel surrounds was stronger than predicted from psychophysics but showed a better match to previous electrophysiological reports.

Identifying separate components of surround suppression

Surround suppression is a well-known phenomenon in which the response to a visual stimulus is diminished by the presence of neighboring stimuli. This effect is observed in neural responses in areas such as primary visual cortex, and also manifests in visual contrast perception. Studies in animal models have identified at least two separate mechanisms that may contribute to surround suppression: one that is monocular and resistant to contrast adaptation, and another that is binocular and strongly diminished by adaptation. The current study was designed to investigate whether these two mechanisms exist in humans and if they can be identified psychophysically using eye-of-origin and contrast adaptation manipulations. In addition, we examined the prediction that the monocular suppression component is broadly tuned for orientation, while suppression between eyes is narrowly tuned. Our results confirmed that when center and surrounding stimuli were presented dichoptically (in opposite eyes), suppression was orientation-tuned. Following adaptation in the surrounding region, no dichoptic suppression was observed, and monoptic suppression no longer showed orientation selectivity. These results are consistent with a model of surround suppression that depends on both low-level and higher level components. This work provides a method to assess the separate contributions of these components during spatial context processing in human vision.

Individually different weighting of multiple processes underlies effects of metacontrast masking

Metacontrast masking occurs when a mask follows a target stimulus in close spatial proximity. Target visibility varies with stimulus onset asynchrony (SOA) between target and mask in individually different ways leading to different masking functions with corresponding phenomenological reports. We used individual differences to determine the processes that underlie metacontrast masking. We assessed individual masking functions in a masked target discrimination task using different masking conditions and applied factor-analytical techniques on measures of sensitivity. Results yielded two latent variables that (1) contribute to performance with short and long SOA, respectively, (2) relate to specific stimulus features, and (3) differentially correlate with specific subjective percepts. We propose that each latent variable reflects a specific process. Two additional processes may contribute to performance with short and long SOAs, respectively. Discrimination performance in metacontrast masking results from individually different weightings of two to four processes, each of which contributes to specific subjective percepts.

Early dissociation between neural signatures of endogenous spatial attention and perceptual awareness during visual masking

The relationship between spatial attention and conscious access has often been pictured as a single causal link: spatial attention would provide conscious access to weak stimuli by increasing their effective contrast during early visual processing. To test this hypothesis, we assessed whether the early attentional amplification of visual responses, around 100 ms following stimulus onset, had a decisive impact on conscious detection. We recorded magnetoencephalographic (MEG) signals while participants focused their attention toward or away from masked stimuli which were physically identical but consciously detected half of the time. Spatial attention increased the amplitude of early occipital responses identically for both detected and missed stimuli around 100 ms, and therefore, did not control conscious access. Accordingly, spatial attention did not increase the proportion of detected stimuli. The earliest neuromagnetic correlate of conscious detection, around 120 ms over the contralateral temporal cortex, was independent from the locus of attention. This early activation combined objective information about stimulus presence and subjective information about stimulus visibility, and was followed by a late correlate of conscious reportability, from 220 ms over temporal and frontal cortex, which correlated exclusively with stimulus visibility. This widespread activation coincided in time with the reorienting of attention triggered by masks presented at the uncued location. This reorienting was stronger and occurred earlier when the masked stimulus was detected, suggesting that the conscious detection of a masked stimulus at an unexpected location captures spatial attention. Altogether, these results support a double dissociation between the neural signatures of endogenous spatial attention and perceptual awareness.

Target-mask shape congruence impacts the type of metacontrast masking

Visual metacontrast masking may depend on the time intervals between target and mask in two qualitatively different ways: in type-A masking the smaller the mask delay from target the stronger the masking while in type-B masking maximal masking effect is obtained with a larger temporal delay of the mask. Variability in the qualitative apperance of masking functions has been explained by variability in stimuli parameters and tasks. Recent research on metacontrast masking has surprisingly shown that both of these types of functions can be found with an identical range of stimulation parameters depending on individual differences between observers. Here we show that obtaining clear-cut type-A masking depends on whether target and mask shapes are congruent or incongruent and whether observers use the cues available due to the congruence factor. Conspicuously expressed type-A masking is selectively associated with incongruent target-mask pairings. In the latter conditions target identification level significantly drops with the shortest target-to-mask delays.

