Isolating neural correlates of conscious perception from neural correlates of reporting one's perception

Abstract

To isolate neural correlates of conscious perception (NCCs), a standard approach has been to contrast neural activity elicited by identical stimuli of which subjects are aware vs. unaware. Because conscious experience is private, determining whether a stimulus was consciously perceived requires subjective report: e.g., button-presses indicating detection, visibility ratings, verbal reports, etc. This reporting requirement introduces a methodological confound when attempting to isolate NCCs: The neural processes responsible for accessing and reporting one's percept are difficult to distinguish from those underlying the conscious percept itself. Here, we review recent attempts to circumvent this issue via a modified inattentional blindness paradigm (Pitts et al., 2012) and present new data from a backward masking experiment in which task-relevance and visual awareness were manipulated in a 2 × 2 crossed design. In agreement with our previous inattentional blindness results, stimuli that were consciously perceived yet not immediately accessed for report (aware, task-irrelevant condition) elicited a mid-latency posterior ERP negativity (~200-240 ms), while stimuli that were accessed for report (aware, task-relevant condition) elicited additional components including a robust P3b (~380-480 ms) subsequent to the mid-latency negativity. Overall, these results suggest that some of the NCCs identified in previous studies may be more closely linked with accessing and maintaining perceptual information for reporting purposes than with encoding the conscious percept itself. An open question is whether the remaining NCC candidate (the ERP negativity at 200-240 ms) reflects visual awareness or object-based attention.

Keywords: P3b; VAN; attention; awareness; masking; task-relevance.

Figure 1

Simplified schematic depicting some of the stages of processing involved in the two main trial types of a typical backward masking experiment in which the stimulus is masked at a constant SOA (e.g., 50 ms) and is visible on roughly 50% of trials. In addition to differences in conscious perception, post-perceptual processing is likely to differ across the two trial types due to the task demands.

Figure 2

Outline of the modified inattentional blindness paradigm used by Pitts et al. (2012). Because the critical stimulus (square shape) is task-irrelevant in phases 1 & 2, post-perceptual processing is avoided while allowing a contrast that better isolates differences in conscious perception. Neural correlates of post-perceptual processing can be evaluated by comparing phases 3 & 2.

Figure 3

Stimuli and methods used in the current experiment. Five types of stimuli (A) were presented in the EEG experiment (C) while awareness was manipulated via short vs. long masking SOAs (16 or 300 ms). Task-relevance of the square and 4-red-line stimuli was manipulated by altering the target detection task. An initial behavioral experiment (B) was conducted to evaluate stimulus visibility at five different masking SOAs.

Figure 4

Detection rates as a function of masking SOA for shape (square) and color (4-red-lines) stimuli in the behavioral experiment. Error bars reflect the standard error of the mean (s.e.m.).

Figure 5

Grand-averaged ERPs elicited by shape (square) and random (control) stimuli across the four main experimental conditions.

Figure 6

Grand-averaged ERPs elicited by color (4-red-lines) and random (control) stimuli across the four main experimental conditions.

Figure 7

Grand-averaged difference waves for shape stimuli (square minus random) and color stimuli (4-red-lines minus random) across the four conditions of interest. Components are labeled at representative electrodes. CIN, contour-integration negativity; SEC, sensory effect of color; VAN, visual awareness negativity; LOP, late occipital positivity; P3b, centro-parietal positivity.

Figure 8

Grand-averaged difference wave topographies (square minus random) plotted over the posterior scalp for the four components of interest (columns) across the four main experimental conditions (rows). Note that the amplitude scale for the CIN component was adjusted to ± 1 μV for the unaware conditions, and for the VAN component ± 2 μV for all conditions except the aware, task-relevant condition. Asterisks indicate significant amplitude differences (p < 0.05) between shape and random ERP as assessed via cluster mass permutation tests for each condition separately.

Figure 9

Grand-averaged difference wave topographies (4-red-lines minus random) plotted over the posterior scalp for the four components of interest (columns) across the four main experimental conditions (rows). Note that the amplitude scales for the SEC and LOP components were adjusted to ± 1 μV for the unaware conditions, and for the P3b component ± 2 μV for all conditions except the aware, task-relevant condition. Asterisks indicate significant amplitude differences (p < 0.05) between shape and random ERP as assessed via cluster mass permutation tests for each condition separately.