Three Cognitive Markers of Unconscious Semantic Activation

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The term subliminal implies a theory of the perceptual threshold, or limen, that has been superseded in modern psychology as a consequence of the influence of signal detection theory [D. M. Green and J. A. Swets, Signal Detection Theory and Psychophysics (Wiley, New York, 1967)]. The term “marginally perceptible” carries less excess meaning in designating the class of stimuli that appear to evade conscious perception. We nevertheless use “subliminal” because of its widespread nontechnical use to designate marginally perceptible stimuli and because it continues to be used routinely in psychology even by those who no longer accept the concept of well-defined thresholds.

Greenwald A. G., Draine S. C., paper presented at the 36th Annual Meeting of the Psychonomic Society; 10 to 12 November, 1995; Los Angeles, CA.

Subjects, all of whom were University of Washington undergraduates, gave consent to participation after having read a preliminary description of experimental procedures.

On each trial of the task, a prime word (either a male or female name, or a pleasant or unpleasant word) was briefly displayed and, after a variable short delay, the target word (a different first name or a different affectively polarized word) was presented. Prime and target words were randomly selected on each trial with two constraints: (i) no target was presented twice in any block of 50 trials, and (ii) the proportion of congruent trials (prime and target having the same affective meaning, or prime and target having the same name gender) was constrained to an average of 50%. One hundred different stimuli (words or names) were used for each categorization task. In each task one subset of 50 served as primes and the remaining 50 served as targets, with these assignments appropriately counterbalanced across subjects. Examples of stimuli are as follows: unpleasant (vomit, kill, bomb), pleasant (honor, happy, kiss), male (mike, david, kevin), and female (kate, mary, sarah). The subject's instructed task was to classify the target word by pressing a key on the left or right side of a computer keyboard (for example, left key to indicate unpleasant and right key for pleasant). After a few blocks of 10 to 20 trials each for practice with the categorization task, subjects started to practice producing their responses during a “response window” that was initially established as the interval from 383 to 517 ms after start of presentation of the target word. Some of the experiments took advantage of speed-accuracy trade-offs [W. A. Wickelgren, Acta Psychol. 41, 67 (1977); B. A. Dosher, Cognit. Psychol. 13, 551 (1981)] by shifting the temporal position of the response window to a shorter or longer value, depending on the subject's ability to perform (for example, shortening it if subjects were making few errors). The response window procedure obliged subjects to respond at speeds that did not permit high levels of accuracy and, consequently, error rates were substantial. (Mean latencies of highly motivated subjects under instructions to respond rapidly are typically between 550 and 650 ms.) The production of relatively high error rates allowed the priming effect—that is, the effect of the prime's congruence versus incongruence with the target's meaning—to be observed in subjects' error rates rather than in their response latencies to targets. With this response window procedure, priming took the form of lower error rates for congruent priming than for incongruent priming, reflecting some combination of facilitation by congruent primes and interference by incongruent primes. Priming was therefore measurable by observing the difference between these error rates. Even though the procedure was designed to constrain response latencies to approximately the range of values that define the response window, response accuracy was analyzed for all trials except for a small percentage with latencies greater than 1500 ms, a value substantially longer than the time elapsed at the end of the window interval. The primary measure used with these data was signal detection theory's d′ measure of sensitivity of the target word's response to the prime word's meaning [see the further explanation in the legend to Fig. 1, and D. M. Green and J. A. Swets, Signal Detection Theory and Psychophysics (Wiley, New York, 1967)]. Results for this sensitivity measure were similar to those obtained with various alternative measures, such as the increase in error rate for incongruent relative to congruent primes or measured information transmitted from prime stimulus to target response. Prime words were presented for 17, 33, or 50 ms at a centered display location to which subjects were instructed to attend. In the visually masked (subliminal) prime condition, the prime was both preceded and followed, at the same screen location, by strings of consonants that served as forward mask (premask) and backward mask (postmask). (An example of a mask stimulus is the letter string GKQHYTPDGFQBYLG.) The premask, prime, postmask, and target stimuli were presented as black letters on a gray background. The premask lasted 100 ms and the postmask 17 ms. (Other pilot studies had shown that masking effectiveness was unaltered by increases in postmask duration beyond 17 ms, the minimum value obtainable with the 60-Hz computer display used in this research.) Subjects viewed the computer video display through a device that presented images from left and right halves of the display screen separately to left and right eyes. Although this dichoptic presentation was not needed for the present procedures (which presented identical stimuli to both eyes at all times), its use has been found to increase mildly the effectiveness of visual masking. The combination of premask and postmask made the prime words difficult or impossible to see for almost all subjects. By contrast, in supraliminal conditions the masking consonant strings were replaced by blanks (that is, the screen background color), as a consequence of which the prime words were easily legible despite their short (50 ms) duration.

