When two pure-tone (2T) stimuli with slightly different frequencies are presented independently to each ear, an auditory illusion, called binaural beats (BB), is perceived as a faint pulsation over a single tone. The frequency of the perceived tone is equal to the mean frequency of 2T and the pulsation has a rate equal to the difference of the two.
The interaction of the 2T stimuli, inside the auditory cortex, can be recorded in the form of auditory steady state responses (ASSR) using conventional electroencephalography (EEG) or magnetoencephalography (MEG). The recorded ASSR usually have small amplitudes and require additional signal processing to separate them from the surrounding cortical activity.
The transient auditory evoked potentials (AEPs) may provide more information about the physiology behind the generation of the BBs. Currently most methods can only generate transient AEPs to binaural phase disparities in random noise, or use amplitude modulating (AM) tones to trigger a binaural frequency difference (BFD).
For this dissertation, a method was developed which uses two frequency modulating (FM) sounds to generate an instantaneous BFD which only lasts for the duration of a single or unitary beat. One major advantage of this method is that it separates the beating rate from the BFD allowing for independent control of the beat occurrences.
This dissertation provides an in depth description of the stimulus generation and acquisition methodology used to evoke transient AEPs to unitary BBs. Several studies were designed to characterize the behavior of the AEPs to some of the key stimulus parameters design and to obtain an optimal set of parameter values that can be used to generate robust transient AEPs.
The result was a method that can be used to generate unitary BBs that have equivalent characteristics to the BBs generated using the 2T method. Furthermore the studies showed that the method is capable of generating BBs that evoke repeatable and robust transient AEPs with large amplitudes and late latencies.
The current electrophysiology research on BBs is limited to ASSR, due to the 2T stimulation. However, the transient AEP in most cases convey additional information that is not contained in the ASSR. For this reason most new research on BBs is focused on the topic of transient AEPs. Most of the methods used for evoking transient BB responses (BBR) rely on some kind of AM or disruption of the continuity of the stimuli to switch between beat and no-beat conditions while using pure-tone stimuli. This, however, is not desirable since each disruption in the continuity results in AEPs.
Stimulus Design and Generation:
Acoustic and Binaural Beats
Acoustic beats are a well-known physical phenomenon that occurs when 2T sounds are presented simultaneously in the same medium and interfere with each other resulting in an AM pure-tone sound. The interference X(t) of the two can be mathematically represented in Equation 3-1 as the sum of two sinusoids with frequencies f1 and f2. Using trigonometric identities Equation 3-1 can then be transformed into Equation 3-2 as the product of a sine wave with frequency equal to the average of the two and a cosine wave with a frequency equal to halve of the difference of the two.
Acquisition of AEPs
The binaural beats generated by dichotic FM stimuli are a relatively new concept and have not been thoroughly investigated. Based on existing BB research, the transient responses can be captured using EEG (AEPs) or MEG, but in both cases the responses are predominantly cortical and appear late after the onset of the stimulus. BBs generated using the 2T method typically have steady state responses or oscillations with frequency equal to the difference of the two stimuli.
Response Waveform Morphology
The AEP responses observed in most cases can be described as a quad-phasic waveform consisting of two positive peaks, labeled as P1 and P2, and two negative peaks N1 and N2. In most cases, the latencies relative to the onset of a beat were around 75 ms for P1, 120 ms for N1, 195 ms for P2, and 330 ms for N2. The amplitudes of the peaks ranged in microvolts and significantly varied in magnitude between subjects and conditions.
The Effects of the Modulation Frequency:
The responses obtained using the new BB stimulation method produce BCR consisting of FMR and BBR. The goal of this study was to characterize the BCRs and determine whether the FMRs can be reduced while still preserving the BBR.
The current stimulus design process was restricted to a certain extent in order to expedite the characterization process and focus only on the parameters of interest. However, the parameters that were not discussed in the studies were the polarity difference between the two stimuli and the direction of modulation. The polarity or phase between the two ears was shown, in the second supplemental study, to have an effect on the response amplitudes. More specifically, the response amplitudes were larger when the binaural phase changed from 0º to 180º relative to the converse.
Source: University of Miami
Author: Todor Mihajloski