Abstract
Perception and encoding of object size is an important feature of sensory systems. In the visual system object size is encoded by the visual angle (visual aperture) on the retina, but the aperture depends on the distance of the object. As object distance is not unambiguously encoded in the visual system, higher computational mechanisms are needed. This phenomenon is termed “size constancy”. It is assumed to reflect an automatic re-scaling of visual aperture with perceived object distance. Recently, it was found that in echolocating bats, the ‘sonar aperture’, i.e., the range of angles from which sound is reflected from an object back to the bat, is unambiguously perceived and neurally encoded. Moreover, it is well known that object distance is accurately perceived and explicitly encoded in bat sonar. Here, we addressed size constancy in bat biosonar, recruiting virtual-object techniques. Bats of the species Phyllostomus discolor learned to discriminate two simple virtual objects that only differed in sonar aperture. Upon successful discrimination, test trials were randomly interspersed using virtual objects that differed in both aperture and distance. It was tested whether the bats spontaneously assigned absolute width information to these objects by combining distance and aperture. The results showed that while the isolated perceptual cues encoding object width, aperture, and distance were all perceptually well resolved by the bats, the animals did not assign absolute width information to the test objects. This lack of sonar size constancy may result from the bats relying on different modalities to extract size information at different distances. Alternatively, it is conceivable that familiarity with a behaviorally relevant, conspicuous object is required for sonar size constancy, as it has been argued for visual size constancy. Based on the current data, it appears that size constancy is not necessarily an essential feature of sonar perception in bats.
Highlights
The representation of object size and its neural encoding is an important function of sensory systems in general
Constancy experiment In the Constancy experiment, five bats were successfully trained to discriminate the VO with a sonar aperture of 23u from the VO with an aperture of 34u, both presented at the same distance of 204 cm
The bat sonar system theoretically provides the explicit information for the unambiguous encoding of an objects’ physical size even when presented at different distances. This hypothesis was tested in the Constancy experiment whether echolocating bats of the species P. discolor spontaneously assigned absolute width information to VOs by combining distance- and aperture information
Summary
The representation of object size and its neural encoding is an important function of sensory systems in general. How organisms perceive the physical object size of distant objects is a fundamental question and not completely answered yet [1,2]. The extent of the image on the retina in terms of its visual aperture is explicitly encoded. When the distance between the observer and the object changes, the visual aperture changes proportionally. ‘‘Size constancy’’ or ‘‘size distance invariance’’ [3,4,5] is assumed to reflect an automatic rescaling of perceived object size with perceived distance. Object distance is not explicitly encoded in the visual system; a mismatch of physical and perceived object distance can lead to a misinterpretation of physical object size and visual illusions [6]
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