Electrophysiological aftereffects of high-frequency transcranial random noise stimulation (hf-tRNS): an EEG investigation.

There is evidence that high-frequency transcranial random noise stimulation (hf-tRNS) is effective in improving behavioural performance in several visual tasks. However, so far there has been limited research into the spatial and temporal characteristics of hf-tRNS-induced facilitatory effects. In the present study, electroencephalogram (EEG) was used to investigate the spatial and temporal dynamics of cortical activity modulated by offline hf-tRNS on performance on a motion direction discrimination task. We used EEG to measure the amplitude of motion-related VEPs over the parieto-occipital cortex, as well as oscillatory power spectral density (PSD) at rest. A time-frequency decomposition analysis was also performed to investigate the shift in event-related spectral perturbation (ERSP) in response to the motion stimuli between the pre- and post-stimulation period. The results showed that the accuracy of the motion direction discrimination task was not modulated by offline hf-tRNS. Although the motion task was able to elicit motion-dependent VEP components (P1, N2, and P2), none of them showed any significant change between pre- and post-stimulation. We also found a time-dependent increase of the PSD in alpha and beta bands regardless of the stimulation protocol. Finally, time-frequency analysis showed a modulation of ERSP power in the hf-tRNS condition for gamma activity when compared to pre-stimulation periods and Sham stimulation. Overall, these results show that offline hf-tRNS may induce moderate aftereffects in brain oscillatory activity.

Limited Attention Diminishes Spatial Suppression From Large Field Glass Patterns.

Glass patterns (GPs) consist of randomly distributed dot pairs (dipoles) whose orientations are determined by specific geometric transforms. We investigated the role of visuospatial attention in the processing of global form from GPs by measuring the effect of distraction on adaptation to GPs. In the nondistracted condition, observers were adapted to coherent GPs. After the adaptation period, they were presented with a test GP divided in two halves along the vertical and were required to judge which side of the test GP was more coherent. In the attention-distracted condition, a high-load rapid serial visual presentation task was performed during the adapting period. The magnitude of the form after-effect was measured using a technique that measures the coherence level at which the test GP appears random. The rationale was that if attention has a modulatory effect on the spatial summation of dipoles, in the attention-distracted condition, we should expect a weaker form after-effect. However, the results showed stronger form after-effect in the attention-distracted condition than in the nondistracted condition, suggesting that distraction during adaptation increases the strength of form adaptation. Additional experiments suggested that distraction may reduce the spatial suppression from large-scale textures, strengthening the spatial summation of local-oriented signals.

The neural basis of form and form-motion integration from static and dynamic translational Glass patterns: A rTMS investigation.

A long-held view of the visual system is that form and motion are independently analysed. However, there is physiological and psychophysical evidence of early interaction in the processing of form and motion. In this study, we used a combination of Glass patterns (GPs) and repetitive Transcranial Magnetic Stimulation (rTMS) to investigate in human observers the neural mechanisms underlying form-motion integration. GPs consist of randomly distributed dot pairs (dipoles) that induce the percept of an oriented stimulus. GPs can be either static or dynamic. Dynamic GPs have both a form component (i.e., orientation) and a non-directional motion component along the orientation axis. GPs were presented in two temporal intervals and observers were asked to discriminate the temporal interval containing the most coherent GP. rTMS was delivered over early visual area (V1/V2) and over area V5/MT shortly after the presentation of the GP in each interval. The results showed that rTMS applied over early visual areas affected the perception of static GPs, but the stimulation of area V5/MT did not affect observers' performance. On the other hand, rTMS was delivered over either V1/V2 or V5/MT strongly impaired the perception of dynamic GPs. These results suggest that early visual areas seem to be involved in the processing of the spatial structure of GPs, and interfering with the extraction of the global spatial structure also affects the extraction of the motion component, possibly interfering with early form-motion integration. However, visual area V5/MT is likely to be involved only in the processing of the motion component of dynamic GPs. These results suggest that motion and form cues may interact as early as V1/V2.

The interaction between orientation and motion signals in moving oriented Glass patterns.

