Changes in the somatosensory N250 and P300 by the variation of reaction time

We investigated the relationship between somatosensory event-related potentials (ERP) and the variation of reaction time (RT). For this purpose, we recorded the ERPs (N250 and P300) in fast- and slow-reaction trials during a somatosensory discrimination task. Strong, standard, and weak target electrical stimuli were randomly delivered to the left median nerve at the wrist with a random interstimulus interval (900–1,100 ms). All the subjects were instructed to respond by pressing a button with their right thumb as fast as possible whenever a target stimulus was presented. We divided all the trials into fast- and slow-RT trials and averaged the data. N250 latency tended to be delayed when the RT was slow, but not significantly. P300 latency was delayed significantly when the RT was slow, but to a much lesser extent than the RT delay, so we concluded that the change of RT was not fully determined by the processes reflected by the somatosensory N250 or P300. Furthermore, the larger and earlier P300 in the fast-RT trials implied that when larger amounts of attentional resources were allocated to a given task, the speed of stimulus evaluation somewhat increased and RT was shortened to a great extent. N250 amplitude did not significantly vary in the two RT clusters. In conclusion, the somatosensory N250 reflects active target detection, which is relatively independent of the modulation of the response speed, whereas the somatosensory P300 could change without manipulation of either the stimulus or the response processing demand. Electronic Publication     

The effect of warning stimulus modality on blink startle modification in reaction time tasks

The present study investigated the effects of lead stimulus modality on modification of the acoustic startle reflex during three reaction time tasks. In Experiment 1, participants (N = 48) were required to press a button at the offset of one stimulus (task relevant) and to ignore presentations of a second (task irrelevant). Two tones that differed in pitch or two lights served as signal stimuli. Blink startle was elicited during some of the stimuli and during interstimulus intervals. Skin conductance responses were larger during task-relevant stimuli in both groups. Larger blink facilitation during task-relevant stimuli was found only in the group presented with auditory stimuli, whereas larger blink latency shortening during task-relevant stimuli was found in both groups. Experiment 2 (N = 32) used only a task-relevant stimulus. Blink magnitude facilitation was significant only in the group presented with tones, whereas blink latency shortening was significant in both groups. Experiment 3 (N = 80) used a go/nogo task that required participants to press a button if one element of a compound stimulus ended before the second, but not if the asynchrony was reversed. The offset asynchrony was varied between groups as a manipulation of task difficulty. Startle magnitude facilitation was larger during acoustic than during visual stimuli and larger in the easy condition. The present data indicate that startle facilitation in a reaction time task is affected by stimulus modality and by task demands. The effects of the task demands seem to be independent of lead stimulus modality.     

Detection of EMG onset in ERP research

Many researchers have used off-line techniques for the automatic detection of electromyogram (EMG) onset. However, very little is known about the accuracy of these methods. In the present study, five such methods are evaluated and their accuracy is reported. Five subjects were asked to produce fast (ballistic) and slow (ramp) contractions with thumb and index finger of the right hand in a simple reaction time task. EMG was recorded from the first dorsal interosseus muscle, and onsets were visually determined in the raw EMG. These onsets were compared with the onsets produced by the automated methods on the rectified and low-pass filtered EMG. Four of the automated methods produced very reliable estimates of the visually determined onsets, at least when additional constraints upon the initial estimates were made. Studies using automated methods for EMG onset detection should report findings about their accuracy.     

Visuospatial attention and motor reaction in children: an electrophysiological study of the "Posner" paradigm

To assess the processing stages involved in attention shifting and response selection tasks in children, we recorded event-related potentials (ERPs) and performance measures during a variant of the Posner paradigm. Subjects responded to visual targets, either preceded by a spatial cue (valid = same side; invalid = opposite side) or presented uncued. Valid targets evoked high-amplitude P1 responses, single-peaked P3s, and the shortest reaction times (RTs). Invalidity cued stimuli evoked delayed RTs, resulting in part from incorrectly oriented attention (decreased P1) leading to delayed target processing (belated N2-P3). Invalid targets also evoked a positive slow wave attributed to prolonged response selection due to cue/target incompatibility. Uncued stimuli elicited the longest RTs, unexplained by deficits in target detection or response selection, which likely resulted from a deficit in motor preparation due to the lack of warning signal. This method may be applied in clinical settings to disentangle selective processing deficits in target detection, response selection, or motor preparation stages.     

