Academic journal article Memory & Cognition

RT Distribution Analysis of Category Congruence Effects with Masked Primes

Academic journal article Memory & Cognition

RT Distribution Analysis of Category Congruence Effects with Masked Primes

Article excerpt

In the number magnitude decision task ("Is the number bigger/smaller than 5?"), response to a target (e.g., 3) is faster following a masked prime congruent with the target (e.g., 1) than it is following an incongruent prime (e.g., 9). This category congruence effect has been reported to be "interference-dominant" relative to a neutral prime (e.g., the # sign, the number 5) on the basis of the analysis of mean response time (RT). Using RT distribution analysis as well as mean RTs, we identified two bases for this pattern. One relates to the choice of neutral baseline: The # prime, unlike the digit prime, does not factor in the cost of perceptual transition between the prime and target, and therefore underestimates facilitation and overestimates the interference effect. The second basis of the interference-dominant pattern is a disproportionate slowdown of congruent trials in the slow RT bins. Furthermore, this slowdown is greater for primes that had been used as targets than it is with "novel" primes that have not been responded to as targets. We interpret the results as suggesting that the category congruence effect has two components with different time courses-one based on stimulus-response mapping, and the other on semantic categorization.

Are stimuli that are not accessible to conscious awareness interpreted semantically? This is a question that has generated much debate. Earlier studies using the lexical decision task and read-aloud (naming) tasks with semantically related primes found only weak and unreliable priming effects with masked primes (e.g., Marcel, 1983; and see Holender, 1986, for a review). In contrast, recent studies using the semantic categorization task have consistently found reliable effects of semantic congruence between the prime and target. In this article, we consider the extent to which masked primes are processed in the semantic categorization task.

In an influential article, Dehaene et al. (1998) suggested that masked primes are processed first semantically, and then all the way to the motor response level. They used a magnitude decision task, in which participants are presented with a number target (either an Arabic numeral, e.g., 6, or a word spelled out, e.g., nine) and are required to decide whether it is bigger or smaller than 5. Each target was preceded by a prime that belonged to the same category as the target (e.g., prime nine, target 6) or the opposite category (e.g., prime one, target 6). Response times (RTs) were faster to targets preceded by congruent primes than to those preceded by incongruent primes. Furthermore, the size of this congruence effect did not differ between prime-target pairs in the same or different notational format (Arabic numerals/words), indicating that the effect occurred at a "notation-independent level of numerical representation" (Dehaene et al., p. 597). In support of the claim that the masked primes are processed down to the motor response level, congruence effects were also demonstrated with electrical (ERP) and hemodynamic (fMRI) measures of brain activity related to preparation of motor responses. Both the lateralized readiness potential (LRP), a component of ERP generally viewed as a marker for response preparation (Coles, Gratton, & Donchin, 1988), and the fMRI signal in the motor cortex showed a small but reliable prime-induced activation on the wrong response side on incongruent trials relative to congruent trials.

Damian (2001), however, questioned Dehaene et al.'s (1998) interpretation, proposing instead that the motor activations may have reflected learned stimulus-response mappings. Damian pointed out that in Dehaene et al.'s bigger-/smaller-than-5 task there was an opportunity to learn the association between perceptual features of the stimuli and the response (i.e., "press the right/left key"), because the masked primes were repeatedly used as (visible) targets. In support of this view, Damian showed that in a size categorization task using a small set of nouns denoting objects (e. …

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