Previous psychophysical evidence suggests that motion and orientation processing systems interact asymmetrically in the human visual system, with orientation information having a stronger influence on the perceived motion direction than vice versa. To investigate the mechanisms underlying this motion-form interaction we used moving and oriented Glass patterns (GPs), which consist of randomly distributed dot pairs (dipoles) that induce the percept of an oriented texture. In Experiment 1 we varied the angle between dipole orientation and motion direction (conflict angle). In separate sessions participants either judged the orientation or motion direction of the GP. In addition, the spatiotemporal characteristics of dipole motion were manipulated as a way to limit (Experiment 1) or favor (Experiment 2) the availability of orientation signals from motion (motion streaks). The results of Experiment 1 showed that apparent GP motion direction is attracted toward dipole orientation, and apparent GP orientation is repulsed from GP motion. The results of Experiment 2 showed stronger repulsion effects when judging the GP orientation, but stronger motion streaks from the GP motion can dominate over the signals provided by conflicting dipole orientation. These results are consistent with the proposal that two separate mechanisms contribute to our perception of stimuli which contain conflicting orientation and motion information: (i) perceived GP motion is mediated by spatial motion-direction sensors, in which signals from motion sensors are combined with excitatory input from orientation-tuned sensors tuned to orientations parallel to the axis of GP motion, (ii) perceived GP orientation is mediated by orientation-tuned sensors which mutually inhibit each other. The two mechanisms are revealed by the different effects of conflict angle and dipole lifetime on perceived orientation and motion direction.

Action Video Games Improve Direction Discrimination of Parafoveal Translational Global Motion but Not Reaction Times.

Playing action video games enhances visual motion perception. However, there is psychophysical evidence that action video games do not improve motion sensitivity for translational global moving patterns presented in fovea. This study investigates global motion perception in action video game players and compares their performance to that of non-action video game players and non-video game players. Stimuli were random dot kinematograms presented in the parafovea. Observers discriminated the motion direction of a target random dot kinematogram presented in one of the four visual quadrants. Action video game players showed lower motion coherence thresholds than the other groups. However, when the task was performed at threshold, we did not find differences between groups in terms of distributions of reaction times. These results suggest that action video games improve visual motion sensitivity in the near periphery of the visual field, rather than speed response.

Suppressive effects on motion discrimination induced by transient flankers are reduced by perceptual learning.

We investigated spatial suppression of a drifting Gabor target of 0.5 c/° induced by adjacent and iso-oriented stationary Gabors (flankers) whose spatial frequency differed by ±1 and ±2 octaves to that of the drifting target. Stimuli (target and flankers) were presented for 33 ms. Results showed greater spatial suppression when the spatial frequency of the stationary but transient flanking Gabors was either equal or 1-2 octaves lower than when it was 1-2 octaves higher than the target's spatial frequency. This asymmetry was evident only for the drifting target, but not for the stationary target. In addition, we investigated whether perceptual learning (PL) reduced the spatial suppression induced by the flankers. We found that PL increased contrast sensitivity for the target, but only when it was reduced by the lateral masking flankers, and its effect did not transfer to an isolated drifting target of equal or higher spatial frequency. These results suggest that PL selectively affects suppressive interactions rather than contrast gain. We suggest that the suppressive effect of low spatial frequency flankers and the lack of suppression with high spatial frequency flankers may reflect two complementary phenomena: camouflage by the transient flankers (i.e., context) and breaking of camouflage by form-motion segmentation. Camouflage may result because both target and flankers activate the motion (magnocellular) system. Breaking of camouflage instead may occur when target and flankers' spatial frequency are more suitable for quasi-independent activation of the form system (by the flankers) and the motion system (by the target).

No priming for global motion in crowding.

