(slow-moving targets presented from directly in front ofthe subject), the MAMA .... This result is not consistent with a simple snapshot view of motion perception,.
Binaural and Spatial Hearing
in Real and Virtual Environments
Edited
by
Robert H. Gilkey v\/right State L/niver5it~ Dayton, Ohio Timothy R. Anderson \V(/~sht-Patterson Air Force Basel Ohio
1997
L:\\VRENCE ERLR\.( I!\I\.SSOCL\.TES. PlfRLISHEI
4
..L
o
o
100 200 300 400 500
0
100 200 300 400 500
Total Duration (ms) FIG. 1 'Ihreshold in degree~ a~ a functIOn of the total duration of stimulm presentation. For the MM1l\ task (filled squares) duration was the presentation time of the single mO\'ing target. For the rviAA task (open __ -~--c,--. /' duration was the time from onset of the first marker until offset of the second marker. Medians and serni-interquartile ranges for three to six replications art' shown separately the four for the four subjects. Where no error bars are shown, the semi-interquartile ranges were [After Grantham, 19851 I
auditory systems do respond directly to the direction or velocity of moving auditory targets (e.g., Altnlan, Syka, and Shmigidina 1970; Ahissar, Ahissar, Bergman, and Vaadia, 1992; Yin and Kuwada, 1983). In contrast to a snapshot mechanism, a motion nlechanism might respond to auditory targets that move in specific directions or at particular velocities, without necessarily being sensitive to the spatial characteristics associated with the target (e.g., its endpoint posi tions). One psychophysical result that might support the existence of such a mechanism would be the finding that a MAMA is smaller than a corresponding MAA. In apparent contrast to the results fronl Grantham (1985), Perrott and Marlborough (1989) reported just such a result: They found that the MAMA for broadband noise was consistently smaller than the minimum detectable angle measured when a nl0ving loudspeaker \\'as activated for 10 nlS only at the end points of its trajectory, This latter condition, called the "marked endpoints" (ME) l
299
I S. Auditory Motion Perception
condition, is analogous to the MAA conditions discussed previously, in that heard two successive markers and had to respond \vhether the second was to the left or right of the first. Velocity was held constant in both conditions at 20 0 /s. The data are plotted in Fig. 2: MAMA is abollt 1°, while the tvlE threshold is 1.5-1.7°, more or less independent of stimulus This result is not consistent with a simple snapshot view of motion perception, which would similar performance in these two tasks (i.e., it would predict that the same information would be available in both . Apparently, subjects were able to use more information in that contained in 10-ms samples at the endpoints. D. Reevaluation of the snapshot hypothesis
One explanation for Perrott and Marlborough's that the snapshot hypothesis is incorrect; for the stimuli and conditions employed, subjects somehow benefited from the midportions of the moving target as well as its endpoints. However, an alternate explanation is that a snapshot analysis is correct and was performed on the moving targets, but the information in the 10-ms ME condition represented a poor and degraded simulation of system's snapshots. Thus, the ME thresholds were elevated because repre poor snapshots.
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• 0
2.0
MAMA
ME
u ~
1 .5 1.0
0.5 0.0
40
50
60
Signal Lev FIG Threshold in degrees a hmction llr signal t'ndpoint (ME) task. IAdapted, with pnmlSSJ(ln, from
70
(dBA)
Filled squares: !\;lAMA ta~k; open ;lnd Marlhorough, I fJHr) J
marked
300
Grantham
Perrott and Marlborough did control for one aspect of the quality of the "simulated" snapshots by varying stin1ulus level. Their finding that thresholds were indeppndent of level in both conditions effectively nlled out the possibility that ME thresholds \vere elevated because the total energy or loudness was in that condition than in the MAJ\1J\ condition. However, another possibility is that the ME thresholds were elevated because the markers were so brief (10 ms). In other words, perhaps the snapshot mechanism requires a longer stimulus on-tin1e than 10 ms to perform optimally, and the ME condition led to poorer performance because it did not faithfully simulate best conditions for sampling the endpoints. This possibility could explain why Granthan1 (1985), v\'ho used 20-ms markers, found no difference between the two conditions, whereas Perrott and Marlborough found the ME condition to be degraded relative to the J\1J\MA condition. The following experiment investigated the effect of marker duration in audi tory spatial resolution.
