Transcranial magnetic stimulation reveals attentional feedback to area V1 during serial visual search

Rhythmic Attention and ADHD: A Narrative and Systematic Review

In recent decades, a growing body of evidence has confirmed the existence of rhythmic fluctuations in attention, but the effect of inter-individual variations in these attentional rhythms has yet to be investigated. The aim of this review is to identify trends in the attention deficit/hyperactivity disorder (ADHD) literature that could be indicative of between-subject differences in rhythmic attention. A narrative review of the rhythmic attention and electrophysiological ADHD research literature was conducted, and the commonly-reported difference in slow-wave power between ADHD subjects and controls was found to have the most relevance to an understanding of rhythmic attention. A systematic review of the literature examining electrophysiological power differences in ADHD was then conducted to identify studies with conditions similar to those utilised in the rhythmic attention research literature. Fifteen relevant studies were identified and reviewed. The most consistent finding in the studies reviewed was for no spectral power differences between ADHD subjects and controls. However, the strongest trend in the studies reporting power differences was for higher power in the delta and theta frequency bands and lower power in the alpha band. In the context of rhythmic attention, this trend is suggestive of a slowing in the frequency and/or increase in the amplitude of the attentional oscillation in a subgroup of ADHD subjects. It is suggested that this characteristic electrophysiological modulation could be indicative of a global slowing of the attentional rhythm and/or an increase in the rhythmic recruitment of neurons in frontal attention networks in individuals with ADHD.

Adaptation and exogenous attention interact in the early visual cortex: A TMS study

Transcranial magnetic stimulation (TMS) to early visual cortex modulates the effect of adaptation and eliminates the effect of exogenous (involuntary) attention on contrast sensitivity. Here we investigated whether adaptation modulates exogenous attention under TMS to V1/V2. Observers performed an orientation discrimination task while attending to one of two stimuli, with or without adaptation. Following an attentional cue, two stimuli were presented in the stimulated region and its contralateral symmetric region. A response cue indicated the stimulus whose orientation observers had to discriminate. Without adaptation, in the distractor-stimulated condition, contrast sensitivity increased at the attended location and decreased at the unattended location via response gain-but these effects were eliminated in the target-stimulated condition. Critically, after adaptation, exogenous attention altered performance similarly in both distractor-stimulated and target-stimulated conditions. These results reveal that (1) adaptation and attention interact in the early visual cortex, and (2) adaptation shields exogenous attention from TMS effects.

Top-down modulation of DLPFC in visual search: a study based on fMRI and TMS

Effective visual search is essential for daily life, and attention orientation as well as inhibition of return play a significant role in visual search. Researches have established the involvement of dorsolateral prefrontal cortex in cognitive control during selective attention. However, neural evidence regarding dorsolateral prefrontal cortex modulates inhibition of return in visual search is still insufficient. In this study, we employed event-related functional magnetic resonance imaging and dynamic causal modeling to develop modulation models for two types of visual search tasks. In the region of interest analyses, we found that the right dorsolateral prefrontal cortex and temporoparietal junction were selectively activated in the main effect of search type. Dynamic causal modeling results indicated that temporoparietal junction received sensory inputs and only dorsolateral prefrontal cortex →temporoparietal junction connection was modulated in serial search. Such neural modulation presents a significant positive correlation with behavioral reaction time. Furthermore, theta burst stimulation via transcranial magnetic stimulation was utilized to modulate the dorsolateral prefrontal cortex region, resulting in the disappearance of the inhibition of return effect during serial search after receiving continuous theta burst stimulation. Our findings provide a new line of causal evidence that the top-down modulation by dorsolateral prefrontal cortex influences the inhibition of return effect during serial search possibly through the retention of inhibitory tagging via working memory storage.

