Informativeness of Auditory Stimuli Does Not Affect EEG Signal Diversity.

Brain signal diversity is a marker of consciousness, being lower in unconscious states and higher during psychedelic states. This study tested whether increasing the information rate of speech would increase signal diversity, reflecting richer experience. Nineteen participants listened to an audiobook at five speeds (65–135% of original) and to backward (unintelligible) speech, plus a resting-state condition. EEG Lempel-Ziv diversity was measured. The main hypothesis was not supported: Bayes Factor showed evidence for no effect of speech speed on diversity. Resting-state diversity was greater than during any speech condition. Diversity also gradually declined over the experiment, possibly due to decreasing vigilance, suggesting that unconstrained rest allows more varied experiences like mind wandering.

Study at a glance

Abstract

Brain signal diversity constitutes a robust neuronal marker of the global states of consciousness. It has been demonstrated that, in comparison to the resting wakefulness, signal diversity is lower during unconscious states, and higher during psychedelic states. A plausible interpretation of these findings is that the neuronal diversity corresponds to the diversity of subjective conscious experiences. Therefore, in the present study we varied an information rate processed by the subjects and hypothesized that greater information rate will be related to richer and more differentiated phenomenology and, consequently, to greater signal diversity. To test this hypothesis speech recordings (excerpts from an audio-book) were presented to subjects at five different speeds (65, 83, 100, 117, and 135% of the original speed). By increasing or decreasing speed of the recordings we were able to, respectively, increase or decrease the presented information rate. We also included a backward (unintelligible) speech presentation and a resting-state condition (no auditory stimulation). We tested 19 healthy subjects and analyzed the recorded EEG signal (64 channels) in terms of Lempel-Ziv diversity (LZs). We report the following findings. First, our main hypothesis was not confirmed, as Bayes Factor indicates evidence for no effect when comparing LZs among five presentation speeds. Second, we found that LZs during the resting-state was greater than during processing of both meaningful and unintelligible speech. Third, an additional analysis uncovered a gradual decrease of diversity over the time-course of the experiment, which might reflect a decrease in vigilance. We thus speculate that higher signal diversity during the unconstrained resting-state might be due to a greater variety of experiences, involving spontaneous attention switching and mind wandering.