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The subliminal presentation of stimuli poses some conceptual problems, however. As Daniel Dennett points out, it can be difficult (or impossible) to distinguish what was experienced and then forgotten from what was never experienced in the first placesee his insightful discussion of Orwellian vs. Stalinesque processes in cognition (D. C. Dennett, 1991. Consciousness explained. Boston: Little, Brown and Co., pp. 116125). This ambiguity is largely attributable to the fact that the contents of consciousness must be integrated over timearound 100 to 200 milliseconds (F. Crick & C. Koch, 2003. A framework for consciousness. Nat. Neurosci. 6[2]: 119126). This period of integration allows the sensation of touching an object and the associated visual perception of doing so, which arrive at the cortex at different times, to be experienced as though they were simultaneous. Consciousness, therefore, is dependent upon what is generally known as working memory. Many neuroscientists have made this same point (J. M. Fuster, 2003. Cortex and mind: Unifying cognition. Oxford: Oxford University Press; P. Thagard & B. Aubie, 2008. Emotional consciousness: A neural model of how cognitive appraisal and somatic perception interact to produce qualitative experience. Conscious. Cogn. 17(3): 811834; B. J. Baars & S. Franklin, 2003. How conscious experience and working memory interact. Trends Cogn. Sci. 7(4): 166172). The principle is somewhat more loosely captured by Gerald Edelmans notion of consciousness as the remembered present (G. M. Edelman, 1989. The remembered present: A biological theory of consciousness. New York: Basic Books).

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B. Libet, C. A. Gleason, E. W. Wright, & D. K. Pearl, 1983. Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential): The unconscious initiation of a freely voluntary act, Brain 106 (Pt 3): 623642; B. Libet, 1985. Unconscious cerebral initiative and the role of conscious will in voluntary action. Behav. Brain Sci. 8: 529566. Another lab has since found that a persons judgment of when he intended to move can be shifted in time by giving him delayed sensory feedback of his actual movements. This suggests that such judgments are retrospective estimates based on the apparent time of movement and not based on an actual awareness of the neural activity that causes the movement (W. P. Banks & E. A. Isham, 2009). We infer rather than perceive the moment we decided to act. (Psychological Science, 20: 1721).

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