As depicted in Fig. 1, it implies that the brain processes underlying conscious experience differ from those that mediate between incoming stimuli and outgoing responses. It implies that information may be transmitted through the organism without ever achieving conscious representation. Three sorts of evidence support this view: that based upon subjective experience, that stemming from neurophysiological studies, and, most specifically, that relying upon the data from behavioural research on subliminal perception. Let us consider these in order of specificity.
Fig. 1.
Since 1948 the existence of these two systems, a sine qua non of subliminal perception, has been confirmed by neurophysiological research (see Moruzzi and Magoun 1949, Samuels 1959, Dixon 1971, 1981). Whereas the receipt and onward transmission of sensory information, initiated by external stimuli, depends upon the classical sensory pathways linking peripheral receptors with their cortical projections, awareness of this sensory traffic — perceptual experience — relies upon sufficient contribution from the ascending fibres of the reticular activating system, that dense network of cells which arises in the brain stem and then spreads upwards and outwards to infiltrate the cortex. If the reticular system is blocked by surgery or drugs, the arrival of sensory information at the cortex still occurs but the owner of the cortex remains oblivious of the fact! This finding, of considerable significance for proponents of subliminal perception, accords with data from research by Libet and his colleagues (1967). Recording from the exposed brains of fully conscious human subjects, they were able to detect electrical potentials initiated by tactile stimuli of which their subjects remained totally unaware. Pari passu with intensifying the external stimulus, the recorded waveform became more complex and the subjects reported, 'I can feel something'. It seems reasonable to assume that the additions to the waveform reflected those contributions from the reticular system upon which consciousness depends.
In a subsequent experiment it was found that the amplitude of an electrical response, recorded from the visual receiving area and initiated by a flash of light to one eye, could be reduced by subliminal presentation of an emotional word to the other eye. Since this effect did not occur for emotionally neutral words, we must assume that the brain could analyse and respond to the meaning of words of whose presence the recipient remained unaware. Yet other studies (Dixon 1971, 1981) have shown, by its electrical response, that the human brain will respond to the meaning of words presented to the ears during sleep. It is interesting to note that even in the deepest sleep, and without awakening the subject, such words may also evoke dreams that are relevant to their meaning.
The researches involving human brain responses to subliminal stimuli have their counterpart in studies of lower animals. Thus it has been shown that a monkey's recognition threshold for a meaningful pattern may be significantly altered by direct electrical stimulation of the animal's reticular system. It has also been found that even the surgically isolated forebrain of a cat will respond to a previously learned pattern when this is presented to the preparation's one remaining eye. Since what is left of the animal in this experiment could hardly be capable of consciousness, this finding illustrates a simple and direct instance of subliminal perception.
One of the most extensively researched examples of subliminal perception occurs in connection with the fact that the awareness threshold for threatening words or pictures may be significantly higher or lower than that for more neutral material. Experiments which involved the simultaneous recording of EEGs (brain rhythms — see electroencephalography), heart rate, and perceptual thresholds suggest that, prior to awareness of a visual stimulus which is gradually increasing in brightness, the brain may analyse the latter's meaning and, as a result, modify its own level of arousal to hasten or retard awareness of the information that it carries.
That the brain monitors and analyses subliminal stimuli receives support from many comparable investigations. Thus, emotional words, presented below threshold to the eye, have been found to change auditory sensitivity, and vice versa. By the same token, during binocular rivalry, in which the subject perceives either with the left eye or with the right but never with the two together, the introduction of a subliminal stimulus to the 'blind' eye produces immediate transfer of perception to that side. Such a mechanism which automatically switches into consciousness a stimulus array that has changed (i.e. one that constitutes a potentially important new stimulus) has obvious survival value.
