George R. Mangun

The neural mechanisms of top-down attentional control

Selective visual attention involves dynamic interplay between attentional control systems and sensory brain structures. We used event-related functional magnetic resonance imaging (fMRI) during a cued spatial-attention task to dissociate brain activity related to attentional control from that related to selective processing of target stimuli. Distinct networks were engaged by attention-directing cues versus subsequent targets. Superior frontal, inferior parietal and superior temporal cortex were selectively activated by cues, indicating that these structures are part of a network for voluntary attentional control. This control biased activity in multiple visual cortical areas, resulting in selective sensory processing of relevant visual targets.

Canonical Microcircuits for Predictive Coding

This Perspective considers the influential notion of a canonical (cortical) microcircuit in light of recent theories about neuronal processing. Specifically, we conciliate quantitative studies of microcircuitry and the functional logic of neuronal computations. We revisit the established idea that message passing among hierarchical cortical areas implements a form of Bayesian inference-paying careful attention to the implications for intrinsic connections among neuronal populations. By deriving canonical forms for these computations, one can associate specific neuronal populations with specific computational roles. This analysis discloses a remarkable correspondence between the microcircuitry of the cortical column and the connectivity implied by predictive coding. Furthermore, it provides some intuitive insights into the functional asymmetries between feedforward and feedback connections and the characteristic frequencies over which they operate.

Intensive training induces longitudinal changes in meditation state related EEG oscillatory activity

The capacity to focus ones attention for an extended period of time can be increased through training in contemplative practices. However, the cognitive processes engaged during meditation that support trait changes in cognition are not well characterized. We conducted a longitudinal wait-list controlled study of intensive meditation training. Retreat participants practiced focused attention (FA) meditation techniques for three months during an initial retreat. Wait-list participants later undertook formally identical training during a second retreat. Dense-array scalp-recorded electroencephalogram (EEG) data were collected during 6 min of mindfulness of breathing meditation at three assessment points during each retreat. Second-order blind source separation, along with a novel semi-automatic artifact removal tool (SMART), was used for data preprocessing. We observed replicable reductions in meditative state-related beta-band power bilaterally over anteriocentral and posterior scalp regions. In addition, individual alpha frequency (IAF) decreased across both retreats and in direct relation to the amount of meditative practice. These findings provide evidence for replicable longitudinal changes in brain oscillatory activity during meditation and increase our understanding of the cortical processes engaged during meditation that may support long-term improvements in cognition.

Visual event-related potentials index focused attention within bilateral stimulus arrays. II. Functional dissociation of P1 and N1 components

Event-related potentials (ERPs) were recorded from 12 subjects as they attended to the left or right hemifield of a visual display while fixating a central point. Stimuli were presented to the left or right visual fields on separate trials (unilateral stimuli) or to both fields simultaneously (bilateral stimuli). In different conditions, the stimulus sequences contained only bilateral stimuli, only unilateral stimuli, or a mixture of unilateral and bilateral stimuli. Bilateral stimuli elicited an enhanced positivity lasting from about 75 to 250 msec that was largest at posterior electrode sites contralateral to the attended hemifield. The early phase of this attention-related positivity appeared to be an enhancement of the exogenous P1 component. In contrast, both the posterior P1 and N1 components were enhanced in response to attended unilateral stimuli. Moreover, the N1 attention effect was reduced when the preceding stimulus contained elements in the attended field. It was concluded that modulations of the N1 and P1 components in these experiments represent different aspects of visual spatial attention: N1 may represent the orienting of attention to a task-relevant stimulus, whereas P1 may represent a facilitation of early sensory processing for items presented to a location where attention is already focused.

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.

Intensive Meditation Training Improves Perceptual Discrimination and Sustained Attention

The ability to focus one’s attention underlies success in many everyday tasks, but voluntary attention cannot be sustained for extended periods of time. In the laboratory, sustained-attention failure is manifest as a decline in perceptual sensitivity with increasing time on task, known as the vigilance decrement. We investigated improvements in sustained attention with training (~5 hr/day for 3 months), which consisted of meditation practice that involved sustained selective attention on a chosen stimulus (e.g., the participant’s breath). Participants were randomly assigned either to receive training first (n = 30) or to serve as waiting-list controls and receive training second (n = 30). Training produced improvements in visual discrimination that were linked to increases in perceptual sensitivity and improved vigilance during sustained visual attention. Consistent with the resource model of vigilance, these results suggest that perceptual improvements can reduce the resource demand imposed by target discrimination and thus make it easier to sustain voluntary attention.

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.

Luminance and spatial attention effects on early visual processing.

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80-110 ms), occipital P1 (110-140 ms), and parietal N1 (130-180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135-220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350-750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect.(ABSTRACT TRUNCATED AT 250 WORDS)

The spatial allocation of visual attention as indexed by event-related brain potentials

The spatial distribution of visual attention was studied using event-related brain potentials (ERPs) to index stimulus processing as the locus of attention was shifted across the visual fields. Stimuli were flashed in random order to one of three locations: one in each of the lateral visual fields and one on the vertical meridian. Selective visual-spatial attention was manifested in the ERPs as an amplitude modulation of the sensory-evoked components over frontal, central, parietal, and occipital scalp areas. Attended stimuli also elicited broader negative components that appeared to be endogenous and could be dissociated from the amplitude enhancement of the earlier sensory-evoked components. A gradient of attention was evident in the progressive decline in amplitude of the sensory-evoked components of the ERPs to the lateral stimuli as attention was focused at increasing distances from the stimulus location. These results are discussed in terms of “spotlight” and “gradient” models of the spatial allocation of visual attention.

Dissociation of brain activity related to syntactic and semantic aspects of language

In psycholinguistic research, there has been considerable interest in understanding the interactions of difFerent types of linguistic information during language processing. For example, does syntactic information interact with semantic or pragmatic information at an early stage of language processing, or only at later stages in order to resolve ambiguities of language? Developing reliable measures of language processes such as syntax and semantics is important to address many of these theoretical issues in psycholinguistics. In the present study, event-related brain potentials (ERPs) were recorded from healthy young subjects while they read pairs of words presented one word at a time. The ERPs for the second word of each pair were compared as a function of whether the preceding word was or was not (1) semantically related (i.e., synonyms; semantic condition) or (2) grammatically correct (syntactic condition). In the semantic condition the ERPs obtained to words preceded by nonsemantically related words elicited an N400 component that was maximal over centroparietal scalp regions. In contrast, in the syntactic condition the ERPs obtained to words preceded by grammatically incorrect articles or pronouns yielded a negativity with a later onset, and a frontopolar, left hemisphere scalp maximum. This replicates our previous findings of a syntactic negativity in a word pair design that was performed in the German language. Further, the present data provide scalp distributional information, which suggests that the syntactic negativity represents brain processes that are dissociable from the centroparietal N400 component. Thus, these findings provide strong evidence for a separate negative polarity ERP component that indexes syntactic aspects of language processing.