Academic journal article Perception and Psychophysics

The Role of Parity, Physical Size, and Magnitude in Numerical Cognition: The SNARC Effect Revisited

Academic journal article Perception and Psychophysics

The Role of Parity, Physical Size, and Magnitude in Numerical Cognition: The SNARC Effect Revisited

Article excerpt

People indicate the physical size or the parity status of small numbers faster by a left-hand key and those of larger numbers by a right-hand key. Because magnitude information is not required for successful performance in these tasks, the presence of a number-space association (the SNARC effect) has been taken to indicate the automatic activation of numerical magnitude in all tasks with numerals. In order to test this account, in a series of five experiments, we derived two consensual markers of automatic activation of irrelevant numerical magnitude, the size congruity effect (for judgments of physical size), and the Garner effect (for judgments of parity). Both markers were found independent of the SNARC effect. Consequently, we question the traditional explanation of the SNARC effect and offer an alternative account in terms of a highly overlearned stimulus-response loop.

Is 3 an odd or even number? The time it takes one to record a response for this simple question is affected by the relative position of the response key used to indicate parity. People respond to 3 (and to other small numbers) faster with a left-hand key, and they respond to 8 (and to other larger numbers) faster with a right-hand key, regardless of the odd-even assignment of the response keys. This spatial-numerical association of response codes (SNARC; Dehaene, Bossini, & Giraux, 1993) is notable because magnitude information is not strictly needed for deciding parity. Consequently, the presence of the SNARC effect has been taken to support the idea that the meaning of numerals (i.e., numerical magnitude) is activated in an automatic fashion when numerals are presented for view for any purpose. It is this explanation of the SNARC effect that we challenge in the present study.

We employed two separate tools to test the traditional explanation of the SNARC effect. The first was the size congruity effect (SCE), a potent marker of automatic activation of semantic information with numerals. The SCE documents the influence of numerical magnitude on judgments of physical size of numerals. When the relative physical sizes of numerals in a pair are compared, people respond faster for congruent pairs (e.g., 8 2) than for incongruent pairs (e.g., 8 2; Fitousi & Algom, 2006). For singly presented numerals, people react to the large physical format of 8 faster than they do to that of 2, but the reverse holds true for 8 versus 2 (Algom, Dekel, & Pansky, 1996; Choplin & Logan, 2005). The task in studies of the SCE entails a nonsemantic attribute of digits, yet irrelevant numerical value cannot be ignored and affects task performance. Do the SCE and the SNARC effect interact in processing?

In order to test the relationship between the two markers of automatic activation, in the first experiment we derived the SCE and the SNARC effect on a common set of numerals for the same group of observers. The task comprised judgments of the physical size (large, small) of numerals conveyed via alternative response key assignments (large size by the left-hand key and small size by the righthand key, or large size by the right-hand key and small size by the left-hand key). If the SNARC effect taps activation of magnitude information, it should interact with the SCE, which registers the inadvertent involvement of numerical magnitude in tasks of physical size. In particular, a larger SNARC effect is expected for congruent (large numbers in large physical formats and small numbers in small physical formats) than for incongruent (numerical and physical size conflict) number stimuli. If the SCE and the SNARC effect are found to be independent of each other, a revision of the traditional interpretation of the latter as a marker of automatic activation is invited.

Our second theoretical device was the Garner effect, psychology's classic vehicle for deciding the separability or integrality of stimulus dimensions in perceptual processing (Garner, 1974; Melara & Algom, 2003; Melara & Mounts, 1993; Pansky & Algom, 1999, 2002). …

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