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This puzzle appeared in the 'Skeptical Adversaria' (the newsletter of ASKE, the Association for Skeptical Enquiry), April 2006.
Early laboratory experiments on ‘subliminal perception’ purported to reveal that people could demonstrate awareness of the emotional nature of a stimulus before they became aware of its meaning. Typically a word would be flashed on a screen (in the olden days experimenters used an instrument called a tachistoscope) initially at exposure times too short for the subject to recognise it. Duration of exposure would be gradually increased until the subject correctly identified the word. It was found that ‘taboo’ words such as ‘penis’ and ‘whore’ required longer exposure times for correct recognition than neutral words. Moreover, at exposures to short for verbal recognition, subjects displayed elevated physiological indices of emotionality for taboo words compared with those for neutral words, the response investigated typically being skin conductivity, which is affected by sweating.
It was hypothesised that these results demonstrated a mechanism termed ‘perceptual defence’ whereby the brain is able to identify the emotional nature of a stimulus prior to conscious recognition. In turn this was interpreted as part of a more general process of pre-conscious awareness of the meaning of a stimulus (‘subliminal perception’ or ‘subception’ as it was occasionally termed).
Although perceptual defence may be a real phenomenon, consideration of the above experiment suggests that (other than experimenter effects and demand characteristics) there are explanations for the results that do not require pre-conscious awareness of the kind suggested. Any ideas?
One of the difficulties with this kind of experiment is that whereas the subject has the opportunity to signal emotional sensitivity to a stimulus over a wide range of amplitudes of the physiological response, verbal recognition can only be scored as correct or incorrect. In other words the first measure is analogue, the second digital. (It may amount to very nearly the same thing to state that physiological responses may be conditioned to the structural properties of the stimulus and may be activated when the subject is partially aware of these but not sufficiently so to correctly perceive the word.)
A second problem is that the defensiveness may be located not in perceiving but in responding. That is, subjects may be reluctant (consciously or otherwise) to offer as guesses taboo words, such as ‘penis’ and ‘whore’, until the evidence is clearer than in the case of neutral words. In that case we would say that their criterion for identifying a taboo word is stricter than for a neutral word, hence the higher recognition thresholds even when their emotional response is indicating that a taboo word is present.
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