Preparation of response force and movement direction: Onset effects on the lateralized readiness potential

Transcription

1 Psychophysiology, 37 ~2000!, Cambridge University Press. Printed in the USA. Copyright 2000 Society for Psychophysiological Research Preparation of response force and movement direction: Onset effects on the lateralized readiness potential HILTRAUT MÜLLER-GETHMANN, GERHARD RINKENAUER, JUTTA STAHL, and ROLF ULRICH Department of General Psychology and Methodology, University of Tübingen, Germany Abstract Two experiments assessed the preparatory effects of advance information about response force and movement direction on the lateralized readiness potential ~LRP!. In a choice reaction time ~RT! task, an imperative stimulus required an isometric flexion or an extension of the left or right index finger. Prior information about response force or about movement direction reduced RT and shortened the interval from the onset of the imperative stimulus up to the onset of the LRP. Advance information, however, about direction but not about force decreased the interval from LRP onset to the onset of the overt response. The identical pattern of results was obtained in a second experiment, in which each participant performed both precue conditions. The findings of both experiments support the notion that response force is specified before movement direction. These results are consistent with the view accordingly different mechanisms are involved in the specification of muscle force and movement direction. Descriptors: Movement preprogramming, Movement precuing technique, Lateralized readiness potential, Reaction time, Movement direction, Response force Covert preparatory processes within the motor system precede overt motor acts and strongly influence the performance of these acts. Because these processes reveal basic principles of the motor system, it is hardly surprising that there is ongoing interest in unraveling them ~Requin, Brener, & Ring, 1991!. A powerful experimental task to assess preparatory processes is Rosenbaum s ~1980, 1983! precuing technique. This technique is a modified choice reaction time ~RT! task, in which a certain imperative stimulus requires a participant to perform a specific movement as quickly as possible. Before the onset of the imperative stimulus a precue provides information about one or several parameters of the forthcoming movement. For instance, the precue may indicate that a finger extension must be performed without specifying the response finger. If the preparatory processes utilize advance information, RT should be shorter with advance information than without it. Such RT shortening has been well documented in the RT literature ~cf. Requin et al., 1991; Rosenbaum, 1983!. Several recent studies have augmented the inferential power of this technique by combining it with the measurement of regional cerebral blood flow ~e.g., Deiber, Ibanez, Sadato, & Hallett, 1996!, the recording of This research was supported by a grant from the Deutsche Forschungsgemeinschaft ~SO ! to Werner Sommer and Rolf Ulrich. We appreciate the discussions with Hartmut Leuthold, Stefan Mattes, Allen Osman, and Werner Sommer. David A. Rosenbaum and three anonymous reviewers provided helpful comments on an earlier version of this article. Address reprint requests to: Hiltraut Müller-Gethmann, Department of General Psychology and Methodology, University of Tübingen, Friedrichstr. 21, Tübingen, Germany. hiltraut.mueller-gethmann@unituebingen.de. single-neuronal activity ~e.g., Riehle, MacKay, & Requin, 1994; Riehle & Requin, 1995!, transcranial magnetic stimulation ~Hasbroucq et al., 1999!, and the recording of event-related brain potentials ~e.g., De Jong, Wierda, Mulder, & Mulder, 1988; MacKay & Bonnet, 1990!. Riehle and collaborators identified neuronal mechanisms involved in movement preparation ~Riehle et al., 1994; Riehle & Requin, 1995!. In both studies monkeys performed a choice RT task. In one study advance information about movement extent and about the level of frictional force opposing the intended movement was provided ~Riehle et al., 1994!, whereas force level and movement direction were precued in the other study ~Riehle & Requin, 1995!. 1 The results provided evidence for the view that different populations of neurons are involved in the specification of force, movement extent, and movement direction. This view is corroborated by the event-related potential investigation of MacKay and Bonnet ~1990!, which showed that the amplitude of the contingent negative variation is sensitive to movement preparation. Specifically, topographical analysis indicated that the parietal cortex is involved in the specification of movement direction, whereas the primary motor cortex is involved in the specification of force. 1 In a physical sense, force is a vector, and by definition, has both magnitude and direction. Because the magnitude of force, however, can be related to muscle activity within a certain muscle group ~extensor or flexor muscles!, it makes sense to distinguish both the magnitude of force and its direction. In a physiological sense, then, the former parameter relates to the amount of muscle activity whereas the latter refers to the muscle group involved in performing the movement. In this paper the term force will relate to the magnitude of force, while the term direction will relate to the direction of the movement. 507

