Attention capture outside the oculomotor range

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.

A dynamic 1/f noise protocol to assess visual attention without biasing perceptual processing

Psychophysical paradigms measure visual attention via localized test items to which observers must react or whose features have to be discriminated. These items, however, potentially interfere with the intended measurement, as they bias observers' spatial and temporal attention to their location and presentation time. Furthermore, visual sensitivity for conventional test items naturally decreases with retinal eccentricity, which prevents direct comparison of central and peripheral attention assessments. We developed a stimulus that overcomes these limitations. A brief oriented discrimination signal is seamlessly embedded into a continuously changing 1/f noise field, such that observers cannot anticipate potential test locations or times. Using our new protocol, we demonstrate that local orientation discrimination accuracy for 1/f filtered signals is largely independent of retinal eccentricity. Moreover, we show that items present in the visual field indeed shape the distribution of visual attention, suggesting that classical studies investigating the spatiotemporal dynamics of visual attention via localized test items may have obtained a biased measure. We recommend our protocol as an efficient method to evaluate the behavioral and neurophysiological correlates of attentional orienting across space and time.

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.

Independence of implicitly guided attention from goal-driven oculomotor control

Location probability learning-the acquisition of an attentional bias toward locations that frequently contained a search target-shows many characteristics of a search habit. To what degree does it depend on oculomotor control, as might be expected if habit-like attention is grounded in eye movements? Here, we examined the impact of a spatially incompatible oculomotor signal on location probability learning (LPL). On each trial of a visual search task, participants first saccaded toward a unique C-shape, whose orientation determined whether participants should continue searching for a T target among L distractors. The C-shape often appeared in one, "C-rich" quadrant that differed from where the T was frequently located. Experiment 1 showed that participants acquired LPL toward the high-probability, "T-rich" quadrant, an effect that persisted in an unbiased testing phase. Participants were also faster finding the target in the vicinity of the C-shape, but this effect did not persist after the C-shape was removed. Experiment 2 found that the C-shape affected search only when it was task-relevant. Experiment 3 replicated and extended the findings of Experiment 1 using eye tracking. Thus, location probability learning is robust in the face of a spatially incompatible saccade, demonstrating partial independence between experience-guided attention and goal-driven oculomotor control. The findings are in line with the modular view of attention, which conceptualizes the search habit as a high-level process abstracted from eye movements.

Pupil size variations reveal covert shifts of attention induced by numbers

The pupil light response is more than a pure reflexive mechanism that reacts to the amount of light entering the eye. The pupil size may also react to the luminance of objects lying in the visual periphery, revealing the locus of covert attention. In the present study, we took advantage of this response to study the spatial coding of abstract concepts with no physical counterpart: numbers. The participants' gaze was maintained fixed in the middle of a screen whose left and right parts were dark or bright, and variations in pupil size were recorded during an auditory number comparison task. The results showed that small numbers accentuated pupil dilation when the darker part of the screen was on the left, while large numbers accentuated pupil dilation when the darker part of the screen was on the right. This finding provides direct evidence for covert attention shifts on a left-to-right oriented mental spatial representation of numbers. From a more general perspective, it shows that the pupillary response to light is subject to modulation from spatial attention mechanisms operating on mental contents.

To look or not to look: dissociating presaccadic and covert spatial attention

Attention is a central neural process that enables selective and efficient processing of visual information. Individuals can attend to specific visual information either overtly, by making an eye movement to an object of interest, or covertly, without moving their eyes. We review behavioral, neuropsychological, neurophysiological, and computational evidence of presaccadic attentional modulations that occur while preparing saccadic eye movements, and highlight their differences from those of covert spatial endogenous (voluntary) and exogenous (involuntary) attention. We discuss recent studies and experimental procedures on how these different types of attention impact visual performance, alter appearance, differentially modulate the featural representation of basic visual dimensions (orientation and spatial frequency), engage different neural computations, and recruit partially distinct neural substrates. We conclude that presaccadic attention and covert attention are dissociable.