Biomedical subjects

G R Mangun

Publications and source records attributed to G R Mangun.

At least 19 recordsLinked to original sources

PubMed1997

Event-related brain potential correlates of two states of conscious awareness in memory.

We report an event-related potential (ERP) experiment of human recognition memory that explored the relation between conscious awareness and electrophysiological activity of the brain. We recorded ERPs from healthy adults while they made "remember" and "know" recognition judgments about previously seen words. These two kinds of judgments reflect "autonoetic" and "noetic" awareness, respectively. The ERP effects differed between the two kinds of awareness while they were similar for "true" and "false" recognition. Noetic awareness was associated with a temporoparietal positivity in the N400 range (325-600 ms) and a late (600-1,000 ms) frontocentral negativity, whereas autonoetic awareness was associated with a widespread, late, bifrontal and left parietotemporal (600-1000 ms) positivity. In the very late (1,300-1, 900 ms) time window, a right frontal positivity was observed for both remember and know judgments of both true and false targets. These results provide physiological evidence for two types of conscious awareness in episodic memory retrieval.

Adult↗

PubMed1996

Estimation of interpolation errors in scalp topographic mapping.

Topographic maps are commonly constructed from electrical scalp recordings (such as EEGs and ERPs) using several different interpolation methods. It is important to determine the accuracy of such maps. Previous assessments of interpolation methods have been based on global error measures and the visual appearance of the topographic maps. However, the relationship of interpolation error to local contributing factors requires a more detailed analysis. In this paper, we use simulations to explore and quantify the relationship of error to global and local factors for different interpolation methods. We find that among the best interpolation methods, adequate electrode density is more important than the method used. For shallow sources, we show that local interpolation error is most correlated with potential gradient, and has a lesser correlation with distance to nearest electrode. The greatest correlation, however, is with the product of gradient and distance. Thus, interpolation error can be controlled locally by making the interelectrode distance inversely proportional to the expected potential gradient. With shallow sources, areas far from any electrode and having high apparent gradient are likely to have high interpolation error. Moreover, all areas far from any electrode may contain high interpolation errors, and should be interpreted with caution.

Brain Mapping↗

PubMed1996

Developmental dyslexia: passive visual stimulation provides no evidence for a magnocellular processing defect.

Livingstone et al. [Livingstone, M. S., Rosen, G. D., Drislane, F. W. and Galaburda, A. M. Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia. Proceedings of the National Academy of Science U.S.A. 88, 7943-7947, 1991] presented evidence for a defect of the magnocellular visual processing stream in developmental dyslexia. We attempted to replicate this effect using transient and steady-state VEPs to checkerboard reversal stimuli in a group of adult developmental dyslexics. Several different reversal rates and contract levels were utilized. No differences were found between the dyslexic and control groups for the low-contrast, rapidly reversing patterns, nor for any combination of stimulus rate or contrast that was tested. Thus, these findings do not support a magnocellular processing deficit in developmental dyslexia.

Adult↗

PubMed1996

Spatial distribution of visual attention: perceptual sensitivity and response latency.

Studies of the spatial distribution of visual attention have shown that attentional facilitation monotonically decreases in a graded fashion with increasing distance from an attended location. However, reaction time (RT) measures have typically shown broader gradients than have signal detection (SD) measures of perceptual sensitivity. It is not clear whether these differences have arisen because the stages of information processing indexed by RT measures are different from those indexed by SD measures, or whether these differences are due to methodological confounds in the SD studies. In the present set of experiments, the spatial distribution of attention was studied by using a luminance detection task in an endogenous cuing paradigm that was designed to permit accurate calculations of SD and RT measures for targets at cued and uncued locations. Subjects made target-present/absent decisions at one of six possible cued or uncued upper visual hemifield locations on each trial. The results from three experiments suggest that the differences between broad and focal attentional distributions are not the result of different stages of information processing indexed by RT measures as opposed to SD measures. Rather, the differing distributions appear to reflect variations in attentional allocation strategies induced by the perceptual requirements typical of RT paradigms as opposed to SD paradigms. These findings support numerous prior studies showing that spatial attention affects perceptual sensitivity and that the strategic allocation of attention is a highly flexible process.

