Covert Shifts Of Visual Attention Research Articles

In pursuit of visual attention: SSVEP frequency-tagging moving targets

Previous research has shown that visual attention does not always exactly follow gaze direction, leading to the concepts of overt and covert attention. However, it is not yet clear how such covert shifts of visual attention to peripheral regions impact the processing of the targets we directly foveate as they move in our visual field. The current study utilised the co-registration of eye-position and EEG recordings while participants tracked moving targets that were embedded with a 30 Hz frequency tag in a Steady State Visually Evoked Potentials (SSVEP) paradigm. When the task required attention to be divided between the moving target (overt attention) and a peripheral region where a second target might appear (covert attention), the SSVEPs elicited by the tracked target at the 30 Hz frequency band were significantly, but transiently, lower than when participants did not have to covertly monitor for a second target. Our findings suggest that neural responses of overt attention are only briefly reduced when attention is divided between covert and overt areas. This neural evidence is in line with theoretical accounts describing attention as a pool of finite resources, such as the perceptual load theory. Altogether, these results have practical implications for many real-world situations where covert shifts of attention may discretely reduce visual processing of objects even when they are directly being tracked with the eyes.

Using task effort and pupil size to track covert shifts of visual attention independently of a pupillary light reflex.

We tested the link between pupil size and the task effort involved in covert shifts of visual attention. The goal of this study was to establish pupil size as a marker of attentional shifting in the absence of luminance manipulations. In three experiments, participants evaluated two stimuli that were presented peripherally, appearing equidistant from and on opposite sides of eye fixation. The angle between eye fixation and the peripherally presented target stimuli varied from 12.5° to 42.5°. The evaluation of more distant stimuli led to poorer performance than did the evaluation of more proximal stimuli throughout our study, confirming that the former required more effort than the latter. In addition, in Experiment 1 we found that pupil size increased with increasing angle and that this effect could not be reduced to the operation of low-level visual processes in the task. In Experiment 2 the pupil dilated more strongly overall when participants evaluated the target stimuli, which required shifts of attention, than when they merely reported on the target's presence versus absence. Both conditions yielded larger pupils for more distant than for more proximal stimuli, however. In Experiment 3, we manipulated task difficulty more directly, by changing the contrast at which the target stimuli were presented. We replicated the results from Experiment 1 only with the high-contrast stimuli. With stimuli of low contrast, ceiling effects in pupil size were observed. Our data show that the link between task effort and pupil size can be used to track the degree to which an observer covertly shifts attention to or detects stimuli in peripheral vision.

Decoding covert shifts of attention induced by ambiguous visuospatial cues.

Simple and unambiguous visual cues (e.g., an arrow) can be used to trigger covert shifts of visual attention away from the center of gaze. The processing of visual stimuli is enhanced at the attended location. Covert shifts of attention modulate the power of cerebral oscillations in the alpha band over parietal and occipital regions. These modulations are sufficiently robust to be decoded on a single trial basis from electroencephalography (EEG) signals. It is often assumed that covert attention shifts are under voluntary control, and that they also occur in more natural and complex environments, but there is no direct evidence to support this assumption. We address this important issue by using random-dot stimuli to cue one of two opposite locations, where a visual target is presented. We contrast two conditions, one in which the random-dot motion is predictive of the target location, and the other, in which it provides ambiguous information. Behavioral results show attention shifts in anticipation of the visual target, in both conditions. In addition, using the common spatial patterns (CSPs) algorithm, we extract EEG power features in the alpha-band (around 10 Hz) that best discriminate the attended location in single trials. We obtain a significant decoding accuracy in 7/10 subjects using a cross-validation procedure applied in the predictive condition. Interestingly, similar accuracy (significant in 5/10 subjects) is obtained when the CSPs trained in the predictive condition are tested in the ambiguous condition. In agreement with this result, we find that the CSPs show very similar topographies in both conditions. These results shed a new light on the behavioral and EEG correlates of visuospatial attention in complex visual environments. This study demonstrates that alpha-power features could be used in brain–computer interfaces to decode covert attention shifts in an environment containing ambiguous spatial information.