Dissociable roles of human frontal eye fields and early visual cortex in presaccadic attention

Shortly before saccadic eye movements, visual sensitivity at the saccade target is enhanced, at the expense of sensitivity elsewhere. Some behavioral and neural correlates of this presaccadic shift of attention resemble those of covert attention, deployed during fixation. Microstimulation in non-human primates has shown that presaccadic attention modulates perception via feedback from oculomotor to visual areas. This mechanism also seems plausible in humans, as both oculomotor and visual areas are active during saccade planning. We investigated this hypothesis by applying TMS to frontal or visual areas during saccade preparation. By simultaneously measuring perceptual performance, we show their causal and differential roles in contralateral presaccadic attention effects: Whereas rFEF+ stimulation enhanced sensitivity opposite the saccade target throughout saccade preparation, V1/V2 stimulation reduced sensitivity at the saccade target only shortly before saccade onset. These findings are consistent with presaccadic attention modulating perception through cortico-cortical feedback and further dissociate presaccadic and covert attention.

Transcranial magnetic stimulation to frontal but not occipital cortex disrupts endogenous attention

Covert endogenous (voluntary) attention improves visual performance. Human neuroimaging studies suggest that the putative human homolog of macaque frontal eye fields (FEF+) is critical for this improvement, whereas early visual areas are not. Yet, correlational MRI methods do not manipulate brain function. We investigated whether rFEF+ or V1/V2 plays a causal role in endogenous attention. We used transcranial magnetic stimulation (TMS) to alter activity in the visual cortex or rFEF+ when observers performed an orientation discrimination task while attention was manipulated. On every trial, they received double-pulse TMS at a predetermined site (stimulated region) around V1/V2 or rFEF+. Two cortically magnified gratings were presented, one in the stimulated region (contralateral to the stimulated area) and another in the symmetric (ipsilateral) nonstimulated region. Grating contrast was varied to measure contrast response functions (CRFs) for all attention and stimulation combinations. In experiment 1, the CRFs were similar at the stimulated and nonstimulated regions, indicating that early visual areas do not modulate endogenous attention during stimulus presentation. In contrast, occipital TMS eliminates exogenous (involuntary) attention effects on performance [A. Fernández, M. Carrasco,Curr. Biol. 30, 4078-4084 (2020)]. In experiment 2, rFEF+ stimulation decreased the overall attentional effect; neither benefits at the attended location nor costs at the unattended location were significant. The frequency and directionality of microsaccades mimicked this pattern: Whereas occipital stimulation did not affect microsaccades, rFEF+ stimulation caused a higher microsaccade rate directed toward the stimulated hemifield. These results provide causal evidence of the role of this frontal region for endogenous attention.

Dissociable roles of human frontal eye fields and early visual cortex in presaccadic attention - evidence from TMS

Shortly before each saccadic eye movement, presaccadic attention improves visual sensitivity at the saccade target 1-5 at the expense of lowered sensitivity at non-target locations 6-11 . Some behavioral and neural correlates of presaccadic attention and covert attention -which likewise enhances sensitivity, but during fixation 12 -are similar 13 . This resemblance has led to the debatable 13-18 notion that presaccadic and covert attention are functionally equivalent and rely on the same neural circuitry 19-21 . At a broad scale, oculomotor brain structures (e.g., FEF) are also modulated during covert attention 22-24 - yet by distinct neuronal subpopulations 25-28 . Perceptual benefits of presaccadic attention rely on feedback from oculomotor structures to visual cortices 29,30 ( Fig. 1a ); micro-stimulation of FEF in non-human primates affects activity in visual cortex 31-34 and enhances visual sensitivity at the movement field of the stimulated neurons 35-37 . Similar feedback projections seem to exist in humans: FEF+ activation precedes occipital activation during saccade preparation 38,39 and FEF TMS modulates activity in visual cortex 40-42 and enhances perceived contrast in the contralateral hemifield 40 . We investigated presaccadic feedback in humans by applying TMS to frontal or visual areas during saccade preparation. By simultaneously measuring perceptual performance, we show the causal and differential roles of these brain regions in contralateral presaccadic benefits at the saccade target and costs at non-targets: Whereas rFEF+ stimulation reduced presaccadic costs throughout saccade preparation, V1/V2 stimulation reduced benefits only shortly before saccade onset. These effects provide causal evidence that presaccadic attention modulates perception through cortico-cortical feedback and further dissociate presaccadic and covert attention.

