Alpha revisited: The role of neural oscillations in visual perception and selective attention
The role of alpha-band oscillatory activity in voluntary attentional control across sensory modalities: An assessment of supramodal attention theory
Snigdha Banerjee, Adam Snyder, Sophie Molholm and John Foxe
Oscillatory alpha-band activity (8-15 Hz) over parieto-occipital cortex in humans plays an important role in voluntary attentional control, with increased alpha-band power observed over cortex contralateral to locations expected to contain distractors. The parietal lobes are prominent generators of alpha oscillations, raising the possibility that alpha is a neural signature of spatial attention across sensory modalities. Here, we asked whether lateralized alpha-band activity was also evident in a purely audio-spatial cueing task and whether it had the same underlying generator configuration as in a purely visuospatial task, which would provide strong support for “supramodal” attention theory. Alternately, alpha-band differences between auditory and visual tasks would support a sensory-specific account. We found lateralized alpha-band activations over parieto-occipital regions for both tasks, yet clear differences in scalp topographies depending on the sensory system within which spatial attention was deployed. Findings suggest that parietally-generated alpha-band mechanisms are central to attentional deployments across modalities, but that they are invoked in a sensory-specific manner. In a following study, we observed that pure voluntary attentional control in the absence of attention-directing cues enhanced early activations in visual cortices. These studies develop robust metrics for voluntary attentional control, and implications for understanding the neural mechanisms of attentional disorders.
Oscillatory markers of perceptual decision formation under vigilant monitoring conditions
Simon Kelly and Redmond O' Connell
We are often required to maintain focus on a continuous task that is perceptually trivial, but involves detection of events that are intermittent, unpredictable, and that lack physical salience – for example, monitoring the proximity of flanking cars on the highway. This cognitive ability, known as vigilant attention, is one of the hardest to study in the laboratory, because the discrete events that we normally use to elicit behavioral and neurophysiological responses are inherently salient, undermining the role of vigilance. We have recently developed a new supramodal continuous-monitoring paradigm that enables continuous electrophysiological (EEG) tracking of perceptual, cognitive and motor processes in parallel over prolonged periods, by exploiting well known oscillatory amplitude effects. While observers monitor for gradually emerging targets defined by changes in a single stimulus feature, we track the encoding of that feature in stimulus-driven steady-state activity (21 Hz), we track accumulated evidence via parietal alpha activity (8-14Hz), and track motor preparation via lateralized beta (22-30 Hz), allowing a unique view on attentional fluctuations at each of these distinct processing stages that have direct consequences for the timing and accuracy of detection decisions.
Alpha rhythms echo the world inside the brain, and make it flicker
The alpha rhythm (8-13Hz) is the largest oscillatory signal that can be recorded from the awake human brain. What is it good for? Current thinking is that it serves an inhibitory purpose: it decreases upon visual stimulation, it is smaller in cortical areas that receive attentional enhancement and higher in those areas that receive suppression. This view implies that when there is alpha, you don't see much, and vice-versa. I will show two recent results from our laboratory that suggest otherwise. First, alpha rhythms were found to echo a random visual stimulation sequence in the brain for about 1 second –alpha was thus positively related to visual processing. Second, alpha can induce an illusion of perceptual flicker for a particular static stimulus (a wheel with 32 spokes, viewed in the periphery) –in other words, the consequences of alpha oscillations can sometimes be perceptually experienced. I will conjecture that alpha rhythms, although inhibitory by nature, do not abolish perception; rather, they temporally shape the stream of perceptual experience.
Dynamic alpha re-mapping during pro- and anti- saccade tasks: common rapid oscillatory mechanisms during both overt and covert attentional deployments
Daniel Belyusar, Adam C Snyder, Hans-Peter Frey, Mark R Harwood and John J Foxe
Previous research on the role of attention in visual tasks has tended to use experimental designs in which alpha rhythms have been shown to slowly modulate over a 1-2 second cue-task interval. However, overt attention, as exemplified by the saccadic eye movement system, can shift several times per second. While some electrophysiological evidence has suggested a common mechanism for shifting both covert attention and eye movements (Kustov AA, Robinson D.L. Nature. 1996 Nov 7;384(6604):74-7) other results favor unique cortical mechanisms (Eimer M., Van Velzen J., Gherri E., Press C. Brain Research, 2007; Mar 02; 1135(1),154-66). To address these conflicting results, we considered a known electrophysiological correlate of covert attention in an anti-saccade paradigm in which participants need to suppress lateralized exogenous cues, in order to quickly move their eyes to the opposite side. Previous research has shown changes in alpha-band (8-14Hz) power correlate with preparatory states, such that increases in alpha levels are associated with active suppression of unattended targets. Our results similarly indicate differential parieto-occipital alpha-band modulations to both cue and target location, to both auditory and visual cues. Results demonstrate rapid shifts in alpha power to cue onset, and later to saccade-related lateralization under 300ms. These phases appear topographically similar across the scalp regardless of stimulus modality suggesting an exciting new role for alpha rhythms in both sensory and motor processes.
Alpha-band rhythms in visual task performance: Phase-locking by sensory stimulation, and relation to encephalographic activity
An event in one sensory modality can phase-reset brain oscillations concerning the same or another modality. This may result in stimulus-locked periodicity in behavioral performance cycling at the frequency of the phase-reset oscillation. My talk will considered this possible impact of sensory events for one of the best-characterized rhythms of the visual system, the alpha-oscillations. In one experiment, we presented rhythmic visual cues at different frequencies and tested their impact on subsequent visual target detection (unimodal impact) at cued and uncued positions. We found a breakdown of cueing benefits for 10Hz-stimulation (in the alpha-band) in comparison to stimulation at flanker frequencies. In addition, 10Hz-stimulation led to an alpha-rhythm in visual task performance post-cueing. In another experiment, we presented a brief sound and found again a periodic pattern in visual task performance post-sound (crossmodal impact) cycling at alpha-frequency. In both experiments, the sinusoidal pattern of visual performance correlated in frequency across individuals with resting encephalographic alpha-oscillations over occipital areas. This indicates that (i) brain oscillations have been entrained/time-locked by the sensory event, and that (ii) this can be used to reveal cyclical influences of brain rhythms on perception to study their functional roles, here in line with rapid alpha-cycles underlying periodic visual sampling.
Cortical cross-frequency coupling dramatically affects performance during a taxing visual-detection task
Ian Fiebelkorn, Adam Snyder, Manuel Mercier, John Butler, Sophie Molholm and John Foxe
Functional networks are comprised of neuronal ensembles bound through synchronization across multiple intrinsic oscillatory frequencies. Various coupled interactions between brain oscillators have been described (e.g., phase-amplitude coupling), but with little evidence that these interactions actually influence perceptual sensitivity. Here, electroencephalographic recordings were made during a sustained-attention task to demonstrate that cross-frequency coupling has significant consequences for perceptual outcomes (i.e., whether participants detect a near-threshold visual target). Our results reveal that phase-detection relationships at higher frequencies are entirely dependent on the phase of lower frequencies, such that higher frequencies alternate between periods when their phase is strongly predictive of visual-target detection and periods when their phase has no influence whatsoever. These data thus bridge the crucial gap between complex oscillatory phenomena and perceptual outcomes. Accounting for cross-frequency coupling between lower (i.e., delta and theta) and higher frequencies (e.g., beta and gamma), we show that visual-target detection fluctuates dramatically as a function of pre-stimulus phase, with performance swings of as much as 80 percent.