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Since the time of Freud, the case study method has been the primary method for developing psychoanalytic theory and treatment. A number of challenges to the scientific validity of this method for building theory have occurred because it is contaminated by observer bias. Kandel (1983) noted that hypotheses could be developed through the case study method, but they needed to be validated by empirical research. To meet the challenge that psychoanalysis was unscientific, one of the earliest methods to obtain empirical data was through subliminal stimulation experiments. Hypotheses could be tested in a controlled laboratory setting. These experiments demonstrated that an unconscious actually exists and also showed that the unconscious influenced normal behavior and psychiatric conditions.
The earliest studies of subliminal stimulation were by Poetzl (1917) and later by Fisher (1954), who exposed individuals to visual stimuli that were below the level of perceptual awareness and then noted their effect on dreaming. In similar studies, Shevrin and Luborsky (1958) found day dreams also to be influenced. Heinemann and Emrich reported that subliminal presentation of emotional words evoked more alpha waves on the EEG than neutral words. However, the scientific methodology pioneered by Silverman (1971) permitted investigators to explore the unconscious. Through controlled, double-blind experiments, the problem of observer bias was eliminated. Here, neither the experimenter nor the subject was aware of which subliminal stimulus was being flashed. This method, which Silverman called “subliminal psychodynamic activation” (SPA), provided the necessary tool to validate in the