It can be argued that a capacity for subliminal perception came about with the evolution of attentional mechanisms. Since the span of consciousness is severely restricted, selective processes evolved whereby only a limited proportion of available sensory information could be admitted to consciousness. Subliminal stimuli constitute some part of the remainder — stimuli which, though insufficiently strong or important to warrant entry into consciousness, may nevertheless be received, monitored, and reacted to. A number of recent pieces of research attest to this view. In one (Corteen and Wood 1972), people were asked to report a stream of prose presented to one ear while words of which they remained unaware were presented to the other ear. It was found that those 'subliminal' words (on the unattended-to ear), which had previously been associated with electric shock, produced an emotional response (i.e. a change in skin resistance due to sweating) without interfering with the attentional task of 'shadowing' prose on the other ear. In another experiment (Henley and Dixon 1974) imagery evoked by music presented to one ear, above the conscious threshold, was shaped by subliminal words to the other ear. Considered together, these two sets of data suggest that, at a preconscious level of processing, the brain can 'decide' whether or not information on a subsidiary or unattended-to channel should be kept isolated from, or used to facilitate, responses evoked by material to which the recipient is devoting his conscious attention.
That a subliminal stimulus can bypass the moderating, rationalizing effects of consciousness has given rise to a number of useful applications of stimulation below awareness. These include the investigating of processes underlying such psychiatric disorders as anorexia nervosa and schizophrenia. Perhaps the most clearly useful application to date has been in the selection of pilots for the Royal Swedish Air Force. In the defence mechanism test (Kragh 1962a, 1962b) the candidate for a flying career has to describe what he sees when flashed a composite picture, consisting of a central human figure which is flanked by a subliminal threatening male face. Numerous applications of this test have shown that those candidates whose responses show characteristic distortions as a result of the subliminal threat are likely to make accident-prone pilots. By using this test to eliminate undesirable trainees, the Air Force has succeeded in making a significant saving in lives and aircraft.
Despite the very great weight of evidence from many disciplines, there are still those who cannot bring themselves to accept the reality of subliminal perception. In the writer's opinion this carefully sustained prejudice is itself a psychological defence against the threat of possible manipulation which is implied by subliminal effects. There is the suggestion here of an unwarranted equation of consciousness with such nebulous properties of mind as will, self-control, and conscience. Evidence of many kinds, including that from studies of behaviour under hypnosis, suggests that this particular conceit is, to say the least, mistaken.
Besides being used in a diagnostic capacity, subliminal stimulation has now been employed in a therapeutic context. For example, by reducing anxiety through the subliminal presentation of reassuring messages it has been found possible to reduce neurotic overeating in cases of obesity (Silverman et al. 1978) and improve performance at mathematics (Ariam 1979). Other techniques involving stimulation below the conscious threshold have proved useful in the treatment of phobias (Tyrer, Lee, and Horn 1978).
From the evidence to date, it seems that all the sensory modalities shown in Fig. 2 have a subliminal range within which excitation can occur without conscious representation. Of particular interest in this connection is the finding (Cowley, Johnson, and BrooksbankM 1977, Kirk-Smith et al. 1978) that even subliminal olfactory stimuli — e.g. pheromones — may have a significant effect. Even though unable consciously to detect the smell of female pheromones, their inhalation by male subjects made the latter perceive photographs of women as more attractive than they would otherwise have been.
The subliminal range for each modality is still not clear but probably depends upon the relative importance, from a survival point of view, of the modality in question and the extent to which its response system can function adequately without the aid of consciousness. Thus, while pain might be expected to have a very short range (i.e. it would be important to be immediately aware of noxious stimuli) the interoceptive senses (i.e. those concerned with internal 'automatic' bodily responses) remain almost entirely subliminal (as indeed we know they are).
Fig. 2. Hypothetical extents of subliminal and phenomenal effects for different sensory modalities.
(Published 1987)
See also attention.
— N. F. Dixon
- Bibliography
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- Cowley, J. J., Johnson, A. L., and Brooksbank, B. W. L. (1977). 'The effect of two odorous compounds on performance in an assessment of people test'. Psychoneuroendocrinology, 2.
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- — — (1981). Preconscious Processing.
- Groeger, J. A. (1984). 'Preconscious influences on language production' (Ph.D. thesis, Belfast).
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- — — Martin, A., Ungaro, R., and Mendelsohn, E. (1978). 'Effect of subliminal stimulation of symbiotic fantasies on behaviour modification treatment of obesity'. Journal of Consultative Clinical Psychology, 46/3.
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