Biomedical subjects

Marty G Woldorff

Publications and source records attributed to Marty G Woldorff.

At least 19 recordsLinked to original sources

PubMed2007

Electrophysiological evidence for notation independence in numerical processing.

BACKGROUND: A dominant view in numerical cognition is that numerical comparisons operate on a notation independent representation (Dehaene, 1992). Although previous human neurophysiological studies using scalp-recorded event-related potentials (ERPs) on the numerical distance effect have been interpreted as supporting this idea, differences in the electrophysiological correlates of the numerical distance effect in symbolic notations (e.g. Arabic numerals) and non-symbolic notations (e.g. a set of visually presented dots of a certain number) are not entirely consistent with this view. METHODS AND RESULTS: Two experiments were conducted to resolve these discrepancies. In Experiment 1, participants performed a symbolic and a non-symbolic numerical comparison task ("smaller or larger than 5?") with numerical values 1-4 and 6-9 while ERPs were recorded. Consistent with a previous report (Temple & Posner, 1998), in the symbolic condition the amplitude of the P2p ERP component (210-250 ms post-stimulus) was larger for values near to the standard than for values far from the standard whereas this pattern was reversed in the non-symbolic condition. However, closer analysis indicated that the reversal in polarity was likely due to the presence of a confounding stimulus effect on the early sensory ERP components for small versus larger numerical values in the non-symbolic condition. In Experiment 2 exclusively large numerosities (8-30) were used, thereby rendering sensory differences negligible, and with this control in place the numerical distance effect in the non-symbolic condition mirrored the symbolic condition of Experiment 1. CONCLUSION: Collectively, the results support the claim of an abstract semantic processing stage for numerical comparisons that is independent of input notation.

Journal Article↗

PubMed2007

Timing and sequence of brain activity in top-down control of visual-spatial attention.

Recent brain imaging studies using functional magnetic resonance imaging (fMRI) have implicated a frontal-parietal network in the top-down control of attention. However, little is known about the timing and sequence of activations within this network. To investigate these timing questions, we used event-related electrical brain potentials (ERPs) and a specially designed visual-spatial attentional-cueing paradigm, which were applied as part of a multi-methodological approach that included a closely corresponding event-related fMRI study using an identical paradigm. In the first 400 ms post cue, attention-directing and control cues elicited similar general cue-processing activity, corresponding to the more lateral subregions of the frontal-parietal network identified with the fMRI. Following this, the attention-directing cues elicited a sustained negative-polarity brain wave that was absent for control cues. This activity could be linked to the more medial frontal-parietal subregions similarly identified in the fMRI as specifically involved in attentional orienting. Critically, both the scalp ERPs and the fMRI-seeded source modeling for this orienting-related activity indicated an earlier onset of frontal versus parietal contribution ( approximately 400 versus approximately 700 ms). This was then followed ( approximately 800-900 ms) by pretarget biasing activity in the region-specific visual-sensory occipital cortex. These results indicate an activation sequence of key components of the attentional-control brain network, providing insight into their functional roles. More specifically, these results suggest that voluntary attentional orienting is initiated by medial portions of frontal cortex, which then recruit medial parietal areas. Together, these areas then implement biasing of region-specific visual-sensory cortex to facilitate the processing of upcoming visual stimuli.

Adolescent↗

PubMed2006

Selective attention and audiovisual integration: is attending to both modalities a prerequisite for early integration?

Interactions between multisensory integration and attention were studied using a combined audiovisual streaming design and a rapid serial visual presentation paradigm. Event-related potentials (ERPs) following audiovisual objects (AV) were compared with the sum of the ERPs following auditory (A) and visual objects (V). Integration processes were expressed as the difference between these AV and (A + V) responses and were studied while attention was directed to one or both modalities or directed elsewhere. Results show that multisensory integration effects depend on the multisensory objects being fully attended--that is, when both the visual and auditory senses were attended. In this condition, a superadditive audiovisual integration effect was observed on the P50 component. When unattended, this effect was reversed; the P50 components of multisensory ERPs were smaller than the unisensory sum. Additionally, we found an enhanced late frontal negativity when subjects attended the visual component of a multisensory object. This effect, bearing a strong resemblance to the auditory processing negativity, appeared to reflect late attention-related processing that had spread to encompass the auditory component of the multisensory object. In conclusion, our results shed new light on how the brain processes multisensory auditory and visual information, including how attention modulates multisensory integration processes.

