Representational Shifts Towards the Prototype in Memory for Hue

Representational Shifts Towards the Prototype in Memory for Hue Laura Jane Kelly (lkelly@ucmerced.edu) Evan Heit (eheit@ucmerced.edu) Cognitive and Information Sciences, University of California, Merced 5200 N. Lake Road, Merced, CA 95343 USA Abstract Representational shifts in memory have been a recent topic of interest and debate (Blanco & Gureckis 2012; Lupyan, 2008; Richler, Gauthier & Palmeri, 2011; Richler, Palmeri & Gauthier, 2012). Whether there are true systematic biases in memory due to a stimulus being labeled has been proposed and contested. The fundamental proposal that representations shift toward the prototype has not previously been demonstrated. In the present experiment, participants judged colored silhouettes by color category or by preference, then were asked to remember the hue of the original silhouette among five narrowly distinct options. By using the single dimension of hue, we are able to show prototypical representational shifts in memory for colored silhouettes after a few minutes. We did not observe a difference between color labeled and preference judged silhouettes, refuting the claim that labeling is the source of prototypical representational shifts. Keywords: Concepts and Categories; Perception; Memory; Labels; Color Representation; Introduction A representational shift is a spatial metaphor for a systematic difference between the representation as measured and the original stimulus that inspired the representation. The representation is said to have shifted from the original sensory input somewhere along its cognitive path before being measured. The representational shift hypothesis (Lupyan, 2008) suggests that when explicit labels are used while perceiving an object, the encoded representation shifts from what it would be without explicit labeling to a more prototypical representation. The original experiments (Lupyan, 2008), described in more detail below, did not directly address the main predictions of the representational shift hypothesis: the existence of a systematic shift or the direction of that shift. The analysis relied on inferring a representational shift from a pattern of non-directional forgetting. The data only directly indicated worse memory for labeled objects. The existence of a systematic shift and the direction of the potential shift is simply not shown by the data collected. In the experiment presented here, we use color to test memory for recently presented silhouettes of animals and objects. In a paradigm similar to Lupyan (2008), the objects are labeled or not labeled followed by a surprise memory task. Instead of a yes-no recognition task, we present an array of 5 hue variations for the participants to choose among. By switching from multidimensional objects to unidimensional hue, and testing fine variations of hue memory rather than course-grained recognition memory, we can see whether there are systematic shifts of hue memory and their direction within the dimension of hue. The pattern of false alarms is used to look for representational shifts. Why do these representational shifts matter? We already know there are top-down influences on memory (e.g., Heit, 1997). Two particularly dramatic examples of the imperfection of memory are eyewitness testimony (Wells & Olson, 2003) and flashbulb memories (Schmolck, Buffalo, & Squire, 2000). Small differences between the experience of an object or color and its representation in memory may seem minor in comparison to changing the race of a shooter based on stereotypes or radically rewriting how you heard about a defining national moment over the course of a few years. However, taking into account how pervasive these small differences would be, representational shifts could have extensive effects on how meaning is build and supported. Prototypes can be conceived of as resulting from the build up of exemplars over time (Nosofsky, 1986; Palmeri & Nosofsky, 2001). As a ramification of the representational shift hypothesis, exemplars in memory would not accurately reflect experience alone but would also have a bias related to when the example of the category was experienced. The stronger the category, the stronger the pull of the prototype, and as a result, the more new exemplars are biased towards categories as they already stand. The Representational Shift Hypothesis Lupyan (2008) proposed that there would be a difference in encoding and retrieving memories of objects if the categorization was explicit rather than only implicit at the time of encoding. Previous work had shown better category learning with labels than without labels (Lupyan, Rakison & McClelland, 2007), suggesting that there are differences in how categories with labels are activated. The difference in activation of the category could influence the encoding of exemplars of the category. Specifically, the representational shift hypothesis asserts that concurrent labels activate a more prototypical representation of the category than would the perceived exemplars activate alone. The stored encoding is hypothesized to be a mixture of the exemplar activation and the label activation; the final representation has shifted toward the prototype as a result of the interaction of perception and semantic memory. The original work (Lupyan, 2008) used an experimental paradigm consisting of a presentation of chairs and lamps followed by a surprise recognition test including both the old objects and matched new object lures that were very