Response processes in information–integration category learning☆
Introduction
Categorization is the act of responding differently to objects or events in separate classes or categories. It is a vitally important skill that allows us to find food and avoid toxins, and to approach friends and escape foes. During the past decade there has been a surge of interest in the neural basis of category learning. Perhaps the most important discovery to come from this research is that humans have multiple category-learning systems, which are each best suited for learning certain types of category structures, and are each mediated by different neural circuits (Ashby et al., 1998, Ashby and O’Brien, 2005, Erickson and Kruschke, 1998, Love et al., 2004, Nosofsky et al., 1994, Reber et al., 2003).
In rule-based category-learning tasks the categories can be learned via some explicit reasoning process. Frequently, the rule that maximizes accuracy (i.e., the optimal strategy) is easy to describe verbally (Ashby et al., 1998). In the most common applications, only one stimulus dimension is relevant, and the subject’s task is to discover this relevant dimension and then to map the different dimensional values to the relevant categories. A variety of evidence implicates the prefrontal cortex (PFC), anterior cingulate, the head of the caudate nucleus, and medial temporal lobe structures in rule-based category learning (e.g., Brown and Marsden, 1988, Filoteo et al., 2005, Filoteo, Maddox, Ing, et al., 2007, Muhammad et al., 2006, Seger and Cincotta, 2006).
In information–integration (II) category-learning tasks, accuracy is maximized only if information from two or more stimulus components (or dimensions) is integrated at some pre-decisional stage (Ashby & Gott, 1988). In many cases the optimal strategy is difficult or impossible to describe verbally (Ashby et al., 1998). An example of an II task is shown in Fig. 1. In this case the two categories are each composed of circular sine-wave gratings that vary in the width and orientation of the dark and light bars. The diagonal line denotes the category boundary. Note that no simple verbal rule correctly separates the disks into the two categories. Nevertheless, many studies have shown that people reliably learn such categories, provided they receive consistent and immediate feedback after each response (for a review, see Ashby & Maddox, 2005).
The search for the neural locus of II category learning has focused on the basal ganglia, and more specifically on the striatum. For example, a number of studies have reported that patients with basal ganglia dysfunction are impaired in II tasks (Filoteo et al., 2001, Filoteo, Maddox, Salmon, et al., 2007, Maddox and Filoteo, 2001) and neuroimaging studies of II learning have reported significant learning-related striatal activation (Cincotta and Seger, 2007, DeGutis and D’Esposito, 2007, Nomura et al., 2007, Seger and Cincotta, 2002). In addition, a large literature implicates the striatum in visual discrimination learning and category learning in non-human animals (for a review, see Ashby & Ennis, 2006). For example, single-cell recording studies in monkeys show that striatal medium spiny cells develop category-specific responses after categorization training (Merchant et al., 1997, Romo et al., 1995, Romo et al., 1997).
II category learning is also indirectly linked to the striatum via its similarities to the serial reaction time (SRT) task (Nissen & Bullemer, 1987), which is among the most widely studied procedural-learning tasks. These similarities are important because there is much evidence that procedural learning is mediated largely within the striatum (e.g., Salmon and Butters, 1995, Willingham, 1998). One notable similarity concerns the learning of response positions, rather than abstract labels. Willingham, Wells, Farrell, and Stemwedel (2000) reported that switching hands on the response keys did not interfere with implicit SRT learning, but switching the location of the response keys caused significant interference. Ashby, Ell, and Waldron (2003) replicated this result with II category learning. More specifically, Ashby et al. (2003) reported that switching the location of the category response keys interfered with II performance, but not with rule-based performance. In this study, participants first learned either rule-based or II categories. Then one group switched the hands that they used to depress the response keys, and for another group the location of the response keys was switched (there was also a control group). Neither of these manipulations had any effect on the performance of participants in the rule-based conditions. In the II conditions however, switching hands had no effect on performance, but switching the response keys interfered significantly with response accuracy. The similarity of these II results to the results of Willingham et al. (2000), provide strong evidence for a procedural-learning contribution to II categorization.
