Probability and interstimulus interval effects on the N140 and the P300 components of somatosensory erps

Event-related potentials (ERPs) evoked by somatosensory stimuli were studied in two experiments with manipulation of the interstimulus interval (ISI) in the range from 1 to 4 seconds and the stimulus probability from 10 to 50% in fine steps. All the stimuli were presented randomly on both index fingers as odd-ball paradigm, and the stimulus intensities on both sides were equally set at 2.5 times the subjective sensory threshold at which the stimuli could easily be ignored when delivered as nontarget stimuli. During the course of two experiments, the N140 was evoked by the target and nontarget stimuli, otherwise the P300 was evoked by the target stimuli alone. In the experiment on the effects of ISIs change, the N140 amplitude showed a tendency to increase with increasing ISI and the P300 amplitude increased with increasing ISI. In the other experiment on the effects of change in stimulus probability, the amplitudes of both the N140 and the P300 decreased with increasing stimulus probability. From the results across two experiments. it is argued that the effects of both probability and ISI changes on the amplitudes of both the N140 and the P300 do not directly reflect the recovery time of psychological functions but may also involve the complicated effects derived from the habituation of the individual generators and the difficulty for performing the task. Moreover, it is indicated that the stimulus intensity used is one of major factors to be considered for comparing results to discuss the effects of ISI and probability on the ERPs.     

Effects of frontal lesions on a selective attention task

We set out to test the hypothesis that patients with frontal damage are specifically disabled in carrying out tasks requiring a high level of controlled attention. A group of patients with frontal lesions and another group of patients with retrorolandic lesions were tested for selective attention on a computerized task designed to produce a conflict situation between automatic and controlled processes. Frontal patients proved to be significantly more prone to errors of commission (false alarms) than retrorolandic patients.

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Sommario Il presente lavoro si propone di verificare l'ipotesi di una specifica perturbazione di soggetti con lesioni frontali nella esecuzione di compiti richiedenti un livello elevato di controllo attentivo volontario. Due gruppi di pazienti, rispettivamente con lesioni frontali e retrorolandiche, sono stati sottoposti ad un esperimento di attenzione selettiva assistito dal computer, costruito in modo da realizzare una situazione conflittuale tra processi di tipo automatico e volontario. I risultati hanno evidenziato una significativa tendenza dei soggetti con lesione frontale a commettere un maggiore numero di errori di commissione (falsi allarmi) rispetto ai soggetti con lesione retrorolandica.

     

Effect of interstimulus interval on attentional modulation of cortical activities in human somatosensory areas.

OBJECTIVE: To investigate the effect of interstimulus interval (ISI) on attentional enhancement of cortical activity in human somatosensory cortices. METHODS: Somatosensory magnetic fields (SEFs) evoked by electrical stimulation of the right median nerve at ISIs of 0.5, 1, 3 and 5 s were recorded in two separate conditions by using whole head type magnetoencephalography. In the attend-condition: subjects directed attention to the stimuli by watching the stimulated body site and mentally counting the stimuli. In the ignore-condition: subjects ignored the stimuli by reading a book. RESULTS: Attention to the tactile stimulation amplified SEFs in contralateral secondary (SIT) at ISIs of 1, 3 and 5 s and in ipsilateral SII at ISI of 5 s. There was no effect of attention on the SEF in primary. The ratios of the equivalent current dipole moments calculated for the attend-condition compared to ignore-condition were 1.49 +/- 0.14 at ISI of 1 s, 1.47 +/- 0.29 at 3 s and 1.34 +/- 0.23 at 5 s in the contralateral SII and 1.40 +/- 0.24 at 5 s in the ipsilateral SII. CONCLUSIONS: Time longer than 0.5 s might be required for active attention genesis and, after it was generated, the cortical activation in the attend condition was proportional to the activity in the ignore condition.     

Passive enhancement of the somatosensory P100 and N140 in an active attention task using deviant alone condition.

