Bioacoustic spatial perception by humans: A controlled laboratory measurement of spatial resolution without distal cues

The angular spatial resolution of a wide-angle air sonar using a continuous transmission frequency-modulated radiation, with the output coupled binaurally to the auditory system of a user, was measured under restrained controlled conditions. This was done to determine the effect of adding a narrow central field of view of 9 deg to a wide-angle sonar. The target objects were three equidistant vertical rods initially spaced apart by 10 deg. This was varied down to a spacing of 4 deg. Ten nonvisual subjects achieved an angular resolution of 6 deg. Four of these ten subjects continued learning to achieve an unexpected spatial resolution of 4 deg within the 9 deg central field. A mean error of approximately 1 deg in direction accuracy was achieved. It is inferred that the unique variations in the octave band ultrasonic echoes within the narrow field, and the invariance of the on-axis echo as one’s head is turned, enables this angular resolution and accuracy to be achieved within the wide binaural field of view of 50 deg. This ability to resolve specula objects within a narrow angular resolution element of 9 deg is linked to the bat’s ability to seemingly resolve object glints within a distal resolution element of less than 2 wavelengths.