Maddox, Bohil, and Ing (2004) reported the results of a related experiment. In A–B training conditions, the stimulus was displayed on each trial along with the query “Is this an A or B?” The observer pressed one key for category A and a separate key for category B, followed by corrective feedback. In the Yes–No training conditions, on half the trials the stimulus was displayed along with the query “Is this an A?” and on the other half of the trials the stimulus was displayed along with the query “Is this a B?” The observer pressed one key to respond “No”, and a separate key to respond “Yes”, and both keys had a fixed location. Thus, in A–B training each category was associated with a consistent response location, whereas in the Yes–No conditions there was no consistent mapping from category label to response location. For rule-based categories, there was no difference between these two training procedures, but with II categories, learning was significantly worse with Yes–No training.
Ashby et al., 2003, Maddox et al., 2004 both interpreted their results as evidence that response locations are learned during II categorization. In the Ashby et al. (2003) study, switching the response keys changed the location of the two category responses and produced an interference. In the Yes–No training condition of the Maddox et al. (2004) study, participants sometimes pressed the Yes key to signal a category A response and sometimes they pressed the No key. The Yes and No keys had fixed locations, so the two category responses had no consistent locations, and impaired learning was observed.
The Ashby et al. (2003) study showed that changing the response locations after learning was essentially complete disrupted performance in II tasks, but it did not address the question of whether a consistent response location is needed during the initial learning. Maddox et al. (2004) partially addressed this issue, but participants in their Yes–No conditions made an extra logical decision that was not required during the more traditional A–B training. This is potentially problematic because it seems possible that working memory and executive attention are needed to answer the query “Is this an A?”. Several studies have shown that II category learning is not significantly affected when participants simultaneously perform a secondary task that requires working memory and executive attention (DeCaro et al., 2008, Waldron and Ashby, 2001, Zeithamova and Maddox, 2006). One interpretation of this result is that executive processes and procedural-learning processes operate (largely) independently. If so, then any condition that requires their integration may be problematic for participants. For this reason, it is difficult to rule out the hypothesis that Maddox et al. (2004) observed an interference because the Yes–No condition required participants to integrate executive and procedural-learning processes, whereas the A–B condition did not.
This article presents a direct test of this response position hypothesis. Experiment 1 tests whether consistent response locations are required for initial II learning and Experiment 2 tests whether the Yes–No interference reported by Maddox et al. (2004) occurred because there was no consistent association between categories and response positions or because an extra logical decision was required.
Section snippets
Experiment 1
Experiment 1 tests whether consistent response locations are required for initial II learning. There is reason to believe that consistent response locations should not be required. Ashby, Ennis, and Spiering (2007) interpreted the results of Ashby et al., 2003, Maddox et al., 2004 to mean that the most critical cortical projection of the II category-learning system was somewhere in premotor cortex. Olson and Gettner (1999) reported that some cells in a prominent premotor area (i.e.,
Experiment 2
Experiment 1 showed that a consistent response location facilitated, but was not necessary for II learning. Experiment 2 tests whether the Yes–No interference reported by Maddox et al. (2004) occurred because there was no consistent association between categories and response positions or because an extra logical decision was required. Maddox et al. (2004) compared performance in the Yes–No condition to performance in a control condition in which the response positions were consistent
General discussion
Experiment 1 showed that consistent response positions are not required for effective II category learning. Instead, a consistent feature association is sufficient. Consistent response positions, in addition to a consistent feature association, appeared to speed early learning, but it did not lead to any asymptotic benefits, either in terms of overall accuracy or in the likelihood that an II strategy was used. These results suggest that the interference observed by Ashby et al. (2003) when they
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This research was supported in part by National Institute of Health Grant R01 MH3760-2.