2 508 H. Müller-Gethmann et al. The view that the human primary motor cortex is engaged in the specification of movement force has been strengthened further by the study of Dettmers et al. ~1995!. These authors measured the relative regional cerebral blood flow while their participants flexed the right index finger with different levels of force. This measure revealed two functional populations within the cortex. The activity within one population increased with the level of peak force whereas the activity of the other population was unaffected by variations in peak force. Although the former population included the primary motor cortex, the primary somatosensory cortex, the dorsal bank of the cingulate sulcus, the ventral part of the posterior supplementary motor area, and the cerebellar vermis, the strongest correlation between regional blood flow and force peak was measured within the contralateral primary motor and somatosensory cortex. By contrast, the latter population was restricted to the contralateral putamen, the bilateral insular cortex, the second somatosensory cortex, and the dorsal part of the posterior supplementary motor area. Hence these results indicate that a forceful finger flexion is reflected in widely distributed cerebral activity that is modulated by movement force. This conclusion is consistent with the growing evidence that the neuroanatomical structure of the motor system appears to be more complex than previously assumed ~for a review see Rizolatti, Luppino & Matelli, 1998!. Chronopsychophysiology ~van der Molen, Bashore, Halliday, & Callaway, 1991! provides a complementary, more abstract approach to unravel less the neuroanatomical structure of the motor system but rather the information processing structure within this system. Several recent studies within this field ~De Jong et al., 1988; Leuthold, Sommer, & Ulrich, 1996; Osman, Moore, & Ulrich, 1995; Ulrich, Leuthold, & Sommer, 1998! combined the precuing technique with the recording of the lateralized readiness potential ~LRP!~cf. Coles, 1989!, the physiological origin of which is located within primary motor cortex ~Sasaki, Gemba, & Tsujimoto, 1990; Kristeva, Cheyne, & Deecke, 1991; Riehle & Requin, 1989!. There is strong evidence that the onset of the LRP indicates the moment in time when the response hand becomes centrally activated ~Gratton et al., 1990; Gratton, Coles, Sirevaag, Eriksen, & Donchin, 1988; Kristeva et al., 1991!. Miller and Hackley ~1992! showed that a voluntary response can be aborted after the onset of the LRP but prior to the onset of EMG. In information-processing terms, these results indicate that the emergence of the LRP indexes the onset of relatively early motoric processes and thus can be used to divide the RT into two intervals ~Osman et al., 1995!. The S-LRP interval reflects the duration of those processes from stimulus onset to the onset of the LRP. By contrast, the LRP-R interval encompasses processes intervening between the onset of the LRP and the onset of the overt response itself. Leuthold et al. ~1996! used the LRP to assess whether advance information about movement direction shortens the LRP-R interval as one might expect if preparation facilitates motoric processes. In their study, four imperative stimuli were mapped one-to-one onto four response alternatives consisting of isometric flexions or extensions of the right or left index finger. Relevant for the present issue are their conditions in which the imperative stimulus was preceded by a precue that either was non-informative or provided information about the requested movement direction ~flexion vs. extension! but not about the responding hand. RT was shorter when advance information was given. The manipulation of precue information not only affected RT but also the onset of the LRP. Consistent with the notion that advance information facilitates motoric processes, the LRP-R interval was shorter with directional advance information than without it. This result provides strong evidence for the idea that advance information about movement parameters shortens the motoric portion of RT. Precue information also shortened the S-LRP interval. This effect may emerge on at least two levels within the information processing system. First, the speed of perceptual processes may benefit from precue information, which reduces stimulus uncertainty. Second, precue information also reduces the number of response alternatives and therefore may increase the speed of response selection ~cf. Rosenbaum, 1980!. The aim of the present study was to extend the approach by Leuthold et al. ~1996!. More specifically, we investigated whether advance information about the magnitude of response force will also shorten the LRP-R and the S-LRP interval. As in the study of Leuthold et al., four imperative stimuli were mapped onto four response alternatives. In contrast to their study, these alternatives consisted of weak or strong isometric flexions with the right or left index finger. In half of the trials advance information about the required force level was provided. We hypothesized that when the level of the force output is specified after the response hand has been activated centrally, preparatory force adjustments should facilitate motor processing and thus shorten the LRP-R interval. Alternatively, if the level of response force is specified before the central activation of the response hand, the duration of this interval should be unaffected by advance information about response force. The former outcome would be expected if force and directional precues operate on the same mechanisms. By contrast, the latter outcome would further strengthen the view that different mechanisms are involved in the specification of response force and movement direction ~MacKay & Bonnet, 1990; Riehle & Requin, 1995!. EXPERIMENT 1 Experiment 1 consisted of two conditions with statistical analyses performed separately. In the force condition different required force levels were precued, whereas the direction condition served as a control condition in which movement direction was precued. Method Participants In the force condition 12 female and 8 male students ~mean