I. EXPERIMENT 1:
THE EFFECT OF MARKER DURATION ON THE MAA
It is known that the MAA increases as interstimulus interval (lSI) decreases (Grantham, 1985; Perrott and Pacheco, 1989). The closer together in time the two markers in an MAA experiment are, the more widely spatially separated they must be to remain discriminable. However, for a given constant lSI the effects of marker duration have not been systematically investigated. If a snapshot mecha nism requires the stimulus to on for a certain minimum time to perform optimally, there should be a measurable degradation in performance as stimulus duration decreases, for a given constant lSI. A. Subjects
Two adults with clinically normal hearing, ages 29 and 33 years, were paid subjects in this experiment. Neither had participated in previous spatial resolution tasks, although both had been subjects in other psychoacoustic experiments. Each was tested three to five times per \veek in sessions that lasted 1-2 h; frequent rest intervals were provided. For subject, practice was provided until perform ance had stabilized prior to formal data collection (two to five sessions). B. Apparatus and stimuli
Subjects were tested individually in a darkened anechoic chamber. During a session the subject was seated in the center of the room and instructed to maintain a steady upright, forward orientation without tilting the head either to the side or forward. No bite bar was provided, although a head rest served to minimize head movements.
301
15. Auditory Motion Perception
The subject faced the center of a horizontal arc of 55 stationary (JBL-811 OH) loudspeakers, positioned at ear level 1.8 m distant from the subject and spanning 164°. Only 11 loudspeakers of this array, separated from each other by ZO and subtending an arc from _4° to + 16°, were employed in this experiment. The stimulus was a wideband (1 00-8Z00 Hz) noise burst with a 5-ms rise-de cay time. Marker duration was either 50 ms (the LONG condition) or 10 ms (the SHORT56 condition; see Fig. 3). Nominal stimulus level in these two cases was 56 dB SPL. A third condition was employed in which marker duration was 10 ms and nominal level was 63 dB SPL (SHORT63 condition). This condition was included to allow an assessment of the effects of stimulus duration for equal-en ergy stimuli. Most previous investigations of the MAA have held lSI constant (say at 1 s) and have derived psychometric functions by varying the angular separation between the two markers. In the present experiment, angular separation was varied, but in such a way as to keep uelocity constant. Note in Fig. 3 that angular separation is shown as 6°, and total duration between onset of the first marker and offset of the second marker is 300 ms; thus, the velocity is 6°/300 ms ZOo/so When presenting a smaller separation (say 4°), the total duration was decreased (to ZOO ms in this example), thus maintaining velocity constant at ZOoIs. In this experiment functions were derived for velocities of Zoo/s and 60 /s. The slower velocity entails timing parameters for which spatial resolution in dynamic tasks approaches an optimum level, whereas at the faster velocity spatial resolution is noticeably impaired (Chandler and Grantham, 199Z). For each presentation of a marker, a stimulus was randomly sampled from a catalog of 30 tokens of the desired duration stored on the disk. These tokens had been pregenerated such that levels in the Z4 adjacent critical bands were inde pendently jittered over a ZO-dB range (e.g., see Wightman and Kistler, 199Z); this spectral scrambling was designed to prevent subjects from learning individual loudspeaker characteristics. 0
C. Procedure
A trial consisted of two noise burst markers presented sequentially from the stationary loudspeaker(s). Both were either 10 ms or 50 ms in duration, depending on the condition under investigation (see 3). The first (reference) marker occurred from a random location between _4° and +4° azimuth. The second marker was presented either from the same loudspeaker as the first (a "nonsignal trial "), or from a loudspeaker to the right of the first (a "signal" trial). The a pri:;ri probability of each type of trial was 0.50. The subject responded by button press to indicate whether the second pulse appeared to be in the same location or a location to the right of the first pulse. Immediate feedback was given.::
that this "same-different" paracugm is Jittl'rent from the more iocities