Contextual effects in speed perception may occur at an early stage of processing

How does nearby motion affect the perceived speed of a target region? When a central drifting Gabor patch is surrounded by translating noise, its speed can be misperceived over a fourfold range. Typically, when a surround moves in the same direction, perceived centre speed is reduced; for opposite-direction surrounds it increases. Measuring this illusion for a variety of surround properties reveals that the motion context effects are a saturating function of surround speed (Experiment I) and contrast (Experiment II). Our analyses indicate that the effects are consistent with a subtractive process, rather than with speed being averaged over area. In Experiment III we exploit known properties of the motion system to ask where these surround effects impact. Using 2D plaid stimuli, we find that surround-induced shifts in perceived speed of one plaid component produce substantial shifts in perceived plaid direction. This indicates that surrounds exert their influence early in processing, before pattern motion direction is computed. These findings relate to ongoing investigations of surround suppression for direction discrimination, and are consistent with single-cell findings of direction-tuned suppressive and facilitatory interactions in primary visual cortex (V1).

Suppression pathways saturate with contrast for parallel surrounds but not for superimposed cross-oriented masks

Contrast masking from parallel grating surrounds (doughnuts) and superimposed orthogonal masks have different characteristics. However, it is not known whether the saturation of the underlying suppression that has been found for parallel doughnut masks depends on (i) relative mask and target orientation, (ii) stimulus eccentricity or (iii) surround suppression. We measured contrast-masking functions for target patches of grating in the fovea and in the periphery for cross-oriented superimposed and doughnut masks and parallel doughnut masks. When suppression was evident, the factor that determined whether it accelerated or saturated was whether the mask stimulus was crossed or parallel. There are at least two interpretations of the asymptotic behaviour of the parallel surround mask. (1) Suppression arises from pathways that saturate with (mask) contrast. (2) The target is processed by a mechanism that is subject to surround suppression at low target contrasts, but a less sensitive mechanism that is immune from surround suppression 'breaks through' at higher target contrasts. If the mask can be made less potent, then masking functions should shift downwards, and sideways for the two accounts, respectively. We manipulated the potency of the mask by varying the size of the hole in a parallel doughnut mask. The results provided strong evidence for the first account but not the second. On the view that response compression becomes more severe progressing up the visual pathway, our results suggest that superimposed cross-orientation suppression precedes orientation tuned surround suppression. These results also reveal a previously unrecognized similarity between surround suppression and crowding (Pelli, Palomares, & Majaj, 2004).

Long-range suppressive interactions between S-cone and luminance channels

Surround suppression (SS) refers to a reduction in the effective stimulus contrast in one visual location produced by a stimulus presented in an adjacent location. This type of suppression is tuned for orientation and spatial frequency and is thought to be a cortical process. In this paper we used psychophysical measurements to determine whether S-cone-driven signals are affected by surround suppression and, if so, whether S-cone and achromatic signals interact at spatially-remote locations. Our results revealed three important aspects of surround suppression. Firstly, we show that S-cone probes are suppressed by simultaneous S-cone contrast surrounds and that this suppression has the characteristics of a cortical mechanism. Secondly, we show that when probes and surrounds are presented simultaneously, there are no suppressive interactions between S-cone and luminance stimuli. Finally, we demonstrate that this apparent independence is an artifact of signal timing: when the S-cone components of the stimuli precede the luminance components by approximately 40 ms, we find a significant interaction between the two pathways. The amplitude of this interaction depends critically upon the relative onset times of the two components. These results indicate that some component of surround suppression depends on neural computations that occur after the S- and luminance pathways are combined in striate cortex. In addition, the strong dependence of the magnitude of surround suppression on temporal ordering suggests that much of the effect is driven by transient signals.

