Signal enhancement and suppression during visual-spatial selective attention.

Selective attention involves the relative enhancement of relevant versus irrelevant stimuli. However, whether this relative enhancement involves primarily enhancement of attended stimuli, or suppression of irrelevant stimuli, remains controversial. Moreover, if both enhancement and suppression are involved, whether they result from a single mechanism or separate mechanisms during attentional control or selection is not known. In two experiments using a spatial cuing paradigm with task-relevant targets and irrelevant distractors, target, and distractor processing was examined as a function of distractor expectancy. Additionally, in the second study the interaction of perceptual load and distractor expectancy was explored. In both experiments, distractors were either validly cued (70%) or invalidly cued (30%) in order to examine the effects of distractor expectancy on attentional control as well as target and distractor processing. The effects of distractor expectancy were assessed using event-related potentials recorded during the cue-to-target period (preparatory attention) and in response to the task-relevant target stimuli (selective stimulus processing). Analyses of distractor-present displays (anticipated versus unanticipated), showed modulations in brain activity during both the preparatory period and during target processing. The pattern of brain responses suggest both facilitation of attended targets and suppression of unattended distractors. These findings provide evidence for a two-process model of visual-spatial selective attention, where one mechanism (facilitation) influences relevant stimuli and another (suppression) acts to filter distracting stimuli.

fMRI evidence for both generalized and specialized components of attentional control.

A central question in the study of selective attention is whether top-down attentional control mechanisms are generalized or specialized for the type of information that is to be attended. The current study examined this question using a voluntary orienting task that cued observers to attend to either one of two locations or to one of two colors. Location (spatial) and color (nonspatial) conditions were presented either randomly intermixed within the same block of trials or in separate blocks. Functional magnetic resonance imaging revealed that directing attention to a location or to a color activated a network of overlapping dorsal frontal and parietal areas, previously implicated in attentional control. The pattern of observed overlap was not affected by the intermixed versus blocked presentation of location and color conditions. Although portions of the frontal-parietal network were more active in response to location cues than to color cues, a secondary analysis also revealed that medial dorsal frontal and parietal cortex were specifically engaged in shifting visual attention regardless of the cued dimension (location or color). Together, the present results support the conclusion that attentional control is the combination of a generalized network that works in concert with subregions of the frontoparietal network that are highly specialized for directing attention based on the content of the to-be-attended information.

Interactions between attention and perceptual grouping in human visual cortex.

Freeman et al. demonstrated that detection sensitivity for a low contrast Gabor stimulus improved in the presence of flanking, collinearly oriented grating stimuli, but only when observers attended to them. By recording visual event-related potentials (ERPs) elicited by a Gabor stimulus, we investigated whether this contextual cueing effect involves changes in the short-latency afferent visual signal from V1 that have a stimulus onset latency between 60 and 80 ms and/or longer-latency changes from visual cortex. Under dual-task conditions, the subjects performed contrast discrimination for a central Gabor and an orientation judgment for a pre-specified subset of the flanking Gabors. On random trials, the central Gabor could be collinearly or orthogonally oriented with respect to the attended flankers. Subjects showed improvements in discriminating the contrast of the central grating when it was oriented collinearly with the attended flankers. The ERP difference between attending to collinear versus orthogonal flankers manifested as a positive polarity response at occipital electrodes with a latency of 180-250 ms after stimulus onset. No shorter-latency contextual cueing differences were observed in the ERPs. The ERP latency profile of the contextual cueing effect argues against the hypothesis that short-latency afferent activity from V1 is the stage of processing at which attention can influence neuronal lateral interactions. However, the scalp voltage distribution of the longer-latency contextual cueing effect is similar to the one generated by the early phasic stimulus onset activity from V1. These findings leave open the possibility that V1 is involved in the attentional modulation of lateral interactions but that this has a longer time course, likely being mediated by re-afferent inputs from later stages of the visual pathway.

Brain regions activated by endogenous preparatory set shifting as revealed by fMRI.

