Simultaneous comparison of two sound localization measures.

Almost all behavioral studies of sound localization have used either an approach-to-target or pointing/orienting task to assess absolute sound localization performance, yet there are very few direct comparisons of these measures. In an approach-to-target task, the subject is trained to walk to a sound source from a fixed location. In an orienting task, finger, head and/or eye movements are monitored while the subject's body is typically constrained. The fact that subjects may also initiate head and eye movements toward the target during the approach-to-target task allows us to measure the accuracy of the initial orienting response and compare it with subsequent target selection. To perform this comparison, we trained cats to localize a broadband noise presented randomly from one of four speakers located ± 30° and ± 60° in azimuth. The cat responded to each sound presentation by walking to and pressing a lever at the perceived location, and a food reward was delivered if the first attempt was correct. In tandem, we recorded initial head and eye orienting movements, via magnetic search coils, immediately following target onset and prior to the walking response. Reducing either stimulus duration or level resulted in a systematic decline in both measurements of localization performance. When the task was easy, localization performance was accurate for both measures. When the task was more difficult, the number of incorrect (i.e., wrong selection) and no-go (i.e., no selection) responses increased. Interestingly, for many of the incorrect trials, there was a dissociation between the orienting response and the target selected, and for many of the no-go trials, the gaze oriented towards the correct target even though the cat did not move to it. This suggests different neural systems governing walking to a target as compared to unconditioned gaze orienting.