age 25 years, range years! took part in one practice session lasting about 30 min and one 2.5-hr experimental session on the next day. They received either course credit or money ~10 DM per hour! for their participation. All participants had normal or correctedto-normal vision and the mean handedness score ~Oldfield, 1971! was 65 ~SD 34!. In the direction condition a fresh sample of 12 female and 8 male students ~mean age 26 years, range years! was recruited. All had normal or corrected-to-normal vision and a mean handedness score of 72 ~SD 47!. Procedure In the force condition each participant was to produce five brisk finger flexions at the beginning of the practice and of the experimental session to establish the participant s average maximal voluntary force. The practice session consisted of five blocks of 40 trials. The experimental session comprised 16 blocks with the first block considered as practice. After each block participants could take a short rest and initiate the next block when they felt ready to proceed. They were told the meaning of the precues and were instructed to use the precue information for response preparation. The digits 1, 2, 3, and 4 served as imperative stimuli and were mapped one-to-one onto the four response alternatives, which con-

3 Preparation of force and movement direction 509 sisted of a weak or strong flexion of the right or left index finger in the force condition. A criterion of 30% maximal voluntary force was defined for each participant. In the low force condition, participants were asked to produce responses with a peak force below this criterion, whereas in the high force condition the peak force had to be above this criterion. In the direction condition, maximal voluntary force was not determined. Here the response alternatives consisted either of a flexion or extension with the right or left index finger. The assignment of the stimuli to the responses was counterbalanced across participants to avoid systematic S-R mapping effects between the ordinal position of the digits and the identity of the responses. Each stimulus and each type of precue appeared equally often within a single block. The presentation order of stimuli and precue types was randomized over the trial sequence in a single block. Each trial started with the presentation of a fixation cross in the center of the computer screen. After 450 ms the fixation cross was replaced by a precue for 300 ms. This precue provided verbal advance information about the required force level ~strong or weak! or about the required direction ~up or down!, respectively, or consisted of seven plus signs when no advance information was given. After a constant foreperiod of 1,400 ms the fixation cross was replaced by the imperative stimulus for 300 ms. Then, 1,100 ms after stimulus offset, feedback was provided on the screen for 400 ms. In the case of a correct response, the word Korrekt replaced the fixation cross. Appropriate feedback was provided in the case of a wrong response ~anticipations, i.e., RTs 100 ms, misses, i.e., RTs 1,000 ms, wrong hand responses, responses with wrong force level, responses in the wrong direction, or bimanual responses!. The next trial started 2,300 ms after feedback offset with the presentation of the fixation cross. Mean RT and the percentage of response errors were computed for each block and given as feedback at the end of each block. Participants were asked to keep their eyes on the fixation cross and not to blink or move their eyes except during the feedback and intertrial period. They were also instructed to respond quickly and to avoid response errors. Stimuli and Precues The white-colored precues and stimuli were presented in the center of the screen on a blue background. The size of the fixation cross was As precues the words stark ~strong, ! or schwach ~weak, ! were displayed in the force condition. In the direction condition the words oben ~up, ! or unten ~down, ! served as precues. The noninformative precue consisted of seven plus signs ~ !. The imperative stimulus ~ ! consisted of a digit from 1 to 4. A constant viewing distance of 80 cm was ensured by a fixed chin rest. Recording of LRP, Electrooculogram (EOG), Electromyogram (EMG), and Response Force Electroencephalogram ~EEG! activity from C39 and C49 ~cf. Coles, 1989!, designating sites 4 cm to the left and right of Cz along the intramural line, and vertical EOG from above the left eye were recorded with the left mastoid as a common reference. 2 Horizontal 2 We also recorded EEG signals at Pz and Cz in each experiment of the present study. Because these data, however, are not particular relevant for assessing the present hypothesis, we will not report the corresponding results in order to save space. EOG was recorded differentially between the left and right outer canthi. EMG activity was recorded at ventral forearm sites 2.5 cm to the left and right of the point that trisected the wrist elbow distance. High-pass filters were set at 0.1 Hz for EMG and vertical EOG recordings and 0.01 Hz for all other channels. Low-pass filters were set at 30 Hz for all channels. The EMG was rectified offline. After averaging, the waveforms were smoothed by lowpass filtering ~cut-off frequency 4 Hz!. Recordings were made with Ag0AgCl electrodes and Acqua-Gel electrode electrolyte. All signals were digitized at a rate of 100 Hz for 3,000 ms starting 200 ms before precue onset. Electrode impedance was kept below 10 kv for EMG leads and below 5 kv for all other sites. For data analysis, all trials with incorrect responses or with ocular artifacts were discarded. A threshold of 70 mv in all EOG channels was chosen for artifact rejection. Because there were no systematic effects of precue information on heog activity, potential precue effects on the LRP cannot