Academic journal article Canadian Journal of Experimental Psychology

Repetition and the SNARC Effect with One- and Two-Digit Numbers

Academic journal article Canadian Journal of Experimental Psychology

Repetition and the SNARC Effect with One- and Two-Digit Numbers

Article excerpt

The SNARC (Spatial Numerical Association of Response Codes) effect is the finding that small numbers elicit faster left than right responses and large numbers elicit faster right than left responses. This effect suggests that numbers activate left-right magnitude-laterality codes and that these codes interact with the selection of left-right responses. In the present research, subjects made parity decisions for one-digit numbers (in Experiment 1) and two-digit numbers (in Experiment 2), and we examined the effect of stimulus repetition on the SNARC effect. With single-digit stimuli, responses were faster and the SNARC effect was eliminated when stimuli were identical on successive trials. With two-digit stimuli, responses were faster when the ones digit was repeated, but the SNARC effect was found regardless of whether the digit was repeated or not. We argue that magnitude-laterality codes are activated in the process of accessing number information in memory and that this process can be short circuited if the visual stimulus matches that on the previous trial. Thus, no SNARC effect is found in Experiment 1 when identical stimuli are presented on successive trials. However, this result is not found in Experiment 2 because successive stimuli do not match even if the ones digit is repeated.

Keywords: SNARC effect, repetition effect

In the present research, we examined the activation of left-right spatial information in the processing of one- and two-digit numbers. Dehaene, Bossini, and Giraux (1993) observed that small numbers yielded faster left than right responses and larger numbers yielded faster right than left responses in a parity judgment task. They labeled this phenomenon the Spatial Numerical Association of Response Codes (SNARC) effect. Dehaene and his colleagues explained the SNARC effect by assuming that digit magnitude was represented in an analog fashion along a mental number line, with small numbers on the left and large numbers on the right. Although some researchers have disputed the notion that an analog representation underlies this effect (e.g., Santens & Gevers, 2008), it seems clear that numerical stimuli can activate what might be termed left-right "magnitude-laterality codes" in which "left" is associated with small digits and "right" with large digits. A critical question is the circumstances under which such representations are generated and how they influence response processes. In the present research, we assessed whether the SNARC effect would be observed when stimuli are repeated from trial to trial, both with one-digit and two-digit numbers. Repeating stimuli has been theorized to short circuit some of the processing needed to produce a response (e.g., Pashler & Baylis, 1991), and no SNARC effect would be expected if the skipped stages are necessary for that effect.

Previous research suggests that there are two components to the SNARC effect. On one hand, Fischer, Castel, Dodd, and Pratt (2003) found that merely presenting numbers could direct attention to spatial locations in a manner consistent with the SNARC effect. In their task, a digit was presented at fixation, and, after a delay, a detection target was presented randomly to either the left or the right. In keeping with a positional, left-to-right association with magnitude, responses to targets on the left were faster when preceded by a small digit and responses to targets on the right were faster when preceded by a larger digit. Related results were obtained by Nicholls, Loftus, and Gevers (2008) using an unspeeded perceptual discrimination task. These results suggest that left-right laterality codes are activated upon presentation of digits and can serve to direct attention under some circumstances.

Other research implies that the activation of such laterality codes can have effects on response selection as well. Keus and Schwarz (2005) presented digits in a parity judgment task to either the left or the right visual field. …

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