Biomedical subjects

V T Nasman

Publications and source records attributed to V T Nasman.

7 recordsLinked to original sources

PubMed1994

Task decision difficulty: effects on ERPs in a same-different letter classification task.

A "same-different" letter pair choice reaction time task was used to compare reaction time, performance accuracy, and P3 amplitude and latency at three levels of task difficulty. Stimulus set was held constant across tasks, and task difficulty was manipulated by instructions to the subject. Subjects delivered a button-press response to designate whether members of each letter pair matched or mismatched on the basis of their physical identity (low difficulty), name identity (medium difficulty), or category identity (vowels/consonants, high difficulty). Task decision difficulty was confirmed by slower reaction times and reduced response accuracy. P3 amplitude was inversely related to task decision difficulty; relatively larger P3s were associated with matched (vs. mismatched) letter pairs. These findings evidence direct effects of task difficulty on P3 amplitude, possibly as the result of "equivocation" related to more difficult task judgments.

Adult↗

PubMed1993

Reduced P3b category response in prefrontal patients.

Several studies have implicated a frontal/prefrontal contributor to P3b. We hypothesized that prefrontal involvement may be necessary to the processing of novel contextual information, as suggested by selective reduction of P3a in prefrontal patients. ERPs were recorded from 12 controls and 12 unilateral prefrontal lesion patients. In two target-detection tasks, subjects/patients viewed randomly-ordered stimuli drawn from differentially probable categories: numbers (standard, 90%) and letters (deviant, 10%). Topographic and midline analyses were applied to examine target and category effects on P3b. Low-probability targets evoked distinct P3bs in each group. Control subjects' P3bs were additionally enhanced to deviant targets and nontargets. Patients' P3s were enhanced to deviant nontargets but were unaffected by deviant targets. This is consistent with the view that target- and nontarget-evoked P300s are functionally distinct, and suggests that a prefrontal contributor to P3b supports attention to contextual attributes among task-relevant stimuli.

Adult↗

PubMed1992

Effect of task decision on P300.

Previous studies have shown that the structure of an experimental task may modulate P300 (P3) responsiveness to task-related test stimuli. In this study, four tasks were compared to study possible effects of decision type on P3s to rare and frequent visual word stimuli. One or three oddballs were designated in two separate experiments. Subjects in each experiment viewed randomized and repeated presentations of nine 5- to 8-letter words, and performed each of four target detection and choice discrimination tasks: (1) silent counting of oddball(s), (2) button-press to oddball(s), (3) choice response ('yes' or 'no') to oddball(s) and frequents, and (4) choice button-press to oddball(s) and frequents. Although oddball stimuli elicited characteristic 'target' P3s over the parietal scalp, there was no significant effect of task type on the oddball-evoked P3. Frequents tended to elicit larger P3s in the choice discrimination tasks (yes/no, go/go) than in target-detection tasks (counting, go/no-go), indicating a selective effect of decision type on P3s to frequent-category events.

Adolescent↗

PubMed1992

Taking attention to task: P300, task response probability, and within-category deviation detection.

Previously, we reported enhanced P3 amplitudes to between-category deviations among high and low probability events. Here, we tested the effects of a within-category difference. Subjects performed a go/no-go button press task as they viewed repeated, randomly-ordered presentations of nine double-digit numbers. Eight numbers fell within a prescribed range (42-49, standards); prior to testing, subjects selected one standard number for later recall. A ninth, out-of-range (91, deviant) number was also included. Subjects were tested under two conditions, in which they responded either to one (low response probability) or to seven (high response probability) standard nonselected numbers, designated as targets. Relatively larger P3s were consistently apparent only when the deviant nontarget was associated with a low probability response to a nontarget. There was a selective effect of nontarget response probability on P3 amplitude to the deviant nontarget. Our results indicate that within-category deviation detection is facilitated by "controlled" attention to the structure of the stimulus field.

Adolescent↗

PubMed1990

Parietal P3 response as an indicator of stimulus categorization: increased P3 amplitude to categorically deviant target and nontarget stimuli.

Two experiments were performed in which we compared the effects of selected non-deviant versus categorically deviant stimuli on parietal P3 under a variety of conditions in which one, both, or neither stimulus was a target of an experimental task. Subjects were repeatedly presented with series of 8 numeric stimuli and 1 alphabetic (Deviant) stimulus. P3 amplitudes to target and nontarget Deviant stimuli were consistently and significantly larger than to other, non-deviant targets and nontargets, respectively. Nontarget Deviant stimuli evoked P3 amplitudes comparable to those evoked by low-probability non-deviant targets. The observed differences indicate that P3 amplitude is a sensitive indicator of perceived category differences, and that the effect of category deviance on parietal P3 amplitude is independent of task response classification (target or nontarget) and of response probability.

Adolescent↗

PubMed1988

A modified, event-related potential-based guilty knowledge test.

Subjects chose and pretended to steal one object from a box of nine. They then watched a visual display of verbal representations of objects including their chosen object or one of eight novel objects on each trial. They were told to count one of the novel objects and that although they were welcome to try to beat our test, they would be unable to avoid noticing the chosen object. P3 responses were obtained only to counted and to chosen objects in 7 of 10 subjects not eliminated for artifact or noncooperation.

Guilt↗

PubMed1987

Late vertex positivity in event-related potentials as a guilty knowledge indicator: a new method of life detection.

Subjects were allowed to choose an item to keep from nine items in a box. They then were shown one of nine words randomly selected on a display screen. One of these words described the chosen item, the others described novel items. The subjects were told to try not to react emotionally to any of the words, but to try to defeat this test of deception. It was found that large positive waves with latencies between 400 and 700 ms poststimulus were present in the ERPS to the chosen but not to the novel words.

Adolescent↗