Less Is More: Semantic Information Survives Interocular Suppression When Attention Is Diverted

Abstract

The extent of unconscious semantic processing has been debated. It is well established that semantic information is registered in the absence of awareness induced by inattention. However, it has been debated whether semantic information of invisible stimuli is processed during interocular suppression, a procedure that renders one eye's view invisible by presenting a dissimilar stimulus to the other eye. Inspired by recent evidence demonstrating that reduced attention attenuates interocular suppression, we tested a counterintuitive hypothesis that attention withdrawn from the suppressed target location facilitates semantic processing in the absence of awareness induced by interocular suppression. We obtained an electrophysiological marker of semantic processing (N400 component) while human participants' spatial attention was being manipulated with a cueing paradigm during interocular suppression. We found that N400 modulation was absent when participants' attention was directed to the target location, but present when diverted elsewhere. In addition, the correlation analysis across participants indicated that the N400 amplitude was reduced with more attention being directed to the target location. Together, these results indicate that inattention attenuates interocular suppression and thereby makes semantic processing available unconsciously, reconciling conflicting evidence in the literature. We discuss a tight link among interocular suppression, attention, and conscious awareness.

Significance statement: Interocular suppression offers a powerful means of studying the extent of unconscious processing by rendering a salient stimulus presented to one eye invisible. Here, we provide evidence that attention is a determining factor for unconscious semantic processing. An electrophysiological marker for semantic processing (N400 component) was present when attention was diverted away from the suppressed stimulus but absent when attention was directed to that stimulus, indicating that inattention facilitates unconscious semantic processing during the interocular suppression. Although contrary to the common sense assumption that attention facilitates information processing, this result is in accordance with recent studies showing that attention modulates interocular suppression but is not necessary for semantic processing. Our finding reconciles the conflicting evidence and advances theories of consciousness.

Keywords: attention; consciousness; event-related potential; interocular suppression.

Figure 1.

The experimental procedure of the congruent (A), incongruent (B), and visible (C) conditions. A prime word (200 ms), arrow cue (200 ms), and CFS stimuli with a target word are presented in sequence, separated by two 300 ms blank screen intervals. The CFS stimuli, composed of seven different frames of Korean letters, are presented to one eye for 700 ms, while the target word is presented to the other eye from the third to fifth frames. A, In the congruent condition, the target word is presented to the cued location. B, In the incongruent condition, the target word is presented opposite to the cued location. C, In the visible condition, the target word is presented to the cued location of both eyes.

Figure 2.

ERP results (N = 24) obtained from the congruent (A), incongruent (B), and visible conditions (C). ERP waveforms obtained from the Cz electrode are locked with the onset of the target. Red and gray lines indicate the ERPs elicited by the unrelated and related target words, respectively. D–F, N400 amplitudes for the three key electrodes (Fz, Cz, and Pz) are created by the ERP difference (unrelated − related) within the 300–700 ms post-target word onset temporal window. Error bars indicate SEM. *p < 0.05 (t test). **p < 0.01 (t test). ***p < 0.001 (t test). N400 amplitudes from all electrodes are topographically plotted, with a significant modulation for all levels (p < 0.05) marked with a filled circle at the corresponding electrode position.

Figure 3.

Correlation between N400 and location judgment performance (N = 24). The N400 amplitude (unrelated − related) is plotted as a function of location judgment performance for the congruent (A) and incongruent conditions (B). Dashed lines indicate linear regression lines. **p < 0.05 (correlation significance). The correlation coefficients (r) from all electrodes are topographically plotted, with a significant correlation for all levels (p < 0.05) marked with a filled circle at the corresponding electrode position.

Figure 4.

Correlation results (N = 24). Semantic judgment performance is plotted as a function of location judgment performance in the congruent (A) and incongruent conditions (B). The VEOG amplitude is plotted as a function of location judgment performance for the congruent (C) and incongruent conditions (D). The VEOG amplitude is obtained from the difference between the cue conditions above and below the fixation within the 300–700 ms post-target word onset. Dashed lines indicate linear regression lines.

Figure 5.

Results of Experiment 2 (N = 22). Location judgment performance of the incongruent condition is plotted as a function of location judgment performance of the congruent condition (A). The dashed line is a unity line. B, C, Semantic judgment performance is plotted as a function of location judgment performance in the congruent condition (B) and incongruent condition (C). Dashed lines indicate linear regression lines.