Brain-computer interfacing using modulations of alpha activity induced by covert shifts of attention

BackgroundVisual brain-computer interfaces (BCIs) often yield high performance only when targets are fixated with the eyes. Furthermore, many paradigms use intense visual stimulation, which can be irritating especially in long BCI sessions. However, BCIs can more directly directly tap the neural processes underlying visual attention. Covert shifts of visual attention induce changes in oscillatory alpha activity in posterior cortex, even in the absence of visual stimulation. The aim was to investigate whether different pairs of directions of attention shifts can be reliably differentiated based on the electroencephalogram. To this end, healthy participants (N = 8) had to strictly fixate a central dot and covertly shift visual attention to one out of six cued directions.ResultsCovert attention shifts induced a prolonged alpha synchronization over posterior electrode sites (PO and O electrodes). Spectral changes had specific topographies so that different pairs of directions could be differentiated. There was substantial variation across participants with respect to the direction pairs that could be reliably classified. Mean accuracy for the best-classifiable pair amounted to 74.6%. Furthermore, an alpha power index obtained during a relaxation measurement showed to be predictive of peak BCI performance (r = .66).ConclusionsResults confirm posterior alpha power modulations as a viable input modality for gaze-independent EEG-based BCIs. The pair of directions yielding optimal performance varies across participants. Consequently, participants with low control for standard directions such as left-right might resort to other pairs of directions including top and bottom. Additionally, a simple alpha index was shown to predict prospective BCI performance.

Prepare for conflict: EEG correlates of the anticipation of target competition during overt and covert shifts of visual attention

When preparing to make a saccadic eye movement in a cued direction, perception of stimuli at the target location is enhanced, just as it is when attention is covertly deployed there. Accordingly, the timing and anatomical sources of preparatory brain activity accompanying shifts of covert attention and saccade preparation tend to exhibit a large degree of overlap. However, there is evidence that preparatory processes are modulated by the foreknowledge of visual distractor competition during covert attention, and it is unknown whether eye movement preparation undergoes equivalent modulation. Here we examine preparatory processes in the electroencephalogram of human participants during four blocked versions of a spatial cueing task, requiring either covert detection or saccade execution, and either containing a distractor or not. As in previous work, a typical pattern of spatially selective occipital, parietal and frontal activity was seen in all task versions. However, whereas distractor presence called on an enhancement of spatially selective visual cortical modulation during covert attention, it instead called on increased activity over frontomedial oculomotor areas in the case of overt saccade preparation. We conclude that, although advance orienting signals may be similar in character during overt and covert conditions, the pattern by which these signals are modulated to ameliorate the behavioral costs of distractor competition is highly distinct, pointing to a degree of separability between the overt and covert systems.

Low-level fixation search in natural scenes by optimal extraction of texture-contrast information

We construct the beginnings of a low-level theory of visual fixations in natural scenes by the formulation and verification a Barlow-type hypothesis for fixation selection—where the fixation patterns are designed to maximally contrast and textural information. We first briefly overview optimum contrast-based fixation strategies in natural scenes [1] and thereafter develop an optimum texture-based fixation selection algorithm based on our computational theory of non-stationarity measurement in natural images. In particular it is shown how a simple relative coding error measure between sub-patches of a window (that defines the image scale of analysis) can effectively measure the non-stationary structure of natural scenes which subsequently can be employed for the optimal extraction of textural information. Finally we propose a simple coupling of the optimal texture-based and contrast-based fixation algorithms which exhibits robust performance for fixation selection in natural images. The performance of the fixation algorithms are evaluated for natural images by comparison to actual human fixations performed on the images. The fixation patterns thus obtained outperform randomized, Gaffe-based [2], and Itti [3] fixation strategies in terms of matching human fixation patterns in terms of mutual information. These results also demonstrate the important role that contrast and textural information play in low-level visual processes in the HVS. [1] Raj, R. G., Geisler, W. S., Frazor, R. A., & Bovik, A. C. (Oct 2005). "Contrast statistics for foveated visual systems: Fixation selection by minimizing contrast entropy," J. Opt Soc Amer A, vol. 22, pp. 2039-2049. [2] Rajashekar, U., van der Linde, I., Bovik, A. C., & Cormack, L. K. (2008). "GAFFE: A gaze-attentive fixation finding engine," IEEE Trans Image Processing, to appear. [3] Itti, L., & Koch, C. (May 2000.). A saliency-based search mechanism for overt and covert shifts of visual attention, Vision Res, vol. 40, no. 10-12, pp. 1489-1506.

Increased effect of target eccentricity on covert shifts of visual attention in patients with neglect

Debate continues regarding the mechanisms underlying covert shifts of visual attention. We examined the relationship between target eccentricity and the speed of covert shifts of attention in normal subjects and patients with brain lesions using a cued-response task in which cues and targets were presented at 2° or 8° lateral to the fixation point. Normal subjects were slower on invalid trials in the 8° as compared to 2° condition. Patients with right-hemisphere stroke with neglect were slower in their responses to left-sided invalid targets compared to valid targets, and demonstrated a significant increase in the effect of target validity as a function of target eccentricity. Additional data from one neglect patient (JM) demonstrated an exaggerated validity × eccentricity × side interaction for contralesional targets on a cued reaction time task with a central (arrow) cue. We frame these results in the context of a continuous ‘moving spotlight’ model of attention, and also consider the potential role of spatial saliency maps. By either account, we argue that neglect is characterized by an eccentricity-dependent deficit in the allocation of attention.