Corticomotor excitability during a choice-hand reaction time task

Fourteen neurologically healthy, right-handed subjects performed a choice-hand reaction time (RT) task, which involved wrist flexion or extension of either the left or right hand to one of three fixed target locations corresponding to 45° flexion, 20° flexion, or 20° extension from the starting position. In each trial, a pre-cue provided information regarding the forthcoming target location. The hand was specified by the imperative signal. Focal transcranial magnetic stimulation (TMS) was delivered over the hand motor area of either the right or left hemisphere at set times during the foreperiod, and at random intervals during the RT interval defined by electromyography onset. As expected, an increase in corticomotor excitability was observed in the agonist of the responding hand over the RT interval. When the cue appeared at a location that required flexion with either hand, an increase in excitability was observed following stimulation over the hemisphere ipsilateral to the responding hand, indicating activation of the homologous muscle. However, when the cue appeared at a location at which the response would require flexion with one hand and extension with the other, the modulation of excitability was also based on the direction of the response. This direction-specific effect was only observed for TMS delivered to the left hemisphere during the left-hand movement, and suggested goal-based preparation in the left hemisphere independent of whether the actual movement is made with the right or left hand. These results indicate that both the homologous-muscle and the directional-specific constraints affect the corticomotor excitability of the non-responding hand.     

Effects of task complexity in young and old adults: reaction time and P300 latency are not always dissociated

Twelve young and 11 elderly men (mean ages 21.1 and 70.1) performed a choice-reaction time (RT) task in which stimulus degradation and stimulus-response (S-R) compatibility were manipulated. The extant literature has suggested that the effects of age on RT are usually augmented (multiplicative) in more difficult task conditions, but also that the effects of age on the latency of the P300 component of the event-related brain potential (ERP) are constant (additive). The results indicated that the effects of age on RT were enhanced in more difficult conditions, whether the difficulty consisted of stimulus degradation or S-R incompatibility. However, the effects of age on P300 latency were enlarged as the stimuli were degraded, but not if the S-R mapping was incompatible. Thus, it appears that task content determines if effects of age on P300 latency are additive or multiplicative. A simple model is proposed that produces the obtained pattern of effects.     

Toward a chronopsychophysiology of mental rotation

In a parity judgment task, the ERPs at parietal electrode sites become the more negative the more mental rotation has to be executed. In two experiments, it was investigated whether a temporal relationship exists between the onset of this amplitude modulation and the moment when mental rotation is executed. Therefore, the duration of processing stages located before mental rotation was manipulated. The amplitude modulation was delayed when either the perceptual quality of the stimulus was reduced (Experiment 1) or when character discrimination was more difficult (Experiment 2). The results suggest that the onset of the rotation-related negativity might be used as a chronopsychophysiological marker for the onset of the cognitive process of mental rotation.     

Electrophysiological correlates of local muscular fatigue effects upon human visual reaction time

Summary A study of the neurophysiological basis of reaction time change was undertaken as a means of exploring the physiological mechanisms of local muscular fatigue effects upon sensorimotor performance. The identification of electrophysiological indices of central and peripheral processes within the human electroencephalogram and electromyogram, enabled a fractionation of total reaction time into component latencies measuring sensory reception time, sensorimotor integration time, central motor outflow time and peripheral motor contraction time. Simple foot dorsiflexion reactions to visual stimuli were observed in 18 male college students. Foot responses under one condition were performed against a resistance which necessitated a moderate degree of muscular tension before movement could occur while a second condition required normal unresisted responses. Two intensities of serial isometric work resulting in the order of a 38% decrement in maximum voluntary contractile capability (MVC) were performed by each subject. While the rate and extent of MVC decrement varied with the inherent strength of the subject and the intensity of the exercise performed, unconditional changes were observed in the spatiotemporal dimensions of reaction time performance following exercise-induced fatigue. The quality of total reaction time was found to deteriorate, particularly when responses were resisted. Peripheral deficiencies, suggestive of a decreased rate of tension development, were evidenced by a marked elongation of resisted motor times, and less vigorous and extensive unresisted responses. Insofar as the energy of response electromyograms was also diminished, central mechanisms were implicated, possibly due to a shift in motor unit recruitment. Concomitant changes were also observed in central processing.This research project was undertaken in the Motor Integration Research Laboratory of the Department of Exercise Science at the University of Massachusetts     

Simple apparatus for generating reaction time histograms

Human reaction times (RT) to sensory stimuli show variations that cannot be explained by peripheral or central neural processes. The statistical distribution of RT's for a given stimulus and differences in RT distributions for different stimuli seem to have received little attention. This report describes a simple apparatus for generating RT histograms while a subject responds to various stimuli. The system has been found useful not only for the on-line generation of RT histograms with light and sound stimuli but also for studying simultaneously the time course of cortical potentials evoked by these stimuli.     

