Implicit Learning: A Demonstration and a Novel SRT Paradigm

Implicit Learning: A Demonstration and a Novel SRT Paradigm Fayme Yeates 1 ( fy212@exeter.ac.uk) F.W. Jones 2 A.J. Wills 1 M.R. F. Aitken 3 I.P.L. McLaren 1 School of Psychology, College of Life and Environmental Sciences, University of Exeter, UK. School of Psychology, Canterbury Christ Church University School of Psychology, University of Cambridge. Abstract Evidence for human learning without awareness of what is learned has been sought in serial reaction time (SRT) tasks in which, unknown to participants, the locations of stimuli follow a particular rule or sequence (Willingham, Nissen & Bullemer, 1989). A number of criticisms have been levelled at such tasks, including a lack of adequate control for sequential effects and a discrepancy in sensitivity between measures of implicit and explicit knowledge about the task (Jones & McLaren, 2009; Shanks & St. John, 1994). In this study we provide a novel, two-choice SRT paradigm whereby the locations of the response stimuli are sometimes predicted by a separate set of stimuli on screen. A color-filled square appears before each stimulus requiring a response, with participants informed this is simply a fixation point to prepare for the next trial. Two out of eight colors are predictive on 80% of trials, and performance on these consistent trials was faster than on the other six colors that were equally likely to result in either of the two possible responses. All these trial types were faster and more accurate than the remaining inconsistent 20% of trials for the predictive colors, which also produce more errors than control colors. A prediction task and interview followed the task, on which participants performed at near (slightly below) chance levels. We suggest that this task is a useful tool for studying associative learning in humans, as it provides reliable effects that appear to demonstrate implicit learning with relatively brief training. Keywords: Associative learning; implicit learning; SRT task Introduction In 1994 Shanks and St. John published their seminal review of implicit learning in Behavioral and Brain Sciences. Their conclusion was that implicit learning in humans had not yet been conclusively demonstrated. One paradigm that seemed to hold some promise in this regard was the SRT task pioneered by Willingham, Nissen and Bullemer (1989). In this task participants had to simply respond to one of four possible stimulus locations by pressing a spatially compatible key. Unknown to them, the sequence in which the stimulus locations were presented was not random but instead repeated a particular ordering. Willingham et al found that participants trained on such a sequence learned to respond faster than controls, yet were unable to say much about the sequence or, importantly, to predict the next stimulus location given a number of preceding trials. Shanks and St. John analysed this result, and others like it, and concluded that whilst the prediction task was potentially a sufficiently sensitive method of assaying participants' knowledge of the sequence, the evidence suggested that the non-significant findings on the prediction task had more to do with a lack of power than a lack of knowledge. Subsequent reviews of more recent evidence (e.g. Lovibond and Shanks, 2002; Shanks and Lovibond, 2002; Mitchell, De Houwer and Lovibond, 2009) have seen no reason to change this conclusion. In this paper we attempt to provide the evidence needed to prove that learning can take place without awareness of what is being learned. We do this using a variant of the SRT task employed by Jones and McLaren (2009) developed by Aitken (1996) which, instead of using the preceding trials to determine the later ones, uses a separate stimulus to predict which of the location stimuli will be presented next. A colored square presented between two stimulus locations is the predictor stimulus, and which of the two stimulus locations changes from an open to a filled circle is what determines the response required. If the left circle fills, then a left key press is needed and if the right-hand circle fills then a right key press is appropriate. The colored square is presented just before a response is required (while both circles are unfilled) so that it can predict which response will shortly be needed. Rather than run a separate control group, we follow Cleeremans (1993) in using a within- subject control. Thus some colors are never predictive (they have a random relationship with the stimulus location) whereas others are correlated with the response needed on the next trial. The detailed design of this paradigm was dictated by our assumption of a dual-process mechanism for human learning. Following McLaren, Green and Mackintosh (1994), we conceptualized learning as being driven by two quite different sets of processes, one Cognitive in nature, employing conscious, controlled, rule-based symbolic processes, the other Associative in nature, automatic in operation and based on simple algorithms that capture the correlations between events. The challenge, then, was to arrive at a paradigm that would readily allow learning on an associative basis but would be much less amenable to rule- based cognition. The Aitken SRT paradigm allows for a parameterization that meets these requirements. We used 8 colors in total, but made only 2 of them predictive (one left, one right). Thus, 6 out of the 8 bore no relationship to the next stimulus location to occur. Following the technique used by Posner and Snyder (1975), the 2 predictive colors were themselves only 80% reliable. Hence, if one of them typically predicted a left response would be the next needed, 20% of the time it was followed by a stimulus location that