Evidence for a theta-band behavioural oscillation in rapid face detection

Theories of rhythmic perception propose that perceptual sampling operates in a periodic way, with alternating moments of high and low responsiveness to sensory inputs. This rhythmic sampling is linked to neural oscillations and thought to produce fluctuations in behavioural outcomes. Previous studies have revealed theta- and alpha-band behavioural oscillations in low-level visual tasks and object categorization. However, less is known about fluctuations in face perception, for which the human brain has developed a highly specialized network. To investigate this, we ran an online study (N = 179) incorporating the dense sampling technique with a dual-target rapid serial visual presentation (RSVP) paradigm. In each trial, a stream of object images was presented at 30 Hz and participants were tasked with detecting whether or not there was a face image in the sequence. On some trials, one or two (identical) face images (the target) were embedded in each stream. On dual-target trials, the targets were separated by an interstimulus interval (ISI) that varied between 0 to 633 ms. The task was to indicate the presence of the target and its gender if present. Performance varied as a function of ISI, with a significant behavioural oscillation in the face detection task at 7.5 Hz, driven mainly by the male target faces. This finding is consistent with a high theta-band-based fluctuation in visual processing. Such fluctuations might reflect rhythmic attentional sampling or, alternatively, feedback loops involved in updating top-down predictions.

Periodic attention operates faster during more complex visual search

Attention has been found to sample visual information periodically, in a wide range of frequencies below 20 Hz. This periodicity may be supported by brain oscillations at corresponding frequencies. We propose that part of the discrepancy in periodic frequencies observed in the literature is due to differences in attentional demands, resulting from heterogeneity in tasks performed. To test this hypothesis, we used visual search and manipulated task complexity, i.e., target discriminability (high, medium, low) and number of distractors (set size), while electro-encephalography was simultaneously recorded. We replicated previous results showing that the phase of pre-stimulus low-frequency oscillations predicts search performance. Crucially, such effects were observed at increasing frequencies within the theta-alpha range (6–18 Hz) for decreasing target discriminability. In medium and low discriminability conditions, correct responses were further associated with higher post-stimulus phase-locking than incorrect ones, in increasing frequency and latency. Finally, the larger the set size, the later the post-stimulus effect peaked. Together, these results suggest that increased complexity (lower discriminability or larger set size) requires more attentional cycles to perform the task, partially explaining discrepancies between reports of attentional sampling. Low-frequency oscillations structure the temporal dynamics of neural activity and aid top-down, attentional control for efficient visual processing.

α Phase-Amplitude Tradeoffs Predict Visual Perception

Spontaneous α oscillations (∼10 Hz) have been associated with various cognitive functions, including perception. Their phase and amplitude independently predict cortical excitability and subsequent perceptual performance. However, the causal role of α phase-amplitude tradeoffs on visual perception remains ill-defined. We aimed to fill this gap and tested two clear predictions from the pulsed inhibition theory according to which α oscillations are associated with periodic functional inhibition. (1) High-α amplitude induces cortical inhibition at specific phases, associated with low perceptual performance, while at opposite phases, inhibition decreases (potentially increasing excitation) and perceptual performance increases. (2) Low-α amplitude is less susceptible to these phasic (periodic) pulses of inhibition, leading to overall higher perceptual performance. Here, cortical excitability was assessed in humans using phosphene (illusory) perception induced by single pulses of transcranial magnetic stimulation (TMS) applied over visual cortex at perceptual threshold, and its postpulse evoked activity recorded with simultaneous electroencephalography (EEG). We observed that prepulse α phase modulates the probability to perceive a phosphene, predominantly for high-α amplitude, with a nonoptimal phase for phosphene perception between -π/2 and -π/4. The prepulse nonoptimal phase further leads to an increase in postpulse-evoked activity [event-related potential (ERP)], in phosphene-perceived trials specifically. Together, these results show that α oscillations create periodic inhibitory moments when α amplitude is high, leading to periodic decrease of perceptual performance. This study provides strong causal evidence in favor of the pulsed inhibition theory.