Repetitive transcranial magnetic stimulation over frontal eye fields disrupts visually cued auditory attention

Voluntary eye movements and covert shifts of visual attention activate the same brain regions. Specifically, the intraparietal sulcus and the frontal eye fields (FEF) appear to be involved both with generating voluntary saccades as well with attending to a peripheral spatial location. Furthermore, these regions appear to be required by both tasks--functional disruption of these regions impairs both tasks. Therefore, it appears that the targeting system that allows us to plan saccades is the same system that allows us to covertly track peripheral visual information. Recent neuroimaging studies suggest that these brain regions are also activated when participants engage in auditory spatial attention tasks. However, it remains unclear whether these regions are required by these tasks. We used repetitive transcranial magnetic stimulation (rTMS) to disrupt the FEF while participants performed an auditory localization task. On each trial, a visual cue directed attention to the probable laterality of the auditory target, and the participant decided whether the subsequent target sound came from an upper or lower speaker. In the absence of TMS, individuals were faster to respond to targets that occurred on the cued side (valid trials) than when the target appears contralaterally to the cued side (invalid side). TMS interfered with this effect, such that the costs associated with ipsilateral invalidly cued targets were substantially reduced. These results suggest that the eye-movement system is needed for normal auditory attention.

A neural network implementation of a saliency map model

The saliency map model proposed by Itti and Koch [Itti, L., & Koch, C. (2000). A saliency-based search mechanism for overt and covert shifts of visual attention. Vision Research, 40, 1489-1506] has been a popular model for explaining the guidance of visual attention using only bottom-up information. In this paper we expand Itti and Koch's model and propose how it could be implemented by neural networks with biologically realistic dynamics. In particular, we show that by incorporating synaptic depression into the model, network activity can be normalized and competition within the feature maps can be regulated in a biologically plausible manner. Furthermore, the dynamical nature of our model permits further analysis of the time course of saliency computation, and also allows the model to calculate saliency for dynamic visual scenes. In addition to explaining the high saliency of pop-out targets in visual search tasks, our model explains attentional grab by sudden-onset stimuli, which was not accounted for by previous models.

Visual saliency as an aid to updating digital maps

Visual attention is considered in the context of a professional computer-based task, using aerial photography for updating topographic mapping data (photogrammetry). There is potential for using visual attention models to help develop various semi-automated ‘attention-aware’ support systems for this task, and these are discussed. An experimental study is described which examined the potential influence of expertise, image type and exposure duration on the role of visual saliency or salience (as calculated by Itti, L., & Koch, C. (2000). A saliency-based search mechanism for overt and covert shifts of visual attention. Vision Research, 40, 1389–1506 saliency maps) in the distribution of visual attention with such imagery. Using a non-intrusive, low-resolution and low-cost method to determine the approximate distribution of visual attention, effects of expertise and landscape type were found. Unexpectedly, saliency appeared to be more relevant to visual attention among expert users than novices, and potential reasons for this are explored. Implications and further research plans are discussed.

Crossmodal coupling of oculomotor control and spatial attention in vision and audition

Fixational eye movements occur involuntarily during visual fixation of stationary scenes. The fastest components of these miniature eye movements are microsaccades, which can be observed about once per second. Recent studies demonstrated that microsaccades are linked to covert shifts of visual attention. Here, we generalized this finding in two ways. First, we used peripheral cues, rather than the centrally presented cues of earlier studies. Second, we spatially cued attention in vision and audition to visual and auditory targets. An analysis of microsaccade responses revealed an equivalent impact of visual and auditory cues on microsaccade-rate signature (i.e. an initial inhibition followed by an overshoot and a final return to the pre-cue baseline rate). With visual cues or visual targets, microsaccades were briefly aligned with cue direction and then opposite to cue direction during the overshoot epoch, probably as a result of an inhibition of an automatic saccade to the peripheral cue. With left auditory cues and auditory targets microsaccades oriented in cue direction. We argue that microsaccades can be used to study crossmodal integration of sensory information and to map the time course of saccade preparation during covert shifts of visual and auditory attention.

