Cortical evoked potentials to an auditory illusion: binaural beats

Abstract

Objective: To define brain activity corresponding to an auditory illusion of 3 and 6Hz binaural beats in 250Hz or 1000Hz base frequencies, and compare it to the sound onset response.

Methods: Event-Related Potentials (ERPs) were recorded in response to unmodulated tones of 250 or 1000Hz to one ear and 3 or 6Hz higher to the other, creating an illusion of amplitude modulations (beats) of 3Hz and 6Hz, in base frequencies of 250Hz and 1000Hz. Tones were 2000ms in duration and presented with approximately 1s intervals. Latency, amplitude and source current density estimates of ERP components to tone onset and subsequent beats-evoked oscillations were determined and compared across beat frequencies with both base frequencies.

Results: All stimuli evoked tone-onset P(50), N(100) and P(200) components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude with the low base frequency and to the low beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left lateral and inferior temporal lobe areas in all stimulus conditions. Onset-evoked components were not different across stimulus conditions; P(50) had significantly different sources than the beats-evoked oscillations; and N(100) and P(200) sources located to the same temporal lobe regions as beats-evoked oscillations, but were bilateral and also included frontal and parietal contributions.

Conclusions: Neural activity with slightly different volley frequencies from left and right ear converges and interacts in the central auditory brainstem pathways to generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats. Cortical potentials recorded to binaural beats are distinct from onset responses.

Significance: Brain activity corresponding to an auditory illusion of low frequency beats can be recorded from the scalp.

Fig. 1

Time courses of the stimuli that evoked 3 Hz binaural beats (top) and the ERP waveforms evoked by them (bottom). Stimuli consisted of tone bursts with frequencies of 250 Hz or 1000 Hz presented to one ear, and frequencies 3 Hz higher to the other. Similarly, 6 Hz beats were evoked by 250 and 256 Hz or 1000 and 1006 Hz tones. Brain potentials were recorded in response to the binaural stimuli and the onset, beat and offset evoked components are indicated on the waveform.

Fig. 2

Scalp distribution of the potentials to tone onsets and the 3 Hz binaural beats with the low (250Hz) and high (1000 Hz) base frequencies. Grand average across 18 subjects. The plots include a baseline of 100 ms before tone onset and the initial components of the offset responses.

Fig. 3

Scalp distribution of the potentials to tone onsets and the 6 Hz binaural beats with the low (250Hz) and high (1000 Hz) base frequencies. Grand average across 18 subjects. The plots include a baseline of 100 ms before tone onset and the initial components of the offset responses.

Fig. 4

Average source current density estimates associated with P₅₀, N₁₀₀ and P₂₀₀ to stimulus onset in the 250 Hz base frequency and 3 Hz beat condition. Six views of the brain are presented for each component, as indicated on each view.

Fig. 5

Average source strength estimates associated with the spectral peak at 3 Hz (top) and with the 3 Hz beats-evoked oscillation peak P₃ with 250 Hz base frequency (bottom). For the spectral peak, strength is given in arbitrary relative units, while for peak P₃, strength is given in current density. Six views of the brain are presented for each display, as indicated on each view.

Fig. 6

Time courses of average current density values in the left and right hemisphere regions that were most involved with 3 Hz binaural beats: Lateral temporal areas (BA 20, 21, 37) anterior-temporal lobe (BA 38) and frontal region (BA11) with the 250 Hz base frequency. The grand-averaged waveform recorded at C_z, including components P₅₀, N₁₀₀, P₂₀₀, P₂₀₀′, N₀, P₁, P₂ and P₃ peaks are plotted at the top. Note the correspondence of current density peaks to the scalp-recorded positive and negative peaks. Time 0 corresponds to stimulus onset.