An ongoing controversy concerns whether executive control mechanisms can actively reconfigure the cognitive system in preparation for switching to a new task set. To address this question, we recorded brain activity from 14 healthy participants, using event-related functional magnetic resonance imaging, while they performed a cued attention task. Critically, in any particular trial, the cued task set was either the same as that in the previous trial or switched. As was hypothesized, cue-related, switch-specific preparatory activity was observed in a network of dorsal frontal and parietal brain areas that are typically associated with cognitive control processes. Moreover, the magnitude of switch-specific preparatory activity varied with the number of possible task sets that could be presented in a given trial block. These findings provide compelling support for the existence of top-down, preparatory control processes that enable set switching. Furthermore, they demonstrate that global task structure is a critical determinant of whether switch-specific preparatory activity is observed.

Electrophysiological correlates of lateral interactions in human visual cortex.

Detection thresholds for visually presented targets can be influenced by the nature of information in adjacent regions of the visual field. For example, detection thresholds for low-contrast Gabor patches decrease when flanked by patches that are oriented collinearly rather than orthogonally with the target. Such results are consistent with the known microanatomy of primary visual cortex, where long-range horizontal connections link cortical columns with common orientation preferences. To investigate the neural bases of collinearity effects, we recorded event-related brain potentials (ERPs) together with psychophysical measures for targets flanked by collinear vs. orthogonal gratings. Human volunteers performed a contrast discrimination task on a target grating presented at a perifoveal location. For targets flanked by collinear stimuli, we observed an increased positive polarity voltage deflection in the occipital scalp-recorded ERPs between 80 to 140 ms after stimulus onset. Such a midline occipital scalp voltage distribution of this ERP collinearity effect is consistent with a generator in primary visual cortex. Two later negative voltage ERP deflections (latencies of 245-295 and 300-350 ms) were focused at lateral occipital scalp sites, a pattern consistent with activity in extrastriate visual cortex. These ERP effects were correlated with improved contrast discrimination for central targets presented with collinear flanks. These results demonstrate that the integration of local flanking elements with a central stimulus can occur as early as 80 ms in human visual cortex, but this includes processes occurring at longer latencies and appears to involve both striate and extrastriate visual areas.

Conflict monitoring in the human anterior cingulate cortex during selective attention to global and local object features.

Parallel processing affords the brain many advantages, but processing multiple bits of information simultaneously presents formidable challenges. For example, while one is listening to a speaker at a noisy social gathering, processing irrelevant conversations may lead to the activation of irrelevant perceptual, semantic, and response representations that conflict with those evoked by the speaker. In these situations, specialized brain systems may be recruited to detect and resolve conflict before it leads to incorrect perception and/or behavior. Consistent with this view, recent findings indicate that dorsal/caudal anterior cingulate cortex (dACC), on the medial walls of the frontal lobes, detects conflict between competing motor responses primed by relevant versus irrelevant stimuli. Here, we used a cued global/local selective attention task to investigate whether the dACC plays a general role in conflict detection that includes monitoring for conflicting perceptual or semantic representations. Using event-related functional magnetic resonance imaging (fMRI), we found that the dACC was activated by response conflict in both the global and the local task, consistent with results from prior studies. However, dACC was also activated by perceptual and semantic conflict arising from global distracters during the local task. The results from the local task have implications for recent theories of attentional control in which the dACC's contribution to conflict monitoring is limited to response stages of processing, as well as for our understanding of clinical disorders in which disruptions of attention are associated with dACC dysfunction.

Neural mechanisms of top-down control during spatial and feature attention.

Theories of visual selective attention posit that both spatial location and nonspatial stimulus features (e.g., color) are elementary dimensions on which top-down attentional control mechanisms can selectively influence visual processing. Neuropsychological and neuroimaging studies have demonstrated that regions of superior frontal and parietal cortex are critically involved in the control of visual-spatial attention. This frontoparietal control network has also been found to be activated when attention is oriented to nonspatial stimulus features (e.g., motion). To test the generality of the frontoparietal network in attentional control, we directly compared spatial and nonspatial attention in a cuing paradigm. Event-related fMRI methods permitted the isolation of attentional control activity during orienting to a location or to a nonspatial stimulus feature (color). Portions of the frontoparietal network were commonly activated to the spatial and nonspatial cues. However, direct statistical comparisons of cue-related activity revealed subregions of the frontoparietal network that were significantly more active during spatial than nonspatial orienting when all other stimulus, task, and attentional factors were equated. No regions of the frontal-parietal network were more active for nonspatial cues in comparison to spatial cues. These findings support models suggesting that subregions of the frontal-parietal network are highly specific for controlling spatial selective attention.