be attributed to heog artifacts. The LRP was determined as suggested by Coles ~1989! and averaging of the mean waveforms was performed either timelocked to the stimulus or to the response to compute the S-LRP and the LRP-R, respectively ~Osman & Moore, 1993!. The averaged waveforms were filtered before their onsets were estimated. The jackknife method ~Miller, Patterson, & Ulrich, 1998! was used to estimate the onset latency differences for the stimulus- and for the response-locked LRP. As recommended by Miller et al. the onset of the S-LRP ~LRP-R! was defined as the point in time when a criterion of 50% ~30%! of the amplitude was reached. The force-time function of a response was recorded in each trial by means of force-sensitive keys, which allowed for near-isometric recordings of index finger flexions and extensions. A cantilever beam ~ mm!with a minimal movement was held by an adjustable clamp at one end, while the other remained free. Strain gauges were attached near the fixed end of the leaf spring. One force key was used for each index finger. The fingertip was located in an adjustable thimble-like holder attached to the free end of the leaf spring. A response was registered as soon as the force of a flexion or an extension exceeded a criterion force of 50 cn from the baseline force level being defined as the average force during a 200-ms interval before precue presentation. Both forearms and palms rested on boards. Results Error Rates and RT In the force condition the overall percentage of errors was 4.1% comprising 1.4% misses ~RTs 1,000 ms!, 0.4% anticipations ~RTs 100 ms!, 1.0% wrong-sided keypresses, and 1.3% keypresses with an incorrect magnitude of force. Analysis of variance ~ANOVA! revealed a strong effect of precue category on RT, F~1,19! 329.2, p.001; with precue information RTs were 95 ms shorter compared with the non-informative precue condition ~mean RTs were 461 and 556 ms, respectively!. In the direction condition the overall error rate was 4.2%, which was due to 0.9% misses ~RTs 1,000 ms!, 0.8% anticipations ~RTs 100 ms!, 0.9% wrong-sided keypresses, and 1.6% keypresses in the wrong direction. The percentage of errors was similar to that of the force condition. Again a significant effect of precue category on mean RT was obtained, F~1,19! 336.4, p.001. Mean RT was 531 ms without directional advance information and decreased to 429 ms when this information was provided. The size of this effect ~102 ms! was similar to the one obtained in the force condition. A between-subject ANOVA comparing the effects between the force

4 510 H. Müller-Gethmann et al. and the direction condition revealed that RT was affected neither by condition, F~1,38! 1.87, p.18, nor by the interaction of condition with precue, F~1,38!.91, p.34. LRP Latencies The stimulus-locked LRPs for the two precue conditions of the force condition can be seen in Figure 1 ~left-hand upper panel!. The mean S-LRP latency was 267 ms with precue information about force and 340 ms without advance information; this difference of ms ~95% confidence interval! was highly significant, t~19! 6.84, p.001. This result extends the finding of Leuthold et al. ~1996!, who reported a similar reduction for precues about movement direction. The response-locked LRPs for the two precue conditions of the force condition are shown in Figure 1 ~right-hand upper panel!. Surprisingly, in both conditions LRP activity emerged at about the same point in time before the onset of the response. More specifically, the duration of the LRP-R interval was 178 ms with precue information, and 186 ms without it; this difference of ms was insignificant, t~19! 1.49, p.1. This result indicates that advance information about response force does not shorten the LRP-R interval. Hence, the finding of Leuthold et al. ~1996! that advance information about movement direction shortens the LRP-R interval does not seem to generalize to advance information about response force. Figure 1 depicts the stimulus-locked LRP ~left-hand lower panel! and the response-locked LRP ~right-hand lower panel! for the two precue conditions of the direction condition. The S-LRP was 307 ms for the non-informative precue and was reduced by ms to 248 ms for the direction precue, t~19! 7.1, p.001. In contrast to the results obtained in the force condition, the LRP-R interval was significantly shorter with precue information about movement direction ~157 ms! than without it ~182 ms!, the effect being ms, t~19! 3.6, p.01. Both results corroborate the findings obtained by Leuthold et al. ~1996!. Motor Time To assess whether advance information on movement direction affected even distal processes of the motor system, we averaged EMG time-locked to the onset of the overt response for the direction condition. The EMG-response interval, namely the motor time, was 91.3 ms when prior information on movement direction was given and 91.8 ms without this information. This difference of ms was not significant, t~19! 0.96, p 0.1. Therefore, advance information on movement direction did not shorten late processes within the motor system. Discussion The two conditions of Experiment 1 replicated the precuing effect of response force and movement direction on RT. Moreover, in both conditions the S-LRP interval was shorter when advance information was provided, which indicates an increase in speed of those processes that precede the central activation of the response Figure 1. Grand mean of the lateralized readiness potentials of Experiment 1. S onset of the imperative stimulus; R onset of the overt response. Left-hand panels: Averages are time locked to the onset of the imperative stimulus. Right-hand panels: Averages are time locked to the onset of the overt response. Upper panels: Force condition. Lower panels: Direction condition.

