Subliminal or unconscious perception refers to the idea that stimuli presented below the threshold for conscious awareness can influence an individual's thoughts, feelings, or actions (2). The possibility that an individual can acquire and act on input without being aware of doing so has implications for the study of consciousness and the larger set of processes which characterize the I-function. It is generally assumed that that conscious perception of a stimulus is necessary in order to act on that stimulus, and this conscious decision to act is one of several processes which characterize the I-function. Thus, it is useful to explore whether the existence of subliminal/unconscious perception negate the role of the I-function. Several psychological and neuropsychological examples of subliminal/unconscious perception, including unconscious semantic priming, memory for unconsciously perceived events, blindsight, and prosopagnosia, can be considered in relation to consciousness and the I-function.
Early psychological studies conducted during the late 1800s and early 1900s were among the first to suggest that unconscious processing of stimuli presented below the level of conscious awareness can be detected in responses to questions which require subjects to make decisions regarding the unperceived stimuli. In such studies, subjects were presented with indiscernible stimuli and asked to make guesses regarding the nature of the stimuli. For example, visual stimuli such as letters, numbers, or geometric shapes were presented at such a great distance from the subjects that they could not be seen clearly, or names of letters would be whispered so faintly that subject could not hear them. Interestingly, it was shown that, when asked to guess whether a given stimuli was a letter or digit, subjects were able to do so with an accuracy which could not be attributed to chance guessing (2).
Unconscious semantic priming in controlled laboratory studies provides additional evidence for the existence of subliminal/unconscious perception. Classic studies conducted in the 1970s by Anthony Marcel demonstrated that a decision regarding a stimulus can be influenced by an unconsciously perceived, preceding stimulus. For example, classification of a letter string stimulus as a word or a nonword occurs more quickly when it is preceded by a semantically related word than when it is preceded by a semantically non-related word (2). The fact that the initial stimulus influences the classification of a proceeding stimulus implies that the initial stimulus has been processed unconsciously. Many additional studies have replicated these findings, establishing the validity of unconscious semantic priming (3).
The existence of memory for unconsciously perceived events provides further evidence of subliminal/unconscious perception. For example, evidence suggests that surgical patients who undergo general anesthesia may have some memory for events which occurred during the surgery if their memory is assessed via indirect methods. Such memory tasks include having the patient listen to word stimuli through head phones during the surgery and then asking them to complete word stems which represent the beginnings of those stimuli. In such cases, patients tend to complete the word stems with words which were presented during the operation (4). Since patients under general anesthesia are unaware of stimuli in the external environment, the fact that stimuli presented during the surgery can influence post-operative tasks suggests that the initial stimuli were perceived and processed unconsciously.
Additional evidence for the existence of unconscious processing comes from the study of neurological disorders in which conscious perception of visual input is specifically impaired, while the ability to process and make use of such input remains preserved. Disorders of this nature include blindsight and prosopagnosia. The study of the specific nature of the impairments seen in these patients provides a unique neurobiological perspective to the study of visual processing as well as conscious and unconscious perception.
Blindsight is a rare phenomenon in which individuals with damage to their striate cortex - the center for the control of vision in the brain - retain the ability to discriminate unseen stimuli in their clinically blind visual field when forced-choice procedures are used (5). Although blindsighted individuals deny any awareness of visual perception in their blind visual field (or the blind regions of their impaired visual field), their ability to process and act on visual input presented to the blind field seems to remain relatively intact. For example, even when blindsight patients say that they can not see anything in their blind field, they can detect, locate, describe, and discriminate between static, flickering, and moving targets (5). While many of the tests used to evaluate the rare abilities of blindsight patients rely heavily the subjective reports of these patients, more recent studies have employed direct measures of saccadic eye movements and pupillary responses (6) to visual stimuli in order to prove that these patients are not using the visual abilities of their good fields in completing blindsight tasks.
While the neurobiological basis of blindsight remains relatively unclear, it has been suggested that the ability to process and make use of unseen visual input may be mediated by a secondary neuronal pathway which branches from the optic nerve, bypasses the main reception site for normal vision - the V1 area of the primary visual cortex, and extends instead to an area of the midbrain known as the super colliculus (7). Studies employing functional magnetic resonance imaging (fMRI) scans in an attempt to localize and compare neural activity in the brain during the execution of conscious and unconscious seeing tasks lend support to this theory. The results of such studies show more activity in the cortex when the patient is aware of presented stimuli and more activity in the superior colliculus when the patient is unaware of the presented stimuli (8). The fact that a specific neuronal pathway may account for the unconscious processing which occurs in blindsight, suggests that it may eventually be possible to develop a neurobiologcal explanation for the more general phenomenon of subliminal/unconscious processing.
Prosopagnosia is another rare and peculiar neurological disorder characterized by a specific inability to recognize faces (9,10). As with blindsight, patients with prosopagnosia demonstrate an ability to perceive, process, and use the visual input of which they claim not to be aware. For example, prosopagnosics are faster at matching simultaneously presented faces when they are familiar (11). In addition they show distinctly different levels of skin conductance in response to familiar and nonfamiliar faces (7). Thus, these patients are yet further evidence for the existence of subliminal/unconscious processing.
While the above psychological and neuropsychological phenomena have important implications for the study of consciousness and the I-function, there is no evidence to suggest that voluntary action can be initiated based unconsciously processed stimuli. Rather, evidence indicates that one must be aware of perceiving stimuli in order to initiate action based on that stimuli (2). In this way, subliminal/unconscious perception is much like the "reflexs" which quadriplegics are able to produce despite their inability to initiate voluntary motor movements. Thus, the I-function and conscious awareness do seem to be necessary for normal functioning despite the existence of unconsciousness perception and processing.
2)Su bliminal Perception, Encyclopedia of Psychology
3)Replicable Unconscious Semantic Priming
4)Memory for Unconsciously Perceived Events: Evidence from Anesthetized Patients 5)Blindsight and Visual Awareness, Conscious and Cognition, 7, 292-311
6)Pupillary Responses With and Without Awareness in Blindsight, Consciousness and Cognition, 7, 324-326
7) Weiskrantz, Lawrence. (1992). Unconscious Vision: The
strange phenomenon of blindsight. The Sciences, 32(5), 23-28.
8)Out of Sight,
Into Mind, The Scientist Planet Sciences
10)Face
Blind!, A personal account of prosopagnosia
11)On a
Confusion About a Function of Consciousness, Unedited preprint version
of: Behavioral and Brain Sciences, 18(2), 227-287
12)Psy
chological Investigations of Unconscious Perception, Journal of
Consciousness Studies, 1998, 5, 5-18
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