Because preliminary findings revealed that direct measure performance was depressed by the requirement to respond rapidly, the response window procedure was not used during blocks of trials that obtained direct measures. Different discriminations on visually masked stimuli were requested in different experiments to allow opportunities to demonstrate that some types of information might penetrate visual masking more readily than others. The basic properties of the results shown in Fig. 1 did not vary with the different direct measures, adding to confidence in generality of conclusions.

The conclusion that unconscious cognition is indicated by the presence of statistically significant intercept effects in the regression analyses of Fig. 1 rests on a methodological analysis by A. G. Greenwald, M. R. Klinger, and E. S. Schuh [J. Exp. Psychol. Gen. 124, 22 (1995)] that extends the logic of an analysis introduced by P. M. Merikle and E. M. Reingold [J. Exp. Psychol. Learn. Mem. Cognit. 17, 224 (1991)]. A concern in interpreting such intercept effects is the possibility that a spurious intercept may be produced when the predictor (in this instance, the direct measure of prime perceptibility) is imperfectly measured. However, the regression analyses in Fig. 1 do not have the properties that can produce such spurious intercept effects. Such properties include both positive regression slopes and average predictor scores substantially above zero. In contrast, the regression slopes that we obtained were approximately flat and predictor scores (that is, direct measures) were noticeably above zero only with prime duration of 50 ms. For a more detailed discussion of the possibility of spurious intercept effects, see (21).

The level of perceptibility of masked 50-ms primes can be read from the horizontal distribution of values in the lower panel of Fig. 1, A and C. Levels of direct measure performance corresponding to d' values <1.0 are commonly associated with self-reports of little or no perceptibility. Findings of SOA effects closely resembling those in Fig. 2B were obtained when the plotted variable was changed to magnitude of intercept effect from regression analysis; that is, statistically significant intercept effects were found only for the 67-ms SOA. The intercept-effect alternative analysis confirms that the pattern in Fig. 2B for subliminal priming as a function of SOA is indeed a pattern for unconscious priming. The plotted analysis in Fig. 2B, which includes all subjects who received masked priming, is properly comparable to the analysis in Fig. 2A for supraliminal priming (for which regression analysis is not an appropriate method).

The result shown in Fig. 3 is related to one previously reported by J. Cheesman and P. M. Merikle [Can. J. Psychol. 40, 343 (1986)]. They showed that supraliminal priming was greater when there was a higher proportion of congruent priming trials in a block of trials, but subliminal priming showed no such effect. Their finding could be explained by the difference in two-trial sequential effects shown in Fig. 3.

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The brevity of unconscious semantic activation measured by the prime-target SOA should not be confused with the latency after masked prime presentation at which semantic information is available. Semantic activation is presumed to occur after preliminary operations that may require a few hundred milliseconds for subliminal prime words (as well as for visible target words).

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Greenwald A. G., Draine S. C., 25th Carnegie Symposium on Cognition: Scientific Approaches to the Question of Consciousness, Cohen J. D., Schooler J. W., Eds. (Erlbaum, Mahwah, NJ) in press.

Cubic polynomial regression functions were used to capture possible nonlinear trends in the data. However, it can be seen in the figure that intercept effects for these nonlinear functions were similar to those estimated by linear regression functions.

Abrams R. L., Draine S. C., Greenwald A. G., paper presented at the 36th Annual Meeting of the Psychonomic Society; 10 to 12 November, 1995; Los Angeles, CA.

Supported by grants from the National Institute of Mental Health (MH-41328) and National Science Foundation (SBR-9422242).