Acoustic Stimulation↗

PubMed2006

Dissociation of event-related potentials indexing arousal and semantic cohesion during emotional word encoding.

Event-related potential (ERP) studies have shown that emotional stimuli elicit greater amplitude late positive-polarity potentials (LPPs) than neutral stimuli. This effect has been attributed to arousal, but emotional stimuli are also more semantically coherent than uncategorized neutral stimuli. ERPs were recorded during encoding of positive, negative, uncategorized neutral, and categorized neutral words. Differences in LPP amplitude elicited by emotional versus uncategorized neutral stimuli were evident from 450 to 1000 ms. From 450 to 700 ms, LPP effects at midline and right hemisphere frontal electrodes indexed arousal, whereas LPP effects at left hemisphere centro-parietal electrodes indexed semantic cohesion. This dissociation helps specify the processes underlying emotional stimulus encoding, and suggests the need to control for semantic cohesion in emotional information processing studies.

Adult↗

PubMed2006

Good times for multisensory integration: Effects of the precision of temporal synchrony as revealed by gamma-band oscillations.

The synchronous occurrence of the unisensory components of a multisensory stimulus contributes to their successful merging into a coherent perceptual representation. Oscillatory gamma-band responses (GBRs, 30-80 Hz) have been linked to feature integration mechanisms and to multisensory processing, suggesting they may also be sensitive to the temporal alignment of multisensory stimulus components. Here we examined the effects on early oscillatory GBR brain activity of varying the precision of the temporal synchrony of the unisensory components of an audio-visual stimulus. Audio-visual stimuli were presented with stimulus onset asynchronies ranging from -125 to +125 ms. Randomized streams of auditory (A), visual (V), and audio-visual (AV) stimuli were presented centrally while subjects attended to either the auditory or visual modality to detect occasional targets. GBRs to auditory and visual components of multisensory AV stimuli were extracted for five subranges of asynchrony (e.g., A preceded by V by 100+/-25 ms, by 50+/-25 ms, etc.) and compared with GBRs to unisensory control stimuli. Robust multisensory interactions were observed in the early GBRs when the auditory and visual stimuli were presented with the closest synchrony. These effects were found over medial-frontal brain areas after 30-80 ms and over occipital brain areas after 60-120 ms. A second integration effect, possibly reflecting the perceptual separation of the two sensory inputs, was found over occipital areas when auditory inputs preceded visual by 100+/-25 ms. No significant interactions were observed for the other subranges of asynchrony. These results show that the precision of temporal synchrony can have an impact on early cross-modal interactions in human cortex.

Acoustic Stimulation↗

PubMed2006

Pre-target activity in visual cortex predicts behavioral performance on spatial and feature attention tasks.

Physiological studies in humans and monkeys have revealed that, in response to an instruction to attend, areas of sensory cortex that code the attributes of the expected stimulus exhibit increases in neural activity prior to the arrival of the stimulus. Models of selective visual attention posit that these increases in activity give attended stimuli a processing advantage over distracting stimuli. Here, we test two key predictions of this view by using functional magnetic resonance imaging to record human brain activity during a cued voluntary orienting task. First, we tested whether pre-stimulus modulations are observed during both cued spatial and cued feature attention. Secondly, we tested whether the magnitude of pre-stimulus modulations predicts behavioral performance. Our results indicate that cue-triggered expectation of targets with particular spatial or nonspatial features activates areas of the visual cortex selective for these features. Furthermore, the magnitude of the cue-triggered modulations correlated with behavioral measures, such that those subjects who exhibited relatively large pre-stimulus modulations of activity performed better on the behavioral task. These findings support the view that top-down control systems bias activity in sensory cortices to favor the processing of expected target features and that this bias is related to behavior.