Form-from-motion: MEG evidence for time course and processing sequence.

The neural mechanisms and role of attention in the processing of visual form defined by luminance or motion cues were studied using magnetoencephalography. Subjects viewed bilateral stimuli composed of moving random dots and were instructed to covertly attend to either left or right hemifield stimuli in order to detect designated target stimuli that required a response. To generate form-from-motion (FFMo) stimuli, a subset of the dots could begin to move coherently to create the appearance of a simple form (e.g., square). In other blocks, to generate form-from-luminance (FFLu) stimuli that served as a control, a gray stimulus was presented superimposed on the randomly moving dots. Neuromagnetic responses were observed to both the FFLu and FFMo stimuli and localized to multiple visual cortical stages of analysis. Early activity in low-level visual cortical areas (striate/early extrastriate) did not differ for FFLu versus FFMo stimuli, nor as a function of spatial attention. Longer latency responses elicited by the FFLu stimuli were localized to the ventral-lateral occipital cortex (LO) and the inferior temporal cortex (IT). The FFMo stimuli also generated activity in the LO and IT, but only after first eliciting activity in the lateral occipital cortical region corresponding to MT/V5, resulting in a 50-60 msec delay in activity. All of these late responses (MT/V5, LO, and IT) were significantly modulated by spatial attention, being greatly attenuated for ignored FFLu and FFMo stimuli. These findings argue that processing of form in IT that is defined by motion requires a serial processing of information, first in the motion analysis pathway from V1 to MT/V5 and thereafter via the form analysis stream in the ventral visual pathway to IT.

A role for top-down attentional orienting during interference between global and local aspects of hierarchical stimuli.

Various models of selective attention propose that greater attention is allocated toward target stimuli when conflicting distracters make selection more difficult, but compelling evidence to support this view is scarce. In the present experiment, 15 participants performed a cued global/local selective attention task while brain activity was recorded with event-related functional magnetic resonance imaging. The presence of conflicting versus nonconflicting distracters during target processing activated regions of frontal, parietal, and visual cortices that were also activated when participants oriented attention in response to global- and local-task cues. These findings support models in which conflict between target and distracter stimuli is resolved by more selectively focusing attention upon target stimuli.

Effects of practice on executive control investigated with fMRI.

Various models of executive control predict that practice should modulate the recruitment of executive brain mechanisms. To investigate this issue, we asked 15 participants to perform a cued global/local attention task while brain activity was recorded with event-related functional magnetic resonance imaging (fMRI). Practice significantly reduced the recruitment of left inferior parietal regions that were engaged when participants oriented attention in response to global and local cue stimuli. In contrast, practice increased the recruitment of midline frontal regions that were engaged by interference between global and local forms during target processing. These findings support models of executive control in which practice increases the tendency for stimuli to automatically evoke task-relevant processes and responses.

Dissociating top-down attentional control from selective perception and action.

Research into the neural mechanisms of attention has revealed a complex network of brain regions that are involved in the execution of attention-demanding tasks. Recent advances in human neuroimaging now permit investigation of the elementary processes of attention being subserved by specific components of the brain's attention system. Here we describe recent studies of spatial selective attention that made use of positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and event-related brain potentials (ERPs) to investigate the spatio-temporal dynamics of the attention-related neural activity. We first review the results from an event-related fMRI study that examined the neural mechanisms underlying top-down attentional control versus selective sensory perception. These results defined a fronto-temporal-parietal network involved in the control of spatial attention. Activity in these areas biased the neural activity in sensory brain structures coding the spatial locations of upcoming target stimuli, preceding a modulation of subsequent target processing in visual cortex. We then present preliminary evidence from a fast-rate event-related fMRI study of spatial attention that demonstrates how to disentangle the potentially overlapping hemodynamic responses elicited by temporally adjacent stimuli in studies of attentional control. Finally, we present new analyses from combined neuroimaging (PET) and event-related brain potential (ERP) studies that together reveal the timecourse of activation of brain regions implicated in attentional control and selective perception.