5 Preparation of force and movement direction 511 hand. These processes may involve early motoric stages such as those involved in response selection and perhaps perceptual stages. The precuing effect on the LRP-R interval of movement direction reported by Leuthold et al. ~1996! was also replicated. Interestingly, however, the response-locked LRP was unaffected by precue information about response force. These results imply that precue information about movement direction speeds up motoric processes, which follow the central activation of the response hand yet precede the EMG onset, as the analysis of the response-locked EMG indicates. However, precue information about response force seems not to facilitate these more distal motoric processes in the stimulus-response chain. It might be argued that force precues did not affect late preparatory stages because the required force level of the response was more difficult to specify than its direction and that therefore participants were less able to preprogram force at a later stage of movement preparation. For example, force but not direction may be regarded as a continuous movement parameter and therefore, in contrast to the specification of movement direction, participants have to select an appropriate force level in accordance with the experimentally predefined force criterion. It seems possible that this more complex selection task hampers the specification of force at a later level of movement preparation. If so, the precue effect should be smaller in the force than in the direction condition. The data, however, do not support this view because the precuing effect did not differ between both conditions. It should also be mentioned that the absence of any effect of the force precue on the LRP-R interval cannot be due to hierarchical, serial programming ~cf. Rosenbaum, 1980!. According to a hierarchical model one may assume that the direction of a movement has to be programmed before force can be specified. However, in the present experiment movement direction was always known before stimulus onset and thus could be programmed as a default parameter. Therefore, the absence of any effect of the force precue on the LRP-R interval cannot be attributed to hierarchical motor programming. According to the results of Experiment 1, precue information about movement direction and response force seems to produce differential effects on the LRP-R interval. However, in face of the insignificant effect of the force precue on the LRP-R interval, one might be tempted to argue that this null effect merely reflects a lack of statistical power that disguises a real effect. This suspicion receives some support from the fact that the LRP-R interval obtained was somewhat shorter with precue information than without it. EXPERIMENT 2 In an attempt to rule out the interpretation of insufficient statistical power, we conducted Experiment 2, in which both experimental conditions were performed by each participant. Thus, if the same pattern of results emerged again, this finding would argue against the idea that the insignificant effect of the force precue on the LRP-R interval merely reflects a lack of statistical power. Method Participants A fresh sample of 18 female and 14 male students participated ~mean age 25 years, range years!. All had normal or corrected-to-normal vision and a mean handedness score of 81 ~SD 37!. Procedure The procedure was virtually identical to that of Experiment 1. The two precue conditions ~force vs. directional precue condition! were performed successively with the order of administration counterbalanced across participants. To further increase the similarity between the two conditions, half of the participants in the direction condition always responded with high force and the other half always with low force; in the force condition half of the participants always responded with an extension and the other half always with a flexion. To avoid cross-over effects from the first to the second condition, a different set of letters served as imperative stimuli in each condition. Participants performed a practice session to familiarize themselves with the mapping of the letters to the responses. The practice session consisted of 8 blocks comprising 40 trials each. The experimental session consisted of 16 blocks; the same letters and the same mapping as in the practice session were employed. Stimuli, Precues, and Recordings Precues and recordings were the same as in Experiment 1. Uppercase letters ~ ! instead of digits were employed as imperative stimuli. For each participant eight letters were randomly selected from the following set of letters, which by inspection appear minimally confusable: A, C, E, H, L, M, P, S, T, U, and X. Four letters were used for the force condition and the remaining ones for the direction condition. Results Error Rates and RT The overall error rate was 8.8%, which was due to 2.8% misses ~RTs 1,000 ms!, 0.8% anticipations ~RTs 100 ms!, 3.2% wrong-sided keypresses, 1.5% keypresses with an incorrect magnitude of force, and 0.5% keypresses in the wrong direction. This higher rate than that found in Experiment 1 can be attributed to the more complex letter-to-response mapping in Experiment 2. A twoway ANOVA with factors precue condition ~force vs. direction! and advance information ~without vs. with! was performed on RT. There was a highly significant main effect of advance information, F~1,31! 405.1, p.001. Mean RT was 577 ms without and 471 ms with advance information. Note that the size of the precue effect ~106 ms! was similar to the one obtained in Experiment 1. There was neither a main effect of condition nor a significant interaction of the two factors, Fs 1. The respective mean RT in the force condition was 582 and 475 ms; in the direction condition it was 573 and 468 ms. LRP Latencies The LRP waveforms for the two conditions are shown in Figure 2. In both precue conditions, the S-LRP interval was again significantly shorter with advance information. In the force condition the effect was ms ~left-hand upper panel!; the mean S-LRP interval was 282 ms with precue information and 368 ms without it, t~31! 9.0, p.001. When advance information about the direction was provided, the S-LRP interval decreased by ms, from 352 ms when no prior information was provided to 285 ms when prior information was available, t~31! 6.32, p.001 ~left-hand lower panel!. As in Experiment 1, the LRP-R interval was shorter when the precue provided information about the direction of the movement ~137 ms! compared with the non-informative precue ~152 ms!; this difference of ms was again statistically reliable, t~31!