OBJECTIVE: We investigated the changes in the somatosensory P100 and N140 during passive (reading) versus active tasks (counting, button pressing) and oddball (target=20%, standard=80%) versus deviant alone conditions (standards were omitted). METHODS: Nine healthy subjects performed the 3 tasks (reading, counting and button pressing) under two conditions. Standard and target electrical stimuli were presented in a random order to the index or middle fingers of the left hand at a constant 800 ms interstimulus interval in the oddball conditions. In the deviant alone conditions, only target stimuli were presented with the same timing as in the oddball conditions. RESULTS: The N140 amplitude increased for the deviant alone stimuli compared with the oddball standard and target stimuli regardless of whether the task was passive or active, indicating passive shifts of attention related to temporal infrequency. The P100 amplitude also increased for the deviant alone stimuli compared with the oddball standard and target stimuli in both passive and active tasks, but the enhancement seemed to be even smaller than that of the N140 amplitude. CONCLUSIONS: The somatosensory N140 passively increased even if subjects tried to attend actively to the stimulus source when the deviant alone condition was used. This change in N140 amplitude may be related to a strong orienting effect against a 'silent' background. SIGNIFICANCE: The present study provided evidence that the N140 is an indicator of passive attention against a silent background when the deviant alone condition or long interstimulus interval was used.     

Event-related potentials during a selective attention task with short interstimulus intervals in patients with schizophrenia.

OBJECTIVE: To study selective attention in schizophrenia by examining event-related potentials during a dichotic listening task with short interstimulus intervals (ISIs). DESIGN: Prospective study. PARTICIPANTS: Twelve patients with schizophrenia in remission and 12 age-matched controls with no history of psychiatric or neurological illness. INTERVENTIONS: Participants were asked to push a button in response to target stimuli in either ear. OUTCOME MEASURES: Reaction time, correct response rate and results of electroencephalography recorded at 3 regions: mean segmental amplitudes between 0 and 200 ms and between 200 and 400 ms after stimuli and processing negativity (Nd), measured by the negative area during these periods. RESULTS: Distinct slow-positive potentials for unattended stimuli, which were elicited in a task with long ISIs in a previous report, did not emerge in either group in this study. Although the 2 groups did not differ significantly in terms of Nd area, in the controls the mean segmental amplitude for attended standard stimuli was significantly greater than that in the patients with schizophrenia. CONCLUSIONS: Impairment of selective attention in patients with schizophrenia is related to a lack of ability to focus attention on attended task-relevant stimuli during a selective attention task with short ISI.     

Relationships between attention effects and intensity effects on the cognitive N140 and P300 components of somatosensory ERPs.

OBJECTIVES: This study attempts to elucidate the relative contributions of exogenous and endogenous components to the N140 and P300 potentials elicited by somatosensory stimulation. METHODS: Somatosensory event-related potentials (ERPs) were evoked using an odd-ball paradigm with the frequent (80%) stimuli delivered to the left index finger and the infrequent (20%) stimuli delivered to the right index finger. Both types of stimuli had the same intensity within each experiment. The experiment was repeated using 6 different stimulus intensities ranging from the sensory threshold to 3 times the threshold. Each experiment was done under two conditions. In one, the subjects were asked to count and respond to the infrequent stimuli. In the other, the subjects were instructed to ignore the stimuli whether frequent or infrequent. In addition, the compound sensory potential of the right median nerve was separately recorded from electrodes at the wrist using the same range of stimulus intensities applied to the right index finger. RESULTS: Amplitudes of the N140 and P300 elicited by both attended and unattended infrequent stimuli increased in a parallel fashion as a function of stimulus intensity, so that the amplitude difference between attended and unattended responses was independent of the stimulus intensity. The amplitude of the compound sensory nerve potential at the wrist exhibited a similar slope to those of the N140 and P300. CONCLUSIONS: Thus, it is concluded that the scalp N140 and P300 consist of two components: an endogenous component, which is independent of the stimulus intensity, and an exogenous component, which increases as a function of stimulus intensity. The relative contribution of these components to the N140 and P300 amplitudes is also discussed.     

Inhibitory effect of intensity and interstimulus interval of conditioning stimuli on somatosensory evoked magnetic fields

Magnetoencephalography (MEG) recordings were performed to investigate the inhibitory effects of conditioning stimuli with various types of interstimulus intervals (ISIs) or intensities on somatosensory evoked magnetic fields (SEFs) using a 306‐ch whole‐head MEG system. Twenty‐three healthy volunteers participated in this study. Electrical stimuli were applied to the right median nerve at the wrist. Six pulse trains with ISIs of 500 ms were presented in Experiment 1. A paired‐pulse paradigm with three kinds of conditioning stimulus (CON) intensities, 500 ms before the test stimulus (TS), was applied in Experiment 2. Finally, three CONs 500 or 1000 ms before TS were presented in Experiment 3. Three main SEF deflections (N20m, P35m, and P60m) were observed, and the source activities of P35m and P60m significantly decreased after the 2nd pulse of a six pulse trains. These source activities also significantly decreased with increasing intensity of CON. In addition, these attenuations of source activities were affected by CON–CON or CON–TS intervals. These results indicated that the source activities were modulated by the intensity and ISIs of CONs. Furthermore, P35m after the stimulation were very sensitive to CONs; however, the attenuation of P60m after the stimulation lasted for a longer period than that of P35m. Our findings suggest that the conditioning stimulation had inhibitory effects on subsequent evoked cortical responses for more than 500 ms. Our results also provide important clues about the nature of short‐latency somatosensory responses in human studies.     

