Biology subjects

VanRullen, R.

Publications and source records attributed to VanRullen, R..

5 recordsLinked to original sources

bioRxiv2018

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, while 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 predictions: (1) high-level, attentional regions and (2) early visual regions should both be iteratively (periodically) involved during the search. Here, we used Transcranial Magnetic Stimulation (TMS) applied over the Frontal-Eye Field (FEF), 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 7 Hz (theta) suggesting that the FEF is periodically involved during this difficult search task. We further compared this result with two previous studies (Dugue 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, for both studies, the same pattern of interference, i.e. V1 is periodically involved during this difficult search task, at the theta frequency. Together, these converging findings confirm our predictions that difficult search is supported by the periodic involvement of both low- and high-level regions, at the theta frequency.\n\nSignificant statementAttention models postulate a two-stage process during visual search in which early visual regions perform feature extraction, while higher-order regions perform attentional selection, these two levels iteratively (periodically) communicating until target recognition. Using TMS, we tested whether there is a causal link between both attentional and early visual regions, and attentional search performance. We showed that a difficult, attentional search is supported by the periodic involvement of both V1 and the FEF, at the theta frequency ([~]6-7 Hz). This finding support the idea that visual search tasks are processed by a hierarchical system involving periodic, iterative connections between low- and high-level regions allowing successful attentional exploration.

neuroscience↗

bioRxiv2017

The hidden spatial dimension of alpha: 10 Hz perceptual echoes propagate as periodic travelling waves in the human brain.

Alpha oscillations play a special role in vision. During sensory processing, reverse-correlation techniques revealed that white-noise luminance sequences elicit a robust occipital [~]10 Hz response that periodically reverberates the input sequence for up to 1 s. These perceptual echoes constitute the impulse response function of the visual system. However, the spatial dimension of perceptual echoes remains unknown: do they reverberate across the cortex simultaneously? Does stimulation over multiple visual coordinates evoke multiple synchronized echoes, or do they show consistent phase differences? Here, we tested the spatial dimension of perceptual echoes in two electroencephalogram (EEG) experiments manipulating the location of the visual stimulation. When a single disc flickered a white-noise luminance sequence in the upper visual field, we observed a single \"echo wave\" originating in posterior sensors and spatially propagating towards frontal ones (i.e. periodic travelling wave). The presentation of two independent flickering discs in separate visual hemifields produced two simultaneous and superimposed echo waves propagating in opposite directions, one in response to each stimulus. Strikingly, at many electrode sites, the phase of the two echoes differed, with a phase advance for the contralateral stimulus location. EEG source reconstruction tentatively located the waves within contralateral parieto-occipital cortex. In conclusion, the alpha rhythm processes stimulus information as a travelling wave that propagates across the cortical representation of retinotopic space in the human brain. In line with the \"cortical scanning\" hypothesis (Pitts & McCulloch, 1947), these results suggest the existence of an additional spatial dimension embedded in the phase of the alpha rhythm.\n\nSignificance statementHow does the spatial dimension of sensory processing relate to the temporal dimension of brain rhythms? Using correlation techniques, we characterized perceptual echoes, the average electroencephalogram response induced by visual stimuli that change luminance randomly. We found that perceptual echoes are actually periodic waves that travel through human visual cortex. Strikingly these periodic waves show consistent phase differences across the visual field, processing screen locations sequentially across distinct phases of the cycle following basic retinotopy. These results suggest the existence of an additional \"hidden\" spatial dimension in sensory cortex, encoded in the phase of the alpha oscillatory cycle. This could mean that perceptual echoes behave like sweeps of a sonar, processing the visual field in cycles of [~]100 ms duration.