Transcranial magnetic stimulation entrains alpha oscillatory activity in occipital cortex

Parieto-occipital alpha rhythms (8-12 Hz) underlie cortical excitability and influence visual performance. Whether the synchrony of intrinsic alpha rhythms in the occipital cortex can be entrained by transcranial magnetic stimulation (TMS) is an open question. We applied 4-pulse, 10-Hz rhythmic TMS to entrain intrinsic alpha oscillators targeting right V1/V2, and tested four predictions with concurrent electroencephalogram (EEG): (1) progressive enhancement of entrainment across time windows, (2) output frequency specificity, (3) dependence on the intrinsic oscillation phase, and (4) input frequency specificity to individual alpha frequency (IAF) in the neural signatures. Two control conditions with an equal number of pulses and duration were arrhythmic-active and rhythmic-sham stimulation. The results confirmed the first three predictions. Rhythmic TMS bursts significantly entrained local neural activity. Near the stimulation site, evoked oscillation amplitude and inter-trial phase coherence (ITPC) were increased for 2 and 3 cycles, respectively, after the last TMS pulse. Critically, ITPC following entrainment positively correlated with IAF rather than with the degree of similarity between IAF and the input frequency (10 Hz). Thus, we entrained alpha-band activity in occipital cortex for ~ 3 cycles (~ 300 ms), and IAF predicts the strength of entrained occipital alpha phase synchrony indexed by ITPC.

Extinguishing Exogenous Attention via Transcranial Magnetic Stimulation

Orienting covert exogenous (involuntary) attention to a target location improves performance in many visual tasks [1, 2]. It is unknown whether early visual cortical areas are necessary for this improvement. To establish a causal link between these areas and attentional modulations, we used transcranial magnetic stimulation (TMS) to briefly alter cortical excitability and determine whether early visual areas mediate the effect of exogenous attention on performance. Observers performed an orientation discrimination task. After a peripheral valid, neutral, or invalid cue, two cortically magnified gratings were presented, one in the stimulated region and the other in the symmetric region in the opposite hemifield. Observers received two successive TMS pulses around their occipital pole while the stimuli were presented. Shortly after, a response cue indicated the grating whose orientation observers had to discriminate. The response cue either matched-target stimulated-or did not match-distractor stimulated-the stimulated side. Grating contrast was varied to measure contrast response functions (CRF) for all combinations of attention and TMS conditions. When the distractor was stimulated, exogenous attention yielded response gain-performance benefits in the valid-cue condition and costs in the invalid-cue condition compared with the neutral condition at the high contrast levels. Crucially, when the target was stimulated, this response gain was eliminated. Therefore, TMS extinguished the effect of exogenous attention. These results establish a causal link between early visual areas and the modulatory effect of exogenous attention on performance.

Visual Size Processing in Early Visual Cortex Follows Lateral Occipital Cortex Involvement

Neural activation in the early visual cortex (EVC) reflects the perceived rather than retinal size of stimuli, suggesting that feedback possibly from extrastriate regions modulates retinal size information in EVC. Meanwhile, the lateral occipital cortex (LOC) has been suggested to be critically involved in object size processing. To test for the potential contributions of feedback modulations on size representations in EVC, we investigated the dynamics of relevant processes using transcranial magnetic stimulation (TMS). Specifically, we briefly disrupted the neural activity of EVC and LOC at early, intermediate, and late time windows while participants performed size judgment tasks in either an illusory or neutral context. TMS over EVC and LOC allowed determining whether these two brain regions are relevant for generating phenomenological size impressions. Furthermore, the temporal order of TMS effects allowed inferences on the dynamics of information exchange between the two areas. Particularly, if feedback signals from LOC to EVC are crucial for generating altered size representations in EVC, then TMS effects over EVC should be observed simultaneously or later than the effects following LOC stimulation. The data from 20 humans (13 females) revealed that TMS over both EVC and LOC impaired illusory size perception. However, the strongest effects of TMS applied over EVC occurred later than those of LOC, supporting a functionally relevant feedback modulation from LOC to EVC for scaling size information. Our results suggest that context integration and the concomitant change of perceived size require LOC and result in modulating representations in EVC via recurrent processing.SIGNIFICANCE STATEMENT How we perceive an object's size is not entirely determined by its physical size or the size of its retinal representation but also the spatial context. Using transcranial magnetic stimulation, we investigated the role of the early visual cortex (EVC) and the higher-level visual area, lateral occipital cortex (LOC), known to be critically involved in object processing, in transforming an initial retinal representation into one that reflects perceived size. Transcranial magnetic stimulation altered size perception earlier over LOC compared with EVC, suggesting that context integration and the concomitant change in perceived size representations in EVC rely on feedback from LOC.