Adolescent↗

PubMed1995

Brain potentials in developmental dyslexia: differential effects of word frequency in human subjects.

The differences of word processing between a group of adult developmental dyslexics and control subjects were examined with the event-related potential (ERP) technique. In particular, the effects of word frequency and word recognition were assessed. The subjects viewed a series of frequently and infrequently used words, most of which were repeated after some intervening items and they discriminated between first and second presentations of the words. It can be shown that in the range from 300 to 550 ms post stimulus the amplitude of the N400 component, an ERP measure of semantic processing, is reduced for high frequency words. This effect is more pronounced in the dyslexic group and the effects of word recognition are also reduced in the dyslexic group for high frequency words. These findings are discussed with respect to current concepts of dyslexia and of semantic processing.

Adult↗

PubMed1995

Electrophysiological signs of sustained and transient attention to spatial locations.

Event-related potentials were elicited by bilateral and unilateral stimulus arrays flashed in rapid sequence in order to investigate both focused attention and attentional orienting. Subjects attended selectively to the stimuli on one side of the bilateral arrays and were required to discriminate infrequent target stimuli on either the attended side (no switch of attentional focus) or unattended side of the array (switch of attentional focus). The ERPs to the bilateral stimuli elicited an occipital P1 component that was larger in amplitude over scalp regions contralateral to the attended visual half-field. The ERPs to the unilateral stimuli on the attended side also showed an amplitude enhancement of early P1 components, followed by a positive shift that lasted until 200 msec latency over the contralateral occipital scalp. No enhancement of the N1 component was observed to attended-side stimuli. These patterns were not different for conditions requiring or not requiring a spatial switch of the attentional focus. In conjunction with ERP signs of focused spatial attention, significant differences in discrimination performance (d') were obtained for the attended vs unattended-side targets; no changes in measures of criterion (beta) were obtained. These data support the idea that the early occipital P1 attention effect represents a facilitation of visual inputs that occur at attended locations in the visual field.

Adolescent↗

PubMed1995

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)

Adolescent↗

PubMed1995

Neural mechanisms of visual selective attention.

Visual selective attention improves our perception and performance by modifying sensory inputs at an early stage of processing. Spatial attention produces the most consistent early modulations of visual processing, which can be observed when attention is voluntarily allocated to locations. These effects of spatial attention are similar when attention is cued in a trial-by-trial, or sustained, fashion and are manifest as changes in the amplitudes, but not the latencies, of evoked neural activity recorded from the intact human scalp. This modulation of sensory processing first occurs within the extrastriate visual cortex and not within the striate or earlier subcortical processing stages. These relatively early spatial filters alter the inputs to higher stages of visual analysis that are responsible for feature extraction and ultimately object perception and recognition, and thus provide physiological evidence for early precategorical selection during visual attention. Moreover, the physiological evidence extends early selection theories by providing neurophysiologically precise information about the stages of visual processing affected by attention.

Arousal↗

PubMed1994

ERP and RT signs of a rightward bias for spatial orienting in a split-brain patient.

Neuropsychological data have shown that the two cerebral hemispheres differ in the control of spatial attention. The present study investigated hemispheric asymmetries and visuomotor integration in a split-brain patient and three control subjects. Simple reaction times (RTs) and event-related potentials (ERPs) were recorded to lateralized stimuli presented at different eccentricities in the left and right visual hemifields. Both electrophysiological and behavioural data showed that, unlike controls, the split-brain patient showed a strong rightward attentional bias resulting in shorter RTs and larger P300 potentials to stimuli falling in the rightmost space. Furthermore, ERPs also showed that while the RH has a bilateral control of visual space, the LH spatial orienting capability is most restricted to the contralateral hemifield.

