Richard Simson

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.

The scalp topography of potentials in auditory and visual Go/NoGo tasks ☆

Abstract The topography of averaged ERP to auditory and visual stimuli was evaluated employing an array of scalp electrodes overlying the left hemicranium in a sequential discrimination (Go-NoGo) task. The discriminanda (S1 and S2) were presented 1 sec apart and the subjects made a delayed motor response when S2 differed from S1 and withheld response when they were the same. Late positive components were associated with both S1 and S2 and slow negative shifts (CNV) occurred during the S1–S2 interval. The negative shifts terminated after the NoGo S2 but persisted after the Go stimuli until the motor response was performed. The CNV topography changed markedly over the 500 msec prior to S2. The early auditory CNV was restricted to the posterior frontal region, whereas the visual CNV was parieto-occipital with a secondary central focus. Just before S2, both the visual and auditory CNV possessed a central maximum with only a small posterior extension persisting in the visual modality. As in the previous topographic studies (Simson et al. 1976; 1977), the late positive components had parietal maxima, but in this study the Go and NoGo LPC differed in size, timing and topography. When the effect of the CNV return following the NoGo S2 was taken into account, however, the S2 LPC topographies were less disparate. Subtraction to the Go and NoGo ERP revealed an early modality specific difference waveform which was comparable in timing to the previously described N2 difference waveform. A modality unspecific parietally maximum difference potential reflected the larger Go LPC and a centrally maximum later wave the CNV resolution following the NoGo stimuli.

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.

Association cortex potentials and reaction time in auditory discrimination

Abstract Averaged evoked responses to auditory stimuli were obtained in simple reaction time and vigilance tasks, as well as a no response control condition. The P2 component of sensory evoked potentials had essentially identical peak latency for the various experimental conditions. By contrast, the P3 component of association cortex potentials was found to be about 100 msec longer in latency for vigilance than for simple reaction time, and to be significantly longer in latency for a harder compared to an easier discrimination in the vigilance task. These changes in P3 latency were accompanied by comparable alterations in mean reaction time. In the vigilance conditions, single trial EEG recordings were scored for peak latency of P3 for each signal. Significant product-moment correlations were obtained between the single trial P3 and reaction time values. The results were interpreted to reflect different functional roles of sensory evoked potentials and association cortex potentials.

An electrophysiologic indication of auditory processing defects in autism

Averaged evoked potentials (EPs) to clicks, random pitch changes (signals), and random deletions of stimuli within a regular train of tones were examined in five autistic and five normal children. Brainstem auditory EPs were abnormal in one of the autistic patients. The early cortical EP components P60 and N100 showed no differences across groups, whereas the P200 component of the cortical responses to clicks, as well as the late positive component (P300) to the pitch changes and deleted stimuli, were significantly smaller in the autistic subjects as a group. Furthermore, when P200 and P300 amplitudes were averaged across conditions for the individual subjects, these components were smaller in every autistic subject than in any of the normal subjects. These results are consistent with the view that there are auditory defects in autism that may sometimes involve lower levels of neural transmission as manifested by abnormalities in the brainstem and auditory EP, but are more consistently manifest in higher aspects of processing that involve the registration and storage of stimulus information. It is suggested that the severe language disorder in childhood autism may be secondary to the basic deficits in higher auditory processing.

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.

The N2 component elicited by stimulus matches and multiple targets

This paper examines two methodological issues concerning the N2 component of human event-related potentials. The first issue concerns the circumstance that the most common way to obtain N2 in discrimination tasks is with an infrequent deviant stimulus that mismatches a frequent, standard stimulus. In these studies it is not possible to disentangle the effects of stimulus probability and stimulus mismatch on N2. In the present study it was found that, if two stimuli regularly alternate, N2 is elicited by infrequent repetitions of either stimulus. Thus, N2 is elicited by infrequent stimulus matches as well as infrequent stimulus mismatches. The second issue concerns the effect of stimulus probability on N2. Whereas previous research has established that the amplitude of N2 is inversely related to stimulus probability, the present study found that the number of possible targets in a visual discrimination task also has effects on N2 amplitude, with the overall probability of targets kept constant. Increasing the number of targets was associated with an increase in the duration of N2 and a differential enhancement of N2 at fronto-central as opposed to posterior-lateral recording sites. The latter results provide further evidence for the existence of two visual N2 components and tentative grounds for differentiating N2 from N400.

Topographic Analysis of Auditory Event-Related Potentials

Publisher Summary This chapter discusses the topographic analysis of auditory event-related potentials (ERPs). The analysis of neural mechanisms underlying human auditory processes depends principally upon the study of scalp recorded potentials. These auditory event-related potentials (ERPs) reflect neural activity at all stages of auditory processing. As yet, however, the intracranial sources of the various auditory ERP components have not been conclusively identified. Knowledge of the anatomical locus of these generators is essential for interpretation of ERPs in terms of the neural processes that generate them, as well as providing a basis for linking psychological theories of auditory processing with neurophysiological mechanisms. Topographic studies of some of the more prominent auditory ERP components have been carried out. These data support the conclusion that the obligatory AEP components up to 200msec in latency are modality specific in topography and are consistent with sources principally within the superior temporal plane. Task-related components, a negativity at about 200 msec in peak latency (N2) and the later positive complex (LPC) exhibit different scalp distributions, with that of the former being modality specific and the latter being coextensive with LPC associated with visual stimuli.

6 On Relating Event-Related Potential Components to Stages of Information Processing

Publisher Summary The successful-attempt to relate event-related potential (ERP) components to specific stages of information processing would provide a means of understanding some aspects of the ways in which the brain works, as well as yield data relevant to the theories of cognitive processing that have been based on behavioral observations. The ERP and behavioral sources of data both have inherent limitations, but taken together they might allow for converging operations to test theories common to both areas. This chapter describes recent experimental results obtained in laboratory on two ERP components thatmay reflect two stages of processing—namely, pattern recognition and stimulus categorization. Although RT data were obtained, the manner by which stages were identified was different than in behavioral studies. The first step was to identify a class of variables that affect the amplitude or latency of a particular component, or complex of components, and on the basis of the nature of the variables infer the functional significance of the associated physiological activity.

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.