Action Potentials↗

PubMed2006

Hemispheric asymmetry of sulcus-function correspondence: quantization and developmental implications.

Spatial covariances between the geometric centers of human occipital sulci and visual functional areas were calculated to reduce the spatial variance of functional-area locations between subjects. Seven visual areas in each occipital hemisphere were retinotopically mapped, using horizontal- and vertical-meridian stimuli and (15)O PET in 11 subjects. Sulcal locations were determined using anatomic brain models derived from high-resolution MRI images. Location variability for sulci and functional areas was similar in magnitude, with average standard deviations of (2.7x, 5.3y, 5.7z) mm and (4.3x, 5.4y, 5.3z) mm, respectively. Sulcal locations were predictive of functional-area locations (i.e., significant spatial covariance) in the minority of structure-function pairings tested (25 of 168). Location variability was reduced by an average of 27% for functional areas showing significant covariation with sulcal features. Early-developing sulci were stronger predictors of functional-area location than late-developing sulci. Sulcus-function covariance was stronger in the left occipital lobe than in the right occipital lobe. Notably, the left calcarine fissure demonstrated powerful covariances with functional areas in both hemispheres, suggesting that it serves as a developmental "anchor" for functional areas in the occipital cortex. These findings support the hypothesis that hemispheric lateralization of function is reflected in the strength of correspondence between cortical surface anatomy and function.

Adolescent↗

PubMed2006

Attentional capacity for processing concurrent stimuli is larger across sensory modalities than within a modality.

One finding in attention research is that visual and auditory attention mechanisms are linked together. Such a link would predict a central, amodal capacity limit in processing visual and auditory stimuli. Here we show that this is not the case. Letter streams were accompanied by asynchronously presented streams of auditory, visual, and audiovisual objects. Either the letter streams or the visual, auditory, or audiovisual parts of the object streams were attended. Attending to various aspects of the objects resulted in modulations of the letter-stream-elicited steady-state evoked potentials (SSVEPs). SSVEPs were larger when auditory objects were attended than when either visual objects alone or when auditory and visual object stimuli were attended together. SSVEP amplitudes were the same in the latter conditions, indicating that attentional capacity between modalities is larger than attentional capacity within one and the same modality.

Acoustic Stimulation↗

PubMed2005

The spread of attention across modalities and space in a multisensory object.

Attending to a stimulus is known to enhance the neural responses to that stimulus. Recent experiments on visual attention have shown that this modulation can have object-based characteristics, such that, when certain parts of a visual object are attended, other parts automatically also receive enhanced processing. Here, we investigated whether visual attention can modulate neural responses to other components of a multisensory object defined by synchronous, but spatially disparate, auditory and visual stimuli. The audiovisual integration of such multisensory stimuli typically leads to mislocalization of the sound toward the visual stimulus (ventriloquism illusion). Using event-related potentials and functional MRI, we found that the brain's response to task-irrelevant sounds occurring synchronously with a visual stimulus from a different location was larger when that accompanying visual stimulus was attended versus unattended. The event-related potential effect consisted of sustained, frontally distributed, brain activity that emerged relatively late in processing, an effect resembling attention-related enhancements seen at earlier latencies during intramodal auditory attention. Moreover, the functional MRI data confirmed that the effect included specific enhancement of activity in auditory cortex. These findings indicate that attention to one sensory modality can spread to encompass simultaneous signals from another modality, even when they are task-irrelevant and from a different location. This cross-modal attentional spread appears to reflect an object-based, late selection process wherein spatially discrepant auditory stimulation is grouped with synchronous attended visual input into a multisensory object, resulting in the auditory information being pulled into the attentional spotlight and bestowed with enhanced processing.

