Biomedical subjects

Gregory V Simpson

Publications and source records attributed to Gregory V Simpson.

10 recordsLinked to original sources

PubMed2006

Rapid targeting followed by sustained deployment of visual spatial attention.

We investigated preparatory attention processes when a spatial discrimination was required at a cued location, by measuring electroencephalography following a central symbolic cue to deploy spatial attention. Electroencephalography activity in response to the cue revealed three cue-related activations: an early-onset positivity following the P1 at posterior scalp sites contralateral to the cued location, followed by cue-related frontal scalp activity and later-onset sustained activity at posterior scalp sites contralateral to the cued location. The early contralateral positivity may reflect rapid targeting of the cued location. Our results also extend the findings of cue-related frontal activity followed by posterior activity contralateral to the cued location, found with nonspatial feature discriminations, to a task requiring a spatial discrimination.

Adult↗

PubMed2006

Brain activation patterns during memory of cognitive agency.

Agency is the awareness that one's own self is the agent or author of an action, a thought, or a feeling. The implicit memory that one's self was the originator of a cognitive event - the sense of cognitive agency - has not yet been fully explored in terms of relevant neural systems. In this functional magnetic resonance imaging (fMRI) study, we examined brain activation patterns differentiating memory for the source of previously self-generated vs. experimenter-presented word items from a sentence completion paradigm designed to be emotionally neutral and semantically constrained in content. Accurate memory for the source of self-generated vs. externally-presented word items resulted in activation of dorsal medial prefrontal cortex (mPFC) bilaterally, supporting an emerging body of work that indicates a key role for this region in self-referential processing. Our data extend the function of mPFC into the domain of memory and the accurate retrieval of the sense of cognitive agency under conditions where agency was encoded implicitly.

Adult↗

PubMed2005

Parallel networks operating across attentional deployment and motion processing: a multi-seed partial least squares fMRI study.

Anticipatory deployment of attention may operate through networks of brain areas that modulate the representations of to-be-attended items in advance of their occurrence through top-down control. Luks and Simpson (2004) (Luks, T.L., Simpson, G.V., 2004. Preparatory deployment of attention to motion activates higher order motion-processing brain regions. NeuroImage 22, 1515-1522) found activations in both control areas and sensory areas during anticipatory deployment of attention to visual motion in the absence of stimuli. In the present follow-up analysis, we tested which network activity during anticipatory deployment of attention is functionally connected with task-related network activity during subsequent selective processing of motion stimuli. Following a cue (anticipatory phase), participants monitored a sequence of complex motion stimuli for a target motion pattern (task phase). We analyzed fMR signal using a partial least squares analysis with previously identified cue- and motion-related voxels as seed regions. The method identified two networks that covaried with the activity of seed regions during the cue and motion-stimulus-processing phases of the task. We suggest that the first network, involving ventral intraparietal sulcus, superior parietal lobule and motor areas, is related to anticipatory and sustained visuomotor attention. Operating in parallel to this visuomotor attention network, there is a second network, involving visual occipital areas, frontal areas as well as angular and supramarginal gyri, that may underlie anticipatory and sustained visual attention processes.

Adult↗

PubMed2005

Biasing the brain's attentional set: II. effects of selective intersensory attentional deployments on subsequent sensory processing.

This study used high-density mapping of human event-related potentials to examine the brain activity associated with selective information processing when subjects were cued on a trial-by-trial basis to perform a discrimination in either the visual or auditory modality. On each trial, word-cues (S1) instructed subjects to attend to features within one sensory-modality of an impending compound auditory-visual stimulus (S2) that arrived approximately 1-second following the cue. Subjects made a discrimination within the cued modality of the S2 stimulus. The spatio-temporal patterns of activity in response to the compound S2 stimulus were examined as a function of the sensory modality being attended. The earliest effects of intersensory attention on visual processing were seen subsequent to the initial activation of visual cortex, beginning at 80 ms and continuing into the P1 and N1 components of the visual ERP. The scalp-topography of this earliest modulation was consistent with modulation of activity in ventral visual stream areas. Thus, the locus of effects on visual S2 processing differed from the anticipatory parieto-occipital biasing activity that preceded S2 presentation. This pattern of effects strongly suggests that the anticipatory activity (following the cue) associated with sustaining the focus of attention during intersensory attention, at least in the context of this paradigm, does not operate as a simple gain mechanism in early visual sensory areas. Rather, attentional biasing can operate through a higher-order process whereby parieto-occipital cortices influence the subsequent flow of visual processing in the ventral stream.