Can processing of face trustworthiness bypass early visual cortex? A transcranial magnetic stimulation masking study

As a highly social species, we constantly evaluate human faces to decide whether we can trust someone. Previous studies suggest that face trustworthiness can be processed unconsciously, but the underlying neural pathways remain unclear. Specifically, the question remains whether processing of face trustworthiness relies on early visual cortex (EVC), required for conscious perception. If processing of trustworthiness can bypass EVC, then disrupting EVC should impair subjective (conscious) trustworthiness perception while leaving objective (forced-choice) trustworthiness judgment intact. We applied double-pulse transcranial magnetic stimulation (TMS) to right EVC, at different stimulus onset asynchronies (SOAs) from presentation of a face in either the left or right hemifield. Faces were slightly rotated clockwise or counterclockwise, and were either trustworthy or untrustworthy. On each trial, participants discriminated 1) trustworthiness, 2) stimulus rotation, and 3) reported subjective visibility of trustworthiness. At early SOAs and specifically in the left hemifield, performance on the rotation task was impaired by TMS. Crucially, though TMS also impaired subjective visibility of trustworthiness, no effects on trustworthiness discrimination were obtained. Thus, conscious perception of face trustworthiness (captured by subjective visibility ratings) relies on intact EVC, while objective forced-choice trustworthiness judgments may not. These results are consistent with the hypothesis that objective trustworthiness processing can bypass EVC. For basic visual features, extrastriate pathways are well-established; but face trustworthiness depends on a complex configuration of features. Its potential processing without EVC is therefore of particular interest, further highlighting its ecological relevance.

Contribution of FEF to Attentional Periodicity during Visual Search: A TMS Study

Visual search, looking for a target embedded among distractors, has long been used to study attention. Current theories postulate a two-stage process in which early visual areas perform feature extraction, whereas higher-order regions perform attentional selection. Such a model implies iterative communication between low- and high-level regions to sequentially select candidate targets in the array, focus attention on these elements, and eventually permit target recognition. This leads to two independent predictions: (1) high-level, attentional regions and (2) early visual regions should both be involved periodically during the search. Here, we used transcranial magnetic stimulation (TMS) applied over the frontal eye field (FEF) in humans, known to be involved in attentional selection, at various delays while observers performed a difficult, attentional search task. We observed a periodic pattern of interference at ∼6 Hz (theta) suggesting that the FEF is periodically involved during this difficult search task. We further compared this result with two previous studies (Dugué et al., 2011, 2015a) in which a similar TMS procedure was applied over the early visual cortex (V1) while observers performed the same task. This analysis revealed the same pattern of interference, i.e., V1 is periodically involved during this difficult search task, at the theta frequency. Past V1 evidence reappraised for this paper, together with our current FEF results, confirm both of our independent predictions, and suggest that difficult search is supported by low- and high-level regions, each involved periodically at the theta frequency.