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bioRxiv2017

Individual alpha peak frequency predicts 10 Hz flicker effects on selective attention

Rhythmic visual stimulation (\"flicker\") is primarily used to \"tag\" processing of low-level visual and high-level cognitive phenomena. However, preliminary evidence suggests that flicker may also entrain endogenous brain oscillations, thereby modulating cognitive processes supported by those brain rhythms. Here we tested the interaction between 10 Hz flicker and endogenous alpha-band (~10 Hz) oscillations during a selective visuospatial attention task. We recorded EEG from human participants (both genders) while they performed a modified Eriksen flanker task in which distractors and targets flickered within (10 Hz) or outside (7.5 or 15 Hz) the alpha band. By using a combination of EEG source separation, time-frequency, and single-trial linear mixed effects modeling, we demonstrate that 10 Hz flicker interfered with stimulus processing more on incongruent than congruent trials (high vs. low selective attention demands). Crucially, the effect of 10 Hz flicker on task performance was predicted by the distance between 10 Hz and individual alpha peak frequency (estimated during the task). Finally, the flicker effect on task performance was more strongly predicted by EEG flicker responses during stimulus processing than during preparation for the upcoming stimulus, suggesting that 10 Hz flicker interfered more with reactive than proactive selective attention. These findings are consistent with our hypothesis that visual flicker entrained endogenous alpha-band networks, which in turn impaired task performance. Our findings also provide novel evidence for frequency-dependent exogenous modulation of cognition that is determined by the correspondence between the exogenous flicker frequency and the endogenous brain rhythms.\n\nSignificanceHere we provide novel evidence that the interaction between exogenous rhythmic visual stimulation and endogenous brain rhythms can have frequency-specific behavioral effects. We show that alpha-band (10 Hz) flicker impairs stimulus processing in a selective attention task when the stimulus flicker rate matches individual alpha peak frequency. The effect of sensory flicker on task performance was stronger when selective attention demands were high, and was stronger during stimulus processing and response selection compared to the pre-stimulus anticipatory period. These findings provide novel evidence that frequency-specific sensory flicker affects online attentional processing, and also demonstrate that the correspondence between exogenous and endogenous rhythms is an overlooked prerequisite when testing for frequency-specific cognitive effects of flicker.

neuroscience↗

bioRxiv2017

The rhythm of attentional stimulus selection during visual competition

Recent research indicates that attentional stimulus selection could in fact be a rhythmic process, operating as a sequence of successive cycles. When two items must be monitored, an intriguing corollary of this \"blinking spotlight\" notion could be that the successive cycles are directed alternately to each target; as a result, each item would effectively be selected at half the intrinsic rate of attentional selection. Here, we tested this prediction in two experiments. In an endogenous attention task, subjects covertly monitored one or two peripheral images in order to detect a brief contrast change. In the sustained occipital EEG power spectrum, selecting two vs. one item resulted in a relative increase around 4Hz and a relative decrease around 10-11Hz. In a second experiment, we tested if comparable oscillations could be observed in the stimulus-evoked EEG visual representational content. Subjects saw a first peripheral image displayed alone for 600ms, before a second one also appeared for the same duration, but at a different peripheral location. Using pattern analysis on EEG evoked-responses, we were able to create item selective classifiers that constantly indicated which stimulus was on the screen. The time-course of single-trial classifier decision values presented a relative spectral peak around 11Hz when only one object was present, and around 4-5Hz when two objects were on the screen. These results are both compatible with an attentional stimulus selection process sampling the visual field at around 10-11Hz, and resulting in a half-frequency effective sampling around 4-5Hz when there are two items to monitor.

neuroscience↗

bioRxiv2016

The triple-flash illusion reveals a driving role of alpha-band reverberations in visual perception

The modulatory role of spontaneous brain oscillations on perception of threshold-level stimuli is well established. Here, we provide evidence that alpha-band (7-14 Hz) oscillations not only modulate but also can drive perception. We used the \"triple-flash\" illusion: Occasional perception of three flashes when only two spatially-coincident veridical ones are presented, separated by ~100 ms. The illusion was proposed to result from superposition of two hypothetical oscillatory impulse response functions (IRF) generated in response to each flash (Bowen, 1989). In Experiment 1, we varied stimulus onset asynchrony (SOA) and validated Bowen's theory: the optimal SOA for illusion to occur was correlated, across subjects, with the subject-specific IRF period. Experiment 2 revealed that pre-stimulus parietal alpha EEG phase and power, as well as post-stimulus alpha phase-locking, together determine the occurrence of the illusion on a trial-by-trial basis. Thus, oscillatory reverberations create something out of nothing - a third flash where there are only two.

neuroscience↗