Acoustic Stimulation↗

PubMed2005

Biasing the brain's attentional set: I. cue driven deployments of intersensory selective attention.

Brain activity associated with directing attention to one of two possible sensory modalities was examined using high-density mapping of human event-related potentials. The deployment of selective attention was based on visually presented symbolic cue-words instructing subjects on a trial-by-trial basis, which sensory modality to attend. We measured the spatio-temporal pattern of activation in the approximately 1 second period between the cue-instruction and a subsequent compound auditory-visual imperative stimulus. This allowed us to assess the flow of processing across brain regions involved in deploying and sustaining inter-sensory selective attention, prior to the actual selective processing of the compound audio-visual target stimulus. Activity over frontal and parietal areas showed sensory specific increases in activation during the early part of the anticipatory period (~230 ms), probably representing the activation of fronto-parietal attentional deployment systems for top-down control of attention. In the later period preceding the arrival of the "to-be-attended" stimulus, sustained differential activity was seen over fronto-central regions and parieto-occipital regions, suggesting the maintenance of sensory-specific biased attentional states that would allow for subsequent selective processing. Although there was clear sensory biasing in this late sustained period, it was also clear that both sensory systems were being prepared during the cue-target period. These late sensory-specific biasing effects were also accompanied by sustained activations over frontal cortices that also showed both common and sensory specific activation patterns, suggesting that maintenance of the biased state includes top-down inputs from generators in frontal cortices, some of which are sensory-specific regions. These data support extensive interactions between sensory, parietal and frontal regions during processing of cue information, deployment of attention, and maintenance of the focus of attention in anticipation of impending attentionally relevant input.

Acoustic Stimulation↗

PubMed2005

Phase locking of single neuron activity to theta oscillations during working memory in monkey extrastriate visual cortex.

Working memory has been linked to elevated single neuron discharge in monkeys and to oscillatory changes in the human EEG, but the relation between these effects has remained largely unexplored. We addressed this question by measuring local field potentials and single unit activity simultaneously from multiple electrodes placed in extrastriate visual cortex while monkeys were performing a working memory task. We describe a significant enhancement in theta band energy during the delay period. Theta oscillations had a systematic effect on single neuron activity, with neurons emitting more action potentials near their preferred angle of each theta cycle. Sample-selective delay activity was enhanced if only action potentials emitted near the preferred theta angle were considered. Our results suggest that extrastriate visual cortex is involved in short-term maintenance of information and that theta oscillations provide a mechanism for structuring the recurrent interaction between neurons in different brain regions that underlie working memory.

Action Potentials↗

PubMed2004

Geometrical interpretation of fMRI-guided MEG/EEG inverse estimates.

Magneto- and electroencephalography (MEG/EEG) and functional magnetic resonance imaging (fMRI) provide complementary information about the functional organization of the human brain. An important advantage of MEG/EEG is the millisecond time resolution in detecting electrical activity in the cerebral cortex. The interpretation of MEG/EEG signals, however, is limited by the difficulty of determining the spatial distribution of the neural activity. Functional MRI can help in the MEG/EEG source analysis by suggesting likely locations of activity. We present a geometric interpretation of fMRI-guided inverse solutions in which the MEG/EEG source estimate minimizes a distance to a subspace defined by the fMRI data. In this subspace regularization (SSR) approach, the fMRI bias does not assume preferred amplitudes for MEG/EEG sources, only locations. Characteristic dependence of the source estimates on the regularization parameters is illustrated with simulations. When the fMRI locations match the true MEG/EEG source locations, they serve to bias the underdetermined MEG/EEG inverse solution toward the fMRI loci. Importantly, when the fMRI loci do not match the true MEG/EEG loci, the solution is insensitive to those fMRI loci.

