Binaural pitch fusion: Effects of sound level in listeners with normal hearing

Abstract

Pitch is an important cue that allows the auditory system to distinguish between sound sources. Pitch cues are less useful when listeners are not able to discriminate different pitches between the two ears, a problem encountered by listeners with hearing impairment (HI). Many listeners with HI will fuse the pitch of two dichotically presented tones over a larger range of interaural frequency disparities, i.e., have a broader fusion range, than listeners with normal hearing (NH). One potential explanation for broader fusion in listeners with HI is that hearing aids stimulate at high sound levels. The present study investigated effects of overall sound levels on pitch fusion in listeners with NH. It was hypothesized that if sound level increased, then fusion range would increase. Fusion ranges were measured by presenting a fixed frequency tone to a reference ear simultaneously with a variable frequency tone to the opposite ear and finding the range of frequencies that were fused with the reference frequency. No significant effects of sound level (comfortable level ± 15 dB) on fusion range were found, even when tested within the range of levels where some listeners with HI show large fusion ranges. Results suggest that increased sound level does not explain increased fusion range in listeners with HI and imply that other factors associated with hearing loss might play a larger role.

Keywords: Binaural; Fusion; Pitch; Sound level.

Fig. A.1.

Simulated example fusion functions. The x-axis represents the variable frequency presented to the comparison ear in kHz. The y-axis represents the proportion of fusion. Each curve (color and pattern) represents a different parameterization of Eq. 1 according to the figure legend. Values of α, σ, and β representing height, width, and skew, were fixed at 2, 432, and 0, respectively in (A). Values of α, μ, and β representing height, frequency offset, and skew were fixed at 2, 2000, and 0, respectively in (B). Values of μ and σ representing frequency offset and skew were fixed at 2000 and 432, respectively in (C).

Fig. A.2.

Correlation between true and estimated fusion range. The x-axis represents the true fusion range calculated from the curve used to simulate experimentally observed fusion functions in kHz. The y-axis represents the estimated fusion range in kHz. The solid black line represents perfect estimation of fusion range. The dashed lines represent 25% above and below the true fusion range (based on 99% confidence intervals for error of fusion range estimates).

Fig. A.3.

Relative frequency histogram for fusion range errors. The x-axis represents the amount of error in the fusion range estimate (expressed in proportion relative to the true fusion range). The y-axis represents the relative frequency of that error in fusion range estimates from the simulations, which sum to one. Each bin of the histogram is defined to include the lower value and exclude the upper value, e.g., bin 1 represents [0,0.05), except for the final bin which represents [0.95,1.0].

Fig. 1.

Mean pure-tone detection thresholds. The x-axis represents the frequency being tested in kHz. The y-axis represents absolute detection threshold in dB hearing level (HL), where circles represent means and error bars represent one standard deviation above or below the mean. The left and right panel correspond to the left and right ear, respectively. Listeners with NH are shown in black and listeners with HI are shown in grey. One listener (HI37) was not tested at 1.5 kHz, so the mean and error bars on 1.5 kHz reflect the other eight listeners.

Fig. 2.

Example fusion function. The blue symbols and solid line represent data gathered from one participant over two experimental blocks. The x-axis represents variable frequencies that were presented in the comparison ear while 2 kHz was presented at the same time to the reference ear. The y-axis represents the proportion of the time that listener NH79 perceived that frequency combination as one pitch. The dotted, black line represents a skewed Gaussian fit to the data (see Eq. 1; values of α, μ, σ, and β were fitted at 1.5, 1914, 235.2, and 1.364, respectively). The fusion range is calculated from the point at which the skewed Gaussian crosses the 0.5 line and is shown by the double-ended purple arrow.

Fig. 3.

Loudness balance levels for listeners with (A) NH or (B) HI. The x-axis represents the frequency being presented in kHz. The y-axis represents the level in dB SPL(C) at which listeners perceived a pure tone as comfortable. Blue crosses and red circles represent the left and right ear, respectively. Each panel corresponds to a different listener whose subject code is given in the top-left corner. Because not every listener perceived 70 dB SPL(C) as comfortable for the 2 kHz tone, level was sometimes adjusted above or below 70 dB. The left ear was the reference ear for listeners NH77, NH79, NH80, and NH82. The age of listeners with HI is given below their subject code.

Fig. 4.

Fusion range by sound level. The x-axis represents sound level. The y-axis represents the fusion range in kHz. (A) The closed, black squares and error bars represent the mean plus or minus one standard deviation. Standard deviations were calculated on log-transformed fusion ranges and then converted back to kHz. Individual data are shown in blue for each of the six listeners with NH whose subject codes are given in the legend. (B) The closed, black circles represent the fusion range for each listener whose subject code is given in the top-left corner of each panel. Offset to the right is a boxplot of the distribution of 17 fusion range estimates, each calculated by removing one data point from the fusion function. Each box represents the median and interquartile range. Whiskers represent the maximum and minimum fusion range estimate.

Fig. 5.

Fusion range by level across listeners with NH or HI. The x-axis represents the mean level between ears in dB SPL(C) presented to each ear at the reference frequency. The y-axis represents the fusion range in kHz. Listeners with NH in the loud condition are shown in blue and listeners with HI in the comfortable condition are shown in black, with each shape representing one individual whose subject codes are given in the legend.