EPFL researchers have identified two successive patterns of brain activity that may distinguish unconscious visual processing from the formation of a unified percept.
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, which suggests that the brain may collect and combine information for a short time before producing the experience that reaches awareness.
This effect is known as "postdiction." Here's an example: two colored dots appear briefly in different places, but people see only a single moving dot that changes color along the way. This means that the second dot must therefore influence how the first is perceived, a typical example of postdiction.
Tracking perception in the brain
Three neuroscientists from the Laboratory of Psychophysics at EPFL, Maëlan Menétrey, Michael Herzog, and David Pascucci, have now examined the brain activity behind this process. Their results show that it involves two successive stages: an early one that encodes individual visual events outside conscious access, followed by a later stage where the integration happens.
The work is published in PLOS Biology.
An illusion that reveals hidden processing
Scientists study this integrative effect with an illusion called the Sequential Metacontrast paradigm (SQM), a test that last less than a second in total. In SQM, participants are shown a rapid sequence of vertical lines on a screen. For a fraction of a second, one line shows a "horizontal offset", which means that its lower half is shifted slightly to the left or right. Immediately after, pairs of straight vertical lines are flashed sequentially, moving outward from the center. This creates the illusion of two moving streams of lines as tilting outward.
The point of the SQM is to show that conscious vision is not a live video feed, but a delayed, edited movie created by the brain. Studies suggest that this integration can continue for roughly 290 to 450 milliseconds before a unified percept emerges.
Watching the brain watch
The EPFL researchers recruited 18 participants aged 18 to 23 and had them watch several versions of the SQM test while recording electrical activity across the scalp with electroencephalography (EEG). After each line sequence, they reported whether the offset appeared to point left or right.
The team used a machine learning method to find patterns in the EEG recordings. This allowed them to test whether brain activity contained information about the presence, timing and location of individual offsets, even when participants couldn't consciously tell them apart.
Two stages of visual processing
The analysis revealed an early pattern of activity over occipital areas at the back of the head. It emerged around 200 milliseconds after each offset appeared. The pattern preserved the order and timing of individual offsets, including when two opposing offsets were combined and couldn't be perceived separately.
A second pattern appeared later over central and parietal areas. It emerged after about 450 milliseconds and was associated with the integrated offset and participants' reports.
A new window into consciousness
The authors propose that the transition between these two patterns marks the shift from unconscious encoding to a unified percept. They caution that the later signal could also include decision-making or other processing that occurs after perception.
The findings provide a way to separate neural activity related to unconscious visual integration from later activity associated with what people perceive and report. This distinction may help researchers test theories of consciousness and understand how the brain turns events spread across time into one coherent visual experience.
Other contributors
- Lausanne University Hospital (CHUV)
- University of Lausanne (UNIL)
- The Sense Innovation and Research Center