Perceptual load and visuocortical processing: event-related potentials reveal sensory-level selection.

Behavioral evidence suggests that the processing of parafoveal stimuli decreases as the perceptual demands of a task at fixation increase. However, it remains unclear whether or not this effect of perceptual load occurs during initial sensory-level processing at early stages of visuocortical analysis. Using event-related potential measures, we found that increasing the perceptual load of foveal targets led to a significant decrease in the sensory-evoked response to parafoveal stimuli. Moreover, these effects were observed using two different operational definitions of perceptual load. This result indicates that perceptual load affects the flow of information during the initial stages of visuocortical processing.

Combined expectancies: event-related potentials reveal the early benefits of spatial attention that are obscured by reaction time measures.

Visual spatial attention has been likened to a "spotlight" that selectively facilitates the perceptual processing of events at covertly attended locations. However, if participants have advance knowledge of the likely location of an impending target and the likely response it will require, facilitation in response performance does not occur for targets at the expected (or attended) location that require an unexpected response. Event-related potentials (ERPs) were recorded during a discrimination task in which the most likely target location and target response were simultaneously cued prior to target onset. The ERPs showed evidence of enhanced perceptual-level processing for all targets at attended locations. These results suggest that the lack of response facilitation for unexpected targets at attended locations is likely due to postperceptual processes that are activated by the inclusion of nonspatial stimulus expectancies, response expectancies, or both.

Integrating electrophysiology and neuroimaging of spatial selective attention to simple isolated visual stimuli.

Visual-spatial attention involves modulations of activity in human visual cortex as indexed by electrophysiological and functional neuroimaging measures. Prior studies investigating the time course and functional anatomy of spatial attention mechanisms in visual cortex have used higher-order discrimination tasks with complex stimuli (e.g. symbol matching in bilateral stimulus arrays, or letter discrimination), or simple detection tasks but in the presence of complex distracting information (e.g. luminance detection with superimposed symbols as distractors). Here we tested the hypothesis that short-latency modulations of incoming sensory signals in extrastriate visual cortex reflect an early spatially specific attentional mechanism. We sought evidence of attentional modulations of sensory input processing for simple, isolated stimuli requiring only an elementary discrimination (i.e. size discrimination). As in prior studies using complex symbols, we observed attention-related changes in regional cerebral blood flow in extrastriate visual cortex that were associated with changes in event-related potentials at a specific latency range. These findings support the idea that early in cortical processing, spatially-specific attentional selection mechanisms can modulate incoming sensory signals based on their spatial location and perhaps independently of higher-order stimulus form.

Tracking the influence of reflexive attention on sensory and cognitive processing.

Previously, we demonstrated that reflexive attention facilitates early visual processing during form discrimination (Hopfinger & Mangun, 1998). In the present study, we tested whether reflexive facilitation of early visual processing will be generated when task load is low (simple luminance detection). Target stimuli that were preceded at short cue-to-target intervals by irrelevant visual events (cues) elicited an enhanced sensory (P1) event-related potential (ERP) component as well as an enhanced longer latency, cognitive ERP component (P300). At long cue-to-target intervals, facilitation in these ERP components was no longer observed, and, although inhibition of return (IOR) was observed in reaction times, the ERPs did not show an inhibition of sensory processing. These results provide converging evidence that reflexive attention transiently facilitates neural processing of visual inputs at multiple stages of analysis (i.e., sensory processing and higher order cognitive processing) but question the view that IOR is manifest at the earliest visual cortical stages of analysis.

Neural sources of focused attention in visual search.

Previous studies of visual search in humans using event-related potentials (ERPs) have revealed an ERP component called 'N2pc' (180-280 ms) that reflects the focusing of attention onto potential target items in the search array. The present study was designed to localize the neuroanatomical sources of this component by means of magnetoencephalographic (MEG) recordings, which provide greater spatial precision than ERP recordings. MEG recordings were obtained with an array of 148 magnetometers from six normal adult subjects, one of whom was tested in multiple sessions so that both single-subject and group analyses could be performed. Source localization procedures revealed that the N2pc is composed of two distinct neural responses, an early parietal source (180-200 ms) and a later occipito-temporal source (220-240 ms). These findings are consistent with the proposal that parietal areas are used to initiate a shift of attention within a visual search array and that the focusing of attention is implemented by extrastriate areas of the occipital and inferior temporal cortex.