Differential modulation of the short- and long-latency somatosensory evoked potentials in a forewarned reaction time task.

OBJECTIVE: We investigated modulation of the short- and long-latency somatosensory evoked potentials (SEPs) in a forewarned reaction time task. METHODS: A pair of warning (auditory) and imperative stimuli (somatosensory) was presented with a 2 s interstimulus interval. In movement condition, subjects responded by grip movement with the ipsilateral hand to the somatosensory stimulation when the imperative stimulus was presented. In counting condition, they silently counted the number of imperative stimuli. The SEPs in response to the imperative stimuli were recorded. RESULTS: Frontal N30 and central N60 amplitudes were significantly smaller in the movement than in the counting or rest conditions. None of the short-latency components differed between the counting and rest conditions. In contrast to the short-latency components, P80 was significantly larger in the counting than in the rest condition, and showed a further increase from the counting to the movement condition. The N140 amplitude was significantly larger in the movement than the rest condition, but was not changed between the counting and the rest conditions. CONCLUSIONS: The attenuation of the frontal N30 and central N60, and the enhancement of the P80 and possibly the N140 resulted from the centrifugal mechanism. The present findings may show the different effects of voluntary movement on the early and subsequent cortical processing of the relevant somatosensory information requiring a behavioral response. SIGNIFICANCE: The present study demonstrated the differential modulation of short- and long-latency components of SEPs in a forewarned reaction time task.     

The effects of interstimulus interval on event-related indices of attention: an auditory selective attention test of perceptual load theory.

OBJECTIVE: We examined the impact of perceptual load by manipulating interstimulus interval (ISI) in two auditory selective attention studies that varied in the difficulty of the target discrimination. METHODS: In the paradigm, channels were separated by frequency and target/deviant tones were softer in intensity. Three ISI conditions were presented: fast (300ms), medium (600ms) and slow (900ms). Behavioral (accuracy and RT) and electrophysiological measures (Nd, P3b) were observed. RESULTS: In both studies, participants evidenced poorer accuracy during the fast ISI condition than the slow suggesting that ISI impacted task difficulty. However, none of the three measures of processing examined, Nd amplitude, P3b amplitude elicited by unattended deviant stimuli, or false alarms to unattended deviants, were impacted by ISI in the manner predicted by perceptual load theory. CONCLUSIONS: The prediction based on perceptual load theory, that there would be more processing of irrelevant stimuli under conditions of low as compared to high perceptual load, was not supported in these auditory studies. SIGNIFICANCE: Task difficulty/perceptual load impacts the processing of irrelevant stimuli in the auditory modality differently than predicted by perceptual load theory, and perhaps differently than in the visual modality.     

Second somatosensory area (SII) plays a significant role in selective somatosensory attention

In order to explore human cortical areas involved in active attention toward a somatosensory modality, somatosensory evoked cortical magnetic fields were recorded in ten healthy adults with a 122-channel whole-head magnetometer while the subjects performed the selective attention task. Two kinds of stimulus modality, somatosensory and auditory, were presented independently in the same session. For the somatosensory modality, a randomized sequence of strong (P=0.45) and weak (P=0.05) electric stimuli was delivered to the right median nerve at the wrist. For the auditory modality, a randomized sequence of 900-Hz (P=0.45) and 950-Hz (P=0.05) tones was delivered to both ears. Subjects were requested to pay attention to the specified stimulus modality (either somatosensory or auditory) and to count the number of rare stimuli of the attended modality (weak stimuli in the somatosensory or 950-Hz tone in the auditory modality). A total of 12 sessions were performed for each subject, among which the order of attended modality was changed alternately and counterbalanced among subjects. In the data analysis, somatosensory evoked fields for frequent stimuli (strong electric stimuli) were compared between the two conditions; attend somatosensory condition (ATS) and attend auditory condition (non-attend somatosensory condition; NATS). In six out of the ten subjects, somatosensory evoked fields showed attention-related change. The magnitude of the estimated generator source in SII, but not in SI, significantly increased from NATS to ATS while keeping the same locations. Moreover, a simulation study using the estimated sources in SII in NATS supported the enhancement of the activity in the SII rather than participation of additional sources in the selective attention task. These results suggest that the SII plays a main role in selective somatosensory attention.     