The time course of contrast masking reveals two distinct mechanisms of human surround suppression

We explored the time course of surround suppression and found clear evidence for two distinct mechanisms: one strong, transient, and largely monocular, the other weaker, sustained, and binocular. We measured detection thresholds for a Gabor target at 8 deg eccentricity surrounded by a large annulus of matching spatial frequency and orientation. At short stimulus durations surround suppression was very strong, but the suppression strength decreased precipitously for durations longer than approximately 100 msec. The strong transient component did not transfer between the eyes and occurred almost instantaneously (<1 frame delay, 12 msec) irrespective of the separation between target and surround. Both suppression components were tightly tuned to orientation, peaking at target orientation, but neither was tuned to target spatial phase. These results are in good agreement with surround suppression properties measured in macaque V1 neurons. The absence of interocular transfer, the strong orientation selectivity, and the high propagation speed incommensurate with slow horizontal connections in V1 suggest that the transient component of suppression originates between input layers and the subsequent layers in V1.

Binding binding: Departure points for a different version of the perceptual retouch theory

In the perceptual retouch theory, masking and related microgenetic phenomena were explained as a result of interaction between specific cortical representational systems and the non-specific sub-cortical modulation system. Masking appears as deprivation of sufficient modulation of the consciousness mechanism suffered by the target-specific signals because of the temporal delay of non-specific modulation (necessary for conscious representation), which explicates the later-coming mask information instead of the already decayed target information. The core of the model envisaged relative magnitudes of EPSPs of single cortical cells driven by target and mask signals at the moment when the nonspecific, presynaptic, excitatory input arrives from the thalamus. In the light of the current evidence about the importance of synchronised activity of specific and non-specific systems in generating consciousness, the retouch theory requires perhaps a different view. This article presents some premises for modification of the retouch theory, where instead of the cumulative presynaptic spike activities and EPSPs of single cells, the oscillatory activity in the gamma range of the participating systems is considered and shown to be consistent with the basic ideas of the retouch theory. In this conceptualisation, O-binding refers to specific encoding which is based on gamma-band synchronised oscillations in the activity of specific cortical sensory modules that represent features and objects; C-binding refers to the gamma-band oscillations in the activity of the non-specific thalamic systems, which is necessary for the O-binding based data to become consciously experienced.

Functional hierarchies of nonconscious visual processing

A number of psychophysical techniques can be used to eliminate the registration of stimuli in visual awareness and to study the dynamics of conscious and nonconscious information processing in the visual system. However, little is known about how these techniques relate to each other. We chose to compare binocular rivalry, induced by orthogonal gratings presented separately to the two eyes, and metacontrast suppression, produced when a target stimulus is followed by a spatially surrounding mask stimulus, to investigate relative levels and correlates of nonconscious processing. Combined with prior results, our findings indicate that binocular rivalry expresses its suppressive effects prior to the level at which the mechanism of metacontrast does. Implications for theories of masking and interpretations of the loss or perceptual effects when stimulus visibility is suppressed by different psychophysical methods are discussed.

Time course of suppression by surround gratings: highly contrast-dependent, but consistently fast

Timing is critical for the effectiveness of a modulating surround signal. In this study, the optimal timing of a suppressing surround signal was measured psychophysically in human subjects. The perceived contrast of a fixated 1-deg circular patch of vertical sinusoidal grating (the target: 4 cpd, Michelson contrast 0.2) was measured as a function of the onset asynchrony between the target and an annular "surround" grating with the same orientation and spatial frequency. The contrast and area of the surround stimulus were varied parametrically. The suppressive signal peaked at earlier times the higher the surround contrast (0.1-0.4), following a function consistent with the contrast-dependence of retinal response dynamics. Increasing the area of the surround grating also moved peak suppression to earlier times. At ca. 2 deg annulus outer diameter the time to peak of the suppressive signal was shortest, although its amplitude grew with annulus area even beyond that. When both the contrast and the area of the centre and surround gratings were equal, suppression was maximal if the surround stimulus was presented ca. 5 ms before the target. Such a short delay of suppression is consistent with a neural implementation based on feedforward-feedback connections, but not with horizontal connections.