Adult↗

PubMed1994

Combined spatial and temporal imaging of brain activity during visual selective attention in humans.

Visual-spatial attention is an essential brain function that enables us to select and preferentially process high priority information in the visual fields. Several brain areas have been shown to participate in the control of spatial attention in humans, but little is known about the underlying selection mechanisms. Non-invasive scalp recordings of event-related potentials (e.r.ps) in humans have shown that attended visual stimuli are preferentially selected as early as 80-90 ms after stimulus onset, but current e.r.p. methods do not permit a precise localization of the participating cortical areas. In this study we combined neuroimaging (positron emission tomography) with e.r.p. recording in order to describe both the cortical anatomy and time course of attentional selection processes. Together these methods showed that visual inputs from attended locations receive enhanced processing in the extrastriate cortex (fusiform gyrus) at 80-130 ms after stimulus onset. These findings reinforce early selection models of attention.

Attention↗

PubMed1994

Sequential operation of disconnected cerebral hemispheres in split-brain patients.

When two concurrent sensorimotor tasks require separate responses, selection of the first response generally delays selection of the second. Dual-task performance was examined in four patients who had undergone surgical transection of the forebrain commissures including the corpus callosum. One light flash was presented to each visual field in succession, and patients made a choice response to each stimulus with the ipsilateral hand, thereby confining the tasks to separate hemispheres. All four showed dual-task interference very similar to that found with normal individuals. Therefore, still-intact subcortical structures must play a critical role in sequencing response selection processes (the 'dual-task bottleneck'), confirming the distinction between the attentional limitations involved in planning actions and those involved in perceptual analysis.

Brain↗

PubMed1994

Electrophysiological and behavioral "costs" and "benefits" during sustained visual-spatial attention.

Event-related potentials and reaction times were recorded to stimuli flashed to the left and right visual hemifields under different conditions of covert spatial attention. In different blocks, subjects were instructed to allocate attention primarily to either the left or to the right stimulus locations, or to divide attention equivalently between left and right locations (i.e., neutral condition). Regardless of attention condition, however, speeded motor responses were required to stimuli in both visual fields. In comparison to the neutral attention condition, reaction times were slower for targets at unattended locations (costs) and faster for the targets at attended locations (benefits), however, only costs were statistically significant. Significant attention-related enhancements were observed for both early and late ERP components. Cost/benefit analysis of the ERPs revealed ERP benefits in the amplitudes of the early sensory-evoked cortical component P1 (100-160 ms). In contrast, ERP costs appeared only at a relatively late stage of perceptual processing (200-280 ms latency post-stimulus). The relationship between RT and ERP costs and benefits is discussed with respect to models of spatial attention.

Adult↗

PubMed1994

Attention to adjacent and separate positions in space: an electrophysiological analysis.

Some theories of visuospatial attention propose that attention can be divided between separated zones of space that exclude the intervening region, whereas other theories state that the focus of attention must encompass a unitary, continuous zone. These contrasting views were evaluated in an experiment in which subjects were required to monitor two of four stimulus locations for targets; the two relevant locations were adjacent in one condition and were separated by an intervening irrelevant location in a second condition. To assess the distribution of attention across the relevant and irrelevant locations, event-related brain potentials (ERPs) were recorded to task-irrelevant "probe" stimuli that were occasionally presented at the individual stimulus locations. When the relevant locations were adjacent, probes presented at irrelevant locations elicited smaller sensory-evoked electrophysiological responses than probes presented at relevant locations, consistent with an attentional suppression of inputs from the unattended locations. When the relevant locations were separated by an irrelevant location, however, the sensory responses evoked by probes presented at this intervening irrelevant location were not suppressed, and target detection performance became slower and less accurate. These results suggest that attention forms a unitary zone that may expand to encompass multiple relevant locations but must also include the area between them; as a result, irrelevant information arising from intervening locations is not suppressed and perceptual processing is compromised.

Adult↗

PubMed1994

[Cerebral lateralization in constitutional dyslexia. Electrophysiologic findings].

