Exp Brain Res (2011) 214:427–435 The RTs of all participants increased in the random trial block of the SRTT relative to the RTs in the preceding Wxed trial block, which reXects implicit sequence learning in this task (Knopman and Nissen 1987). Likewise, the TTs of all participants increased in the Wnal random block after the Wxed-sequence blocks in the PLT, which reXects implicit learning. Further analysis of TT as a combination of RT and MT showed that the RT pattern corresponded with the TT pattern, while the MTs did not increase in the Wnal ran- dom block (they even decreased). Both groups learned to perform the required movements faster over the course of all trial blocks. Implicit learning was exclusively reXected by the increase in RT in the Wnal blocks. We also analyzed the number of errors made in the two tasks. On the PLT, the number of directional errors (DEs) in the Wnal random block sharply increased in the controls but not in the patients with KorsakoV’s syndrome, showing that the implicit learning of the patients was worse than that of the controls. Additional analyses of other error measures on the PLT (i.e., distance from the ideal straight line and detour of the pen trajectory) revealed similar results, show- ing signiWcant group by block (L4-R2) interactions. There were no diVerences in error measures on the SRTT between the two groups. This diVerence in outcome between the two tasks can be explained by the spatial response component of the PLT. The Wnding that implicit spatial learning was compromised in the patients with KorsakoV’s syndrome compared to the controls is consistent with the earlier results of Chun and Phelps (1999), who reported intact implicit skill learning and impaired implicit spatial learning in amnesic patients. Another explanation for the diVerence in results between the two tasks, demonstrated by the error measures, is that the frequency and type of errors made in the PLT are diVer- ent from those made in the SRTT. In the PLT, an error reX- ects the choice of a nonoptimal movement direction in the Wrst phase of the movement toward the target, which can be corrected during the later stages of the movement. In the SRTT, an error is the result of pressing the wrong button, which only can be corrected (and had to be corrected) by pressing another button. Because our participants were allowed to leave the start circle in the PLT before the new target circle lit up, they favored starting speed over accu- racy, resulting in high DE rates during the random- and Wxed-sequence blocks, and strong negative correlations between RT and error percentage. Possibly, these high DE rates might provide a more sensitive measure of implicit learning. However, this diVerence between the two tasks in the sensitivity of their error measures cannot explain the diVerences between the groups in the amount of implicit learning shown in the PLT. The diVerences between the SRTT and PLT tasks regard- ing the magnitude of the movements made, very small dis- 433 crete Wnger presses in the former and pen movements of 6– 10 cm in the latter, and the ability to measure directional errors that could be corrected in the following movement in the PLT could underlie the diVerences on the tasks, rather than the diVerence in the spatial response component between both tasks. But this argument does not answer the question why patients with KorsakoV’s syndrome learned the PLT less well than the control group did. The PLT is somewhat similar to a maze task, and the Wnding of impaired learning of patients with KorsakoV’s syndrome in a maze task (Nissen et al. 1989) strengthens our argument that it is the spatial character of the planning and execution of the response in the PLT that was responsible for the poor learning of the PLT by the patients with KorsakoV’s syn- drome in the present study. Moreover, group diVerences in speed-accuracy trade-oV are unlikely to explain the diVer- ences in results between the controls and KorsakoV groups on the PLT because correlation analyses of PLT data revealed that RT and DE were signiWcantly and substantially (negatively) correlated in both groups. Controls made more errors in the last random block than did the KorsakoV group. This indicates that the implicit learning in this task was worse in the KorsakoV group than in the control group. The comprehensive neuropsychological evaluation of the patient group showed that most patients also had non- memory cognitive deWcits that may have interfered with their ability to learn a skill. However, the majority of patients with KorsakoV ‘s syndrome did not have visuospa- tial disabilities, and thus such deWcits cannot explain the diVerence in the results on the two tasks in the patients with KorsakoV’s syndrome. Although these patients had execu- tive deWcits in the Tower of London test, which measures planning abilities, neither task required planning abilities for its performance. Mental Xexibility and sensitivity to interference were intact in the majority of patients with KorsakoV’s syndromes, but their psychomotor speed was slower than that of the controls; however, all patients could complete the Trail Making Test in accordance with the test’s instruction. The overall decreased psychomotor speed of the patients with KorsakoV’s syndrome is also reXected by the signiWcant between-group diVerences in the time measures in both tasks. However, this overall slowing can- not explain the diVerence in error measure between the two tasks. In this study, several participants remarked that the sequences were not totally random. This indicates that they had some explicit knowledge of the sequence and thus that their test results were not the result of purely implicit learn- ing. However, participants reported this feeling only when questioned about it: none spontaneously reported sequence knowledge. Furthermore, the remark that the sequences were not totally random after an explicit question might be the result of a positive response bias, which has been 123