Unconscious semantic priming extends to novel unseen stimuli
Introduction
A debated issue in cognitive neuroscience concerns the existence of an unconscious semantic processing of masked primes. There are many convincing demonstrations that a masked visual stimulus, which is presented too briefly to be consciously seen, can nevertheless be processed at various sub-semantic levels. Recent examples include visual word identification (Ferrand et al., 1994, Forster, 1999), processing of facial emotions (Whalen et al., 1998), and stimulus–response associations (Eimer and Schlaghecken, 1998, Neumann and Klotz, 1994). Several demonstrations of unconscious access to the meaning of masked primes have also been put forward (Dell'Acqua and Grainger, 1999, Draine and Greenwald, 1998, Greenwald, 1996, Luck et al., 1996, Marcel, 1983).
Most of this literature focuses exclusively on the existence of unconscious access to semantics, leaving open the more general scientific issue of whether semantic manipulations can be performed unconsciously. Can the unconscious meaning of a word be mentally manipulated and used to bias behavior? We recently addressed this issue using a number comparison task with masked primes (Dehaene et al., 1998). Subjects pressed a right or left key to decide whether a visually presented target number was larger or smaller than 5. Unbeknownst to them, another number was presented for 43 ms just before the target, surrounded by masks that made it invisible. Behavioral and brain imaging (ERPs and fMRI) measures revealed that this numerical prime nevertheless yielded a covert motor interference effect. Subjects were faster when the prime and target numbers fell on the same side of 5, and therefore called for the same motor response (congruent trials), than when they did not (incongruent trials). Furthermore, measures of motor activity showed directly that subjects were programming a covert motor response appropriate to the prime before implementing the overt motor response appropriate to the target. We concluded that information extracted from the unseen prime was processed through an entire series of processing stages that included the semantic categorization of the numerical prime as larger or smaller than 5, and that proceeded all the way down to a motor level.
Recently, however, an alternative non-semantic interpretation of masked priming experiments has been gaining strength (Abrams and Greenwald, in press, Damian, in press, Eimer and Schlaghecken, 1998, Neumann and Klotz, 1994). Abrams and Greenwald asked subjects to evaluate the valence of consciously seen targets words as positive or negative. The prior presentation of a masked word, whose valence could be congruent or incongruent with the upcoming target, was shown to facilitate or interfere with the subjects' response. This seemed to prove that the masked prime was categorized semantically as positive or negative. Crucially, however, Abrams and Greenwald went on to demonstrate that the priming effect was entirely due to the fact that the prime words were also presented as conscious targets in other trials. When they examined generalization to novel primes that were never seen consciously, priming was obtained only inasmuch as some of their letter fragments matched those of a word from the target list. For instance, after repeated conscious classification of the words smut and bile as negative, the subliminal prime smile primed the negative response, not the positive one. This result suggests that the priming effect, in this situation, was not due to a subliminal access to semantics. Rather, subjects had learned to respond rapidly to fragments of the target strings with specific left or right key presses, and this sensorimotor learning generalized to other primes made of the same fragments. A similar conclusion has recently been reached by Damian on the basis of a lack of generalization of subliminal priming to novel primes in a semantic categorization task. Those results can be captured by the Neumann and Klotz (1994) direct motor specification hypothesis (hereafter DMS), according to which subjects unconsciously associate each visual stimulus with the adequate response, thus bypassing semantic access.
Could it be the case that under conditions of unconscious perception, the only available pathway is such a non-semantic associative response chain? Our number priming results (Dehaene et al., 1998) can certainly be reinterpreted non-semantically within the DMS framework. Rote stimulus–response learning was possible in our experiment because only eight stimuli (the numbers 1, 4, 6 and 9 in Arabic or in verbal notation) were used repeatedly. Furthermore, and crucially, the same numerical stimuli were used as both primes and targets. Although the instructions were changed in the middle of the experiments, subjects could have rapidly learned to associate each visual stimulus (or a fragment of it) with the corresponding motor response. Following this learning period, the response priming effect we observed could have arisen entirely from a sub-semantic visuomotor association process.
Because the DMS framework seems potentially capable of explaining away most, if not all, previous experiments thought to demonstrate unconscious semantic priming, it becomes crucial to obtain direct evidence for or against it. The purpose of the present paper is to provide such evidence by examining whether the numerical priming effect generalizes to novel unseen primes. We replicate our number priming experiment with two sets of numerical primes. Some are number words or Arabic numerals that can also appear as conscious targets (old set primes). Others are novel numbers that never appear as targets, and thus are never consciously perceived during the entire experiment (new set primes). The semantic and DMS hypotheses make contrasting predictions as to whether those primes should cause response priming. If the DMS hypothesis is correct, response priming should be obtained with the old set primes, but it should not be significant with the new set primes because the subjects have had no occasion to associate their visual shapes with a specific motor response. If, however, the semantic interpretation is correct and the response priming effect reflects a genuine unconscious categorization of the primes at the semantic level, then this effect should extend to the new set primes. Subjects should unconsciously apply the semantic number comparison task to all the numerical primes. This should result in an interference effect for all primes, whether new or old.
Section snippets
Subjects
Eighteen right-handed undergraduate students (nine males) ranging in age from 18 to 31 years (mean age 21.4 years, median age 21 years) volunteered for this study. All of them were naive as to the purpose of the experiment, gave their informed consent and received 30 FF (4.6 Euros).
Instructions
Subjects were told that they would see a target number between 1 and 9, excluding 5, and that they would have to compare it to a fixed standard of 5 by pressing one of two response buttons with their index fingers as
Materials and methods
Eighteen right-handed subjects (five males) ranging in age from 20 to 34 years (mean age 25.5 years, median age 24 years) volunteered for this study. All of them were naive as to the purpose of the experiment, and gave their informed consent. Instructions and stimuli were identical to Experiment 1. We assessed prime visibility using both the subjective method described above (see Section 2) and a new objective measure. At the end of the main experiment, subjects were fully informed of the
General discussion
The aim of the present study was to test two alternative interpretations of the semantic response interference effects with masked primes. One interpretation is that those effects stem from an unconscious categorization of primes at a semantic level. Another proposes that they are caused by direct motor specification via a non-semantic stimulus–response chain. The results were clear-cut in supporting the semantic hypothesis.
First, we observed a significant response priming effect for prime
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