6 512 H. Müller-Gethmann et al. Figure 2. Grand mean of the lateralized readiness potentials of Experiment 2. S onset of the imperative stimulus; R onset of the overt response. Left-hand panels: Averages are time locked to the onset of the imperative stimulus. Right-hand panels: Averages are time locked to the onset of the overt response. Upper panels: Force condition. Lower panels: Direction condition. 2.2, p.05 ~right-hand lower panel!. More interestingly and consistent with Experiment 1, the LRP-R interval did not decrease with precue information in the force condition, t~31! 1.2, p.1 ~right-hand upper panel!; the corresponding intervals were 150 and 144 ms with and without advance information, respectively. The 95% confidence interval for this difference was ms. It is important to note that the precue effect of 15 ms obtained on the LRP-R interval in the direction condition was virtually unaffected by the degree of low-pass filtering. Reanalysis of the data with cut-off frequencies of 4, 8, and 12 Hz revealed an effect size of 14.7, 14.2, and 13.5 ms, respectively. This outcome is consistent with the notion that the difference between the S-LRP onsets in the two precue conditions are not influenced by any factor that systematically biases both LRP onsets to the same extent ~e.g., cut-off frequency!. Nevertheless the t-value associated with each difference decreased with increasing cut-off frequency, though each t-value was significant. Such a decrease in the significance level is expected, because the signal-to-noise ratio worsens as the cut-off frequency of the low-pass filter is increased. Motor Time As in Experiment 1, we assessed whether precue information on movement direction shortened the motor time. The EMG-response interval was 71.3 ms when prior information on movement direction was given and 72.6 ms without advance information. This difference of ms was not significant, t~31! 0.94, p 0.1. Thus, consistent with Experiment 1, advance information about movement direction did not influence the speed of late motoric processes. Discussion Experiment 2 replicated the pattern of results obtained in Experiment 1. In particular, the findings concerning the LRP-R interval confirm the former outcome. Precue information about movement direction again reduced the LRP-R interval. In contrast, the LRP-R interval was not shortened by advance information about response force. GENERAL DISCUSSION Like Leuthold et al. ~1996!, we combined the precuing technique with the recording of the LRP. We not only successfully replicated but also considerably extended their findings. As in their study, our participants received prior information about the forthcoming response alternative. In contrast to Leuthold et al., we identified the effects of advance information not only about movement direction but also about the required force level of the response. Consistent with the bulk of behavioral studies, advance information about either movement parameter greatly shortened RT ~cf. Requin et al., 1991!. This RT shortening supports the notion that covert prepa-

7 Preparation of force and movement direction 513 ratory processes benefit from movement advance information to preprogram the response. Because RT encompasses all internal processes from stimulus input to the onset of the associated response, it is impossible to infer from the RT shortening alone whether precue information affects early or late processes. Therefore, we used the LRP to narrow down the possible loci of the RT savings. The results of both experiments revealed clearly that advance information about movement direction and about response force exerts a differential precuing effect on the LRP. Although precue information about both parameters shortened the S-LRP interval, only advance information concerning movement direction also shortened the LRP-R interval. From this pattern of results we can infer that precue information speeds up early processes ~e.g., perceptual and presumably early motoric processes! due to the reduction of stimulus and response uncertainty, but not always late processes, which begin after the central activation of the response hand. Most important for this study, the present results clearly suggest that these late processes gain a speed advantage from advance information about movement direction but not from advance information about force. This differential precuing effect on the LRP-R interval is compatible with the view that distinct neuronal mechanisms are involved in the preparation of different movement parameters such as direction and force ~Riehle & Requin, 1995!. It may appear surprising that the duration of the LRP-R interval is insensitive to prior information about response force. To account for the null-effect, one may argue that force is not specified in advance but rather achieved by a feedback-controlled mechanism ~e.g., Gordon & Ghez, 1987!, which cannot utilize advance information to enhance performance. Such an account, however, seems to be incompatible with neurophysiological evidence suggesting that