Acoustic Stimulation↗

PubMed2005

Multisensory processing and oscillatory gamma responses: effects of spatial selective attention.

Here we describe an EEG study investigating the interactions between multisensory (audio-visual) integration and spatial attention, using oscillatory gamma-band responses (GBRs). The results include a comparison with previously reported event-related potential (ERP) findings from the same paradigm. Unisensory-auditory (A), unisensory-visual (V), and multisensory (AV) stimuli were presented to the left and right hemispaces while subjects attended to a designated side to detect deviant target stimuli in either sensory modality. For attended multisensory stimuli we observed larger evoked GBRs approximately 40-50 ms post-stimulus over medial-frontal brain areas compared with those same multisensory stimuli when unattended. Further analysis indicated that the integration effect and its attentional enhancement may be caused in part by a stimulus-triggered phase resetting of ongoing gamma-band responses. Interestingly, no such early interaction effects (<90 ms) could be found in the ERP waveforms, suggesting that oscillatory GBRs may be more sensitive than ERPs to these early latency attention effects. Moreover, no GBR attention effects could be found for the unisensory auditory or unisensory visual stimuli, suggesting that attention particularly affects the integrative processing of audiovisual stimuli at these early latencies.

Acoustic Stimulation↗

PubMed2005

Electrophysiological activity underlying inhibitory control processes in normal adults.

In a recent ERP study of inhibitory control using the Stop-Signal Task [Pliszka, S., Liotti, M., Woldorff, M. (2000). Inhibitory control in children with attention-deficit/hyperactivity disorder: Event-related potentials identify the processing component and timing of an impaired right-frontal response-inhibition mechanism. Biological Psychiatry, 48, 238-246], we showed that in normal children (age 10-12 years) the Stop Signals elicited a robust, right-frontal-maximal N200 (latency approximately 200 ms) that was strongly reduced in children with ADHD. To further investigate the mechanisms of response inhibition, this paradigm was applied to 11 healthy young adults. To better distinguish response-inhibition-related activity from early attentional effects, a "Stop-Signal-Irrelevant" condition was added, in which subjects performed the task while ignoring the Stop Signals. In the Stop-Signal-Relevant condition, the right frontal N200 to the Stop Signals was larger for Successful inhibition (SI) than for Failed inhibition (FI) trials. The timing and distribution of this effect was strikingly similar to that of the right-frontal ADHD deficit reported in Pliszka et al. (2000), supporting this activity being related to successful normal inhibitory control processes. In contrast, a posterior N200 was larger for Stop-Relevant than for Stop-Irrelevant trials, likely reflecting enhanced early sensory attention to the Stop Signals when relevant. Two longer-latency failure-specific ERP effects were also observed: a greater frontopolar negative wave (370-450 ms) to Failed than Successful inhibitions, and a greater parietal positive slow wave (450-650 ms) for Failed inhibitions than ignore-stop trials, likely reflecting differential recruitment of error detection and correction mechanisms following Failed attempts to inhibit a response.

Adult↗

PubMed2005

Control networks and hemispheric asymmetries in parietal cortex during attentional orienting in different spatial reference frames.

Neuropsychological research has consistently demonstrated that spatial attention can be anchored in one of several coordinate systems, including those defined with respect to an observer (viewer-centered), to the gravitational vector (environment-centered), or to individual objects (object-centered). In the present study, we used hemodynamic correlates of brain function to investigate the neural systems that mediate attentional control in two competing reference frames. Healthy volunteers were cued to locations defined in either viewer-centered or object-centered space to discriminate the shape of visual targets subsequently presented at the cued locations. Brain responses to attention-directing cues were quantified using event-related functional magnetic resonance imaging. A fronto-parietal control network was activated by attention-directing cues in both reference frames. Voluntary shifts of attention produced increased neural activity bilaterally in several cortical regions including the intraparietal sulcus, anterior cingulate cortex, and the frontal eye fields. Of special interest was the observation of hemispheric asymmetries in parietal cortex; there was significantly greater activity in left parietal cortex than in the right, but this asymmetry was more pronounced for object-centered shifts of attention, relative to viewer-centered shifts of attention. Measures of behavioral performance did not differ significantly between the two reference frames. We conclude that a largely overlapping, bilateral, cortical network mediates our ability to orient spatial attention in multiple coordinate systems, and that the left intraparietal sulcus plays an additional role for orienting in object-centered space. These results provide neuroimaging support for related claims based on findings of deficits in object-based orienting in patients with left parietal lesions.

