Asymmetric temporal envelope encoding: Implications for within- and across-ear envelope comparison

Abstract

Separating sound sources in acoustic environments relies on making ongoing, highly accurate spectro-temporal comparisons. However, listeners with hearing impairment may have varying quality of temporal encoding within or across ears, which may limit the listeners' ability to make spectro-temporal comparisons between places-of-stimulation. In this study in normal hearing listeners, depth of amplitude modulation (AM) for sinusoidally amplitude modulated (SAM) tones was manipulated in an effort to reduce the coding of periodicity in the auditory nerve. The ability to judge differences in AM rates was studied for stimuli presented to different cochlear places-of-stimulation, within- or across-ears. It was hypothesized that if temporal encoding was poorer for one tone in a pair, then sensitivity to differences in AM rate of the pair would decrease. Results indicated that when the depth of AM was reduced from 50% to 20% for one SAM tone in a pair, sensitivity to differences in AM rate decreased. Sensitivity was greatest for AM rates near 90 Hz and depended upon the places-of-stimulation being compared. These results suggest that degraded temporal representations in the auditory nerve for one place-of-stimulation could lead to deficits comparing that temporal information with other places-of-stimulation.

FIG. 1.

95% confidence interval for change in threshold between the 20% and 50% AM depth. The x axis corresponds to the three possible standard AM rate (i.e., the AM rate repeated twice in the three-interval, two-alternative forced-choice task). The y axis corresponds to the difference in threshold between the 20% and 50% AM depth conditions. Confidence intervals were computed assuming a t distribution. Confidence intervals for standard AM rates of 10 and 90 Hz that fall outside of zero imply that there was a significant increase in threshold as AM depth was reduced from 50% to 20%.

FIG. 2.

Illustration of AM pairing configurations. The x axis represents time (each stimulus had a duration of 600 ms). The y axis represents carrier frequency (of either 4000 or 7260 Hz). The z-axis represents relative amplitude of the stimulus. Each row represents a different AM rate pairing configuration. The left and right column correspond to the left and right ear, respectively. Same Place, Across Ears: Carrier frequencies were equal in both ears (either 4000 or 7260 Hz). This configuration may have resulted in the perception of binaural beats and represents temporal envelope comparisons that occur for matched place-of-stimulation across the ears. Different Place, Across Ears: Different carrier frequencies were used in each ear and this configuration represents temporal envelope comparisons completed across spectrum and the ears. Different Place, Within Ears: Different carrier frequencies were used within the same ear and this configuration represents comparisons completed across spectrum but within the same ear.

FIG. 3.

Example raw data from two listeners in the same place, across ears condition. Listener codes for each individual are given in the top left corner. Open shapes and dotted lines correspond to the 20%:50% AM depth condition, and closed shapes and solid lines correspond to the 50%:50% AM depth condition. Performance for 10 Hz standard rate is shown in black and 90 Hz standard rate is shown in grey. The y axis corresponds to the proportion of “different” responses across all trials. The x axis corresponds to the difference in AM rate between the standard and variable AM rate (see Table I for values of AM rates in Hz). The small panel on the left [a log(Δf/f) of −∞] represents the proportion of “different” responses when AM rates were equal. Ideal performance occurs when the proportion of “different” responses is 0 for the small left panel, and 1 for the larger, right panel. Sensitivity (d′) can be calculated directly from the raw data. For raw data for all listeners, please see Supplementary Fig. 2 (Footnote 1).

FIG. 4.

Mean ± one standard deviation sensitivity across listeners for all three conditions. Open shapes and dotted lines correspond to the 20%:50% AM depth condition, and closed shapes and solid lines correspond to the 50%:50% AM depth condition. Performance for 10 Hz standard rate is shown in black and 90 Hz standard rate are shown in grey. Each panel corresponds to a different AM pairing condition (see Fig. 2). The y axis corresponds to sensitivity in d′ (Green and Swets, 1966). The x axis corresponds to the difference in AM rate between the standard and variable AM rate (see Table I for values of AM rates in Hz).

FIG. 5.

Mean ± one standard deviation thresholds for each standard AM rate. Top and bottom rows correspond to standard AM rates of 10 and 90 Hz, respectively. Open and closed shapes correspond to the 20%:50% and 50%:50% AM depth conditions. The y axis corresponds to threshold (defined as 0.707 proportion correct). The x axis corresponds to the AM pairing condition (see Fig. 2). Thresholds were highly variable across listeners (for raw data for each listener, see Supplementary Fig. 2, Footnote 1).

FIG. 6.

(Color online) 95% confidence intervals and individual results for change in threshold (0.707 proportion correct) between the 20%:50% and 50%:50% AM depth conditions. The top and bottom rows correspond to standard AM rates of 10 and 90 Hz, respectively. The left and right columns correspond to 95% confidence intervals and individual results, respectively. The x axis corresponds to the AM pairing configuration (see Fig. 2). The y axis corresponds to the change in threshold between the 20%:50% and 50%:50% AM depth conditions. Values above zero indicated that listeners worsened on the task when AM depth was reduced from 50% to 20% in one place-of-stimulation. Confidence intervals were computed assuming a t distribution. Values above zero imply a significant increase in threshold from the 50%:50% to 20%:50% AM depth conditions. Thresholds for listeners were excluded if they fell above or below the values for log(Δf/f) tested in the experiment. Listener codes and corresponding symbols are given on the far right.

FIG. 7.

Estimated loudness by AM rate. Loudness was estimated using the model from Moore et al. (2016). Results are plotted as in Fig. 4 for comparison, but each point represents a single AM rate (for specific AM rates, see Table II). The y axis corresponds to loudness in sones. The top and bottom panels correspond to SAM tones with carrier frequencies of 4000 and 7260 Hz, respectively. Please note the difference in scale for each panel (7260 Hz resulted in much less loudness overall). Different scales were used to make the change in loudness across AM rate visually apparent.