Shifting visual attention in space: an electrophysiological analysis using high spatial resolution mapping.

OBJECTIVES: Evidence from cortical electrophysiology and functional imaging converges on the view that visual spatial selective attention results in a facilitation of early sensory processing in visual cortical structures. Little is known, however, about the neural control processes that lead to this facilitation. The present study was aimed at further investigating these control processes and their neural correlates by analyzing high spatial resolution maps of brain activity that were evoked by attention-directing cues, but occurred prior to presentation of the target stimulus. METHODS: Subjects (n=14) were presented with central arrow cues that instructed them to attend covertly to either a left or right field location in order to compare two subsequent target stimuli simultaneously presented to the location. On half of the trials, targets were presented to the cued location, while in the other half, targets were presented to the opposite visual field location. Subjects had to respond via button press on 16% of the trials when target stimuli were identical. Event-related potentials (ERPs) were recorded from 92 scalp electrodes which allowed a sufficiently finegrained analysis of the regional specificity of the ERP components. RESULTS: In response to the cues, an initial component over occipital-parietal electrode sites was consistent with an early involvement of the posterior-parietal cortex, perhaps in the initial step of attentional orienting. A second component over the lateral-prefrontal cortex is consistent with the voluntary control and maintenance of attention, a function known to be subserved by frontal cortical structures. A late component narrowly focussed over occipital-temporal electrode sites is most plausibly related to activation of parts of the ventral extrastriate cortex. CONCLUSIONS: The data support the current view that voluntarily orienting visual attention in space leads to top-down modulations in cortical excitability of ventral extrastriate regions initiated by posterior-parietal and mediated by lateral-prefrontal cortical structures.

On the processing of spatial frequencies as revealed by evoked-potential source modeling.

OBJECTIVES AND METHODS: Visually evoked potentials (VEPs) are known to be sensitive to spatial frequency, especially in the time range between 50 and 100 ms post-stimulus. In two experiments we localized the cortical activity elicited by stimuli of varying spatial frequency in scalp-recorded brain potentials, using multi-electrode recordings and dipole-source analysis. RESULTS: Low spatial frequencies (<1 c/d) activated relatively lateral occipital areas, the orientation of the neural ensembles involved being predominantly perpendicular to the scalp surface. In contrast, high spatial frequencies (>4 c/d) induced activation of more medial occipital areas with the predominant orientation of the sources being much more parallel to the scalp surface. Furthermore, at about 100 ms latency the lateral-occipital response to low spatial frequencies was stronger in the right hemisphere; no such asymmetry was found for the responses to the high spatial frequencies. These findings were consistent across varying recording conditions, individual subjects, subject populations, stimulus characteristics (grating orientation, grating vs. checkerboard), and task conditions (active vs. passive). CONCLUSION: The results indicate that there are differences in sensitivity to specific spatial frequencies between primary and secondary visual areas, as well as between the right and the left hemispheres.

Attention and spatial selection: electrophysiological evidence for modulation by perceptual load.

Behavioral data have suggested that perceptual load can modulate spatial selection by influencing the allocation of attentional resources at perceptual-level processing stages (Lavie & Tsal, 1994). To directly test this hypothesis, event-related potentials (ERPs) were recorded for both low- and high-perceptual-load targets in a probabilistic spatial cuing paradigm. The results from three experiments showed that, as measured by the lateral occipital P1 and N1 ERP components, the magnitude of spatially selective processing in extrastriate visual cortex increased with perceptual load. Furthermore, these effects on spatial selection were found in the P1 at lower levels of perceptual load than in the N1. The ERP data thus provide direct electrophysiological support for proposals that link perceptual load to early spatial selection in visual processing. However, our findings suggest a relatively broader model--where perceptual load is but one of many factors mediating early selection.