Which factors determine whether a stimulus is consciously perceived or unconsciously processed?
Here, I investigate how previous experience on two different time scales – long term experience
over the course of several days, and short term experience based on the previous trial – impact
conscious perception. Regarding long term experience, I investigate how perceptual learning does not
only change the capacity to process stimuli, but also the capacity to consciously perceive them. To
this end, subjects are trained extensively to discriminate between masked stimuli, and concurrently rate
their subjective experience. Both the ability to discriminate the stimuli as well as subjective awareness
of the stimuli increase as a function of training. However, these two effects are not simple byproducts
of each other. On the contrary, they display different time courses, with above chance discrimination
performance emerging before subjective experience; importantly, the two learning effects also rely on
different circuits in the brain: Moving the stimuli outside the trained receptive field size abolishes the
learning effects on discrimination ability, but preserves the learning effects on subjective awareness.
This indicates that the receptive fields serving subjective experience are larger than the ones serving
objective performance, and that the channels through which they receive their information are arranged
in parallel. Regarding short term experience, I investigate how memory based predictions arising from
information acquired on the trial before affect visibility and the neural correlates of consciousness. To
this end, I vary stimulus evidence as well as predictability and acquire electroencephalographic data.
A comparison of the neural processes distinguishing consciously perceived from unperceived trials
with and without predictions reveals that predictions speed up processing, thus shifting the neural
correlates forward in time. Thus, the neural correlates of consciousness display a previously unappreciated
flexibility in time and do not arise invariably late as had been predicted by some theorists.
Admittedly, however, previous experience does not always stabilize perception. Instead, previous experience
can have the reverse effect: Seeing the opposite of what was there, as in so-called repulsive
aftereffects. Here, I investigate what determines the direction of previous experience using multistable
stimuli. In a functional magnetic resonance imaging experiment, I find that a widespread network of
frontal, parietal, and ventral occipital brain areas is involved in perceptual stabilization, whereas the
reverse effect is only evident in extrastriate cortex. This areal separation possibly endows the brain
with the flexibility to switch between exploiting already available information and emphasizing the new.
Taken together, my data show that conscious perception and its neuronal correlates display a remarkable
degree of flexibility and plasticity, which should be taken into account in future theories of consciousness.