Discussion

Figure 26: Response-locked LRP for for all 28 subjects for the first and second half of the experiment and for standards (std; solid line), perceptual deviants (pd; dotted line), motor deviants (md; dashed line) and random stimuli (dotted and dashed line), respectively.

(R-R-learning) in a variant of the SRT-task was explored. Furthermore, differences in the neuronal structures involved in explicit and implicit learning were analyzed.

Subjects performed a four-choice reaction time task. Eight locations were arranged so that two locations each shared the same horizontal position and differed only with respect to their vertical location. Subjects had to respond according to the horizontal position of the stimulus with a finger lift, i.e. two locations each were mapped onto one response finger. Unknown to subjects, the stimuli followed a sequence of eight elements which was interrupted occasionally by one of two types of deviant stimuli. Perceptual deviants violated the perceptual sequence but required the same response as a regular stimulus whereas motor deviants required a response with the opposite hand, and thus violated both the response as well as the perceptual sequence. Several tests of explicit sequence knowledge were administered after completion of the SRT-task to assess the amount of verbalizable knowledge about the stimulus regularities. On the basis of these results two groups of subjects were formed. Implicit learners did not exhibit sequence knowledge different from the probability of guessing correctly, whereas explicit learners were able to recall at least three consecutive elements of the sequence correctly.

The basic findings obtained by other researchers in the SRT-paradigm were replicated: Both groups of subjects learned the regularities inherent in the stimulus sequence. This is reflected in a decrease of errors for responses to standard stimuli in the course of the experiment. Furthermore, subjects made more erroneous responses to random stimuli compared to standards in the second half of the SRT-task. Likewise, RT to standard stimuli decreased from the first to the second half and was faster than responses to random stimuli. At least for those subjects who are categorized as implicit, this learning most likely took place without the development of concurrent awareness of the stimulus structure (see, for example, Nissen &

Bullemer, 1987). However, the present study differed from the standard SRT-task in several respects: First, rather than using one block of random stimuli as a control condition which can be compared to the structured stimulus blocks, random stimuli were introduced at the beginning and at the end of each block (see Frensch &

Miner, 1994). This manipulation served to diminish sequence learning effects as it is more difficult for subjects to detect the beginning and the end of the structured parts

in each stimulus block. Indeed, the learning effect in the present study was small compared to other SRT-studies. Second, the introduction of deviant stimuli should further enhance the difficulty of sequence learning. Finally, the subjects had much more experience with the task than in most other studies (each subject was confronted with 256 replications of the sequence, compared to 100 in Nissen &

Bullemer (1987)). Using probabilistic rather than deterministic sequences, Cleeremans & McClelland (1991) found implicit learning of structural regularities after subjects practiced for 60000 trials (see also Jiménez & Méndez, 1999). Thus, it seems to be justified to draw conclusions concerning the mechanisms involved in sequence learning from the present results.

Response- or stimulus- based learning ?

Behavioral results indicate that both explicit as well as implicit learners were sensitive to the violation of the response sequence. Error rate as well as RT did not differ for standard stimuli and perceptual deviants but were enhanced for motor deviants. These effects emerged in the first half of SRT-performance and increased towards the end of the experiment. These results replicate and extend those of Nattkemper & Prinz (1997) who found prolonged RTs for deviants violating the response sequence for symbolic stimulus material (letters) in a group of implicit learners. These results are compatible with the idea that R-R associations might be the major component of implicit sequence learning (Hoffmann & Koch, 1997;

Nattkemper & Prinz, 1997).

