Psychological Investigations of Unconscious Perception

This paper reviews the history of psychological investigations of unconscious perception and summarizes the current status of experimental research in this area of investigation. The research findings described in the paper illustrate how it is possible to distinguish experimentally between conscious and unconscious perception. The most successful experimental strategy has been to show that a stimulus can have qualitatively different consequences on cognitive and affective reactions depending on whether it was consciously or unconsciously perceived. In addition, recent studies of patients undergoing general anaesthesia have shown that the effects of stimuli perceived unconsciously during surgery can last for approximately 24 hours. Taken together, the results of these recent psychological investigations provide empirical support for the importance of unconsciously perceived information in determining cognitive and affective reactions. Investigations of unconscious perception have a long history in psychology. In fact, some of the very earliest studies conducted in psychology laboratories in North American involved demonstrations of unconscious perceptual influences. However, despite this interest in unconscious perception since the late 1800s, it has only been within the past fifteen to twenty years that our understanding of unconscious perceptual processes has advanced considerably. A major reason it took so long to make significant progress is that initially an unanswerable question was asked. The question that most research studies addressed was ‘Are stimuli unconsciously perceived?’ In other words, the primary goal of these studies was to prove the existence of unconscious perceptual processes. Two different experimental approaches were followed in these attempts to prove the existence of unconscious perceptual processes. One approach was based on introspective measures of awareness and the other approach was based on behavioural measures of awareness. Neither approach was successful, and in the first part of this paper, both approaches are reviewed to illustrate why they were unsuccessful. Once it was realized that it is impossible to either prove or disprove the existence of unconscious perceptual processes, another more interesting and more productive question was asked. This question assumes that the conceptual distinction between conscious and unconscious perception is meaningful and asks ‘Are the consequences of unconscious perception qualitatively different from the consequences of conscious perception?’ In the second part of this paper, we describe some of the qualitative differences between unconscious and conscious perceptual processes that have been established. These differences involve both cognitive and affective reactions to stimuli. Taken together, the results of these studies provide rather compelling evidence for the importance of unconscious perceptual processes. In fact, by establishing Journal of Consciousness Studies, 5, No. 1, 1998, pp. 5–18 * Name and address for correspondence. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada to P.M. Merikle and M. Daneman. how unconscious and conscious perceptual processes differ, it has been possible to obtain stronger evidence for the existence of unconscious perceptual processes than it was possible to obtain via direct attempts to demonstrate that stimuli are unconsciously perceived. The final issue we consider in this paper concerns the duration of the influence of unconsciously perceived stimuli. To date, much of the psychological research on unconscious perception has only considered relatively short temporal intervals lasting no more than a few seconds. Obviously, if unconscious perception has an important influence on cognitive and affective reactions, then the effects of unconsciously perceived stimuli must last for considerably longer than a few seconds. Evidence to suggest that unconsciously perceived stimuli can have effects over longer temporal intervals comes from research examining whether patients have memory for events that occurred while they were under general anaesthesia. In general, the results of this research show that unconsciously perceived stimuli can have effects over periods of time measured in hours and days. Does Unconscious Perception Exist? Many psychological studies of unconscious perception have attempted to prove the existence of unconscious perceptual processes by demonstrating that stimuli are perceived when subjects are not consciously aware of the stimuli. The basic strategy followed in these studies is to establish conditions under which conscious perception does not occur and then to demonstrate that stimuli can nevertheless be perceived under these conditions. The success of these studies depends completely on the acceptability of the method used to establish the absence of conscious perception. In the earliest studies, inferences concerning the absence of awareness were based on subjects’ introspective reports. In general, if the subjects’ statements indicated an absence of relevant conscious perceptual experiences, it was assumed that the subjects were in fact unaware of the stimuli. In more recent studies, the absence of relevant conscious experiences has been defined in terms of behavioural measures that indicate an inability to discriminate between alternative stimuli. Studies based on both types of measures have not led to completely convincing results because it is always possible to question whether the measure of conscious perception was successful in guaranteeing a complete absence of ALL relevant conscious experiences. Introspective measures of awareness Studies of unconscious perceptual processes based on introspective measures of awareness date from the very beginning of experimental psychology in North America (see Adams, 1957, for a review of many early studies). As an example of this general approach, consider an experiment conducted in the Psychological Laboratory at Harvard by Boris Sidis and reported in his 1898 monograph, The Psychology of Suggestion: A Research into the Subconscious Nature of Man and Society. Sidis showed subjects cards containing a single printed digit or letter. ‘The subject was placed at such a distance from the card that the character was far out of his range of vision. He saw but a dim, blurred spot or dot’ (p. 170). In fact, ‘the subjects often complained that they could not see anything at all; that even the black, blurred, dim spot often disappeared from their field of vision’ (p. 171). However, when Sidis asked UNCONSCIOUS PERCEPTION 6