The perceptual consequences of the attentional bias: evidence for distractor removal

Mapping the anatomy of perceptual pseudoneglect. A multivariate approach

Fundamental to the understanding of the functions of spatial cognition and attention is to clarify the underlying neural mechanisms. It is clear that relatively right-dominant activity in ventral and dorsal parieto-frontal cortex is associated with attentional reorienting, certain forms of mental imagery and spatial working memory for higher loads, while lesions mostly to right ventral areas cause spatial neglect with pathological attentional biases to the right side. In contrast, complementary leftward biases in healthy people, called pseudoneglect, have been associated with varying patterns of cortical activity. Notably, this inconsistency may be explained, at least in part, by the fact that pseudoneglect studies have often employed experimental paradigms that do not control sufficiently for cognitive processes unrelated to pseudoneglect. To address this issue, here we administered a carefully designed continuum of pseudoneglect and control tasks in healthy adults while using functional magnetic resonance imaging (fMRI). Data submitted to partial least square (PLS) imaging analysis yielded a significant latent variable that identified a right-dominant network of brain regions along the intra-occipital and -parietal sulci, frontal eye fields and right ventral cortex in association with perceptual pseudoneglect. Our results shed new light on the interplay of attentional and cognitive systems in pseudoneglect.

Evidence for a common mechanism of spatial attention and visual awareness: Towards construct validity of pseudoneglect

Present knowledge of attention and awareness centres on deficits in patients with right brain damage who show severe forms of inattention to the left, called spatial neglect. Yet the functions that are lost in neglect are poorly understood. In healthy people, they might produce “pseudoneglect”—subtle biases to the left found in various tests that could complement the leftward deficits in neglect. But pseudoneglect measures are poorly correlated. Thus, it is unclear whether they reflect anything but distinct surface features of the tests. To probe for a common mechanism, here we asked whether visual noise, known to increase leftward biases in the grating-scales task, has comparable effects on other measures of pseudoneglect. We measured biases using three perceptual tasks that require judgments about size (landmark task), luminance (greyscales task) and spatial frequency (grating-scales task), as well as two visual search tasks that permitted serial and parallel search or parallel search alone. In each task, we randomly selected pixels of the stimuli and set them to random luminance values, much like a poor TV signal. We found that participants biased their perceptual judgments more to the left with increasing levels of noise, regardless of task. Also, noise amplified the difference between long and short lines in the landmark task. In contrast, biases during visual searches were not influenced by noise. Our data provide crucial evidence that different measures of perceptual pseudoneglect, but not exploratory pseudoneglect, share a common mechanism. It can be speculated that this common mechanism feeds into specific, right-dominant processes of global awareness involved in the integration of visual information across the two hemispheres.

Intra- and Inter-Task Reliability of Spatial Attention Measures in Pseudoneglect

Healthy young adults display a leftward asymmetry of spatial attention ("pseudoneglect") that has been measured with a wide range of different tasks. Yet at present there is a lack of systematic evidence that the tasks commonly used in research today are i) stable measures over time and ii) provide similar measures of spatial bias. Fifty right-handed young adults were tested on five tasks (manual line bisection, landmark, greyscales, gratingscales and lateralised visual detection) on two different days. All five tasks were found to be stable measures of bias over the two testing sessions, indicating that each is a reliable measure in itself. Surprisingly, no strongly significant inter-task correlations were found. However, principal component analysis revealed left-right asymmetries to be subdivided in 4 main components, namely asymmetries in size judgements (manual line bisection and landmark), luminance judgements (greyscales), stimulus detection (lateralised visual detection) and judgements of global/local features (manual line bisection and grating scales). The results align with recent research on hemispatial neglect which conceptualises the condition as multi-component rather than a single pathological deficit of spatial attention. We conclude that spatial biases in judgment of visual stimulus features in healthy adults (e.g., pseudoneglect) is also a multi-component phenomenon that may be captured by variations in task demands which engage task-dependent patterns of activation within the attention network.