Gustatory reaction time in human adults

Reaction times to varying concentrations of the four basic taste stimuli (sucrose, NaCl, tartaric acid and quinine-HCl) were measured in twenty human adults. Reaction times decreased with increasing concentrations of solutions, and the relationships between the reaction time and the concentration are described by a rectangular hyperbola for all the four tastes. The reaction times to the strongest solutions used in this experiment were about 400 msec for NaCl and tartaric acid and about 700 msec for sucrose and quinine solutions. Since the reaction time to flow of water on the tongue surface was around 200 msec, we concluded that the minimum taste encoding time was 200 msec or less for the high concentrations of salt and acid solutions.     

Postperceptual effects and P300 latency

P300 latency is commonly thought to provide a chronometric index of the duration of perceptual processing. Because the evidence in favor of this assumption is controversial, we examined whether P300 latency is influenced by perceptual processes, response selection, and by motoric processes in two experiments using a two-choice spatial stimulus-response compatibility (SRC) task. Both experiments revealed additive effects of perceptual difficulty with spatial SRC in reaction time and P300 latency. In addition, Experiment 2 showed that P300 latency measured in average waveforms is insensitive to motoric processes. The influence of spatial SRC on P300 latency disagrees with the view that P300 latency is sensitive only to stimulus evaluation processes. However, P300 latency may be used to discriminate between influences on premotoric and motoric processing stages. A response conflict account for the SRC effect on P300 latency is suggested.     

The attentional mechanism of temporal orienting: determinants and attributes

A review of traditional research on preparation and foreperiod has identified strategic (endogenous) and automatic (exogenous) factors probably involved in endogenous temporal-orienting experiments, such as the type of task, the way by which temporal expectancy is manipulated, the probability of target occurrence and automatic sequential effects, yet their combined impact had not been investigated. These factors were manipulated within the same temporal-orienting procedure, in which a temporal cue indicated that the target could appear after an interval of either 400 or 1,400 ms. We observed faster reaction times for validly versus invalidly cued targets, that is, endogenous temporal-orienting effects. The main results were that the probability of target occurrence (catch-trial proportion) modulated temporal orienting, such that the attentional effects at the short interval were independent of catch trials, whereas at the long interval the effects were only observed when catch trials were present. In contrast, the interval duration of the previous trial (i.e., exogenous sequential effects) did not influence endogenous temporal orienting. A flexible and endogenous mechanism of attentional orienting in time can account for these results. Despite the contribution of other factors, the use of predictive temporal cues was sufficient to yield attentional facilitation based on temporal expectancy.

Specific and non-specific effects of transcranial magnetic stimulation on simple and go/no-go reaction time

The effects of subthreshold transcranial magnetic stimulation (TMS) on simple and go/no-go reaction time (RT) tasks were studied in seven healthy volunteers. Subjects were asked to respond by abducting the thumb in a warning-imperative signal paradigm. TMS was randomly delivered at variable delays to the imperative signal (IS). Simple RT was significantly shortened when TMS was delivered to the left motor cortex and parietal regions simultaneously with IS. In the go/no-go paradigm, a similar trend to shorter RT was seen at a delay of 0 ms. Additionally, a significant shortening was observed at a delay of 90 ms with TMS over the contralateral motor cortex only. Movement-related potentials (MRPs) in the two paradigms showed a predominantly contralateral negativity approximately 80 ms preceding EMG onset. Our findings support the existence of two differentiated effects of TMS on RT: (1) one non-specific effect, evidenced in both the simple and go/no-go paradigms at a 0 ms delay, which can be at least partially explained by intersensory facilitation; and (2) a motor-specific effect of TMS, unveiled in the go/no-go paradigm at a 90 ms delay. Received: 25 August 1998 / Accepted: 24 March 1999     

Early processing stages are modulated when auditory stimuli are presented at an attended moment in time: an event-related potential study

The present study investigated with event-related potentials whether attending to a moment in time modulates the processing of auditory stimuli at a similar early, perceptual level as attending to a location in space. The participants listened to short (600 ms) and long (1,200 ms) intervals marked by white noise bursts. The task was to attend in alternating runs either to the short or to the long intervals and to respond to rare offset markers that differed in intensity from the frequent standard offset markers. Prior to the to-be-attended moment, a slow negative potential developed over the frontal scalp. Stimuli presented at the attended compared to the unattended moments in time elicited an enhanced N1 and an enhanced posteriorly distributed positivity (300-370 ms). The results show that attention can be flexibly controlled in time and that not only late but also early perceptual processing stages are modulated by attending to a moment in time.     

