Laterality of basic auditory perception

Abstract

Laterality (left-right ear differences) of auditory processing was assessed using basic auditory skills: (1) gap detection, (2) frequency discrimination, and (3) intensity discrimination. Stimuli included tones (500, 1000, and 4000 Hz) and wide-band noise presented monaurally to each ear of typical adult listeners. The hypothesis tested was that processing of tonal stimuli would be enhanced by left ear (LE) stimulation and noise by right ear (RE) presentations. To investigate the limits of laterality by (1) spectral width, a narrow-band noise (NBN) of 450-Hz bandwidth was evaluated using intensity discrimination, and (2) stimulus duration, 200, 500, and 1000 ms duration tones were evaluated using frequency discrimination. A left ear advantage (LEA) was demonstrated with tonal stimuli in all experiments, but an expected REA for noise stimuli was not found. The NBN stimulus demonstrated no LEA and was characterised as a noise. No change in laterality was found with changes in stimulus durations. The LEA for tonal stimuli is felt to be due to more direct connections between the left ear and the right auditory cortex, which has been shown to be primary for spectral analysis and tonal processing. The lack of a REA for noise stimuli is unexplained. Sex differences in laterality for noise stimuli were noted but were not statistically significant. This study did establish a subtle but clear pattern of LEA for processing of tonal stimuli.

Figure 1

Mean GDTs by stimulus for left and right ears and difference scores. Error bars indicate standard error. Significance of left-right differences on paired t-test is indicated at bottom. NS= p>0.05.

Figure 2

Mean frequency discrimination scores in Hz for left and right ears and difference scores. Error bars indicate standard error. Significance of left-right differences on paired t-test is indicated at bottom. NS= p>0.05.

Figure 3

Mean frequency discrimination in Hz for 3 tonal stimuli (500, 1000 and 4000 Hz) collapsed, showing the effect of stimulus duration only. Error bars indicate standard error. In all cases a slight left laterality for performance was seen and is indicated by negative left-right threshold values at bottom. Individual left-right difference scores were not significant (ns=p>0.05) but the overall difference, collapsed across duration, shows a significant laterality on paired t-test. Laterality did not change with stimulus duration which was confirmed by repeated measures ANOVA (see text).

Figure 4

Mean intensity discrimination in dB plotted for pure tone stimuli at left and for narrow-band and wide-band noise at the right by ear of stimulation. Error bars indicate standard error. Collapsed scores for tones in the left ear are significantly better than the right (p=0.0119) but laterality for narrow-band and wide-band noise is non-significant (ns=p>0.05).

Figure 5

Laterality as indicated by mean left-right ear scores plotted for female subjects (left) and males (right). Data for all three experiments is shown collapsed in each by stimulus type (tones v. noise). Female subjects show a pattern of REA for noise stimuli and LEA for processing of tonal stimuli although the REA is non-significant. In contrast, males show a consistent LEA regardless of stimulus type. Error bars indicate standard error.