Lower-right and upper-left biases within upper and lower visual fields in a circular array task

Visuospatial performance varies along the horizontal and vertical dimensions, resulting in behavioral biases such as pseudoneglect. The interaction between the horizontal and vertical attentional biases was investigated using a novel circular array task capable of conveying relative brightness information across vertical and horizontal dimensions simultaneously. In a novel circular array task comprised of six discs, the grayscale gradient was disrupted by switching two grayscale values within the array. Leftward biases were observed in the lower visual fields and rightward biases in the upper visual fields. More importantly, the magnitude of bias within the upper/lower horizontal dimension altered depending on the relative position of the stimuli along horizontal and vertical axes within each dimension. Manipulating the upper-most and leftward discs yield stronger biases than manipulating rightward discs. Furthermore, stronger biases were observed during bottom and rightward disc manipulation. The upper-left and lower-right biases within the horizontal dimension indicate that the interactions between the horizontal and vertical biases may not rely simply on the dichotomy within the horizontal and vertical dimensions, but also on the relative spatial distribution of stimuli within these dimensions.

A toggle switch of visual awareness?

Major clues to the human brain mechanisms of spatial attention and visual awareness have come from the syndrome of neglect, where patients ignore one half of space. A longstanding puzzle, though, is that neglect almost always comes from right-hemisphere damage, which suggests that the two sides of the brain play distinct roles. But tests of attention in healthy people have revealed only slight differences between the hemispheres. Here we show that major differences emerge if we look at the timing of brain activity in a task optimized to identify attentional functions. Using EEG to map cortical activity on a millisecond timescale, we found transient (20-30 ms) periods of interhemispheric competition, followed by short phases of marked right-sided activity in the ventral attentional network. Our data are the first to show interhemispheric interactions that, much like a toggle switch, quickly allocate neural resources to one or the other hemisphere.

The role of apparent size in building- and object-specific regions of ventral visual cortex

Images of buildings and manipulable objects have been found to activate distinct regions in the ventral visual pathway. Yet, many non-categorical properties distinguish buildings from common everyday objects, and perhaps the most salient of these is size. In this fMRI study, we investigated whether or not changes in perceived scale can account for some of the differences in category-specific responses, independent of the influence of semantic or retinotopic image properties. We used independent scans to localize object-specific ROIs in lateral occipital cortex (LO) and scene-specific ROIs in the parahippocampal place area (PPA) and posterior collateral sulcus. We then contrasted the effects of stimulus category and perceived size/distance in these regions in a factorial design. Participants performed an oddball detection task while viewing images of objects, buildings, and planar rectangles both with and without a background that indicated stimulus size/distance via simple pictorial cues. The analyses of fMRI responses showed effects of perceived size/distance in addition to effects of category in LO and the PPA. Interestingly, when simple rectangles were presented in a control condition against the background that indicated size/distance, LO in the right hemisphere responded significantly more to the small/close rectangles than to the large/far ones, in spite of the fact that the rectangles themselves were identical. These findings suggest that ventral stream regions that show category specificity are modulated by the perceived size and distance of visual stimuli.

Spatial frequency-specific effects on the attentional bias: Evidence for two attentional systems

Using a gratingscales task as a sensitive measure of the attentional bias, we have recently observed a new form of frequency-specific cross-over; people showed left-biased preferences when comparing the high spatial frequency (HiSF) components of the task and rightward biases when comparing low spatial frequencies (LoSFs). Here we investigated which mechanisms underlie the cross-over. (1) We found that leftward and rightward biases were positively correlated, suggesting that the same set of mechanisms are involved in both versions of the task. (2) When we cued attention to the left or right side we found transient effects on gratingscales biases that were symmetrical for the LoSF condition but asymmetrical for the HiSF condition. This indicates that the HiSF condition itself biased stimulus-driven attention more to the left side than the LoSF condition. (3) When we lowered the contrast of the HiSF or the LoSF stimulus components, specifically the latter case made HiSF and LoSF conditions more different. This suggests that HiSF and LoSF conditions differ because HiSF components are more salient and more likely stir stimulus-driven attention. Our data are consistent with the idea that the attentional bias results from right-dominant control mechanisms of stimulus-driven attention potentially interacting with voluntary control mechanisms.