Object-Oriented Echo Perception and Cortical Representation in Echolocating Bats

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Abstract
Echolocating bats can identify three-dimensional objects exclusively through the analysis of acoustic echoes of their ultrasonic emissions. However, objects of the same structure can differ in size, and the auditory system must achieve a size-invariant, normalized object representation for reliable object recognition. This study describes both the behavioral classification and the cortical neural representation of echoes of complex virtual objects that vary in object size. In a phantom-target playback experiment, it is shown that the bat Phyllostomus discolor spontaneously classifies most scaled versions of objects according to trained standards. This psychophysical performance is reflected in the electrophysiological responses of a population of cortical units that showed an object-size invariant response (14/109 units, 13%). These units respond preferentially to echoes from objects in which echo duration (encoding object depth) and echo amplitude (encoding object surface area) co-varies in a meaningful manner. These results indicate that at the level of the bat's auditory cortex, an object-oriented rather than a stimulus-parameter–oriented representation of echoes is achieved. Bats can orientate and hunt for prey in complete darkness using echolocation. Bats use this extraordinary ability, not only to localize objects in space, but also to identify them. The same object, however, can come in different sizes. Here, we use a combination of psychophysical phantom-target experiments and electrophysiological recordings to investigate how echolocating bats perceive objects of different sizes, and how the echoes reflected from these objects are represented in the bat auditory cortex. We trained the neotropical bat Phyllostomus discolor to identify virtual objects, and found that these bats spontaneously associated scaled versions of these objects with the corresponding trained object. Interestingly, we identified neurons in the bat auditory cortex that respond to specific objects irrespective of object size. These findings highlight the sensory capabilities and sophisticated neural processes underlying bat echolocation. This suggests that like the visual system, echolocation meets an important requirement of an effective object-recognition system in that it allows the identification of objects independent of object size.