A novel experimental paradigm to study conscious perception

A new journal article published today in Consciousness and Cognition by Max Levinson and Sylvain Baillet introduces an experimental paradigm that disentangles between the conscious processes and the sensory brain signals of perception.

Conscious perceptual experiences are expected to correlate with content-specific brain activity. However, we argue that a veridicality problem arises when attempting to separate neural signals strictly induced by sensory inputs (unconscious) from the brain processes of the subjective, yet accurate, conscious experience of the latter.

We propose that perceptual filling-in, a phenomenon whereby visual information inaccurately spreads across visual space, as a promising approach to circumvent the veridicality problem of the study of conscious perception.

Perceptual filling-in. A: Example of a traditional ‘peripheral fading’ stimulus. When fixating at the center cross, a peripheral red disk will eventually appear to be filled in by the blue background. B: The radial uniformity illusion stimulus used in the present study. The center circle has a lower luminance than the periphery; both areas have the same blue hue. When fixating at the center cross, the entire image will eventually appear to have the same luminance. C: Illustration of the retinotopic correspondence between the visual field regions shown in A (top) or B (bottom) and neural activity in early visual cortex. Signals produced by peripheral fading targets (red dot, top) are difficult to resolve with non-invasive electrophysiology. The radial uniformity illusion improves signal strength by expanding cortical coverage (bottom). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Filling-in generates non-veridical although unambiguous percepts dissociated from stimulus input. In particular, the radial uniformity illusion induces a filling-in experience between a central disk and the surrounding periphery.

This new article discusses how this illusion facilitates both the detection of neurophysiological responses and subjective phenomenological monitoring.

In addition to theoretical and conceptual considerations, the paper also reports behavioral results from a large (n = 200) psychophysics study to examine key stimulus parameters that drive the conscious experience of filling-in.

Behavioral experiment paradigm. A: experimental session outline. The experiment began with informed consent. Next, the display was calibrated to ensure consistent stimulus sizing across participants. To measure screen resolution, the participant held a credit card up to the screen and adjusted a box to match its size. To measure viewing distance, the participant covered one eye, fixated at a black square, and indicated when a leftwards moving red dot entered the retinal blind spot. Following calibration, the participant was given instructions and a practice task to confirm understanding. The main task (23 trials) was completed at the end. B: Trial outline. The filling-in stimulus disappeared from the screen either after a mouse click or after 15 s elapsed without a click. If the mouse was clicked, participants were presented with a series of questions as shown in C. If the mouse was not clicked, participants did not answer any questions and instead viewed three seconds of dynamic noise to alleviate after-images. C: Flowchart of questions asked after each trial in which filling-in occurred. (For interpretation of the references to color in this figure legend, please see the web version of this article.)

Phenomenological reports from trials with a defined filling-in direction: either periphery-filled (center spreads outwards into the periphery) or center-filled (periphery spreads inwards into the center). A: Ratio of periphery-filled trials to center-filled trials, as a function of both border eccentricity (degrees of visual angle) and periphery luminance. B: Subjective reports of how filling-in was experienced across the filled-in region.

Filling-in behavior as a function of both periphery luminance and border eccentricity (degrees of visual angle). A: Median reaction time for filled-in trials. Bars extend to 1st quartile (below) and 3rd quartile (above). B: Percentage of trials reportedly filled-in, out of 200 total trials per stimulus condition.

The authors propose that these data underpin future hypothesis-driven studies of filling-in to further delineate the neural mechanisms of conscious perception.