THE RELATION BETWEEN IMPLICIT AND EXPLICIT LEARNING : Evidence for Parallel Development

Much research has focused on the separability of implicit and explicit learning, but less has focused on how they might interact. A recent model suggests that in the motor-skill domain, explicit knowledge can guide movement, and the implicit system learns in parallel, based on these movements. Functional imaging studies do not support that contention, however; they indicate that learning is exclusively implicit or explicit. In the experiment reported here, participants learned a motor sequencing task either implicitly or explicitly. At transfer, most of the stimuli were random, but the sequence occasionally appeared; thus, it was not obvious that explicit knowledge could be applied to the task. Nevertheless, participants with explicit training showed sequence knowledge equivalent to those with implicit training, implying that implicit knowledge had been acquired in parallel with explicit knowledge. This result has implications for the development of automaticity and of motor-skill learning. Much of the research on learning and memory over the past two decades has focused on the neural and cognitive separability of implicit and explicit learning (for a number of perspectives, see Schacter & Tulving, 1994). Less research has examined the relationship between implicit and explicit learning. Their relationship is of particular interest in motor-skill learning because it holds stark examples of the importance of both implicit and explicit learning. At advanced levels of skill, explicit access to the knowledge supporting the skill becomes unnecessary or even difficult (Fitts, 1964); an advanced tennis player need not consciously direct the movements of a serve, and in fact may be unable to describe the movement components. Indeed, some components of motor skills remain completely implicit throughout training. For example, in the serial response time (SRT) task, participants perform a four-choice response time task in which the stimuli appear in a repeating 12-unit sequence. Participants may demonstrate implicit learning of the sequence through faster response times even though they never learn it explicitly (Willingham, Nissen, & Bullemer, 1989). Nevertheless, conscious explicit processes are also important in motor-skill learning. Participants can use explicit knowledge of the sequence to support skilled performance in the SRT task (Curran & Keele, 1993). Thus, there is evidence that both implicit and explicit knowledge are useful in producing skilled behavior. Do they interact, and if so, how? One of us (Willingham, 1998) recently proposed a model, COBALT, that posits that implicit motor-skill learning takes place in parallel with explicit learning, so long as physical responses to the stimuli are made. For example, in the SRT task, if participants were pushing buttons while they learned the sequence explicitly, they should simultaneously learn the sequence implicitly. However, studies using positron emission tomography (PET; Grafton, Hazeltine, & Ivry, 1995; Hazeltine, Grafton, & Ivry, 1997; Rauch et al., 1995) support s parate, not parallel, implicit and explicit learning. Grafton and colleagues reported implicit learning was associated with metabolic changes in primary and supplementary motor cortices and the putamen, whereas explicit learning caused changes in prefrontal and premotor cortices (there was not simultaneous activity in the areas associated with implicit learning). Rauch and colleagues also reported separate sites supporting implicit learning (premotor cortex, caudate, and thalamus) and explicit learning (primary visual cortex, perisylvian cortex, and cerebellar vermis). In the experiment reported here, we sought to test whether implicit knowledge is acquired in parallel with explicit knowledge in a motor-skill task. We trained participants to explicitly learn the sequence in the SRT task. We then administered an implicit test of the sequence to see if implicit knowledge had been acquired in parallel.