Abstract
We studied the neuronal correlates of consciousness by characterizing the role of attention and awareness in three psychophysical experiments. First, we investigated the role of visual awareness in the formation of afterimages, phenomena believed to occur in the retina. Visibility of the afterimage-inducing stimuli was manipulated by a powerful dichoptic suppression technique, continuous flash suppression, which allows us to project visual stimuli onto the retina without subjects noticing them at all, sometimes longer than three minutes. We found that reliably suppressing the inducer weakens afterimage strength. Paradoxically, trial-to-trial variability in visibility did not correlate with the intensity of afterimage. As afterimages are enhanced when attention is withdrawn from the adaptor, the opposite effects between awareness and attention were demonstrated. Second, we examined visual motion processing outside the focus of spatial, top-down attention using a dual-task paradigm. Attentional effects in motion processing were characterized by our novel wavelet motion stimuli. Our stimuli effectively activate neurons in the first stage of motion processing, while they are poor stimuli for higher motion processing. Using a contrast-masking paradigm, we found that attention mainly affected the strength of inhibition for high-contrast motion stimuli in an orientation-specific, but not direction-specific manner, presumably reflecting the physiological properties for divisive inhibition within the primary visual cortex. Third, we characterized the role of awareness in classical aversive conditioning. Subjects associated previously neutral auditory stimuli (CS) with aversive mild electric shocks (US). We used skin conductance response, an index for autonomic arousal, as implicit measure for the conditioned response. In delay conditioning, CS was paired with delayed but overlapping US, while in trace conditioning CS was followed by US after a three-second temporal gap. We intermixed these two CSs with another control CS that never predicted US to examine whether awareness plays different roles depending on the temporal relationships between CS and US. Subjects expressed their shock expectancy using their gaze direction, from which we inferred the onset of awareness of CS-US contingency. By aligning the skin conductance response with the onset of awareness, we found that trace, but not delay, conditioning coincided with the onset of awareness.
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