Feature-based attention is not confined by object boundaries: Spatially global enhancement of irrelevant features

Unit of attentional refreshing in multiple identity tracking: object and Boolean map

Tracking multiple distinct moving objects requires periodically refreshing the objects' identity-location bindings. Classic theories assume that individual objects are the units involved in refreshing. Considering that features and locations can be represented as Boolean maps, here we investigated whether Boolean maps can also serve as the units of refreshing in multiple identity tracking (MIT). We used an adapted MIT paradigm, in which every two targets shared the same identity and thus could be represented in one Boolean map. We examined whether only the cued target was refreshed (i.e., object-based refreshing) or the other target sharing the same identity was also refreshed (i.e., Boolean map-based refreshing). In Experiment 1, where objects moved randomly, results showed that while the cued target was tracked better than uncued targets, no significant advantage was observed for the same-identity target, indicating no Boolean map-based refreshing. In Experiment 2, same-identity targets moved in parallel, maintaining a stable spatial relationship within each Boolean map. Both same-identity and cued targets were tracked better than other targets, demonstrating a same-Boolean-map advantage. In Experiment 3, where different-identities targets moved in parallel, no significant advantage was found for either the target moved parallel to or sharing the same identity as the cued target. Taken together, the results indicate that a Boolean map can serve as the unit for attentional refreshing in MIT, but only when targets with the same identity are moving in parallel, so that the spatial relationship within each Boolean map remains the same while the objects are moving; otherwise the refreshing is based on individual objects.

Representational structures as a unifying framework for attention

Our visual system consciously processes only a subset of the incoming information. Selective attention allows us to prioritize relevant inputs, and can be allocated to features, locations, and objects. Recent advances in feature-based attention suggest that several selection principles are shared across these domains and that many differences between the effects of attention on perceptual processing can be explained by differences in the underlying representational structures. Moving forward, it can thus be useful to assess how attention changes the structure of the representational spaces over which it operates, which include the spatial organization, feature maps, and object-based coding in visual cortex. This will ultimately add to our understanding of how attention changes the flow of visual information processing more broadly.

The time course of feature-selective attention inside and outside the focus of spatial attention

Research on attentional selection of stimulus features has yielded seemingly contradictory results. On the one hand, many experiments in humans and animals have observed a "global" facilitation of attended features across the entire visual field, even when spatial attention is focused on a single location. On the other hand, several event-related potential studies in humans reported that attended features are enhanced at the attended location only. The present experiment demonstrates that these conflicting results can be explained by differences in the timing of attentional allocation inside and outside the spatial focus of attention. Participants attended to fields of either red or blue randomly moving dots on either the left or right side of fixation with the task of detecting brief coherent motion targets. Recordings of steady-state visual evoked potentials elicited by the flickering stimuli allowed concurrent measurement of the time course of feature-selective attention in visual cortex on both the attended and the unattended sides. The onset of feature-selective attentional modulation on the attended side occurred around 150 ms earlier than on the unattended side. This finding that feature-selective attention is not spatially global from the outset but extends to unattended locations after a temporal delay resolves previous contradictions between studies finding global versus hierarchical selection of features and provides insight into the fundamental relationship between feature-based and location-based (spatial) attention mechanisms.

Sustained selective attention to chromatic information enhances visuocortical gain at the population level

Prior work in selective attention research has shown that colour-selective attention enhances neural activity in visuocortical areas sensitive to the attended colour while suppressing activity in areas sensitive to ignored colours. However, it is currently unclear whether this effect is limited to attending to specific colour hues or extends to chromatic information more broadly. To investigate this question, we used steady-state visual evoked potentials (ssVEPs) frequency tagging to quantify participants' visuocortical responses to specific elements embedded in arrays of flickering, randomly moving mid-complex patterns. Participants were instructed to attend to either coloured or greyscale patterns while ignoring the others. We found that attending to either coloured or greyscale patterns produced robust increases in ssVEP amplitudes both compared to ignored stimuli and to baseline. There was however no evidence of suppressed responses to ignored patterns. These findings demonstrate that attentional selection based on the presence or absence of chromatic information prompts selectively enhanced visuocortical processing but this selective amplification is not accompanied by suppression of unattended stimuli. Findings are consistent with theoretical notions that predict strong competition between specific exemplars within a given feature dimension, such as red or green, but weak competition between broadly defined stimulus categories, such as chromatic versus non-chromatic.

