Free-field release from masking was studied as a function of the spatial separation of a signal and masker in a two-interval, forced-choice (2IFC) adaptive paradigm. The signal was a 250-ms train of clicks (100/s) generated by filtering 50-/•s pulses with a TDH-49 speaker (0.9 to 9.0 kHz). The masker was continuous broadband (0.7 to 11 kHz) white noise presented at a level of 44 dBA measured at the position of the subject's head. In experiment I, masked and absolute thresholds were measured for 35 signal source locations ( 10 ø increments) along the horizontal plane as a function of seven masking source locations (30 ø increments). In experiment II, both absolute and masked thresholds were measured for seven signal locations along three vertical planes located at azimuthal rotations of 0 ø (median vertical plane), 45 ø, and 90 ø. In experiment III, monaural absolute and masked thresholds were measured for various signal-masker configurations. Masking-level differences (MLDs) were computed relative to the condition where the signal and mask were in front of the subjects after using absolute thresholds to account for differences in the signal's sound-pressure level (SPL) due to direction. Maximum MLDs were 15 dB along the horizontal plane, 8 dB along the vertical, and 9 dB under monaural conditions.

Visual search performance was examined in a two-alternative, forced-choice paradigm. The task involved locating and identifying which of two visual targets was present on a trial. The location of the targets varied relative to the subject's initial fixation point from 0 to 14.8 deg. The visual targets were either presented concurrently with a sound located at the same position as the visual target or were presented in silence. Both the number of distractor visual figures (0-63) present in the field during the search (Experiments 1 and 2) and the distinctness of the visual target relative to the distractors (Experiment 2) were considered. Under all conditions, visual search latencies were reduced when spatially correlated sounds were present. Aurally guided search was particularly enhanced when the visual target was located in the peripheral regions of the central visual field and when a larger number of distractor images (63) were present. Similar results were obtained under conditions in which the target was visually enhanced. These results indicate that spatially correlated sounds may have considerable utility in high-information environments (e.g., piloting an aircraft).

Two experiments investigated how listeners allocate their attention to different segments of a temporal pattern. The experiments allowed a direct test of the predictions of the Proportion of Total Duration (PTD) rule and the Component Relative Entropy (CoRE) model. Listeners had to decide whether two sequences of nine tones had the same or different temporal patterns (tone duration = 25 ms, tone frequency = 1000 Hz). A sequence's temporal pattern was determined by the time intervals between each tone's offset and the next tone's onset. On same trials, the time intervals at corresponding temporal positions in the two sequences were identical. On different trials, the corresponding time intervals were randomly varied. Listener attention to different temporal positions within a sequence was assessed by calculating the decision weights at each position. The results supported the CoRE model and were inconsistent with the PTD rule. Manipulating the mean of the time intervals within the sequence had no consistent effect on the pattern of weights (or on overall performance), indicating that listener attention was not affected by either the proportion of total duration or the perceptual salience of a longer or shorter time interval. However, manipulating the variance of the time intervals had a significant effect: the highest weight was given to the highest variance segment. This weighting strategy leads to better performance because higher variance segments are more diagnostic of whether the sequences are the same or different.

This experiment tested listeners' ability to discriminate between two temporal patterns as a function of the time interval between the pattern onsets. The listener's task was to decide whether two arrhythmic sequences of nine tones had the same or different temporal patterns; the patterns were defined by the time intervals between the tones. According to the temporal pattern correlation model [R. D. Sorkin, J. Acoust. Soc. Am. 87, 1695-1701 (1990)], listeners extract information about the series of time intervals in each sequence and then compute the correlation between the two series. In the present experiment, the tones in the second sequence were presented at a different frequency than the tones in the first sequence. In one condition, all time intervals in the second sequence were compressed or expanded by a factor that varied randomly over trials. Performance was very good when the sequences did not overlap in time, but was poor when the sequences overlapped. Performance was generally consistent with a discrimination mechanism that cannot process more than one pattern at a time.

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Listeners were presented with two successive 9-tone sequences. The task was to discriminate between the temporal patterns defined by the intertone times in each sequence (tone duration=25 ms, tone frequency=1000 Hz). The listener had to indicate whether the two patterns had the same or different (partially correlated) temporal envelopes. A technique suggested by Lutfi [R. Lutfi, 1339 (1995)], was used to determine the importance of each temporal position on the listener’s decision. In the first experiment, one of the intertone times was assigned a different (either higher or lower) mean duration than the others. This intertone time occurred either at an early temporal position (2nd) or at a late position (6th). Results indicated that two positions, the first temporal position and the position with the different mean, had more influence on the listener’s decision than other positions. In the second experiment, the first 4 intertone times were assigned a different variance than the last occurring 4 intertone times. Initial results suggest that listeners give higher weights to the temporal positions with the lower variance. [Work supported by AFSOR.]

The effect of a temporal gap on detecting a 1000-Hz tone in the presence of 60-dB SPL simultaneous maskers was examined. Ten-component, random-frequency maskers and broadband-noise maskers were used in a 2-AFC adaptive task. Random-frequency components were drawn from 300 to 3000 Hz, excluding a 160-Hz band around the signal. Temporal gaps of 10, 20, 40, 80, and 160 ms were tested, positioned either at the onset, center, or offset of either the signal or the masker. Without gaps, both signal and masker durations were 200 ms. To maintain equal energy across all conditions, level compensation was applied when gaps were employed. For temporal gaps in either the multicomponent or noise masker, masked thresholds consistently decreased as gap duration increased. Gaps in the masker appeared to provide a temporal window for detection of the signal. However, for gaps in the signal, masked thresholds decreased with the multicomponent masker, but remained constant with broadband noise masker. With multicomponent maskers, the gaps appeared to reduce informational masking by perceptually segregating the signal from the masker. With broadband noise maskers, there was little informational masking and therefore the temporal gaps did not improve performance. [Work supported by NIDCD.]

Observers were presented with two successive 8-tone sequences; the task was to discriminate between the temporal patterns defined by the inter-tone intervals in each sequence (average inter-tone interval=50 ms, tone duration=25 ms, tone frequency=1000 or 2500 Hz). The observers had to indicate whether the two patterns on each trial were the same or different. A conditional-on-a-single-stimulus (COSS) technique [B. C. Berg, J. Acoust. Soc. Am. 86, 1743–1746 (1991)] was used to evaluate the salience of each temporal position. The probability of responding DIFFERENT is computed given the magnitude of ‖t1,i−t2,i‖, the absolute difference between the inter-tone intervals at each serial position (i=1,2,...,7). The temporal information in the first and last positions had the greatest influence on the observer’s response. These results have implications for models of temporal pattern discrimination. [Work supported by AFOSR.]