Transcranial direct current stimulation (tDCS) facilitates overall visual search response times but does not interact with visual search task factors

Whether transcranial direct current stimulation (tDCS) affects mental functions, and how any such effects arise from its neural effects, continue to be debated. We investigated whether tDCS applied over the visual cortex (Oz) with a vertex (Cz) reference might affect response times (RTs) in a visual search task. We also examined whether any significant tDCS effects would interact with task factors (target presence, discrimination difficulty, and stimulus brightness) that are known to selectively influence one or the other of the two information processing stages posited by current models of visual search. Based on additive factor logic, we expected that the pattern of interactions involving a significant tDCS effect could help us colocalize the tDCS effect to one (or both) of the processing stages. In Experiment 1 (n = 12), anodal tDCS improved RTs significantly; cathodal tDCS produced a nonsignificant trend toward improvement. However, there were no interactions between the anodal tDCS effect and target presence or discrimination difficulty. In Experiment 2 (n = 18), we manipulated stimulus brightness along with target presence and discrimination difficulty. Anodal and cathodal tDCS both produced significant improvements in RTs. Again, the tDCS effects did not interact with any of the task factors. In Experiment 3 (n = 16), electrodes were placed at Cz and on the upper arm, to test for a possible effect of incidental stimulation of the motor regions under Cz. No effect of tDCS on RTs was found. These findings strengthen the case for tDCS having real effects on cerebral information processing. However, these effects did not clearly arise from either of the two processing stages of the visual search process. We suggest that this is because tDCS has a DIFFUSE, pervasive action across the task-relevant neuroanatomical region(s), not a discrete effect in terms of information processing stages.

Transcranial Magnetic Stimulation Reveals Intrinsic Perceptual and Attentional Rhythms

Oscillatory brain activity has functional relevance for perceptual and cognitive processes, as proven by numerous electrophysiology studies accumulating over the years. However, only within the past two decades have researchers been able to study the causal role of such oscillations using transcranial magnetic stimulation (TMS) technology. Two complementary approaches exist. A majority of research employs rhythmic TMS (rTMS) to entrain oscillatory activity and investigate its effect on targeted brain functions. On the other hand, single pulses of TMS (spTMS) that can be delivered with a high spatio-temporal resolution, can be used to precisely probe the state of the system. In this mini-review, we concentrate on this second approach. We argue that, with no a priori hypothesis on the oscillatory frequency of the targeted cortical regions, spTMS can help establish causal links between spontaneous oscillatory activity and perceptual and cognitive functions. Notably, this approach helped to demonstrate that the occipital cortex is periodically involved during specific attentional tasks at the theta (~5 Hz) frequency. We propose that this frequency reflects periodic inter-areal communication for attentional exploration and selection. In the future, clever combination of non-invasive recording and stimulation with well-controlled psychophysics protocols will allow us to further our understanding of the role of brain oscillations for human brain functions.

Attention Reorients Periodically

Reorienting of voluntary attention enables the processing of stimuli at previously unattended locations. Although studies have identified a ventral fronto-parietal network underlying attention [1, 2], little is known about whether and how early visual areas are involved in involuntary [3, 4] and even less in voluntary [5] reorienting, and their temporal dynamics are unknown. We used transcranial magnetic stimulation (TMS) over the occipital cortex to interfere with attentional reorienting and study its role and temporal dynamics in this process. Human observers performed an orientation discrimination task, with either valid or invalid attention cueing, across a range of stimulus contrasts. Valid cueing induced a behavioral response gain increase, higher asymptotic performance for attended than unattended locations. During subsequent TMS sessions, observers performed the same task, with high stimulus contrast. Based on phosphene mapping, TMS double pulses were applied at one of various delays to a consistent brain location in retinotopic areas (V1/V2), corresponding to the evoked signal of the target or distractor, in a valid or invalid trial. Thus, the stimulation was identical for the four experimental conditions (valid/invalid cue condition × target/distractor-stimulated). TMS modulation of the target and distractor were both periodic (5 Hz, theta) and out of phase with respect to each other in invalid trials only, when attention had to be disengaged from the distractor and reoriented to the target location. Reorientation of voluntary attention periodically involves V1/V2 at the theta frequency. These results suggest that TMS probes theta phase-reset by attentional reorienting and help link periodic sampling in time and attention reorienting in space.

