Auditory-motor integration of subliminal phase shifts in tapping: better than auditory discrimination would predict

Abstract

Unilateral tapping studies have shown that adults adjust to both perceptible and subliminal changes in phase or frequency. This study focuses on the phase responses to abrupt/perceptible and gradual/subliminal changes in auditory-motor relations during alternating bilateral tapping. We investigated these responses in participants with and without good perceptual acuity as determined by an auditory threshold test. Non-musician adults (nine per group) alternately tapped their index fingers in synchrony with auditory cues set at a frequency of 1.4 Hz. Both groups modulated their responses (with no after-effects) to perceptible and to subliminal changes as low as a 5° change in phase. The high-threshold participants were more variable than the adults with low threshold in their responses in the gradual condition set. Both groups demonstrated a synchronization asymmetry between dominant and non-dominant hands associated with the abrupt condition and the later blocks of the gradual condition. Our findings extend previous work in unilateral tapping and suggest (1) no relationship between a discrimination threshold and perceptible auditory-motor integration and (2) a noisier sub-cortical circuitry in those with higher thresholds.

Figure 1

Experimental design. The top panel (A) illustrates the phase relationship during baseline and the phase shift, and shows a graphical explanation of the main variables. The bottom panel (B) shows the details of the sequence of the different trial blocks as they were presented during the experiment.

Figure 2

Individual perceptual thresholds, defined as the highest phase change that could not be perceived as different from the 180° phase relationship, whereas the next higher change could be perceived (e.g., participant 10 could not detect 5°, but needed a minimum of 10° for detection: the threshold would be 5°). The y-axis represents the auditory thresholds in degrees, and the x-axis the individual participants. The dashed line demarcates the boundary between high threshold group (light bars), and the low threshold group (dark bars).

Figure 3

Abrupt condition: Mean relative phase (MRP) for the low threshold group (top panel) and high threshold group (bottom panel). The empty circles represent the ‘ideal’ relative phase values, reflecting the stimulus phase, and the filled circles represent the participants’ response.

Figure 4

Gradual condition: Combined relative phase (MRP) for the low and high threshold group. The empty squares represent the ‘ideal’ relative phase (=stimulus phase), and the filled squares represent the participants’ response.

Figure 5

Variability of mean relative phase for the low and high threshold groups in the abrupt condition (top panel), and the gradual condition (bottom panel). The asterisks denote a significant difference between the respective trial block and baseline (trial blocks 1&6); ***p=0.001, *p=0.04.

Figure 6

Absolute error for the low and high threshold groups in the abrupt condition (top panel), and the gradual condition (bottom panel). The asterisks denote a significant difference between the respective trial block and baseline (trial blocks 1&6); ***p=0.001, *p=0.04.