Concurrent motion detection based on dynamic changes in interaural delay

A Comparison of Auditory and Visual Apparent Motion Presented Individually and with Crossmodal Moving Distractors

Unimodal auditory and visual apparent motion (AM) and bimodal audiovisual AM were investigated to determine the effects of crossmodal integration on motion perception and direction-of-motion discrimination in each modality. To determine the optimal stimulus onset asynchrony (SOA) ranges for motion perception and direction discrimination, we initially measured unimodal visual and auditory AMs using one of four durations (50, 100, 200, or 400 ms) and ten SOAs (40–450 ms). In the bimodal conditions, auditory and visual AM were measured in the presence of temporally synchronous, spatially displaced distractors that were either congruent (moving in the same direction) or conflicting (moving in the opposite direction) with respect to target motion. Participants reported whether continuous motion was perceived and its direction. With unimodal auditory and visual AM, motion perception was affected differently by stimulus duration and SOA in the two modalities, while the opposite was observed for direction of motion. In the bimodal audiovisual AM condition, discriminating the direction of motion was affected only in the case of an auditory target. The perceived direction of auditory but not visual AM was reduced to chance levels when the crossmodal distractor direction was conflicting. Conversely, motion perception was unaffected by the distractor direction and, in some cases, the mere presence of a distractor facilitated movement perception.

Velocity Selective Networks in Human Cortex Reveal Two Functionally Distinct Auditory Motion Systems

The auditory system encounters motion cues through an acoustic object’s movement or rotation of the listener’s head in a stationary sound field, generating a wide range of naturally occurring velocities from a few to several hundred degrees per second. The angular velocity of moving acoustic objects relative to a listener is typically slow and does not exceed tens of degrees per second, whereas head rotations in a stationary acoustic field may generate fast-changing spatial cues in the order of several hundred degrees per second. We hypothesized that these two types of systems (i.e., encoding slow movements of an object or fast head rotations) may engage functionally distinct substrates in processing spatially dynamic auditory cues, with the latter potentially involved in maintaining perceptual constancy in a stationary field during head rotations and therefore possibly involving corollary-discharge mechanisms in premotor cortex. Using fMRI, we examined cortical response patterns to sound sources moving at a wide range of velocities in 3D virtual auditory space. We found a significant categorical difference between fast and slow moving sounds, with stronger activations in response to higher velocities in the posterior superior temporal regions, the planum temporale, and notably the premotor ventral-rostral (PMVr) area implicated in planning neck and head motor functions.

Cross-modulation interference with lateralization of mixed-modulated waveforms

PURPOSE

This study investigated the ability to use spatial information in mixed-modulated (MM) sounds containing concurrent frequency-modulated (FM) and amplitude-modulated (AM) sounds by exploring patterns of interference when different modulation types originated from different loci as may occur in a multisource acoustic field.

METHOD

Interaural delay thresholds were measured from 5 normal-hearing adults for an AM sound in the presence of interfering FM and vice versa as a function of interferer modulation rate. In addition, the effects of near versus remote interferer rates, and fixed versus randomized interferer interaural delay, were investigated.

RESULTS

AM interfered with lateralization of FM at all modulation rates. However, the FM interfered with AM lateralization only when the FM rate was higher than the AM rate. This rate asymmetry was surprising given the prevalence of low-frequency dominance in lateralization, but was predicted by a cross-correlation model of binaural interaction. Effects were similar for fixed and randomized interferer interaural delays.

CONCLUSIONS

The results suggest that in multisource environments, sources containing different modulation types significantly interfere with localization in complex ways that reveal interactions between modulation type and rate. These findings contribute to the understanding of auditory object formation and localization.