Feature similarity is non-linearly related to attentional selection: Evidence from visual search and sustained attention tasks

Although many theories of attention highlight the importance of similarity between target and distractor items for selection, few studies have directly quantified the function underlying this relationship. Across two commonly used tasks-visual search and sustained attention-we investigated how target-distractor similarity impacts feature-based attentional selection. Importantly, we found comparable patterns of performance in both visual search and sustained feature-based attention tasks, with performance (response times and d', respectively) plateauing at medium target-distractor distances (40°-50° around a luminance-matched color wheel). In contrast, visual search efficiency, as measured by search slopes, was affected by a much more narrow range of similarity levels (10°-20°). We assessed the relationship between target-distractor similarity and attentional performance using both a stimulus-based and psychologically-based measure of similarity and found this nonlinear relationship in both cases. However, psychological similarity accounted for some of the nonlinearities observed in the data, suggesting that measures of psychological similarity are more appropriate when studying effects of target-distractor similarities. These findings place novel constraints on models of selective attention and emphasize the importance of considering the similarity structure of the feature space over which attention operates. Broadly, the nonlinear effects of similarity on attention are consistent with accounts that propose attention exaggerates the distance between competing representations, possibly through enhancement of off-tuned neurons.

The early attentional pancake: Minimal selection in depth for rapid attentional cueing

There have been conflicting findings on the degree to which rapidly deployed visual attention is selective for depth, and this issue has important implications for attention models. Previous findings have attempted to find depth-based cueing effects on such attention using reaction time (RT) measures for stimuli presented in stereo goggles with a display screen. Results stemming from such approaches have been mixed, depending on whether target/distractor discrimination was required. To help clarify the existence of such depth effects, we have developed a paradigm that measures accuracy rather than RT in an immersive virtual-reality environment, providing a more appropriate context of depth. Three modified Posner Cueing paradigms were run to test for depth-specific rapid attentional selectivity. Participants fixated a cross while attempting to identify a rapidly masked black letter preceded by a red cue that could be valid in depth, side, or both. In Experiment 1a, a potent cueing effect was found for lateral cueing validity, but a weak effect was found for depth despite an extreme difference in virtual depth (1 vs. 300 m). In Experiment 1b, a near-replication of 1a, the lateral effect replicated while the depth effect did not. Finally, in Experiment 2, to increase the depth cue's effectiveness, the letter matched the cue's color, and the presentation duration was increased; however, again only a minimal depth-based cueing effect - no greater than that of Experiment 1a - was observed. Thus, we conclude that rapidly deployed attention is driven largely by spatiotopic rather than depth-based information.

No evidence for proactive suppression of explicitly cued distractor features

Visual search benefits from advance knowledge of nontarget features. However, it is unknown whether these negatively cued features are suppressed in advance (proactively) or during search (reactively). To test this, we presented color cues varying from trial-to-trial that predicted target or nontarget colors. Experiment 1 (N = 96) showed that both target and nontarget cues speeded search. To test whether attention proactively modified cued feature representations, in Experiment 2 (N = 200), we interleaved color probe and search trials and had participants detect the color of a briefly presented ring that could either match the cued color or not. People detected positively cued colors better than other colors, whereas negatively cued colors were detected no better or worse than other colors. These results demonstrate that nontarget features are not suppressed proactively, and instead suggest that anticipated nontarget features are ignored via reactive mechanisms.

Dimension-selective attention and dimensional salience modulate cortical tracking of acoustic dimensions

Some theories of auditory categorization suggest that auditory dimensions that are strongly diagnostic for particular categories - for instance voice onset time or fundamental frequency in the case of some spoken consonants - attract attention. However, prior cognitive neuroscience research on auditory selective attention has largely focused on attention to simple auditory objects or streams, and so little is known about the neural mechanisms that underpin dimension-selective attention, or how the relative salience of variations along these dimensions might modulate neural signatures of attention. Here we investigate whether dimensional salience and dimension-selective attention modulate the cortical tracking of acoustic dimensions. In two experiments, participants listened to tone sequences varying in pitch and spectral peak frequency; these two dimensions changed at different rates. Inter-trial phase coherence (ITPC) and amplitude of the EEG signal at the frequencies tagged to pitch and spectral changes provided a measure of cortical tracking of these dimensions. In Experiment 1, tone sequences varied in the size of the pitch intervals, while the size of spectral peak intervals remained constant. Cortical tracking of pitch changes was greater for sequences with larger compared to smaller pitch intervals, with no difference in cortical tracking of spectral peak changes. In Experiment 2, participants selectively attended to either pitch or spectral peak. Cortical tracking was stronger in response to the attended compared to unattended dimension for both pitch and spectral peak. These findings suggest that attention can enhance the cortical tracking of specific acoustic dimensions rather than simply enhancing tracking of the auditory object as a whole.