There is psychophysical evidence that low-level priming, e.g., from oriented gratings, as well as high-level semantic priming, survives crowding. We investigated priming for global translational motion in crowded and noncrowded conditions. The results indicated that reliable motion priming occurs in the noncrowded condition, but motion priming does not survive crowding. Crowding persisted despite variations in the direction of the flankers with respect to the prime's direction. Motion priming was still absent under crowding when 85% of the flankers moved in the same direction as the prime. Crowding also persisted despite variations in the speed of the flankers relative to the prime even when the flankers' speed was four times slower than the speed of the prime. However, a priming effect was evident when the prime's spatial location was precued and its distance to the flankers increased, suggesting a release from crowding. These results suggest that transient attention induced by precueing the spatial location of the prime may improve subjects' ability to discriminate its direction. Spatial cueing could act to decrease the integration field, thereby diminishing the influence of nearby distracters. In an additional experiment in which we used fewer flankers, we found a priming effect under conditions in which the interelement distance varied between flankers and prime. Overall, the results suggest that motion priming is strongly affected by crowding, but transient attention can partially retrieve such facilitation.

Modelling fast forms of visual neural plasticity using a modified second-order motion energy model.

The Adelson-Bergen motion energy sensor is well established as the leading model of low-level visual motion sensing in human vision. However, the standard model cannot predict adaptation effects in motion perception. A previous paper Pavan et al.(Journal of Vision 10:1-17, 2013) presented an extension to the model which uses a first-order RC gain-control circuit (leaky integrator) to implement adaptation effects which can span many seconds, and showed that the extended model's output is consistent with psychophysical data on the classic motion after-effect. Recent psychophysical research has reported adaptation over much shorter time periods, spanning just a few hundred milliseconds. The present paper further extends the sensor model to implement rapid adaptation, by adding a second-order RC circuit which causes the sensor to require a finite amount of time to react to a sudden change in stimulation. The output of the new sensor accounts accurately for psychophysical data on rapid forms of facilitation (rapid visual motion priming, rVMP) and suppression (rapid motion after-effect, rMAE). Changes in natural scene content occur over multiple time scales, and multi-stage leaky integrators of the kind proposed here offer a computational scheme for modelling adaptation over multiple time scales.

Perception of complex Glass patterns through spatial summation across unique frames.

When processing visual information from the surroundings, human vision depends on the constant integration of form and motion cues. Dynamic Glass patterns (GPs) may be used to study how such visual integration occurs in the human visual system. Dynamic GPs are visual stimuli composed of two or more unique frames consisting of different configurations of dot pairs, called dipoles, presented in rapid succession. Previous psychophysical studies showed that the discrimination of translational and circular dynamic GPs is influenced by both the number of unique frames and the pattern update rate. In this study, we manipulated these two variables to assess their influence on the discrimination threshold of circular, radial, and spiral GPs, partially replicating previous findings on circular GPs. Our results indicate that circular GPs are more easily perceived than radial and spiral GPs, showing lower discrimination thresholds. Furthermore, we found that discrimination thresholds vary as a function of the number of unique frames but not as a function of the pattern update rate. Specifically, coherence thresholds decreased with increasing the number of unique frames. In conclusion, our findings support the existence of spatial summation of form signals coming from the unique frames that generate complex GPs. On the other hand, they do not support temporal integration of local form-motion signals based on the pattern update rate.

Linguistic representations of motion do not depend on the visual motion system.

Embodied semantics proposes that constructing the meaning of motion verb phrases relies on representations of motion in sensory cortex. However, the data reported by earlier studies as evidence for this claim are also explained by a symbolic-semantics view proposing interactions between dissociable systems. In the experiments reported here, participants were visually adapted to real and implied leftward or rightward motion, which produced a motion aftereffect opposite to the direction of the adapting stimulus. Participants then decided whether a directionally ambiguous or a leftward- or rightward-directional verb phrase implied leftward or rightward motion. Because the visual system is engaged in the motion aftereffect, embodied semantics predicts that responses in the motion-aftereffect direction (opposite to the direction of the adapting stimulus) are facilitated, whereas symbolic semantics predicts response facilitation in the direction of the adapting stimulus (opposite to the direction of the motion aftereffect). We found response facilitation in the direction of real- and implied-motion adapting stimuli in ambiguous and directional verb phrases. These results suggest that visual and linguistic representations of motion can be dissociated.

The role of stationary and dynamic test patterns in rapid forms of motion after-effect.

