Toktam Sadralodabai

Free‐field release from masking

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-microseconds 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 subjects head. In experiment I, masked and absolute thresholds were measured for 36 signal source locations (10 degree increments) along the horizontal plane as a function of seven masking source locations (30 degree 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 degrees (median vertical plane), 45 degrees, and 90 degrees. 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 signals 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.

Aurally aided visual search in the central visual field: effects of visual load and visual enhancement of the target

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 subjects 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 highinformation environments (e.g., piloting an aircraft).

Effect of temporal position, proportional variance, and proportional duration on decision weights in temporal pattern discrimination

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 sequences temporal pattern was determined by the time intervals between each tones offset and the next tones 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.

Judgments of lateral distance using transients presented with interaural differences of time

Measurements of perceived lateral distance were made using transients in a lateralization paradigm. Two binaural clicks (referred to as boundary clicks) presented with interaural differences of time (IDT) were used to indicate a lateral “distance” along the interaural axis. IDT for the first click was zero (diotic) and for the second, a fixed and predetermined value (dichotic). The location of an additional click (pointer click) was controlled by the subjects, who were instructed to bisect the perceived lateral distance between the two boundary clicks with this pointer click. The procedure was repeated for three distances. Results indicate a linear relation between judgments of distance and changes in IDT (r2 = 0.999).

Effect of temporal gaps on informational and energy‐based masking

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 mas...

Range of performance for two tasks with random‐frequency context tones

Forty listeners were tested on two tasks known to show large individual differences in performance. In one task, termed sample discrimination, pairs of target tones were drawn from each of two overlapping frequency distributions, and listeners indicated which pair came from the higher distribution. Target tones were presented alone and in the presence of two flanking context tones that were either fixed at 500 and 4000 Hz or had Gaussian variation centered at these two frequencies. In the second task, masked thresholds were determined for a 1000‐Hz tone in the presence of ten‐component, random‐frequency, simultaneous maskers. The maskers were drawn on each presentation from a pool of either 200 or 10 waveforms. For sample discrimination, the range of performance was large for both the no‐context and fixed‐frequency context conditions. For the Gaussian‐context condition, few listeners exceeded chance performance. Significant effects of testing order were found, with exposure to Gaussian‐context conditions ...

The effect of different component means and variances on the discrimination of temporal patterns

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 interton...

Serial position effects in temporal pattern discrimination

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.]

Effects of spatially correlated auditory information on eye motor movements

The present study investigated the effects of spatially correlated auditory information on eye motor movements using a two‐alternative forced‐choice reaction time paradigm. A visual target (0.8 deg) was located at 1 of 17 locations along the horizontal and vertical axes in the frontal plane. The targets were positioned within 17 deg of visual angle relative to the point of fixation. These visual targets were presented alone or they were spatially correlated or uncorrelated with the auditory information (click train). Results indicate a significant three‐way interaction between condition orientation (i.e., horizontal or vertical) and distance of visual target from fixation point [F(8,16) = 4.14, p < 0.01]. The implication of the results will be discussed

Free‐field masking‐level differences

Free‐field release from masking was studied in a two‐alternative, forced‐choice adaptive paradigm. Both absolute and masked thresholds were obtained in these experiments. The signal was a train (250 ms) of broadband clicks generated by filtering 50‐μs pulses with TDH‐49 speakers and adjusting the output with a harmonic equalizer to produce equal dBA/oct over the range 0.9–9.0 kHz. The speaker was placed 1.0 m from the subject. The masker was continuous broadband (0.7 to 11 kHz) white noise presented 1.6 m from the subject. Its level was 44 dBA. Study I tested 36 signal source locations at 10° increments along the horizontal plane as a function of seven masker locations at 30° increments. Study II used seven signal locations along three vertical planes located at azimuthal rotations of 0° (median plane), 45°, and 90°. Study III tested monaural conditions by occlusion of one ear. MLDs were computed after using the absolute thresholds to account for differences in the signals SPL due to direction. In all th...