Abstract:

The natural speech rate is from two to six syllables per second. Listeners can understand speech accelerated up to three times. That is, the temporal bottleneck of speech is 8~12 syllables per second. This temporal bottleneck closely aligns with the alpha band of neural oscillations. Moreover, alpha oscillation may dictate the temporal resolution of perception. The faster alpha oscillation, that is, the shorter alpha cycle, leads to a shorter threshold of the fusion of two stimuli. Thus, the current study investigates whether and how accelerated speech comprehension depends on the speed of alpha oscillation.
Firstly, we aim to ascertain whether the temporal bottleneck of accelerated speech comprehension is consistent with the speed of alpha oscillation. This relationship will be investigated both between and within subjects. In the between-subject experiment, we will test whether individuals with faster alpha oscillations show a shorter threshold in accelerated speech comprehension. In the within-subject experiment, we will examine the consistency between prestimulus alpha speed and speech comprehension in each trial. We’d like to find that, while listeners have a faster alpha oscillation, they comprehension of accelerated speech is better.
Secondly, we will investigate how the speed of alpha oscillation modulates accelerated speech comprehension. To be specific, we will manipulate alpha oscillation at a slower (8 Hz) or faster (12 Hz) speed, and then observe the following speech comprehension performance at a syllable rate of 10 Hz. Especially, we aim to find whether this influence occurs at a higher processing beyond the sensory level. Therefore, we design two experiments to manipulate the speed of alpha oscillation. In one experiment, we will use pure tones with different amplitude modulations to induce different alpha oscillations. In the other experiment, we will use transcranial alternating current stimulation (tACS) to modulate oscillatory activity. To investigate the alpha band impact on higher-level processing, we will place electrodes of tACS not only in the temporal cortex but also in the frontal cortex. We’d like to find that: 1) The speech comprehension will be better under the faster (12 Hz) alpha oscillation than under the slower (8 Hz) alpha oscillation; 2) This influence will be detected under alpha modulations in both temporal and frontal cortex.
Finally, we will explore how the speed of alpha oscillation affects the neural representation of speech. The neural representation of speech will be analyzed by the temporal response function (TRF), which shows how speech envelope is encoded in neural response. We will analyze how the speed of alpha oscillation modulates the TRF, and how this neural modulation will predict the behavioural performance of speech comprehension. We'd like to find the underlying neural mechanism of how alpha oscillation affects the neural processing of speech comprehension.
In sum, we propose a hypothesis to explain how alpha oscillations influence the bottleneck of accelerated speech comprehension. It is hard to process two or more stimuli simultaneously within one cycle of alpha oscillation. Therefore, when the speech rate is slower than the speed of alpha oscillations, one syllable lasts across multiple alpha cycles, allowing for speech to be fully processed and recognized. However, as speech speed gradually increases beyond the frequency of alpha oscillations, multiple syllables exist within one alpha oscillation cycle, leading to competition for cognitive resources and incomplete processing, resulting in difficulty with speech comprehension. Together, we will utilize accelerated speech to investigate the modulation of alpha oscillations on the temporal bottleneck of comprehension. We’d like to find whether alpha oscillation is attributed to higher cognitive processes and explored how alpha oscillation manifests in neural representations of stimuli. We aim to gain a better understanding of how neural oscillations regulate the temporal resolution of perception and to further investigate the general mechanisms underlying the impact of neural oscillations on rapid temporal processing.

Key words: auditory perception, accelerated speech comprehension, temporal resolution, alpha band