Adolescent↗

PubMed2005

Abnormal brain activity related to performance monitoring and error detection in children with ADHD.

Brain electrical activity associated with inhibitory control was recorded in ten ADHD and ten healthy children using high density event related potentials (ERPs) during the Stop Signal Task (SST). SST is a two-choice reaction time (RT) paradigm, in which subjects are required, on 25% of the trials, to withdraw their response upon presentation of a "Stop Signal". In the healthy children, the ERP evoked by the Stop Signal differed for successful inhibitions (SI) compared to failed inhibitions (FI), with greater amplitude of a positive wave peaking around 320 msec over anterior medial frontal scalp (P3a). Such success-related P3a activity was significantly reduced in amplitude in the ADHD group. In addition, the error related negativity (ERN), a sharp negative wave that is present selectively on error trials in choice RT experiments, peaking 100 ms after motor onset, and distributed over anterior medial frontal scalp, was also markedly reduced in the ADHD group. The scalp distribution of the group differences in P3a and the ERN is consistent with a reduction of activity of sources in dorsal anterior cingulate cortex (dACC), suggesting a global deficit in cognitive control operations subserved by dACC in ADHD.

Attention Deficit Disorder with Hyperactivity↗

PubMed2005

Selective attention and multisensory integration: multiple phases of effects on the evoked brain activity.

We used event-related potentials (ERPs) to evaluate the role of attention in the integration of visual and auditory features of multisensory objects. This was done by contrasting the ERPs to multisensory stimuli (AV) to the sum of the ERPs to the corresponding auditory-only (A) and visual-only (V) stimuli [i.e., AV vs. (A + V)]. V, A, and VA stimuli were presented in random order to the left and right hemispaces. Subjects attended to a designated side to detect infrequent target stimuli in either modality there. The focus of this report is on the ERPs to the standard (i.e., nontarget) stimuli. We used rapid variable stimulus onset asynchronies (350-650 msec) to mitigate anticipatory activity and included "no-stim" trials to estimate and remove ERP overlap from residual anticipatory processes and from adjacent stimuli in the sequence. Spatial attention effects on the processing of the unisensory stimuli consisted of a modulation of visual P1 and N1 components (at 90-130 msec and 160-200 msec, respectively) and of the auditory N1 and processing negativity (100-200 msec). Attended versus unattended multisensory ERPs elicited a combination of these effects. Multisensory integration effects consisted of an initial frontal positivity around 100 msec that was larger for attended stimuli. This was followed by three phases of centro-medially distributed effects of integration and/or attention beginning at around 160 msec, and peaking at 190 (scalp positivity), 250 (negativity), and 300-500 msec (positivity) after stimulus onset. These integration effects were larger in amplitude for attended than for unattended stimuli, providing neural evidence that attention can modulate multisensory-integration processes at multiple stages.

Adolescent↗

PubMed2004

The BOLD fMRI refractory effect is specific to stimulus attributes: evidence from a visual motion paradigm.

