The human brain processes syntax in the absence of conscious awareness

Abstract

Syntax is the core computational component of language. A longstanding idea about syntactic processing is that it is generally not available to conscious access, operating autonomously and automatically. However, there is little direct neurocognitive evidence on this issue. By measuring event-related potentials while human observers performed a novel cross-modal distraction task, we demonstrated that syntactic violations that were not consciously detected nonetheless produced a characteristic early neural response pattern, and also significantly delayed reaction times to a concurrent task. This early neural response was distinct from later neural activity that was observed only to syntactic violations that were consciously detected. These findings provide direct evidence that the human brain reacts to violations of syntax even when these violations are not consciously detected, indicating that even highly complex computational processes such as syntactic processing can occur outside the narrow window of conscious awareness.

Figure 1.

Example stimuli and analysis strategy. A, Sequence of events in a typical violation trial. One auditory tone was played during each trial, either immediately before the critical word (Pre-Tone condition) or well after the critical word (Late-Tone condition). B, Schematic diagram showing how trials in each condition were divided for analysis.

Figure 2.

Mean performance on the grammaticality judgment task (d′), as a function of tone condition (n = 24). Error bars represent SEM.

Figure 3.

ERP waveforms, averaged across participants (n = 24) at electrodes F1 and POZ. Scalp distributions of the difference between violation words and congruent control words, averaged across specified time windows, are shown on the right. Approximate electrode locations are denoted with black dots on the uppermost scalp map. Asterisks denote a significant effect (p < 0.05). A, ERPs elicited by correctly detected violations and correctly endorsed congruent control words in the Late-Tone condition. B, ERPs elicited by correctly detected violations and correctly endorsed congruent control words in the Pre-Tone condition. The auditory evoked potential to the tones can be observed prior to stimulus onset. C, ERPs elicited by undetected violations and correctly endorsed correct congruent words in the Pre-Tone condition. D, Table summarizing results by condition and time window. Values indicate the amplitude of the violation effect (violation − canonical), while shaded colors indicate the polarity (blue, negative; red, positive).