advance information about force modulates single-cell activity within the motor system ~e.g., Riehle & Requin, 1995!. As reviewed by Ashe ~1997!, the activity of cells within the primary motor cortex is correlated with the magnitude of force, suggesting an involvement of the primary motor cortex in force control. It seems plausible to assume that this control process benefits from advance information about force, reducing the time taken up by relatively late motoric processes. The present results, however, render this view unlikely. One possibility is that preparatory processes in the primary cortex concerning response force control are not responsible for the RT speedup. Another possibility is that preparatory processes within the primary motor cortex can operate only on certain movement parameters. The present results suggest that these preparatory processes are sensitive to prior information about movement direction but not about the magnitude of force. Consistent with this view, Riehle and Requin ~1995! reported that activity changes of many more neurons within the primary and premotor cortex were related to movement direction than to response force during the preparatory period. Consequently, the present results provide indirect evidence for the idea that the activity changes reported by Riehle and Requin also contribute to the precuing effect on the late RT portion. Because the LRP-R interval encompasses the duration of all processes between the central activation of the response hand and the overt response itself, one could argue that the RT speedup emerging from the directional precues resides within the executionrelated part of the motor system. The directional precue allows the specification of the appropriate muscle groups and thus tensing of these muscles, which in turn may shorten motor time, namely the interval between EMG onset and the onset of the overt response. Motor time, however, was virtually unaffected by directional advance information. The present finding, of course, does not preclude the possibility that the RT benefit of the directional precue is mediated by neuronal structures within the cortico-spinal tract. A recent study ~Hasbroucq et al., 1999!, however, using transcranial magnetic stimulation, renders this possibility rather unlikely. According to this study movement preparation ~such as the selection of muscle groups! is more centrally located than the cortico-spinal tract. Force and direction precues may exert differential effects on the nonlateralized and the lateralized portion of the readiness potential. Before the LRP emerges both hemispheres become activated and this is reflected in a negative going shift of the readiness potential ~cf. Coles, 1989!. It seems likely that this bilateral activation represents a more central preprogramming process that does not affect the interval from the onset of the LRP to the onset of the overt response ~Ulrich et al., 1998!. Central programming processes may especially benefit from advance information about movement force. Consequently, one should expect that force precues facilitate these processes more than direction precues. Accordingly, the S-LRP interval should be reduced more by force than by direction precues. An additional analysis of Experiment 2, however, provides no support for this view. The S-LRP interval was not reduced differentially albeit that an effect in the predicted direction could be observed but failed to be significant. 3 In conclusion, then, the present results suggest the following model of the specification of movement direction and response force, which makes certain assumptions about information processing within the motor system. According to this model, the response is selected in compliance with stimulus information in premotoric processing stages. Furthermore, the ensuing motor processes are divided into an early processing stage that is not limbspecific and into a late stage that is limb-specific ~cf. Miller, Coles, & Chakraborty, 1996; Ulrich et al., 1998!. The LRP onset indicates the onset of the limb-specific motor system. Movement direction, and thus the selection of the appropriate muscle groups is specified, at least partially, within this late system. In contrast, the amount of required muscle activation is specified entirely within the early motor stage. In the case of precue information about response force, the processing time within the non limb-specific system is reduced whereas in the case of precue information about movement direction, the processing time within both systems is shortened. 3 Precues shortened the S-LRP interval ms more in the force than in the direction condition. This difference, however, was not significant, t~31! 1.42, p.17. REFERENCES Ashe, J. ~1997!. Force and the motor cortex. Behavioral Brain Research, 86, Coles, M. G. H. ~1989!. Modern mind brain reading: Psychophysiology, physiology, and cognition. Psychophysiology, 26, Deiber, M. P., Ibanez, V., Sadato, N., & Hallett, M. ~1996!. Cerebral structures participating in motor preparation in humans: A positron emission tomography study. Journal of Neurophysiology, 75, De Jong, R., Wierda, M., Mulder, G., & Mulder, L. J. M. ~1988!. Use of