Interstimulus interval effect on event-related potential N270 in a color matching task.

Event-related brain potentials were recorded in a matching task, in which subjects were asked to discriminate if the color of the second stimulus (S2) was the same as the first stimulus (S1). The interstimulus interval between the two stimuli of a pair was of three levels (150 ms, 500 ms and 1000 ms). A negative component about 270 ms after the presentation of S2 was elicited when the color of the two stimuli was not identical for the interstimulus interval of 500 ms and 1000 ms, but not for the interval of 150 ms. This may suggest that N270 represented the response of the brain to conflicting information between different cortical levels.     

Attentional modulation of the human somatosensory evoked potential in a trial-by-trial spatial cueing and sustained spatial attention task measured with high density 128 channels EEG

We investigated the modulation of the somatosensory evoked potential (SEP) elicited by mechanical stimuli in a spatial sustained attention and a spatial trial-by-trial cueing design by means of high density electrode array EEG recordings. Subjects were instructed to detect rare tactile target stimuli at the to-be-attended hand while ignoring stimuli at the other hand. Analysis of the SEP revealed a highly complex pattern of results. The P50 component was significantly increased for attended stimuli in the sustained attention as opposed to the trial-by-trial cueing condition. However, no difference in amplitude was found for attended as opposed to unattended stimuli. High density electrode array recordings revealed a centero-frontal N140 component (N140c), which preceded the parietal N140 (N140p) by about 20 ms. The N140c exhibited an attention effect in particular in the trial-by-trial spatial cueing condition. The N140p was significantly enlarged with attention across both experimental conditions, but a closer inspection demonstrated that this was mainly due to the great attention effect in the trial-by-trial spatial cueing condition. The late positive component (190-380 ms after stimulus onset) exhibited a significant attention effect in both experimental conditions. The present experiment provides evidence that the attentional modulation of the SEP is different when tactile as opposed to electrical stimuli were used and when only somatosensory stimuli are presented with no further sensory stimulation in other modalities. Furthermore, transient as opposed to sustained spatial attention affected various components of the SEP in a different way.     

Selective temporal attention enhances the temporal resolution of visual perception: Evidence from a temporal order judgment task

We investigated whether attending to a particular point in time affects temporal resolution in a task in which participants judged which of two visual stimuli had been presented first. The results showed that temporal resolution can be improved by attending to the relevant moment as indicated by the temporal cue. This novel finding is discussed in terms of the differential effects of spatial and temporal attention on temporal resolution.     

Differential effect of side of temporal lobe epilepsy on lateralization of hippocampal, temporolateral, and inferior frontal activation patterns during a verbal episodic memory task

The encoding of verbal stimuli elicits left-lateralized activation patterns within the medial temporal lobes in healthy adults. In our study, patients with left- and right-sided temporal lobe epilepsy (LTLE, RTLE) were investigated during the encoding and retrieval of word-pair associates using functional magnetic resonance imaging. Functional asymmetry of activation patterns in hippocampal, inferior frontal, and temporolateral neocortical areas associated with language functions was analyzed. Hippocampal activation patterns in patients with LTLE were more right-lateralized than those in patients with RTLE (P<0.05). There were no group differences with respect to lateralization in frontal or temporolateral regions of interest (ROIs). For both groups, frontal cortical activation patterns were significantly more left-lateralized than hippocampal patterns (P<0.05). For patients with LTLE, there was a strong trend toward a difference in functional asymmetry between the temporolateral and hippocampal ROIs (P=0.059). A graded effect of epileptic activity on laterality of the different regional activation patterns is discussed.     

Effects of variations in interstimulus interval on activation-flow coupling response and somatosensory evoked potentials with forepaw stimulation in the rat.

In functional neuroimaging studies, the hemodynamic response to functional activation is used as a surrogate marker for neuronal activity, typically in response to task paradigms that use periodic stimuli. With use of a model system of electrical forepaw stimulation in rats (n = 14) with laser-Doppler (LD) monitoring of cerebral blood flow (CBF) changes in the somatosensory cortex, the effects of variations in the interstimulus interval (ISI) on the hemodynamic response to periodic stimuli were examined. A characteristic peak flow response was seen for 4-second stimuli and a peak and plateau response were seen for all 8-second stimuli regardless of ISI. However, both the amplitude of the LD(CBF) response and the integrated response were significantly reduced for shorter ISIs, whereas the baseline flow was not altered. Somatosensory evoked potential responses were also recorded in some rats (n = 8) and remained unchanged for the various ISIs for a particular stimulus duration. These results suggest that the decrease in the LD(CBF) responses observed with shorter ISIs likely represents a refractoriness of the hemodynamic response and not neuronal function. These results may have important implications for the optimization and interpretation of functional activation paradigms that use periodic stimuli.     