Functional magnetic resonance imaging (fMRI) studies have demonstrated that the blood oxygenation level-dependent (BOLD) hemodynamic response (HDR) to a stimulus is reduced by the previous presentation of a similar stimulus. We investigated the dependence of this refractory effect upon stimulus characteristics using a novel adaptation paradigm while scanning subjects using fMRI at 4 T. The stimuli were composed of horizontal stripes that scrolled up, scrolled down, or remained static, randomly presented for 1-s duration with stimulus-onset asynchronies (SOAs) of 2-7 s. We identified regions of interest (ROI) in lateral temporal--occipital cortex that were activated by motion stimuli, regardless of direction or SOA. We found strong evidence for direction specificity in motion-sensitive lateral temporal-occipital (LTO) cortex. For stimuli whose direction of motion reprised that of the previous stimulus (e.g., up preceded by up), the fMRI response was attenuated at short SOAs (2-4 s) compared to long SOAs (5-7 s). However, for stimuli whose direction of motion was opposite that of the previous stimulus (e.g., up preceded by down), little or no refractory effect was observed. Additionally, examination of activity in pericalcarine cortex indicated a similar pattern. We conclude that the fMRI refractory effect predominantly reflects local stimulus-specific neuronal or neurovascular adaptation and is unlikely to be a nonspecific response of large vessels that support broad functional regions.

Adult↗

PubMed2004

Mechanisms of moving the mind's eye: planning and execution of spatial shifts of attention.

The usefulness of attentional orienting, both in the real world and in the laboratory, depends not only on the ability to attend to objects or other inputs but also on the ability to shift attention between them. Although understanding the basic characteristics of these shifts is a critical step toward understanding the brain mechanisms that produce them, the literature remains unresolved on a very basic and potentially revealing characteristic of these shifts-namely, whether attention takes longer to shift a farther distance across the visual field. We addressed this question using a series of behavioral tasks involving the voluntary orienting of attention to locations in the visual field. The findings support a model in which attentional shifts include separate "planning" and "execution" stages and in which only the planning stage requires more time for shifts of a greater distance. These results offer resolution to the longstanding debate concerning the effect of attentional shift distance on shift time and provide insight into the fundamental mechanisms of attentional shifting.

Adolescent↗

PubMed2003

The ERP omitted stimulus response to "no-stim" events and its implications for fast-rate event-related fMRI designs.

A major difficulty in fast-rate event-related fMRI experiments is the extensive overlap from adjacent trials in the stimulus sequence. One approach to address this problem is to include "no-stim" or "null" events as a trial type. These are randomized as if they were true stimulus events but no stimulus actually occurs. Assuming that no response is elicited by the null events, their time-locked average reflects only the averaged overlap. Thus, contrasting the averages for the other trial types versus the null-event average subtracts out the overlap, enabling the extraction of the response functions for these other trial types. ERP studies, however, have indicated that an endogenous brain response, the omitted stimulus response (OSR), can be evoked by a missing event in a stream of regularly occurring stimuli. To the extent that this response is elicited by null events in an event-related fMRI experiment, the null-event subtraction or contrast would falsely introduce the inverse of the OSR into the averaged responses to the other trial types. Using high-density ERP recordings, we investigated the effect of different percentages of omitted stimuli (11, 22, and 33%) on the auditory OSR at stimulus rates of one event per second or one event per 2 s. Significant OSRs were found for each percentage in the 1-s condition as well as in the 11% 2-s condition. The responses consisted of an early posterior negative wave (180-280 ms) followed by a larger anterior positive wave. These results have important implications for fast-rate fMRI designs, while also providing new data on the brain's response to omitted stimuli.

Acoustic Stimulation↗

PubMed2002

Delayed striate cortical activation during spatial attention.

Recordings of event-related potentials (ERPs) and event-related magnetic fields (ERMFs) were combined with functional magnetic resonance imaging (fMRI) to study visual cortical activity in humans during spatial attention. While subjects attended selectively to stimulus arrays in one visual field, fMRI revealed stimulus-related activations in the contralateral primary visual cortex and in multiple extrastriate areas. ERP and ERMF recordings showed that attention did not affect the initial evoked response at 60-90 ms poststimulus that was localized to primary cortex, but a similarly localized late response at 140-250 ms was enhanced to attended stimuli. These findings provide evidence that the primary visual cortex participates in the selective processing of attended stimuli by means of delayed feedback from higher visual-cortical areas.

Adult↗