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Zusammenfassung:
A key survival strategy of animals is the detection and adaptive response to rapidly approaching danger. Accordingly, behavioral experiments have shown that animals have a bias towards approaching motion signals as opposed to receding signals in both the visual [e.g., Schiff et al., 1962 Science 136:982] and auditory [e.g., Ghazanfar et al., 2002 PNAS 99:15755] domain. In the auditory domain, primates (including humans) appear to detect approaching motion using dynamic intensity cues. Dynamic intensity can also give information about behaviorally-relevant variables such as the time-to-contact of approaching sound sources, independent of absolute intensity. In the present study, we investigated the representation of looming and receding sounds in rhesus monkey auditory cortex. We recorded local field potential (LFP) activity in the auditory cortex of an awake, behaving monkey while the subject was presented with amplitude modulated complex tones either unimodally or in combination with visual motion-in-depth signals. Auditory stimuli were rising and falling intensity complex tones, 1, 2 or 4 seconds in duration, simulating sound sources moving in depth at a constant velocity. Visual stimuli were expanding/contracting black disks. We independently varied strength and speed of the simulated source. Preliminary spectral analysis of LFP data shows a bigger sustained increase in gamma band (30-50 Hz) activity in response to rising intensity sounds compared to falling intensity sounds in 10 out of 19 cortical sites. This response is not triggered by the stimulus reaching an absolute intensity threshold. Because of the close link between auditory intensity and distance, and the behavioral relevance for detecting approaching motion, we hypothesize that dynamic auditory intensity is encoded as dynamic distance for the following reasons: 1) responses were biased towards rising intensity, in parallel with known behavioral biases; 2) responses were independent of absolute intensity; and 3) responses were enhanced when auditory stimuli were paired with congruent but not incongruent visual motion-in-depth signals.