Task-relevant selective modulation of somatosensory afferent paths from the lower limb

Leg movement attenuates initial somatosensory evoked potentials (SEPS) from both cutaneous and muscle afferent origin. To date, as different sensory inputs become relevant for task performance, selective facilitation from such movement-related gating influences has not been shown. We hypothesized that initial SEP amplitudes from cutaneous (sural nerve, SN) and muscle afferent (tibial nerve, TN) sources are dependent on the relevance of the specific afferent information to task performance. SEPs were obtained at rest and during three movement conditions. In each movement condition, the left foot was passively moved episodically and additional cutaneous 'codes' of sensory information were applied to the dorsum of the left foot. Subjects were instructed to: simply relax (passive), or to make a response following the cessation of movement, dependent either on the cutaneous code (cutaneous task), or the passive movement trajectory of the left foot (position task). Passive movement, with no required subsequent response, attenuated initial TN and SN SEPs to approximately 40% of that at rest (p < 0.05). Versus passive movement, when cutaneous inputs provided the relevant cue for the task, mean SN SEPs significantly increased (p < 0.05), and when the proprioceptive inputs provided the relevant cue for the task, mean TN SEPs significantly increased (p < 0.05). We conclude that specific relevancy of sensory information selectively facilitates somatosensory paths from movement-related attenuation.     

Habituation of the auditory evoked potential in a short interstimulus interval paradigm

The present experiment was carried out to investigate elicitation and habituation of the auditory event related potentials with stimulus trains utilizing a short interstimulus intervals (ISI) of 1500 ms. Scalp event related potentials elicited by auditory stimuli were recorded in 10 male subjects. Thirty auditory stimuli were presented binaurally over headphones to every subject with a duration of 1000 ms, each with a constant ISI of 1500 ms. No task relevance was given to the stimuli. Wave-forms were collected using a Pentium 100 computer. All analyses were carried out over the 30 trials. In each single trial event related potentials, latencies and amplitudes of N1-P2 components were analyzed separately for four frequency bands (0.3-70, 0.3-4, 4-7, 7-13 Hz). Trend effects were tested with linear-regression analyses (N1-P2 amplitude x stimuli number). We found that the amplitude from the first stimulus decreased reliably across trial blocks of the N1-P2 components and that it was directly affected by ISI. The relevance of these results for the habituation is discussed.     

Mapping early somatosensory evoked potentials in selective attention: critical evaluation of control conditions used for titrating by difference the cognitive P30, P40, P100 and N140

Detailed procedures are described for the study of somatosensory event-related potentials (ERPs) to electric stimulation of fingers. Control responses to homogeneous (100%) series of identical stimuli (thus eliminating input mismatch) while the subject reads a novel (thus providing a distinct attention-capturing activity and maintaining vigilance level) are validated as reflecting the exogenous obligatory profiles required for assessing cognitive component in ERPs to target relevant stimuli. With these 'neutral' conditions, the control responses have a similar profile even at larger ISIs such as those separating the infrequent targets in Attention runs. Conversely, series of stimuli identical to those in control runs can elicit cognitive components in a 'Lie' experiment when the subject is induced to treat the stimuli like targets even though there is no discrimination involved. On this basis, the somatosensory P30, P40, P100 and N140 components appearing in the target profiles are considered genuine cognitive components. They have been analyzed with scatter displays, electronic subtraction, bit-mapped displays and with calculation of Z and dilation factors. The cognitive P30 and P40 reflect selective attention-related enhancements of the neural generators in receiving somatosensory cortex. The early parietal positivity P27 can thus be modulated separately from the frontal N30 component and is thought to be generated by a radial dipole in area 1. The later cognitive P100 and N140 reflect the invocation of distinct processors in conjunction with the behavioral use of the sensory input. The evolving topographical patterns of the P100 and N140 electrogeneses, revealed by bit-mapped data, suggest complex interactions between posterior parietal and prefrontal cortex whereby the sensory information is placed into spatial coordinate systems and matched with representations of relevant objects or relationships in space for target processing in the sequential tasks.