Theta oscillations modulate attentional search performance periodically

Visual search--finding a target element among similar-looking distractors--is one of the prevailing experimental methods to study attention. Current theories of visual search postulate an early stage of feature extraction interacting with an attentional process that selects candidate targets for further analysis; in difficult search situations, this selection is iterated until the target is found. Although such theories predict an intrinsic periodicity in the neuronal substrates of attentional search, this prediction has not been extensively tested in human electrophysiology. Here, using EEG and TMS, we study attentional periodicities in visual search. EEG measurements indicated that successful and unsuccessful search trials were associated with different amounts of poststimulus oscillatory amplitude and phase-locking at ∼6 Hz and opposite prestimulus oscillatory phase at ∼6 Hz. A trial-by-trial comparison of pre- and poststimulus ∼6 Hz EEG phases revealed that the functional interplay between prestimulus brain states, poststimulus oscillations, and successful search performance was mediated by a partial phase reset of ongoing oscillations. Independently, TMS applied over occipital cortex at various intervals after search onset demonstrated a periodic pattern of interference at ∼6 Hz. The converging evidence from independent TMS and EEG measurements demonstrates that attentional search is modulated periodically by brain oscillations. This periodicity is naturally compatible with a sequential exploration by attention, although a parallel but rhythmically modulated attention spotlight cannot be entirely ruled out.

The chronometry of visual perception: review of occipital TMS masking studies

Transcranial magnetic stimulation (TMS) continues to deliver on its promise as a research tool. In this review article we focus on the application of TMS to early visual cortex (V1, V2, V3) in studies of visual perception and visual awareness. Depending on the asynchrony between visual stimulus onset and TMS pulse (SOA), TMS can suppress visual perception, allowing one to track the time course of functional relevance (chronometry) of early visual cortex for vision. This procedure has revealed multiple masking effects ('dips'), some consistently (∼+100ms SOA) but others less so (∼-50ms, ∼-20ms, ∼+30ms, ∼+200ms SOA). We review the state of TMS masking research, focusing on the evidence for these multiple dips, the relevance of several experimental parameters to the obtained 'masking curve', and the use of multiple measures of visual processing (subjective measures of awareness, objective discrimination tasks, priming effects). Lastly, we consider possible future directions for this field. We conclude that while TMS masking has yielded many fundamental insights into the chronometry of visual perception already, much remains unknown. Not only are there several temporal windows when TMS pulses can induce visual suppression, even the well-established 'classical' masking effect (∼+100ms) may reflect more than one functional visual process.

The flickering wheel illusion: when α rhythms make a static wheel flicker

α oscillations (8-14 Hz) greatly influence brain activity, yet we generally do not experience them consciously: the world does not appear to oscillate. Dedicated strategies must exist in the brain to prevent these oscillations from disrupting normal processing. Could suitable stimuli fool these strategies and lead to the conscious experience of our own brain oscillations? We describe and explore a novel illusion in which the center of a static wheel stimulus (with 30-40 spokes) is experienced as flickering when viewed in the visual periphery. The key feature of this illusion is that the stimulus fluctuations are experienced as a regular and consistent flicker, which our human observers estimated at ~9 Hz during a psychophysical matching task. Correspondingly, the occipital α rhythm of the EEG was the only oscillation that showed a time course compatible with the reported illusion: when α amplitude was strong, the probability of reporting illusory flicker increased. The peak oscillatory frequency for these flicker-induced modulations was significantly correlated, on a subject-by-subject basis, with the individual α frequency measured during rest, in the absence of visual stimulation. Finally, although the effect is strongest during eye movements, we showed that stimulus motion relative to the retina is not necessary to perceive the illusion: the flicker can also be perceived on the afterimage of the wheel, yet by definition this afterimage is stationary on the retina. We conclude that this new flickering illusion is a unique way to experience the α rhythms that constantly occur in the brain but normally remain unnoticed.