Algorithms↗

PubMed2004

Preparatory deployment of attention to motion activates higher-order motion-processing brain regions.

We used event-related fMRI to test the hypothesis that preparatory attention modulations occur in higher-order motion-processing regions when subjects deploy attention to internally driven representations in a complex motion-processing task. Using a cued attention-to-motion task, we found preparatory increases in fMRI activity in visual motion regions in the absence of visual motion stimulation. The cue, a brief enlargement of the fixation cross, directed subjects to prepare for a complex motion discrimination task. This preparation activated higher-order and lower-order motion regions. The motion regions activated included temporal regions consistent with V5/MT+, occipital regions consistent with V3+, parietal-occipital junction regions, ventral and dorsal intraparietal sulcus, superior temporal sulcus (STS), posterior insular cortex (PIC), and a region of BA 39/40 superior to V5/MT+ involving the angular gyrus and supramarginal gyrus (A-SM). Consistent with our hypothesis that these motion sensory activations are under top-down control, we also found activation of an extensive frontal network during the cue period, including anterior cingulate and multiple prefrontal regions. These results support the hypothesis that anticipatory deployment of attention to internally driven representations is achieved via top-down modulation of activity in task-relevant processing areas.

Adult↗

PubMed2002

Evidence for anterior cingulate cortex involvement in monitoring preparatory attentional set.

An important cognitive function underlying unified, voluntary behavior is attentional control. Two frontal regions, anterior cingulate cortex (ACC) and dorsolateral prefrontal cortex (DLPFC), appear to be particularly involved in attentional control and monitoring. In this study, we investigated whether ACC is involved in monitoring the preparatory allocation of attention during task switching, or whether ACC is active only when subjects are processing target stimuli and selecting a response, via a cued-attention design. Event-related BOLD fMRI activity was examined using a cue-target paradigm in which subjects performed task switches that selectively required reallocation of attention when tasks changed. There were three cue conditions: informative switch, informative repeat, and neutral. There were four target conditions: informatively cued switch, informatively cued repeat, neutrally cued switch, and neutrally cued repeat. Significant ACC activity was observed following both informative switch and informative repeat cues, but not after neutral cues. No significant ACC activity was observed following any of the target conditions. Significant DLPFC activity was observed following all three cue conditions and following neutrally cued switch targets. Overall, our results suggest that ACC is involved in monitoring the preparatory allocation of attention for conflict at the level of activation of competing attentional sets. The results also support the role of DLPFC in holding cognitive goals in working memory and allocating attention to the appropriate processing systems to meet those goals.

Adult↗

PubMed2001

Flow of activation from V1 to frontal cortex in humans. A framework for defining "early" visual processing.

This study provides a time frame for the initial trajectory of activation flow along the dorsal and ventral visual processing streams and for the initial activation of prefrontal cortex in the human. We provide evidence that this widespread system of sensory, parietal, and prefrontal areas is activated in less than 30 ms, which is considerably shorter than typically assumed in the human event-related potential (ERP) literature and is consistent with recent intracranial data from macaques. We find a mean onset latency of activity over occipital cortex (C1(e)) at 56 ms, with dorsolateral frontal cortex subsequently active by just 80 ms. Given that activity in visual sensory areas typically continues for 100-400 ms prior to motor output, this rapid system-wide activation provides a time frame for the initiation of feedback processes onto sensory areas. There is clearly sufficient time for multiple iterations of interactive processing between sensory, parietal, and frontal areas during brief (e.g., 200 ms) periods of information processing preceding motor output. High-density electrical mapping also suggested activation in dorsal stream areas preceding ventral stream areas. Our data suggest that multiple visual generators are active in the latency range of the traditional C1 component of the ERP, which has often been taken to represent V1 activity alone. Based on the temporal pattern of activation shown in primate recordings and the evidence from these human recordings, we propose that only the initial portion of the C1 component (approximately the first 10-15 ms; C1(e)) is likely to represent a response that is predominated by V1 activity. These data strongly suggest that activity represented in the "early" ERP components such as P1 and N1 (and possibly even C1) is likely to reflect relatively late processing, after the initial volley of sensory afference through the visual system and involving top-down influences from parietal and frontal regions.

Adult↗