Learned Categorical Perception for Natural Faces

Learned Categorical Perception for Natural Faces Daniel J. Navarro (daniel.navarro@adelaide.edu.au) Department of Psychology, University of Adelaide, SA 5005, Australia Michael D. Lee (michael.lee@adelaide.edu.au) Department of Psychology, University of Adelaide, SA 5005, Australia Hannah Nikkerud (hannah.nikkerud@adelaide.edu.au) Department of Psychology, University of Adelaide, SA 5005, Australia Abstract We present an experiment involving learned categorical perception for natural faces, using four different cat- egory learning tasks. In the four tasks, participants learned to classify faces divided on the basis of gen- der, hair color, a subjective determined level of ‘trust’, or at random. After category learning, participants rated the similarity of each pair of faces, and their judg- ments were compared to a previously collected ‘base rate’ set of similarities for the same stimuli. Evidence for learned categorical perception was then sought, in the form of increased differences between intra-category and inter-category similarities. Over the four conditions, we observed no learned categorical perception when cate- gories were based on obvious properties (gender and hair color), nor when the category structure was intentionally random. However, when the loosely defined category structure of trust was employed, a learned categorical perception effect emerged. If every stimulus were perceived as a unique event, we would be rapidly inundated with pointless informa- tion. So we organize our perceptions into categories, allowing us to describe the world in a simpler manner, and to generalize better to novel situations. As a conse- quence of this restructuring, items belonging to the same categories tend to be perceived as more similar to one another, while items belonging to different categories ap- pear less similar. These categorical perception phenom- ena have been observed in a range of domains such as phonemic categories (e.g., Liberman, Harris, Hoffman & Griffith, 1957), color categories (e.g., Bornstein & Ko- rda, 1984; Ozgen & Davies, 2002), musical pitch cat- egories (Burns & Ward, 1978), and facial expressions (e.g., Bimler & Kirkland, 2001; Etcoff & Magee, 1992). Categorical perception effects rely on an interaction be- tween the perceptual representation of the environment and the conceptual representations used to interpret it, and so are a central issue in cognitive modeling. In this paper we consider the acquisition of categorical perception for naturalistic faces. We present an experi- ment in which a prior category learning task is shown to influence subsequent similarity judgements, but only for some category structures. Learned Categorical Perception While categorical perception effects may be partially in- nate (e.g., Barrera & Maurer, 1981), much of human conceptual structure is likely to be learned, so there has been some interest in the notion of learned categorical perception. Such studies typically measure the effect of a prior learning task on a subsequent discrimination task. For instance, Goldstone (1994) demonstrated that prior learning of categories based on stimulus size or bright- ness improved performance on subsequent same-different judgements based on the same dimension. The goal in studies of this kind is to see how the act of learning the category structure affects the subsequent judgements, and specifically to see if the recently learned category is reflected in these judgements. Two main theoretical explanations have been pro- posed to account for learned categorical perception (Goldstone, Lippa & Shiffrin, 2001). According to the altered object description account, simply learning that a set of stimuli share a label is sufficient to change the way that they are perceived. In its strongest form, the altered object description account can be viewed as a variant of the linguistic relativity hypothesis (Whorf, 1964), in which linguistic labels are taken to form the underlying basis of perception itself. In a weaker form, it asserts that the prior learning experience allows people to (perhaps temporarily) observe or attend to different properties of the stimuli, adapting their representations to suit the context. The alternative account is the strategic judgement bias account, which argues that categorical perception effects can be viewed as a response to task demands, in which people adopt a new decision strategy, but do not alter the representational description of stimuli. As with the altered object account, the strategic judgement bias ac- count has strong versions and weak versions. A strong version of this theory states that the judgements are a deliberate response to the experimental task, and do not reflect any change in the way stimuli are perceived. For instance, if participants are told that two stimuli belong to the same class, then they will assume that they are supposed to rate them as more similar. Accordingly, they alter their similarity ratings, but do not alter the stimulus representation. In a weaker form, it asserts that prior category learning informs people that some prop- erties are more likely to be relevant to the task than others, so people will focus their decision processes on these properties. It seems clear that the weak form of the altered object description account is remarkably similar to the weak form of the strategic judgement bias account. In both cases, the role of category learning is to allow people