The Interaural Time Delay (ITD) is an important binaural cue for sound source localization. Calculations of ITD values are obtained either from measured time domain Head-Related Impulse Responses (HRIRs) or from their frequency transform Head-Related Transfer Functions (HRTFs). Numerous methods exist in current literature, based on a variety of definitions and assumptions of the nature of the ITD as an acoustic cue. This work presents a thorough comparative study of the degree of variability between some of the most common methods for calculating the ITD from measured data. Thirty-two different calculations or variations are compared for positions on the horizontal plane for the HRTF measured on both a KEMAR mannequin and a rigid sphere. Specifically, the spatial variations of the methods are investigated. Included is a discussion of the primary potential causes of these differences, such as the existence of multiple peaks in the HRIR of the contra-lateral ear for azimuths near the inter-aural axis due to multipath propagation and head/pinnae shadowing.

Studies of the nature of the neural mechanisms involved in goal-directed movements tend to concentrate on the role of vision. We present here an attempt to address the mechanisms whereby an auditory input is transformed into a motor command. The spatial and temporal organization of hand movements were studied in normal human subjects as they pointed towards unseen auditory targets located in a horizontal plane in front of them. Positions and movements of the hand were measured by a six infrared camera tracking system. In one condition, we assessed the role of auditory information about target position in correcting the trajectory of the hand. To accomplish this, the duration of the target presentation was varied. In another condition, subjects received continuous auditory feedback of their hand movement while pointing to the auditory targets. Online auditory control of the direction of pointing movements was assessed by evaluating how subjects reacted to shifts in heard hand position. Localization errors were exacerbated by short duration of target presentation but not modified by auditory feedback of hand position. Long duration of target presentation gave rise to a higher level of accuracy and was accompanied by early automatic head orienting movements consistently related to target direction. These results highlight the efficiency of auditory feedback processing in online motor control and suggest that the auditory system takes advantages of dynamic changes of the acoustic cues due to changes in head orientation in order to process online motor control. How to design an informative acoustic feedback needs to be carefully studied to demonstrate that auditory feedback of the hand could assist the monitoring of movements directed at objects in auditory space.

International audienceThis study evaluates several methods for reporting the perceived location of real sound sources. It is well known that the method used for collecting judgments in auditory-localization experiments has a strong influence on the accuracy of a subject’s response. Previous works on auditory-localization tasks revealed that egocentric pointing methods (which are based on a body-centered coordinate system) allow for more accurate judgments than verbal reporting or exocentric pointing techniques (which are based on a 2D or 3D reporting device). Three different egocentric methods are compared: the most commonly applied “manual pointing” and “head pointing” methods, and the “proximal pointing” method, which forces the participants to indicate the apparent direction by pointing in the proximal region of the head with a marker held at the fingertips. The two first methods involve a rotation of the body of the participant, whereas the third method only involves movements of the arm(s) and hand(s) with a fixed head. Sound stimuli were presented randomly over 24 loudspeakers that were uniformly distributed on the upper hemisphere around the subject. The merits of the different methods are compared and discussed with regard to localization errors and to practical considerations. Although they show similar trends, each of the different methods affects the pointing accuracy in a specific way. The proximal pointing method, for example, is more accurate for sources located at high elevation angles. However, at rear locations close to the median plane an increased bias appears due to difficulties in performing the motor task to reach these positions. The proximal pointing method shows faster response times, which may be advantageous when planning 3D sound localization experiments