Page 49 - Volume 12, Issue 2 - Spring 2012

Fig. 6. Localization of a stationary acoustic target (mean performance from four cats) (A) prior to, and following, cooling deactivation (warm), (B) during bilateral cooling of the anterior auditory field (AAF), (C) during bilateral cooling of A1, and D) during bilateral cooling of the posterior auditory field (PAF). The two concen- tric semicircles represent 50% and 100% response levels and the length of each bold line corresponds to the percentage of correct responses at each location tested. Stimulus was a 100-ms broadband noise burst presented at 20 dB(A) above back- ground levels.
48 Acoustics Today, April 2012
motion processing. Specifically, area MT (middle temporal visual area) or area V5 in the primate (posteromedial lateral suprasylvian area (PMLS) in the cat) is an area uniquely iden- tified to be critical for the discrimination of visual motion (velocity, direction, etc.; see Born and Bradley, 2005). In the auditory system, the ability to discriminate the direction of a moving sound is also a critical survival feature, whether on the part of a predator or prey. Therefore, we have been investigat- ing whether there is a locus in auditory cortex that is essential for the accurate discrimination of motion in the absence of involvement in static auditory target localization. If such a cor- tical area exists, it would strongly support the concept of an acoustic MT. However, it may also be the case that for an area to be involved in acoustic motion direction discrimination that it might be a prerequisite for it to also contribute to the local- ization of stationary acoustic targets.
In cat auditory cortex, four areas have been identified to be critical for accurately determining the spatial location of a static acoustic stimulus (Malhotra and Lomber, 2007; Malhotra et al., 2008). These areas include primary auditory cortex (A1), the posterior auditory field (PAF), the dorsal zone of auditory cortex (area DZ), and the field of the anteri- or ectosylvian sulcus. Is there an auditory cortical area spe- cialized for acoustic motion processing, or are areas involved in static spatial localization also critical for acoustic motion processing? Cats were trained to perform two tasks: a spatial localization task using a static stimulus and a task that required the animals to discriminate leftward from rightward apparent acoustic motion. Focal, reversible cooling deactiva- tion was then used to bilaterally deactivate the anterior audi- tory field (AAF), A1, or PAF.
Both tasks were conducted in an acoustic orienting arena (Lomber et al., 2007) and broad-band noise was delivered at 20 dB SPL above a background level of 58 dB SPL as a stim- ulus. To determine the contribution of the three cortical areas to the accurate spatial localization of a static sound source, the cats were first trained in a semicircular arena to identify the location of a 100-ms broad-band noise burst randomly emitted from one of 13 speakers placed at 15°-intervals across 180° of azimuth. Before and after each cortical deacti- vation, acoustic spatial localization accuracy and precision were excellent, with performance above 80% correct for all locations (Fig. 6A). During bilateral deactivation of AAF cor- tex, sound-localization performance was unimpaired (Fig. 6B). Bilateral deactivation of A1 resulted in a spatial-local- ization impairment throughout the entire field examined to between 40-50% correct across all tested positions (Fig. 6C). In contrast, bilateral deactivation of PAF profoundly impaired the ability of all the cats to accurately and precisely localize the acoustic stimulus (Fig. 6D) to levels just above chance (7.7%). These findings were in agreement with earli- er studies (Malhotra and Lomber, 2007; Lomber and Malhotra, 2008; Malhotra et al., 2008).
Next, acoustic-motion discrimination during bilateral deactivation of the same three areas (AAF, A1, and PAF) was examined in the same four cats used in the static acoustic tar- get localization task. While each animal was fixating on a central LED, apparent motion of 90°/s was generated by