This idea is further supported from stimulus-locked LRP data. Selective activation of the correct response started immediately after the onset of a standard stimulus. LRPs for perceptual deviants (which violated the stimulus-, but not the response sequence) showed by and large the same pattern as those to standards, that is, an early activation of the correct response. In contrast, a significant activation of the correct response for randomly presented stimuli emerged approximately 100 ms after stimulus onset (see table 16). This indicates that selective anticipation may influence the motor system very early. Furthermore, for both explicit as well as implicit learners an activation of the incorrect but expected

response hand was found for motor deviants prior to the execution of the correct response (positive 'dip' in the stimulus-locked LRP). In the second half of the experiment, this started to develop as early as 100 ms prior to stimulus onset indicating that subjects expected the upcoming response already before the imperative stimulus was presented. Taken together, these findings show that specific anticipations about the upcoming response may have been induced by the presence of the sequence. These expectations seem to have an immediate influence on the response execution stage. Similar findings were reported by Eimer and colleagues (1996) for symbolic stimulus material (letters). However, in their study, participants who were unable to recognize any regularities in the stimulus material did not show an activation of the incorrect response hand. In contrast, in the present study, anticipatory response activation was present for subjects who did not exhibit explicit knowledge about the response sequence in a movement sequence recall task (see fig. 25). Thus, it seems reasonable to conclude that conscious awareness of the sequence structure or of the movement sequence is not necessary for the development of response anticipation.

Onset latencies of the stimulus-locked LRP were longer for motor deviants than for standards in the second, but not in the first half of the experiment. This indicates that response anticipation developed with increasing experience of the task.

Learning in the SRT-task, whether accompanied by the development of accessible knowledge of the sequence or not, could consist of changes in the motor related systems itself. This is consistent with PET data which show that procedural learning of a motor skill involves modifications in the same brain areas as those mediating the execution of the skill (Grafton, Mazziotta, Presty, Friston, Frackowiak,

& Phelps, 1992). Furthermore, sequence learning has been found to be impaired in patients with degenerative changes in brain structures mediating motor behavior such as Parkinson's disease (Ferraro, Balota, & Connor, 1993) or Huntington's disease (Knopman & Nissen, 1991).

In contrast to the stimulus-locked LRP, no difference in amplitude or onset-latency was evident in response-locked LRPs. This indicates that response execution processes were most likely not influenced by sequence learning.

Differences between explicit and implicit learning

ERPs were also influenced by stimulus deviance, and this effect differed as a function of accessible sequence knowledge. For implicit learners, mean ERP-amplitude 250 - 450 ms poststimulus was more negative for stimuli presented at deviant locations compared to those at standard and random positions. This effect could reflect either a confirmation or a violation of sequential expectancy of the upcoming stimulus. However, several aspects of the data suggest that this effect was not related to sequential learning itself. First, the ERP-effect of stimulus deviance was not reliably affected by the amount of training (no STIMULUS TYPE by HALF interaction). However, RT and errors were larger for motor deviants and random stimuli compared to standards, and this behavioral effect was affected by the amount of practice. Thus, if the ERP-effect were learning related, one would also expect a statistically reliable interaction with factor half, i.e. with the amount of practice. This is not in line with the data. Secondly, visual inspection of fig. 23 reveals an unexpected trend: If at all, the deviance effect for ERP-amplitude for implicit learners was larger in the first half of the experiment, not in the second as one would expect from RT data. Finally, ERPs were of the same amplitude for standards and random stimuli. However, behavioral data (RT and errors) were significantly different for these two stimulus types. If the ERP-effect would reflect sequence learning, and if sequential and non-sequential stimuli are processed differently in the human brain, one would expect an ERP for random stimuli which is similar to that of deviant events. This is not in line with the data. Thus, it is concluded that the ERP-effect (250-450 ms poststimulus) for implicit learners is most likely not related to sequence learning.

For both, implicit as well as explicit learners, amplitude of the negative-going flank of the P300 (500-600 ms poststimulus) was reliably more negative for perceptual deviants compared to the three other stimulus types (see fig.23).

However, this effect did not vary as a function of training.

Due to the lack of an ERP-effect related to sequence learning it was not possible to look at topographical differences in learning of a symbolic and learning of a spatial sensorimotor sequence.

Taken together, behavioral and ERP-data of the present experiment are more in line with a model which does not assume different cortical structures to be involved in explicit and implicit sensorimotor sequence learning, at least if spatial stimulus sequences are involved.

V. GENERAL DISCUSSION