Sequential Stream Segregation Affects Localisation of Diotic Tones among Tones with Time-Varying Interaural Time Difference

In this study, I examined how sequential stream segregation contributes to the detection of diotic tones among tones with time-varying interaural time differences (ITDs). Target (T) and distractor (D) tones, and a silent duration (–) formed a sequence (DTD–) and this sequence was presented repeatedly. A frequency difference was introduced between target and distractor tones. The distractor tones were also given time-varying ITDs to produce a percept of smooth auditory motion along the interaural axis. In half of the trials, the target tones were not given time-varying ITDs, and thus were diotically presented. The task of the listeners was to determine whether the repeated sequences of DTD–had target tones without motion. The sensitivity d′ for the detection of diotic target tones was higher with larger frequency differences. On the other hand, the criterion c was lower with larger frequency differences. In another session, I confirmed that proportions of reports “two streams” was positively and negatively correlated with d′ and c, respectively. The results indicate that the localisation of a sound image could be influenced by sequential stream segregation in complex sound environments.

An event-related fMRI study of auditory motion perception: no evidence for a specialized cortical system

The existence of a specialized human cortical area for the processing of auditory motion is still a matter of debate. Initial functional imaging studies identified the planum temporale as being motion selective. Recent data contrasting spatially varying stationary stimuli with moving stimuli found no difference in the amount of activation between the two types of stimuli in the planum temporale. The present study re-examines this issue using an event-related paradigm. Ten subjects were scanned while listening to pairs of stimuli that were either both moving or both stationary. Consistent with the aforementioned study, we found no difference in the activation levels in the planum temporale when comparing motion and stationary conditions.

Spatial acuity and prey detection in weakly electric fish

It is well-known that weakly electric fish can exhibit extreme temporal acuity at the behavioral level, discriminating time intervals in the submicrosecond range. However, relatively little is known about the spatial acuity of the electrosense. Here we use a recently developed model of the electric field generated by Apteronotus leptorhynchus to study spatial acuity and small signal extraction. We show that the quality of sensory information available on the lateral body surface is highest for objects close to the fish's midbody, suggesting that spatial acuity should be highest at this location. Overall, however, this information is relatively blurry and the electrosense exhibits relatively poor acuity. Despite this apparent limitation, weakly electric fish are able to extract the minute signals generated by small prey, even in the presence of large background signals. In fact, we show that the fish's poor spatial acuity may actually enhance prey detection under some conditions. This occurs because the electric image produced by a spatially dense background is relatively "blurred" or spatially uniform. Hence, the small spatially localized prey signal "pops out" when fish motion is simulated. This shows explicitly how the back-and-forth swimming, characteristic of these fish, can be used to generate motion cues that, as in other animals, assist in the extraction of sensory information when signal-to-noise ratios are low. Our study also reveals the importance of the structure of complex electrosensory backgrounds. Whereas large-object spacing is favorable for discriminating the individual elements of a scene, small spacing can increase the fish's ability to resolve a single target object against this background.

Neural cross-correlation and signal decorrelation: insights into coding of auditory space

The auditory systems of humans and many other species use the difference in the time of arrival of acoustic signals at the two ears to compute the lateral position of sound sources. This computation is assumed to initially occur in an assembly of neurons organized along a frequency-by-delay surface. Mathematically, the computations are equivalent to a two-dimensional cross-correlation of the input signals at the two ears, with the position of the peak activity along this surface designating the position of the source in space. In this study, partially correlated signals to the two ears are used to probe the mechanisms for encoding spatial cues in stationary or dynamic (moving) signals. It is demonstrated that a cross-correlation model of the auditory periphery coupled with statistical decision theory can predict the patterns of performance by human subjects for both stationary and motion stimuli as a function of stimulus decorrelation. Implications of these findings for the existence of a unique cortical motion system are discussed.

Fast Fourier-based DSP algorithm for auditory motion experiments

A digital signal-processing (DSP) technique for rapid generation of complex auditory motion stimuli based on dynamic linear changes in interaural delay is described. In this technique, a pair of complementary discrete Fourier transforms (DFTs) for which the component spacing in one series is different than that of the other is used. The appeal of this technique is its wide applicability, since it can generate real-time motion stimuli of any velocity and starting interaural delay for complex broadband or filtered noise waveforms and nonstationary sounds such as speech, music, and other natural sounds.