Academic journal article Memory & Cognition

Effects of Verbal and Nonverbal Interference on Spatial and Object Visual Working Memory

Academic journal article Memory & Cognition

Effects of Verbal and Nonverbal Interference on Spatial and Object Visual Working Memory

Article excerpt

We tested the hypothesis that a verbal coding mechanism is necessarily engaged by object, but not spatial, visual working memory tasks. We employed a dual-task procedure that paired n-back working memory tasks with domain-specific distractor trials inserted into each interstimulus interval of the n-back tasks. In two experiments, object n-back performance demonstrated greater sensitivity to verbal distraction, whereas spatial n-back performance demonstrated greater sensitivity to motion distraction. Visual object and spatial working memory may differ fundamentally in that the mnemonic representation of featural characteristics of objects incorporates a verbal (perhaps semantic) code, whereas the mnemonic representation of the location of objects does not. Thus, the processes supporting working memory for these two types of information may differ in more ways than those dictated by the "what/where" organization of the visual system, a fact more easily reconciled with a component process than a memory systems account of working memory function.

Appreciation of the functional organization of the mammalian visual system (Ungerleider & Haxby, 1994; Ungerleider & Mishkin, 1982) has led to the widely accepted view that working and short-term memory1 for objects ("what") and locations ("where") are computed by at least partially discrete neural systems in monkeys (Wilson, Ó Scalaidhe, & Goldman-Rakic, 1993) and humans (e.g., Courtney, Ungerleider, Keil, & Haxby, 1996; McCarthy et al., 1996; Mecklinger & Muller, 1996). And behavioral studies in humans confirm that working memory for these two domains of information are supported by distinct visually based mental codes representing object identity and location, respectively (e.g., Delia Sala, Gray, Baddeley, Allamano, & Wilson, 1999; Hecker & Mapperson, 1997; Smith et al., 1995; Tresch, Sinnamon, & Seamon, 1993). The purpose of the experiments presented in this report, however, was to explore whether spatial and object working memory may also differ along an axis orthogonal to that dictated by the functional organization of the visual system: Object working memory may automatically, obligatorily engage verbal coding mechanisms, whereas spatial working memory may not.

This theoretical proposition arose from our experience with spatial/object manipulations in memory tasks-delayed recognition (Postle & D'Esposito, 1999; Postle, Jonides, Smith, Corkin, & Growdon, 1997), conditional-associative learning (Postle, Locascio, Corkin, & Growdon, 1997), and the n-back task (Postle, Stern, Rosen, & Corkin, 2000). This research has indicated that participants often adopt a strategy of verbally encoding stimuli in the object condition, although our tasks have used relatively "nonverbalizable" Attneave shapes. Participants in these studies of object memory were never instructed to use verbal coding to mediate performance on the tests, yet post-test debriefing suggested that the tendency to do so was strong and was consistent across age groups (from late teens to 80s), neurological status (healthy, Parkinson's disease, stroke, and medial temporal-lobe amnesia), and testing environment (behavioral laboratory and fMRI laboratory). We had not observed a similar tendency in tests of spatial memory that were procedurally identical to the object tasks and that were administered in the same session.

The idea that the representations of objects in working memory include a semantic code can be seen as an extension of the simultaneous multiple encoding theory of Wickens (1972, 1973), which holds that words can be encoded according to their semantic attributes, the physical characteristics of their presentation at the time of encoding, and other attributes (such as language, frequency, representing symbol, and imageability). Thus, Wickens argued that words are encoded not only according to their central function-the conveyance of meaning-but according to attributes and contextual factors that can vary independent of a word's semantic content. …

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