Beat processing is pre-attentive for metrically simple rhythms with clear accents: an ERP study

Abstract

The perception of a regular beat is fundamental to music processing. Here we examine whether the detection of a regular beat is pre-attentive for metrically simple, acoustically varying stimuli using the mismatch negativity (MMN), an ERP response elicited by violations of acoustic regularity irrespective of whether subjects are attending to the stimuli. Both musicians and non-musicians were presented with a varying rhythm with a clear accent structure in which occasionally a sound was omitted. We compared the MMN response to the omission of identical sounds in different metrical positions. Most importantly, we found that omissions in strong metrical positions, on the beat, elicited higher amplitude MMN responses than omissions in weak metrical positions, not on the beat. This suggests that the detection of a beat is pre-attentive when highly beat inducing stimuli are used. No effects of musical expertise were found. Our results suggest that for metrically simple rhythms with clear accents beat processing does not require attention or musical expertise. In addition, we discuss how the use of acoustically varying stimuli may influence ERP results when studying beat processing.

Figure 1. Schematic illustration of the rhythmic patterns used in the experiment.

The pattern consisted of eight sounds and was designed to induce a rhythm with a hierarchical metrical structure (see tree-structure at the top; beats are marked with dots). The omissions occurred in positions varying in metrical salience, with the omissions in D1 on the first beat, the omissions in D2 on the second beat and the other omissions in equally weak metrical positions.

Figure 2. Acoustic analyses of stimulus S1.

A) Waveform, B) spectrogram, C) amplitude envelope, and D) diagram of stimulus S1 (cf. Fig. 1). The spectrogram was calculated with a Short Time Fourier Transform, Gaussian window, window size 2 ms, time resolution 5 ms, frequency resolution 20 Hz, and 50 dB dynamic range. The amplitude envelope was calculated using a loudness model as described in .

Figure 3. ERP responses for D1, D2 and D3 for musicians (N = 14, left) and non-musicians (N = 15, right).

The panels labeled D1, D2 and D3 show the group averaged ERPs for electrode FCz elicited by omissions, the corresponding position in S1, the derived difference waves and the scalp distributions of the difference waves. The panel labeled All shows all difference waves combined. Time 0 is the onset of the omission, or, in the case of S1, the onset of the corresponding sound. The omissions in D1, D2 and D3 were equally rare in occurrence (0.033) and in all cases, a bass drum sound was omitted.

Figure 4. ERP responses for S2, S3 and S4 for musicians (N = 14, left) and non-musicians (N = 15, right).

The panels labeled S2, S3 and S4 show the group averaged ERPs for electrode FCz elicited by omissions in the standards, the corresponding position in S1, the derived difference waves and the scalp distributions of the difference waves. The panel labeled All shows all difference waves combined. Time 0 is the onset of the omission, or, in the case of S1, the onset of the corresponding sound. The omissions in S2, S3 and S4 were equally rare in occurrence (0.225) and in all cases, a hi-hat sound was omitted. For clarity, here we add the difference wave for D3 (see Figure?3for the separate ERPs) to make a comparison with the difference waves derived for the standards possible. The omissions in D3 were in equally weak metrical positions as in S2, S3 and S4.