Event-related brain potentials were recorded from a group of developmental dyslexics using a simple visual discrimination task and compared to the results of a control group. A large P3 component was elicited which did not differ in overall amplitude between groups. While the distribution was virtually symmetrical in the dyslexic group, there was a left hemispheric preponderance in the control group. The latencies of the P3 were longer in the dyslexic group than in the control group. The results are compared to findings of neuroanatomical and neuroimaging studies which revealed less hemispheric asymmetries in dyslexics compared to control subjects.

Adult↗

PubMed1991

Modulations of sensory-evoked brain potentials indicate changes in perceptual processing during visual-spatial priming.

Reaction time (RT) differences to visual stimuli as a function of expectancy have been attributed to changes in perceptual processing or entirely to shifts in decision and response criteria. To help distinguish between these competing interpretations, event-related brain potentials (ERPs) were recorded to lateralized flashes delivered to visual field locations precued by a central arrow (valid stimuli) or not precued (invalid stimuli). Validly cued stimuli in both simple and choice RT tasks elicited consistent amplitude enhancements of the early, sensory-evoked PI component of the ERP recorded at scalp sites overlying lateral prestriate visual cortex (90-130 ms poststimulus). In contrast, the subsequent N1 component (150-200 ms) was enhanced by validly cued stimuli in the choice RT task condition only. These electrophysiological findings support models proposing that the behavioral effects of precuing expected target locations are due, at least in part, to changes in sensory-perceptual processing. Furthermore, these data provide specific information regarding the neural mechanisms underlying such effects.

Adolescent↗

PubMed1990

Visual event-related potentials index focused attention within bilateral stimulus arrays. I. Evidence for early selection.

Event-related potentials (ERPs) were recorded from the scalp while subjects attended to sequences of bilaterally symmetrical arrays of 4 letters (2 in each visual half-field) that were flashed briefly at intervals of 280-520 msec. These sequences also included randomized presentations of unilateral 'probe' stimuli consisting of irrelevant bars (experiment 1) or potentially relevant letter pairs (experiment 2). The task was to pay attention to the letter pairs in either the left or the right half-field on a given run and to press a button when the two letters matched one another (targets). The ERPs to the bilateral arrays included an early positive wave (P1, peaking at 135 msec) that was enhanced over posterior scalp sites contralateral to the attended visual field. Both types of probe stimulus also elicited a larger early positivity in the P1 latency range (100-200 msec) when delivered to the attended half-field, followed in some cases by a more prolonged positive deflection. Notable for its absence was any sign of an enlarged posterior N1 component (160-200 msec), which was prominent in the ERP to attended-field stimuli in previous studies using randomized sequences of unilateral stimuli. Attended-field targets elicited large N2 and P3 (P300) components, which were greatly reduced or absent when targets occurred in the unattended field. The observed ERP effects were interpreted in terms of early sensory selection during visual spatial attention.

Adolescent↗

PubMed1990

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.

Adolescent↗

PubMed1990

Allocation of visual attention to spatial locations: tradeoff functions for event-related brain potentials and detection performance.

Event-related brain potentials (ERPs) were recorded in response to unilateral arrays of letters flashed in rapid, randomized sequences to left and right visual field locations. Subjects were required to focus attention exclusively on either left or right field stimuli, or to divide attention in different proportions between the two fields, with the aim of detecting infrequent target letters. Both d' and percent hits for target detections increased significantly as attentional allocation to a stimulus location increased. Attention operating characteristic (AOC) curves for the target detection scores were highly similar in form to those for the amplitudes of the long-latency, endogenous ERP components--N350-650 and P400-800 (P300). All of these measures showed gradual, nearly rectangular tradeoff functions. In contrast, the AOC curves for the early sensory-evoked components displayed steep, nearly linear amplitude tradeoffs as attention was increasingly allocated to one visual field at the expense of the other. The early and late ERP components were considered as indices of separate but interacting levels of attentional selection having different operating principles.

Adolescent↗