A saliency-based search mechanism for overt and covert shifts of visual attention

Most models of visual search, whether involving overt eye movements or covert shifts of attention, are based on the concept of a saliency map, that is, an explicit two-dimensional map that encodes the saliency or conspicuity of objects in the visual environment. Competition among neurons in this map gives rise to a single winning location that corresponds to the next attended target. Inhibiting this location automatically allows the system to attend to the next most salient location. We describe a detailed computer implementation of such a scheme, focusing on the problem of combining information across modalities, here orientation, intensity and color information, in a purely stimulus-driven manner. The model is applied to common psychophysical stimuli as well as to a very demanding visual search task. Its successful performance is used to address the extent to which the primate visual system carries out visual search via one or more such saliency maps and how this can be tested.

Overt orienting in the rat: parametric studies of cued detection of visual targets.

Covert shifts of visual attention in space have been quantified by measuring the effects of visual cues on the detection of visual targets in humans and monkeys maintaining visual fixation. These observations of "covert orienting" have provided important information regarding the neurobiology of visual attention in primates. This article describes a cued spatial target detection task for physically unrestrained rats. Valid cues (spatially contiguous with the target) enhanced target detection, and invalid cues (spatially discontiguous with the target) degraded target detection. Both visual and auditory cues were effective. These validity effects could not be explained by stimulus additivity or response preparation mechanisms, whereas a cue-independent "alerting effect" appeared to reflect response preparation. The effects compare favorably with primate work and suggest that this method may enable assessment of visual attention shifts in rats.

Inhibition of return produced by covert shifts of visual attention in 6-month-old infants

Posner and Cohen (1984) demonstrated that it attention is oriented to a peripheral spatial location by a visual cue and returns to a central fixation point, processing of subsequent visual stimuli at that location is inhibited relative to the contralateral position. To examine this phenomenon in infants, 25 six-month-olds were tested on a saccadic refixation task designed to produce covert shirts of attention. While fixating a central target, a peripheral cue was presented for 180 ms, followed immediately by the onset of a similar target at either the same or opposite location. Inhibition of return was demonstrated in that saccadic latencies to targets appearing at the same peripheral location were longer than latencies to targets appearing at the contralateral position even though no eye movement had been made to the cue.

A developmental study of covert orienting to peripheral visual cues

Observers aged 6, 8, and 20 years participated in a speeded classification task designed to measure covert shifts of visual attention. On each trial observers responded to a target which appeared in one of three locations arrayed horizontally across the display. Three prestimulus cues were employed: neutral (the cue was present in all three possible target locations), unpredicatable (the cue was present in the target location on a random basis), and predictable (the cue was present in the target location on 80% of the trials). There were four developmental findings: (1) Observers of all ages oriented attention automatically to the cued locations, (2) children processed targets in non-cued locations more slowly than adults, (3) only adults took advantage of the predictability of the cue, and (4) all age groups provided evidence that strategic orienting competes with response priming for cognitive resources. The implications of these findings for theories of attentional development are discussed.

Lateralized deficits in covert shifts of visual attention in chronic and never-medicated schizophrenics compared to normal controls

Lateralized deficits in covert shifts of visual attention in chronic and never-medicated schizophrenics compared to normal controls

Deficits in human visual spatial attention following thalamic lesions.

There has been speculation concerning the role that thalamic nuclei play in directing attention to locations in visual space [Crick, F. (1984) Proc. Natl. Acad. Sci. USA 81, 4586-4590]. We measured covert shifts of visual attention in three patients with unilateral thalamic hemorrhages shortly after the lesion and after a 6-month recovery period. The experiment measured reaction time to targets that occurred at locations to which attention had been cued (valid trials) or at a currently unattended location (invalid trials). Although the patients showed no deficits in visual fields with perimetry and no neglect in the 6-month follow-up, we found slow reaction times for targets on the side contralateral to the lesion whether or not attention had been cued to that location. Deficits have also been found in this task with cortical and midbrain lesions, but the patterns of performance are quite different. The results with thalamic patients suggest they have a specific deficit in the ability to use attention to improve the efficiency of processing visual targets contralateral to the lesion (engage operation). This finding is in accord with hypotheses of a thalamic link between cortical visual attention and pattern recognition systems proposed by Crick.

Isolating attentional systems: A cognitive-anatomical analysis

Recently our knowledge of the mechanisms of visual-spatial attention has improved because of studies employing single cell recording with alert monkeys and others using performance analysis of neurological patients. These studies suggest that a complex neural network that includes parts of the posterior parietal lobe and midbrain is involved in covert shifts of visual attention. Is this system an isolated visual attentional module or is it part of a more general attentional system? Our studies employ the dual-task technique to determine whether covert visual orienting can take place while a person’s attention is engaged in a language processing task. We find clear evidence of interference between the two tasks, which suggests some common operations. However, the results also indicate that whatever is common to the two tasks does not have the same anatomical location as that of visual-spatial attention.