Effects of forward masking on sound localization in cats: basic findings with broadband maskers

Abstract

Forward masking is traditionally measured with a detection task in which the addition of a preceding masking sound results in an increased signal-detection threshold. Little is known about the influence of forward masking on localization of free-field sound for human or animal subjects. Here we recorded gaze shifts of two head-unrestrained cats during localization using a search-coil technique. A broadband (BB) noise masker was presented straight ahead. A brief signal could come from 1 of the 17 speaker locations in the frontal hemifield. The signal was either a BB or a band-limited (BL) noise. For BB targets, the presence of the forward masker reduced localization accuracy at almost all target levels (20 to 80 dB SPL) along both horizontal and vertical dimensions. Temporal decay of masking was observed when a 15-ms interstimulus gap was added between the end of the masker and the beginning of the target. A large effect of forward masking was also observed for BL targets with low (0.2-2 kHz) and mid (2-7 kHz) frequencies, indicating that the interaural timing cue is susceptible to forward masking. Except at low sound levels, a small or little effect was observed for high-frequency (7-15 kHz) targets, indicating that the interaural level and the spectral cues in that frequency range remained relatively robust. Our findings suggest that different localization mechanisms can operate independently in a complex listening environment.

Keywords: ILD; ITD; forward masking; free-field; localization.

Fig. 1.

A: behavioral setup inside the soundproof chamber. A total of 17 speakers located in the frontal hemifield were used in the study. B: stimuli in each trial. The masker sound and the fixation light-emitting diode (LED) were always presented from the center (0°, 0°) with a duration of 600–800 ms. After the LED and the masker were terminated, a short target sound (25 ms) was presented from 1 of the 17 speakers on or off center following a gap (0 or 15 ms). C: frequency ranges of the broadband (BB) and band-limited (BL) sound, as well as the corresponding localization cues. ILD, interaural level difference; ITD, interaural time difference.

Fig. 2.

Scatter plots of the final gaze positions (top) and gaze shifts vs. motor error (bottom) for a broadband target (25 ms, 35 dB SPL) obtained with cat 33. A: no forward masker was presented (control). B: a 50-dB forward masker was presented during fixation, and there was a 15-ms gap between the end of the masker and the beginning of the target (FM15). C: a 50-dB forward masker was presented, and the target was presented immediately after the termination of the masker (FM0). The large open symbols at top represent target locations, and the small filled symbols represent gaze positions with the same color and shape as the corresponding targets. The gain and precision (δ) derived from the regression analysis (straight lines, bottom) are indicated by the 2 values in brackets computed separately for horizontal (1st value; Hori) and vertical (2nd value; Vert) gaze movements. Only 11 of the 17 target locations are plotted for clarity.

Fig. 3.

Localization accuracy measured with the gain for BB targets (25 ms) as a function of target sound level. Con, control with no masker presented. The error bars are 95% confidence intervals derived with the bootstrap method. Filled symbols indicate a significant decrease in the gain from the control (P < 0.05; bootstrapping). Seven sound levels of the targets were tested (overall levels were 20, 30, 35, 40, 50, 65, and 80 dB SPL). Each group of conditions (masked and unmasked) was obtained with the same sound level with each condition having n > 68 trials across all locations.

Fig. 4.

A: localization precision measured with δ for BB targets (25 ms) as a function of target sound level, in the same format as Fig. 3. A large δ value is an indication of large scatter, and thus lower precision. Filled symbols indicate a significant change (increase or decrease) in δ from the control (P < 0.05; bootstrapping). B: scatter plots of gain vs. δ combining all target sound levels and conditions (i.e., control, FM0, and FM15) for BB targets for both cats. r_x and r_y are correlation coefficients between gain and δ obtained separately for horizontal and vertical gaze movements; n = 21 conditions.

Fig. 5.

Response latencies (means and SDs) for BB targets. Filled symbols indicate a significant increase in the latency compared with the control (P < 0.05).

Fig. 6.

Gaze shifts vs. motor error plotted separately for horizontal (blue circles) and vertical (red squares) movements for low-frequency BL targets (80 dB SPL) without (A) or immediately after (B) a BB forward masker. Freq, frequency.

Fig. 7.

Localization accuracy measured with the gain for BL targets (25 ms) as a function of target sound level, in the same format as Fig. 3. Target levels plotted are the overall sound levels. An 80-dB SPL overall sound level corresponds to a spectrum level of 47, 43, and 41 dB SPL, for the low-, mid-, and high-frequency conditions, respectively; n > 68 trials for each condition (masked or unmasked).

Fig. 8.

Localization precision measured with the δ for BL targets (25 ms) as a function of target sound level, in the same format as Fig. 7.

Fig. 9.

Response latencies for BL targets (25 ms) as a function of target sound level, in the same format as Fig. 6.

Fig. 10.

Correlation coefficients between gain and δ across all target sound levels and conditions (i.e., control, FM0, and FM15) for BB and BL targets. Examples for deriving the bar values are shown in Fig. 4B as r_x and r_y for horizontal and vertical gaze movements. A positive correlation between gain and δ indicates a negative correlation between accuracy and precision, and vice versa.

Fig. 11.

Localization accuracy measured with the gain for BL targets (25 ms, 50 dB SPL) using new tokens of frozen noise.

Fig. 12.

Localization accuracy measured with the gain (left) for low-frequency BL targets with additional stimulus manipulations (right) obtained with cat 36. The FM0 (25 ms), FM15 (25 ms), and control are replotted from Fig. 7A, right, at 65 dB SPL. FM0 (50 ms) was obtained with a new noise (50-ms duration, no gap; top right). FM25 (25 ms) was obtained with the same 25-ms noise used before with a gap of 25 ms (bottom right).