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UniversitΣt Tⁿbingen - Lehrstuhl Tierphysiologie |
Human listening experiments on the psychophysics of ranging paradigms.
Proccedings of the 24th G÷ttingen Neurobiology Conference, 1996
The ranging capabilities of the bats' sonar receiver have been studied extensively in behavioral 2-AFC - experiments employing the playback of artificial echoes to the animals' echolocation pulses. Such procedures are thought to generate the impression of phantom targets located at different distances. These tests made use of two paradigms: range difference (RD), the discrimination between different echo delays, and jitter versus non-jitter (JNJ), the detection of a regularly alternating, 'jittering' delay against a reference of constant delay values. The perceptional equivalence of these paradigms hypothesized by some workers has been doubted by others and a third paradigm, jitter-versus-jitter (JJ), which would require the discrimination of different jitter amplitudes, has been proposed for the purpose of clarifying this unresolved issue. In order to obtain a first glimpse of the psychophysical properties of the stimuli delivered in the ranging experiments, passive listening tasks with sequences of repeated artificial 'pulse-echo-pairs' where designed for human subjects according to all three mentioned rules and thresholds were measured in an adaptive 3-AFC procedure. It could be demonstrated, that the subjects' performance depended on a variety of stimulus parameters (e.g., repetition rate, reference value of the pulse-echo-delay and the characteristics of the individual pulses). The nature of these relationships depended on the tested paradigm, hinting at possible differences in the involved mechanisms. Each paradigm seems to constitute an individual perceptual task, with a somewhat closer relationship existing between JNJ and JJ. The JNJ-performance depended on the pulse-echo-delay of the reference stimulus, which weakens the view that a grossly similar effect found in bats can be regarded as proof for a specific internal representation of the stimuli along a range axis in these animals. In several experiments qualitative, individual differences among the subjects surfaced, which might be interpreted as a hint towards the existence of individual strategies. This would imply, that the solution of these tasks allows for different approaches. In the tasks studied here, several evidences from the performance as well as from it's subjective corollaries seem to point to the involvement of higher cognitive functions, such as pattern recognition and stream segregation, which have not been considered in the study of ranging so far. The subjects of this study were carrying out passive hearing tasks, whereas ranging bats are making use of an active sensory system. Furthermore, modifications to the stimuli were necessary for adapting to the requirements of the human subjects. Therefore, no attempt is made to purport that problems associated with experiments in bats have been solved by means of our experiments once for all. Rather, we regard our results as hypotheses, which we consider worth for further testing with bats.
range difference, jitter detection, jitter discrimination, psychoacoustical properties