And in additional review to Merikle and Cheeseman (1984) in relations to this issue, they identified two classes of threshold measures: subjective and objective (Experimental 2). Within the scope of dissociation, this subjective/objective threshold paradigm shows two different ways to evaluate whether a participant is consciously aware of a stimulus. The subjective measure relies on the participant's self report of the existence of a stimulus. In other words, the participant simply indicates whether or not they were aware of the stimulus. Essentially, a subject has to decide whether a word or nothing appeared over a number of blocks of trials, each having different exposure time. The exposure time for the first block is usually long enough for subjects to perform well above chance. After each block, the subject’s detection performance is assessed and if it is found to be above sixty percent, then in the next block of trials, the exposure time will be shortened by approximately five milliseconds. This process is then repeated until the subject performs below sixty percent for a set of trials, after which a final block of trials will be administered at the same exposure time to in order to confirm that performance is below sixty percent. This exposure time is then set as the subject’s detection threshold, and stimuli shown at this time are assumed to be subliminal, by which in their opinion, “a distinction must be made between the subjective threshold, the level of discriminative responding at which observers claim not to be able to detect perceptual information at better than a chance level of performance, and the objective threshold, the level of discriminating responding corresponding to chance-level performance” (Merikle and Cheeseman, 1986., p. 42 re-visited).
However, a disadvantage of the subjective measure is that a response bias may lead the participant to choose against reporting a stimulus when the participant feels ambivalence. As a result, each person may gauge “awareness” using their own terms, meaning inconsistencies among experiments. An objective measure, on the other hand, is obtained when the subject is forced to choose between fixed alternatives or discriminate between several options – even if they believe the options are equivalent.
A further book review of Kihlstrom, Barnhardt, and Tataryn, 1992, with regards to the research works of Mike and Reingold’s experiment on subliminal perception in general, it argues that apart from being methodologically superior, the Mike and Reingold experiment also represents a conceptual shift away from looking at subliminal perception as a unique phenomenon, to a perspective which places subliminal perception under the larger umbrella of unconscious perception, which includes perception during hypnosis-included blindness, when attention is diverted or divided, or in cases of blind sight. In a sense, the key phenomenon being investigated with this method may not be perception of stimulus presented below the threshold of detection, but rather, the unconscious perception, by which Kihlstrom, Barnhardt, & Tataryn, 1992, criticized that the level of detect ability where perceptual information may be discriminated at a chance level. This may perhaps suggest that this objective measure provides a lower threshold for conscious awareness, leading to more conservative evaluations of when subliminal perception occurs.
In spite of its popularity within Merikle and Cheeseman’s research on subliminal perception, one problem with their method of experiment just explained is that it assumes that if the rates of detection are no different from chance, then on the trials where the subject reported seeing something, he was merely guessing correctly. However, it may be the case that absolute exposure time has some significant effect on the amount of perception independent of awareness, such that individuals with high thresholds may show more subliminal perception than those with low thresholds because of the difference in exposure time.
Here, as we can see, there is disagreement among researchers on whether the subjective or objective threshold measure should be used, or whether the dissociation paradigm as a whole is suitable. No doubt, with the apparent uncertainty of the dissociation paradigm, researchers have looked for new ways to establish whether perception without awareness occurs at all. The “exclusion paradigm” (Debner and Jacoby, 1994) offers a different way of looking at the effect by pitting conscious processing against unconscious processing which may be able to avoid the disputed aspects of the dissociation paradigm. The word completion experiment illustrates how the exclusion paradigm puts conscious and unconscious processes in conflict with each other. For example, subjects in Debner and Jacoby's experiments were presented with a priming word but were told to avoid using the prime in a word completion exercise. The intent of this experimental design was that if subjects were consciously aware of the stimulus, they would avoid using the stimulus to complete words. Subjects were first briefly flashed priming words for durations that ranged from subliminal to clearly perceptible. Subjects were then given word stems of partially completed words, and asked to complete the words. However, subjects were instructed to not use the priming word in order to complete the partial word. But if however, the subjects were not aware of the priming word but had nevertheless perceived it, then the priming word might influence their word completion task suggesting then that, there would be no conscious effort against using that word. Thus, ruling out the problems with arbitrary thresholds and response bias, since subjects could no longer decide to report awareness on their own terms. They found that indeed, priming words presented for very brief durations – the subliminal stimuli – were much more likely to be used in completing words than the priming words shown for longer durations. This provides an interesting new experimental format in contrast to the usual dissociation, since the exclusion paradigm shows distinct results for consciously versus unconsciously perceived stimuli.