Subsecond adaptation to directional motion can induce a rapid form of motion after-effect (rMAE). Unlike the characteristics of the classic motion after-effect (MAE), produced by adaptation of several seconds or minutes, the properties of the rMAE have been less well explored. In a series of experiments, we assessed the role of stationary and dynamic test patterns (counterphase flickering gratings) in generating rMAE. In particular, we varied the duration, temporal frequency, and spatial phase of the adapting stimuli. Our results show that rMAE is only generated by dynamic test patterns, exhibiting a strong dependence on the adaptation duration and temporal frequency but not on the spatial phase. Similarly to the classic dynamic MAE, the temporal frequency tuning of the dynamic rMAE suggests the involvement of both low-pass and band-pass visual channels. Unexpectedly, our results do not show evidence for static rMAE. We speculate that a stationary test pattern presented immediately (or very soon) after the adapting pattern could interfere with the effects of adaptation by disrupting weak motion signals that arise from adapted and unadapted motion detectors (Ledgeway & Smith, 1994a, 1994b).

Motion-form interactions beyond the motion integration level: evidence for interactions between orientation and optic flow signals.

Motion and form encoding are closely coupled in the visual system. A number of physiological studies have shown that neurons in the striate and extrastriate cortex (e.g., V1 and MT) are selective for motion direction parallel to their preferred orientation, but some neurons also respond to motion orthogonal to their preferred spatial orientation. Recent psychophysical research (Mather, Pavan, Bellacosa, & Casco, 2012) has demonstrated that the strength of adaptation to two fields of transparently moving dots is modulated by simultaneously presented orientation signals, suggesting that the interaction occurs at the level of motion integrating receptive fields in the extrastriate cortex. In the present psychophysical study, we investigated whether motion-form interactions take place at a higher level of neural processing where optic flow components are extracted. In Experiment 1, we measured the duration of the motion aftereffect (MAE) generated by contracting or expanding dot fields in the presence of either radial (parallel) or concentric (orthogonal) counterphase pedestal gratings. To tap the stage at which optic flow is extracted, we measured the duration of the phantom MAE (Weisstein, Maguire, & Berbaum, 1977) in which we adapted and tested different parts of the visual field, with orientation signals presented either in the adapting (Experiment 2) or nonadapting (Experiments 3 and 4) sectors. Overall, the results showed that motion adaptation is suppressed most by orientation signals orthogonal to optic flow direction, suggesting that motion-form interactions also take place at the global motion level where optic flow is extracted.

A comparison of equivalent noise methods in investigating local and global form and motion integration.

Static and dynamic cues within certain spatiotemporal proximity are used to evoke respective global percepts of form and motion. The limiting factors in this process are, first, internal noise, which indexes local orientation/direction detection, and, second, sampling efficiency, which relates to the processing and the representation of global orientation/direction. These parameters are quantified using the equivalent noise (EN) paradigm. EN has been implemented with just two levels: high and low noise. However, when using this simplified version, one must assume the shape of the overall noise dependence, as the intermediate points are missing. Here, we investigated whether two distinct EN methods, the 8-point and the simplified 2-point version, reveal comparable parameter estimates. This was performed for three different types of stimuli: random dot kinematograms, and static and dynamic translational Glass patterns, to investigate how constant internal noise estimates are, and how sampling efficiency might vary over tasks. The results indicated substantial compatibility between estimates over a wide range of external noise levels sampled with eight data points, and a simplified version producing two highly informative data points. Our findings support the use of a simplified procedure to estimate essential form-motion integration parameters, paving the way for rapid and critical applications to populations that cannot tolerate protracted measurements.

Lack of orientation specific adaptation to vertically oriented Glass patterns in human visual cortex: an fMRI adaptation investigation.

