Brain activity reveals two stages of visual processing before conscious perception
Seeing feels immediate; we open our eyes and the world seems to appear as a continuous stream. But some visual illusions show that events occurring later can change how we perceive what happened earlier, suggesting that the brain may collect and combine information for a short time before producing the experience that reaches awareness.
A pair of neuroscientists from EPFL's Laboratory of Psychophysics has uncovered two stages of visual processing before conscious perception occurs. The brain encodes individual visual events immediately after they are seen, followed by a later stage where integration of these events takes place. This is demonstrated through the Sequential Metacontrast paradigm (SQM) test, which involves a rapid sequence of vertical lines with a brief horizontal offset.
Participants are then shown sequential pairs of straight vertical lines moving outward, creating an illusion of two moving streams of lines tilting outward. The study recruited 18 participants, ages 18 to 23, who watched several versions of the SQM test while their brain activity was recorded via electroencephalography (EEG). After each line sequence, participants reported whether the offset appeared to point left or right.
The researchers used machine learning methods to analyze the EEG recordings, revealing early occipital activity (~200 milliseconds) encoding individual visual offsets and their temporal order. Later central-parietal activity (~450 milliseconds) represented the integrated percept and behavioral reports, indicating a transition from unconscious encoding to conscious visual integration.
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