Stimulus presentation rate dissociates sequential effects in event-related potentials and reaction times

The present study explored the impact of the stimulus presentation rate on sequential effects in event-related potentials (ERPs) and reaction times (RTs). Random series of equiprobable tones were presented at interstimulus intervals (ISIs) of 1.3, 2.1, and 2.9s. Fast and accurate choice responses to the tones were required. Although sequential effects in RTs were stable across all ISIs, the common first-order repetition effect in P300 amplitude was only observed at the 1.3-s ISI and not at the slower presentation rates. This dissociation between the first-order effects in RTs and ERPs speaks against an explanation of both effects by a common expectancy mechanism. In addition, sequential effects were observed as early as about 100 ms after stimulus onset in the lateralized readiness potential. Together with similar sequential effects in P300 latency, this finding supports a continuous flow model of information processing.     

The structure of motor programming: Evidence from reaction times and lateralized readiness potentials

There is a widely accepted notion that movement elements are assembled prior to movement execution in a central motor programming stage. However, it is not clear how this stage is structured—whether it is a unitary stage allowing different motor parameters to cross talk or whether there are several independent processes dealing with each motor parameter. We addressed this question by orthogonally manipulating two movement‐related factors: response sequence complexity and movement duration. Both factors yielded main effects on reaction time but no interaction. Additive effects of both factors on the onsets of response‐ but not stimulus‐synchronized lateralized readiness potentials suggest separable motoric loci of sequence complexity and duration. These findings are at variance with the notion of a unitary movement programming stage.     

Effects of aging on event-related brain potentials and reaction times in an auditory oddball task

Auditory event-related potentials (ERPs) were recorded from 71 healthy individuals between 18 and 82 years of age during performance of a disjunctive reaction time task in an auditory oddball paradigm. The effects of aging on reaction times and on the latencies, amplitudes, and distributions of each of the main ERP components were examined. No significant slowing of the reaction times of the elderly subjects was observed in relation to the younger ones. The peak latencies of both the N1 and P2 components elicited by standard tones were slightly but significantly slowed with age. In the ERPs of target tones, the later, endogenous components (N2, P3, and SW) showed linear increases in latency as a function of age; the later the component, the longer the age-related delay. In general, aging was associated with less negativity (both N2 and SW) and more positivity (P3) over the anterior scalp, together with a smaller P3 and a more pronounced N2 over posterior scalp areas. Most of the effects observed in target ERPs were also evident in the difference waves derived from subtraction of the standard from the target ERPs, although the slope of the age-related latency increase of N2 was shallower and that of the P3 was steeper in the difference ERPs. These findings are discussed in relation to previous accounts of ERP changes with aging.     

Electroencephalographic and reaction time asymmetries to musical chord stimuli

EEG was recorded over left and right hemispheres at temporal leads (T3, T4) referred to the vertex (Cz) in 16 right-handed male subjects. Musical chords were presented randomly in monaural sequence, during a task which required a selective motor response to stimuli presented to one ear. The integrated amplitude of the 8-13 Hz alpha rhythm was measured when subjects listened passively. Under all conditions, a lower mean alpha amplitude was recorded over the right hemisphere than the left, regardless of which ear was stimulated. Alpha suppression over the right temporal area was accentuated when the performance task directed attention to the stimulus. Reaction time to left ear stimulation was shorter than that for the right ear. With monaural stimulus presentation behavioral asymmetry, and various EEG asymmetries can be observed. There is hemispheric asymmetry associated with attention to task relevant stimuli indicated by reduction in the alpha rhythm over the right temporal area and asymmetry in reaction time with greater efficiency of the left ear to muscial chords.     

The functional significance of ERP effects during mental rotation

In a parity judgment task, the ERPs at parietal electrode sites become more negative as more mental rotation has to be executed. This article provides a review of the empirical evidence regarding this amplitude modulation. More specifically, experiments are reported that validate both the functional relationship between mental rotation and the amplitude modulation as well as the temporal relationship both in single- and in dual-task situations. Additionally, ERP effects are reported in the psychological refractory period (PRP) paradigm with mental rotation as the second task. Finally, unresolved issues are discussed that, I hope, might stimulate future research.     

Locus of the effect of temporal preparation: evidence from the lateralized readiness potential

It is well established that reaction time (RT) is shorter when a response signal is preceded by a warning signal, because the warning signal causes the participant to prepare for the upcoming response. A review of chronometric and psychophysiological studies reveals the prevailing view that this temporal preparation operates mainly at a motor level speeding up rather late processes. To assess the locus of this preparation effect, we conducted two experiments employing the lateralized readiness potential (LRP). Contrary to this prevailing view, the results of both experiments clearly indicate that temporal preparation enhances the processing speed of relatively early processes, because a manipulation of temporal uncertainty affected RT, the P300 latency, and the stimulus-to-LRP interval but not the LRP-to-keypress interval.