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R Simson

Publications and source records attributed to R Simson.

9 recordsLinked to original sources
PubMed1988

Effects of the amount of stimulus information processed on negative event-related potentials.

Event-related potentials were recorded during simple reaction time and 3 discrimination conditions which varied in the amount of stimulus information that needed to be processed. It was found that NA became longer in duration as the amount of stimulus information that required processing was increased. Using sequential topographic mapping, it was concluded that there are at least 3 overlapping deflections that comprise NA. The experimental effect appeared to be mainly on the third deflection of NA. The problem of overlap between NA and later positive-going components, P380 and P3b, is discussed in terms of their relative latencies and scalp distributions.

Adult↗
PubMed1986

Topography of visual event-related potentials during geometric and phonetic discriminations.

Event-related potentials (ERPs) were obtained to letters during 3 tasks that involved a SIMPLE RESPONSE (SR) to each letter presentation, a FORM discrimination of the letters that formed a closed loop, and a RHYME discrimination of the letters that rhymed with the letter 'v'. The first task only required detection of the letters, the FORM task required a visual-spatial analysis, and the RHYME task, a grapheme-phoneme conversion of the letters followed by a determination of rhyming characteristics. The SR ERPs were morphologically different from the discriminative ERPs, notably by the absence of N2 and P3. The difference wave forms between the discrimination and SR conditions and between the targets and non-targets indicated differential topographies of components associated with the FORM and RHYME tasks in the 300 msec latency region. In both of these tasks, components exhibited distributions localized primarily in the occipital regions, whereas in the RHYME task they extended more anteriorly and encompassed temporo-parietal regions. Thus, although the stimulus presentation was visual, the requirement of a visual-auditory conversion in the RHYME task resulted in activity that was more proximal to auditory regions than when only a FORM analysis of the letters was required.

Adult↗
PubMed1985

Effects of expectation on negative potentials during visual processing.

ERPs were recorded during several RT tasks: simple RT; oddball choice RT; a LIE condition in which subjects were told stimuli would infrequently change, but did not; differential responding to two equiprobable stimuli that were randomized in one condition and alternated in another condition. Subtracting ERPs elicited during simple RT from those elicited during the other conditions, it was found that a negative component, NA, was enhanced, relative to simple RT, in all the other RT tasks. The data of the LIE condition indicated that NA was enhanced by the expectation that unpredictable stimulus changes would occur, even when they did not. The data of the 50/50 alternating RT condition indicated that stimulus changes by themselves enhance NA, even when they are predictable. There appear to be several deflections that comprise NA. NA was obtained with a variety of subtractions that balanced stimulus probability, the structure of the stimulus sequence and task instructions. Similar results were obtained whether subjects made a finger lift response or counted stimuli.

Adult↗
PubMed1979

A brain event related to the making of a sensory discrimination.

Event-related potentials associated with detected targets in a vigilance task were analyzed in two ways: (i) by sorting the potentials in terms of sequential reaction time bins of 50 milliseconds and (ii) by examining the single trial waveforms. A negative component (N2) covaried in latency with reaction time. These results support the hypothesis that N2 reflects a decision process which controls behavioral responses in sensory discrimination tasks.

Behavior↗
PubMed1977

The scalp topography of potentials in auditory and visual discrimination tasks.

Averaged event-related cortical potentials (ERPs) were obtained from an array of scalp electrodes overlying the left hemicranium in response to regularly presented visual or auditory stimuli (non-signals)and to infrequent random replacements by different stimuli (signals) in the same modality. A motor response was required to the signals. Non-signal ERPs were subtracted from signal ERPs and the topographic distributions of the negative (N2 delta) and positive (P3 delta) components were plotted as isopotential maps. N2 delta distributions differed for the auditory and visual modalities, whereas P3delta was modality unspecific. These topographic data were compared to those from the previous study of missing stimulus potentials (Simson et al. 1976) using maps representing the contributions from unilateral cerebral sources. The N2 delta and negative missing stimulus potential distributions ascribed to cortical activity within the secondary auditory and visual regions, whereas the late positive component (positive missing stimulus potential or P3 delta) were considered to derive principally from inferior parietal association cortex.

Adult↗
PubMed1976

The scalp topography of potentials associated with missing visual or auditory stimuli.

Averaged potentials time-locked to regularly presented visual and auditory stimuli and to the occasional random deletion of a stimulus were recorded from a scalp electrode array overlying the left hemicranium. The major components of visual and auditory evoked potentials and of the potentials associated with missing stimuli (MSP) were measured and their amplitude distributions depicted in the form of isopotential maps. The N1 components of the VEP and AEP had distributions compatible with sources in and near the respective primary cortical projection areas. The P2 components were more widely distributed and could be attributed in part to generators within modality specific association areas. The MSP comprised two main components, an initial negativity (NMSP) and a later positive wave (PMSP). The NMSP distributions were different in the visual and auditory modalities, and were similar to the respective EP topographies. The NMSP appeared to reflect a more powerful contribution of association areas than did the evoked responses. The PMSP topography was modality unspecific with distributions which were maximal over the parietal region. The possible functional significance of the NMSP and PMSP was considered in the light of their timing and topography.

Adult↗
PubMed1975

Cortical evoked potentials elicited by real speech words and human sounds.

Averaged evoked potentials were recorded from PZ and left and right temporo-parietal electodes to real speech words and human sounds in 8 right-handed subjects. Stimuli were presented in a "no task" condition where the subject was instructed to listen attentively, and a vigilance condition where the subject responded to a particular word or sound during a run of such stimuli. The vigilance condition produced two classes of stimuli:signals and non-signals. Evoked potentials to physically identical words or sounds were examined when they were "no task", non-signal and signal stimuli. P300 amplitude increased significantly as a function of increasing task demands going from "no task" to non-signal to signal. When a strict statistical criterion for multiple comparisons (Bonferroni test) was applied in looking for asymmetries between hemispheres, only 2 isolated left greater than right differences turned out to be significant. Review of the literature concerning evoked potential correlates of differential hemispheric processing pointed up flaws in design, statistical technique, and inconsistencies in reported findings which suggested that while evoked potentials may sometimes reflect differences in hemispheric functioning, this effect is marginal at best.

Adult↗
PubMed1975

The late positive component (P300) and information processing in sentences.

Averaged visual evoked potentials to sequentially flashed words comprising a sentence were recorded from vertex and left and right temporoparietal electrodes in 8 right-handed subjects. In condition 1 the sentence took the form: The -eel is on the shoe, in which the first grapheme was omitted from the second word, so that the subject did not know the meaning of the second word until he viewed the last word. In condition 2, the sentence took the form: The heel is on the shoe, in which the second word was given and the last word provided no further information. P300 latency to words which delivered information (last word of condition 1, second word of condition 2) were significantly longer than P300 latency to any of the other words in the sentence, as well as to the same position word in the other condition. Comparisons of P300 latencies to redundant words (the, is, on) within and between conditions showed no significant differences. P300 amplitude to the last word was significantly larger than P300 amplitude to any of the other words within the sentence, even in condition 2 where the second word delivered information. The major effect of information delivery was on P300 latency, while "syntactic closure" had its major effect on P300 amplitude. The fact that evoked potentials to all words had P300 components was attributed to the engagement of the P300 system whenever task-related language stimuli are used.

Adolescent↗