The perception of coherent form configurations in natural scenes relies on the activity of early visual areas that respond to local orientation cues. Subsequently, high-level visual areas pool these local signals to construct a global representation of the initial visual input. However, it is still debated whether neurons in the early visual cortex respond also to global form features. Glass patterns (GPs) are visual stimuli employed to investigate local and global form processing and consist of randomly distributed dots pairs called dipoles arranged to form specific global configurations. In the current study, we used GPs and functional magnetic resonance imaging (fMRI) adaptation to reveal the visual areas that subserve the processing of oriented GPs. Specifically, we adapted participants to vertically oriented GP, then we presented test GPs having either the same or different orientations with respect to the adapting GP. We hypothesized that if local form features are processed exclusively by early visual areas and global form by higher-order visual areas, then the effect of visual adaptation should be more pronounced in higher tier visual areas as it requires global processing of the pattern. Contrary to this expectation, our results revealed that adaptation to GPs is robust in early visual areas (V1, V2, and V3), but not in higher tier visual areas (V3AB and V4v), suggesting that form cues in oriented GPs are primarily derived from local-processing mechanisms that originate in V1. Finally, adaptation to vertically oriented GPs causes a modification in the BOLD response within early visual areas, regardless of the relative orientations of the adapting and test stimuli, indicating a lack of orientation selectivity.

Touching motion: rTMS on the human middle temporal complex interferes with tactile speed perception.

Brain functional and psychophysical studies have clearly demonstrated that visual motion perception relies on the activity of the middle temporal complex (hMT+). However, recent studies have shown that hMT+ seems to be also activated during tactile motion perception, suggesting that this visual extrastriate area is involved in the processing and integration of motion, irrespective of the sensorial modality. In the present study, we used repetitive transcranial magnetic stimulation (rTMS) to assess whether hMT+ plays a causal role in tactile motion processing. Blindfolded participants detected changes in the speed of a grid of tactile moving points with their finger (i.e. tactile modality). The experiment included three different conditions: a control condition with no TMS and two TMS conditions, i.e. hMT+-rTMS and posterior parietal cortex (PPC)-rTMS. Accuracies were significantly impaired during hMT+-rTMS but not in the other two conditions (No-rTMS or PPC-rTMS), moreover, thresholds for detecting speed changes were significantly higher in the hMT+-rTMS with respect to the control TMS conditions. These findings provide stronger evidence that the activity of the hMT+ area is involved in tactile speed processing, which may be consistent with the hypothesis of a supramodal role for that cortical region in motion processing.

Motion streaks do not influence the perceived position of stationary flashed objects.

In the present study, we investigated whether motion streaks, produced by fast moving dots Geisler 1999, distort the positional map of stationary flashed objects producing the well-known motion-induced position shift illusion (MIPS). The illusion relies on motion-processing mechanisms that induce local distortions in the positional map of the stimulus which is derived by shape-processing mechanisms. To measure the MIPS, two horizontally offset Gaussian blobs, placed above and below a central fixation point, were flashed over two fields of dots moving in opposite directions. Subjects judged the position of the top Gaussian blob relative to the bottom one. The results showed that neither fast (motion streaks) nor slow moving dots influenced the perceived spatial position of the stationary flashed objects, suggesting that background motion does not interact with the shape-processing mechanisms involved in MIPS.

Attentional modulations of audiovisual interactions in apparent motion: Temporal ventriloquism effects on perceived visual speed.

The timing of brief stationary sounds has been shown to alter different aspects of visual motion, such as speed estimation. These effects of auditory timing have been explained by temporal ventriloquism and auditory dominance over visual information in the temporal domain. Although previous studies provide unprecedented evidence for the multisensory nature of speed estimation, how attention is involved in these audiovisual interactions remains unclear. Here, we aimed to understand the effects of spatial attention on these audiovisual interactions in time. We utilized a set of audiovisual stimuli that elicit temporal ventriloquism in visual apparent motion and asked participants to perform a speed comparison task. We manipulated attention either in the visual or auditory domain and systematically changed the number of moving objects in the visual field. When attention was diverted to a stationary object in the visual field via a secondary task, the temporal ventriloquism effects on perceived speed decreased. On the other hand, focusing attention on the auditory stimuli facilitated these effects consistently across different difficulty levels of secondary auditory task. Moreover, the effects of auditory timing on perceived speed did not change with the number of moving objects and existed in all the experimental conditions. Taken together, our findings revealed differential effects of allocating attentional resources in the visual and auditory domains. These behavioral results also demonstrate that reliable temporal ventriloquism effects on visual motion can be induced even in the presence of multiple moving objects in the visual field and under different perceptual load conditions.

