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- 17/04/2009: Attention 2
- 14/04/2009: Attention 1
- 11/04/2009: Visual memory
- 08/04/2009: Conscious computer
- 05/04/2009: Biology and culture
- 02/04/2009: No conflict with science
- 30/03/2009: Reductionism is necessary
- 27/03/2009: The P3 wave
- 24/03/2009: The global workspace
- 21/03/2009: Neural correlates of consciousness
The P3 wave
A paper published a year and a half ago is similar to the Gaillard paper that has been discussed in the last two posts. The older paper is titled Brain Dynamics Underlying the Nonlinear Threshold for Access to Consciousness and its lead author is Antoine Del Cul (here). The methods used have less resolution in some ways then the Gaillard group but have the advantage of varying the masking in small steps between unconscious and conscious events. In the transition region between ‘seen’ and ‘not seen’ it is very informative.
Using scalp electrode they recorded ERPs (event related potentials) during testing. A target was shown briefly (16ms) and was followed by a mask. The time between the target and the mask was varied and this SOA (stimulus onset asynchrony) crossed a threshold from subliminal to conscious perception. The transition was documented in two ways: a forced choice and a subjective report.
The effect that occurred with the same threshold curve as consciousness was called the P3 wave. It was not a local potential but spread across both hemispheres and from the front to the back of the cortex. It occurred about 400 ms after the target. Up until the P3 wave there were only smaller differences between the data for subliminal and conscious processing and these differences did not show a threshold effect but varied in a linear way.
This gives some information to settle a question in my mind. Which of the following is closer to what happens?
1. Subliminal and conscious perception are entirely different processes – ruled out.
2. Subliminal and conscious perception start out the same but then differ progressively – seems indicated.
3. Subliminal and conscious perception are the same process and consciousness occurs (or not) after perception is complete – not ruled out or indicated.