A further review to the experiment of Merikle and Reingold 1990, which looks at unconscious perception of words versus non-words, they found significant differences between detect and non-detect trials, indicating that subjects were not merely guessing when they said they saw the stimulus. In addition, these researchers claimed that the patterns of responses indicated qualitative differences between conscious and unconscious perception. Specifically, the results showed that when subjects reported detecting the stimuli, performance on the forced choice task was significantly above chance for both words and non-words, but when they failed to detect the stimulus, responses to word stimuli were still significantly better than chance whereas those for non-word stimuli were no different from chance? The hypothesis to this explanation to their findings may be that although there is a significant difference between detect and non-detect trials, meaning that awareness does make a difference; this difference may be quantitative rather than qualitative. In this sense, conscious awareness may be seen as a more sensitive index of amount of processing done on the stimulus (meaning that the subject reports detecting the stimulus after a number amount of processing is done on the stimulus) rather than as some mechanism which allows for control or manipulation of the information. Suggesting that the assertion in Merikle and Reingold’s experiment is that regardless of whether the criteria for detection changes, or where the criteria is, there are real consequences for subjectively being aware or not-aware of the stimulus. And while Merikle and Cheeseman (1984) may have addressed the issues by classifying types of self-reports, Reingold and Toth in 1996, described one area of the fundamental issues: “factors unrelated to awareness, such as demand characteristics and preconceived biases, may lead subjects to adopt a conservative response criterion and report null perceptual awareness even under conditions in which conscious perceptual information is available. Response bias represents a threat not only to the validity of the subjective report measure of awareness, but also to its reliability. In particular, variability in response criteria makes it difficult to compare reports of null subjective confidence across-subjects, or within-subjects across conditions”. (p. 162)
While there are much evidence supporting the existence of some kind of perception without awareness, controversy has underscored research in the field as we can see after a few critical reviews, experts disagree on what should be called subliminal perception, and how such effects can be measured experimentally. Certainly, research has overwhelmingly demonstrated that people can perceive stimuli and act on them without reporting being consciously aware of the stimuli. However, whether or not this qualifies as true subliminal perception is still a matter of debating issues. Last but not least, while the controversy will no doubt continue, the overwhelming body of evidence from a diverse set of independent experiments is hard to ignore. It seems that subliminal perception, in some form, does exist and can be reliably measure, but is still a matter for future research.
References
Debner, J. A., and Jacoby, L. L. (1994). Unconscious Perception: Attention, Awareness and Control. Journal of Experimental Psychology: Learning, Memory, and Cognition, 20, 304 – 317.
Kihlstrom, J. F., Barnhardt, T. M., and Tataryn, D. J. (1992). Implicit Perception. In R. F. Bornstein, T. S. Pittman, (Eds.), Perception Without Awareness: Subliminal perception (pp. 17 – 20). New York: Guildford Press.
Merikle, P. M. and Cheeseman, J. (1984). Priming With and Without Awareness. Perception and Psychophysics, 36, 387 – 395.
Merikle, P. M. and Cheeseman, J. (1986). Distinguishing Conscious from Unconscious Perceptual Processes. Canadian Journal of Psychology, 40, 343 – 367.
Merikle, P. M. and Reingold, E. M. (1990). Recognition and Lexical Decision without Detection: Unconscious Perception? Journal of Experimental Psychology: Human Perception and Perfomance, 16, 574 – 583.
Merikle, P. M., and Joordens, S. (1997). Measuring Unconscious Influences. In J. D. Cohen and J. W. Schooler, (Eds.), Scientific Approaches to Consciousness. (pp. 109 – 123). New York: Guildford Press.
Reingold, E. M., and Toth, J. P. (1996). Process Dissociations versus Task Dissociations: A Controversy in Progress. In G. Underwood (Ed.), Implicit Cognition. (pp. 159 – 202). Oxford University Press.
Wilhelmsen, A. and Carr, T. H. and (1989). Task Induced Strategies and Near Threshold Priming: Conscious Influences on Unconscious Perception. Journal of Memory and Learning, 28, 412 – 443.