Mechanisms Underlying Directional Motion Processing and Form-Motion Integration Assessed with Visual Perceptual Learning.

Dynamic Glass patterns (GPs) are visual stimuli commonly employed to study form-motion interactions. There is brain imaging evidence that non-directional motion induced by dynamic GPs and directional motion induced by random dot kinematograms (RDKs) depend on the activity of the human motion complex (hMT+). However, whether dynamic GPs and RDKs rely on the same processing mechanisms is still up for dispute. The current study uses a visual perceptual learning (VPL) paradigm to try to answer this question. Identical pre- and post-tests were given to two groups of participants, who had to discriminate random/noisy patterns from coherent form (dynamic GPs) and motion (RDKs). Subsequently, one group was trained on dynamic translational GPs, whereas the other group on RDKs. On the one hand, the generalization of learning to the non-trained stimulus would indicate that the same mechanisms are involved in the processing of both dynamic GPs and RDKs. On the other hand, learning specificity would indicate that the two stimuli are likely to be processed by separate mechanisms possibly in the same cortical network. The results showed that VPL is specific to the stimulus trained, suggesting that directional and non-directional motion may depend on different neural mechanisms.

Motion processing impaired by transient spatial attention: Potential implications for the magnocellular pathway.

Spatial cues presented prior to the presentation of a static stimulus usually improve its perception. However, previous research has also shown that transient exogenous cues to direct spatial attention to the location of a forthcoming stimulus can lead to reduced performance. In the present study, we investigated the effects of transient exogenous cues on the perception of briefly presented drifting Gabor patches. The spatial and temporal frequencies of the drifting Gabors were chosen to mainly engage the magnocellular pathway. We found better performance in the motion direction discrimination task when neutral cues were presented before the drifting target compared to a valid spatial cue. The behavioral results support the hypothesis that transient attention prolongs the internal response to the attended stimulus, thus reducing the temporal segregation of visual events. These results were complemented by applying a recently developed model for perceptual decisions to rule out a speed-accuracy trade-off and to further assess cueing effects on visual performance. In a model-based assessment, we found that valid cues initially enhanced processing but overall resulted in less efficient processing compared to neutral cues, possibly caused by reduced temporal segregation of visual events.

Detection of first- and second-order coherent motion in blindsight.

Blindsight patients can detect fast moving stimuli presented within their blind field even when they deny any phenomenal visual experience. Although mounting evidence suggests the presence of different mechanisms and separate neural substrates underlying the processing of first-order (luminance-defined) and second-order (contrast-defined) motion, the perception of second-order motion in blindsight has scarcely been explored. In the present study, we investigated whether two blindsighted patients (GY and MS) can detect a variety of first- and second-order moving stimuli, and by using repetitive transcranial magnetic stimulation (rTMS), we assessed the role of V5/MT(+) and V3(+) in coherent motion processing. The hemianopes and four control subjects performed a two-interval forced-choice task in which they judged whether a pattern of coherently moving first-order or second-order textured squares moved in the first or second interval. They were not asked to report the direction of motion because neither of them could do so better than expected by chance. The results showed that MS, who has extensive destruction of the ventral cortical visual pathway as well as his V1 lesion, could not process second-order motion at all, whereas GY could perform second-order tasks but only at high-contrast modulation. This may have introduced first-order components in second-order moving stimuli and provided artifactual cues to motion. Moreover, rTMS delivered over area V5/MT(+) impaired detection of both first- and second-order motion in undamaged control subjects, whereas rTMS over V3(+) did not impair their performance in any of the stimuli employed. On the other hand, rTMS over V3(+) did impair GY's detection of first-order motion and high-contrast second-order moving textured squares that are likely to contain artifactual luminance cues. rTMS over V5/MT(+) impaired first-order motion detection in MS. Overall, the results suggest that neither of the blindsight patients can detect artifact-free second-order motion.