Covert attention increases contrast sensitivity: psychophysical, neurophysiological and neuroimaging studies

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523

An important effect of sustained attention is the facilitation of perception. Although the term "sustained" suggests that this beneficial effect endures continuously as long as something is attended, we present electrophysiological evidence that perception at attended locations is actually modulated periodically. Subjects detected brief light flashes that were presented peripherally at locations that were either attended or unattended. We analyzed the correlation between detection performance for attended and unattended stimuli and the phase of ongoing EEG oscillations, which relate to subsecond fluctuations of neuronal excitability. Although on average, detection performance was improved by attention-indicated by reduced detection thresholds at attended locations-we found that detection performance for attended stimuli actually fluctuated over time along with the phase of spontaneous oscillations in the θ (≈7 Hz) frequency band just before stimulus onset. This fluctuation was absent for unattended stimuli. This pattern of results suggests that "sustained" attention in fact exerts its facilitative effect on perception in a periodic fashion.O ur senses are constantly confronted with a flow of information that exceeds the limits of our sensory systems. One mechanism that limits this input to a manageable amount is selective attention-the ability to enhance processing of a particular object or location. The investigation of attention has been central to our understanding of cognition. Here we present evidence for a largely unexplored feature of selective attention: its periodic rather than continuous operation.The most prominent effect of attention is the facilitation of perception-selected information is processed more efficiently (1, 2) or faster (3). One example of this facilitation is enhanced contrast sensitivity at attended locations (4, 5). Two types of selective attention are usually distinguished: transient automatic attention and sustained voluntary attention (6, 7). Transient attention is captured automatically by sudden salient events, whereas sustained attention can be controlled at will. Although the term "sustained" suggests that the facilitative effect of attention endures continuously as long as attention is deployed at a particular location, it is conceivable that attention operates in a periodic rather than a continuous fashion (8,9). This periodicity of attention is implicit in many visual search tasks in which attention is assumed to switch successively between the different display elements (10-12). On the basis of modeling of human psychometric functions in a signal detection task, VanRullen et al. (8) recently proposed that attention could sample information at a rate of approximately seven items per second-no matter whether attention was focused on multiple targets or on just a single item. A similar attention-driven periodic sampling mechanism (albeit at a different frequency of ≈13 Hz) has been suggested to account for illusory motion reversals in the continuous wagon-wheel illusion (9,...

Section: Discussionsupporting

confidence: 91%

“…One example of this facilitation is enhanced contrast sensitivity at attended locations (4,5). Two types of selective attention are usually distinguished: transient automatic attention and sustained voluntary attention (6,7).…”

mentioning

confidence: 99%

Spontaneous EEG oscillations reveal periodic sampling of visual attention

Busch

VanRullen

2010

543

523

“…SDT posits that performance can be summarized by two statistics: d′, indexing sensitivity to signal occurrence in signal-to-noise units, and c, reflecting a bias to report signal occurrence (modeled as a decision criterion). Using this approach, it is now well established that cues that predict the location of a behaviorally relevant signal increase sensitivity (3)(4)(5)(6) by improving the precision of visual processing (7)(8)(9)(10). By contrast, cues that predict a greater probability of signal occurrence alone are believed to have no influence on sensitivity (11,12) but instead bias observers to report signal occurrence by adopting a more liberal decision criterion.…”

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confidence: 99%

Dissociable prior influences of signal probability and relevance on visual contrast sensitivity

Wyart

Nobre

Summerfield

2012

245

262

According to signal detection theoretical analyses, visual signals occurring at a cued location are detected more accurately, whereas frequently occurring ones are reported more often but are not better distinguished from noise. However, conventional analyses that estimate sensitivity and bias by comparing true- and false-positive rates offer limited insights into the mechanisms responsible for these effects. Here, we reassessed the prior influences of signal probability and relevance on visual contrast detection using a reverse-correlation technique that quantifies how signal-like fluctuations in noise predict trial-to-trial variability in choice discarded by conventional analyses. This approach allowed us to estimate separately the sensitivity of true and false positives to parametric changes in signal energy. We found that signal probability and relevance both increased energy sensitivity, but in dissociable ways. Cues predicting the relevant location increased primarily the sensitivity of true positives by suppressing internal noise during signal processing, whereas cues predicting greater signal probability increased both the frequency and the sensitivity of false positives by biasing the baseline activity of signal-selective units. We interpret these findings in light of “predictive-coding” models of perception, which propose separable top-down influences of expectation (probability driven) and attention (relevance driven) on bottom-up sensory processing.

“…Psychophysical studies have found that covert attention alters color contrast sensitivity (19), as it does for luminance contrast sensitivity (20). Yet, color contrast sensitivity is largely unaltered during saccades, at least at nontarget locations (21).…”

Section: Discussionmentioning

confidence: 99%

Dynamic sensitivity of area V4 neurons during saccade preparation

Han

Xian

Moore

2009

During the preparation of saccadic eye movements, visual attention is confined to the target of intended fixation and there is a corresponding diminution of visual sensitivity at nontarget locations. Neurons within the macaque visual cortex exhibit correlates of these perceptual changes, such as in area V4, where neuronal responses are enhanced during the preparation of saccades to stimuli within the receptive field (RF), and responses are suppressed during the preparation of saccades to other locations. Both the perceptual and neurophysiological effects suggest that the sensitivity of visual cortical neurons to input is dynamic during saccade preparation. We probed the contrast sensitivity of area V4 neurons to nontarget stimuli at varying times during the preparation of saccades to locations outside of the neuron's receptive field. We found that the contrast sensitivity of many neurons is profoundly altered within 50 ms of saccade onset. The luminance or color contrast sensitivity of individual V4 neurons could increase, decrease, or remain unchanged before saccade onset. For luminance contrast sensitivity, decreases in sensitivity were more frequent and larger in magnitude, resulting in an overall decrement in sensitivity across the population. For color contrast, the effects were smaller and more heterogeneous, resulting in little or no overall change in sensitivity across the population. Our results demonstrate the dynamic influence that saccade preparation has on the sensitivity of visual cortical neurons and suggest a basis for the changes in perception known to occur during saccade preparation.oculomotor ͉ saccadic suppression ͉ transaccadic integration ͉ visuomotor integration ͉ perceptual constancy V isual spatial attention is typically time locked to the onset of saccades (1-2). Although attention is sometimes deployed to peripheral locations without concomitant saccades (covert attention), more often it is deployed along with them (overt attention). As a result of heightened attention at the saccade target, saccades are accompanied by diminished perception at nontarget locations (3). Neurophysiological studies have established that covert attention alters the responses of neurons in the visual cortex (4). For example, covert attention enhances the responses or contrast sensitivity of neurons in macaque area V4 to visual stimuli within their receptive fields (RFs) (5-9). The perceptual and neurophysiological effects of covert attention are thought to result at least in part from a direct influence of saccade-related signals on visual cortical representations (10). For example, the attentional modulation of visually driven responses of area V4 neurons can be recreated by electrical microstimulation within the frontal eye field (FEF) (11-13). Specifically, microstimulation of the FEF at sites that spatially overlap RFs of recorded V4 neurons transiently enhances the response to stable RF stimuli. In contrast, microstimulation of the FEF at sites that do not overlap with V4 RFs suppresses V4 responses. T...