8 514 H. Müller-Gethmann et al. partial stimulus information in response processing. Journal of Experimental Psychology: Human Perception and Performance, 14, Dettmers, C., Fink, G. R., Lemon, R. N., Stephan, K. M., Passingham, R. E., Silbersweig, D., Holmes, A., Ridding, M. C., Brooks, D. J., & Frackowiak, R. S. ~1995!. Relation between cerebral activity and force in the motor areas of the human brain. Journal of Neurophysiology, 74, Gordon, J., & Ghez, C. ~1987!. Trajectory control in targeted force impulses: II. Pulse height control. Experimental Brain Research, 67, Gratton, G., Bosco, C. M., Kramer, A. F., Coles, M. G., Wickens, C. D., & Donchin, E. ~1990!. Event-related brain potentials as indices of information extraction and response priming. Electroencephalography and Clinical Neurophysiology, 75, Gratton, G., Coles, M. G. H., Sirevaag, E. J., Eriksen, C. W., & Donchin, E. ~1988!. Pre- and poststimulus activation of response channels: A psychophysiological analysis. Journal of Experimental Psychology: Human Perception and Performance, 14, Hasbroucq, T., Osman, A., Possamaï, C. A., Burle, B., Carron, S., Latour, S., & Mouret, I. ~1999!. Cortico-spinal inhibition reflects time but not event preparation: Neuronal mechanisms of preparation dissociated by transcranial magnetic stimulation. Acta Psychologica, 100, Kristeva, R., Cheyne, D., & Deecke, L. ~1991!. Neuromagnetic fields accompanying unilateral and bilateral voluntary movements: Topography and analysis of cortical sources. Electroencephalography and Clinical Neurophysiology, 81, Leuthold, H., Sommer, W., & Ulrich, R. ~1996!. Partial advance information and response preparation: Inferences from the lateralized readiness potential. Journal of Experimental Psychology: General, 125, MacKay, W. A., & Bonnet M. ~1990!. CNV, stretch reflex, and reaction time correlates of preparation for movement direction and force. Electroencephalography and Clinical Neurophysiology, 85, Miller, J., Coles, M. G. H., & Chakraborty, S. ~1996!. Dissociation between behavioral and psychophysiological measures of response preparation. Acta Psychologica, 94, Miller, J., & Hackley, S. A. ~1992!. Electrophysiological evidence for temporal overlap among contingent mental processes. Journal of Experimental Psychology: General, 121, Miller, J., Patterson, T., & Ulrich, R. ~1998!. Jackknife-based method for measuring LRP onset latency differences. Psychophysiology, 35, Oldfield, R. C. ~1971!. The assessment and analysis of handedness: The Edinburgh inventory. Neuropsychologia, 9, Osman, A., & Moore, C. M. ~1993!. The locus of dual-task interference: Psychological refractory effects on movement-related brain potentials. Journal of Experimental Psychology: Human Perception and Performance, 19, Osman, A., Moore, C. M., & Ulrich, R. ~1995!. Bisecting RT with lateralized readiness potentials: Precue effects after the LRP onset. Acta Psychologica, 90, Requin, J., Brener, J., & Ring, C. ~1991!. Preparation for action. In J. R. Jennings & M. G. H. Coles ~Eds.!, Handbook of cognitive psychophysiology: Central and autonomic nervous system approaches ~pp !. New York: Wiley. Riehle, A., & Requin, J. ~1989!. Monkey primary motor and premotor cortex: Single-cell activity related to prior information about direction and extent of an intended movement. Journal of Neurophysiology, 61, Riehle, A., MacKay, W. A., & Requin, J. ~1994!. Are extent and force independent movement parameters? Preparation- and movement-related neuronal activity in the monkey cortex. Experimental Brain Research, 9, Riehle, A., & Requin, J. ~1995!. Neuronal correlates of the specification of movement direction and force in four cortical areas of the monkey. Behavioral Brain Research, 70, Rizolatti, G., Luppino, G., & Matelli, M. ~1998!. The organization of the cortical motor system: New concepts. Electroencephalography and Clinical Neurophysiology, 106, Rosenbaum, D. A. ~1980!. Human movement initiation: Specification of arm, direction, and extent. Journal of Experimental Psychology: General, 109, Rosenbaum, D. A. ~1983!. The movement precuing technique: Assumptions, applications, and extensions. In R. A. Magill ~Ed.!, Memory and control in motor behavior ~pp !. Amsterdam: North-Holland. Sasaki, K., Gemba, H., & Tsujimoto, T. ~1990!. Cortical field potential associated with hand movement on warning-imperative visual stimulus and cerebellum in the monkey. Brain Research, 519, Ulrich, R., Leuthold, H., & Sommer, W. ~1998!. Motor programming of response force and movement direction: An ERP analysis. Psychophysiology, 35, van der Molen, M. W., Bashore, T. R., Halliday, R., & Callaway, E. ~1991!. Chronopsychophysiology: Mental chronometry augmented by psychophysiological time markers. In J. R. Jennings & M. G. H. Coles ~Eds.!, Handbook of cognitive psychophysiology: Central and autonomic nervous system approaches ~pp !. New York: Wiley. ~Received January 22, 1999; Accepted August 13, 1999!