Intense low-frequency sound transiently biases human sound lateralisation

Abstract

Intense low-frequency (LF) sound exposure transiently alters hearing thresholds and other markers of cochlear sensitivity, and for these changes the term 'Bounce phenomenon' (BP) has been coined. Under the BP, hearing thresholds slowly oscillate for several minutes involving both stages of hyper- and hyposensitivity and it is reasonable to assume that the perception of sounds at levels well above threshold will also be affected. Here, we evaluated the effect of the BP on auditory lateralisation in healthy human subjects. Sound lateralisation crucially depends on the processing of either interaural level- or time differences (ILDs and ITDs, respectively), depending on the spectral content of the sound. The ILD needed to perceive a virtual sound source in the middle of the head was tracked across time. Measurements were carried out without and with a previous exposure to an intense LF-sound in the left ear, to elicit the BP. In 65% of the recordings, significant time-variant deviations from the perceived midline were observed after cessation of the LF-sound. In other words, a binaural stimulus perceived in the middle moved perceptually to the side and often back to the middle after presentation of the intense LF-sound. This means that intense LF-sound exposure can lead to a biasing of ILD-based sound localisation.

Fig 1. cILD tracking procedure.

The stimulus ILD was adjusted by changing the relative levels of the left- and right-ear signals (L and R, respectively) in opposite directions, which made the sound source image perceptually shift from left-to-right or right-to-left (see filled circles and arrows inside the head). The displayed wave amplitudes are shown for an instance when the stimulus ILD (green squares) was 10 dB. The ILD of subsequent binaural probe tones was increased in a given direction (see x-axis) until the subject pressed a button, shifting the direction of change (green crosses denote turnpoints). The average of subsequent ILD turnpoints defined the “centering ILD” (blue). The tracking was carried out both without and with previous exposure to an intense low-frequency stimulus on the left ear.

Fig 2. Representative examples of interaural level differences required to centre a sound source image (cILDs).

In each case, the cILDs obtained before (blue) and right after exposure (red) to an intense low-frequency stimulus are shown. For each probe-tone frequency (upper left), the most typical pattern is shown (see subject ID in bottom left). As time progressed (vertical axis), systematic changes were observed in the cILDs after the low-frequency sound exposure. The left and right vertical dashed lines are respectively upper and lower outlier limits of the baseline cILD distribution (see data analysis for details). For ease in the comparison, the across-time average of the baseline cILD was subtracted from the baseline and LF-stimulation cILDs (individual and mean un-normalized cILDs are shown in Fig 4). Relative to baseline levels, positive cILDs correspond to higher level in the left-side probe tone and vice versa for negative cILDs (see labels on the horizontal axis). Under the BP, perceived lateralisation of the sound image for a stimulus presented with baseline ILD (zero normalized ILD) is in the opposite direction to the cILD, as shown in the above diagrams.

Fig 3. Example individual data (see details in upper left) that presented a significant BP in both cILD (red) and detection threshold (black).

Both mirror-inverse (panels (a) and (c)) and symmetrical patterns (panels (b) and (d)) could be identified among the subset of cases that presented a valid BP in both measures. To facilitate the comparison, cILDs and thresholds were subtracted by the across-time average level of their corresponding baseline.

Fig 4. Individual and mean cILDs as a function of time obtained for each probe-tone frequency (upper labels).

The blue and red curves correspond to conditions without and with a previous exposure to an intense low-frequency sound stimulus, respectively. Dashed blue lines show outlier limits of the average baseline cILDs (see data analysis for details). For each probe-tone frequency, only the subset of cases that passed the significance criterion have been included (N = 10, 10, 11 and 13 for f = 500, 1000, 2000 and 4000 Hz, respectively). Black dashed-dotted lines show harmonic oscillator fits to the average cILD of each probe-tone frequency.