Springer Online Journal Archives 1860-2000
Summary 1. Recordings were made from single inferior colliculus neurons ofMolossus ater andMolossus molossus (Molossidae) to pure tone pulses and FM-pulses with different modulation heights, durations (20 ms, 2 ms) and directions (upward, downward sweeps). 2. Minimum thresholds to pure tone pulses at best frequency and FM-pulses centered at best frequency of equal duration were compared. Using stimuli of 20 ms duration, 58% of the neurons inM. ater (49% inM. molossus) had equal threshold values within ±5 dB, 28% inM. ater (37% inM. molossus) responded with more than 5 dB higher thresholds to FM-stimuli. With stimulus durations of 2 ms, the distribution changed: 27% of the neurons inM. ater (41% inM. molossus) responded 5 to 60 dB more sensitively to the FM-stimulus that mimics the FM-component of the orientation call, only 17% inM. ater (26% inM. molossus) responded more sensitively to pure tone stimuli. Comparison of thresholds to FM-stimuli of different sweep rates also revealed a population of neurons more sensitive to the fast sweep rate (22%M. ater, 29%M. molossus). 3. Specialized neurons were found that did not respond to a particular stimulus configuration (e.g. neurons not responding to upward sweeping FM-stimuli, neurons not responding to slowly sweeping FM-stimuli, etc.). 4. Response patterns of single neurons were found to depend on the frequency-time-course of the stimulus. Especially with slowly sweeping FM-signals, neuronal response activity (measured in number of spikes per stimulus) could be greater than to any other stimulus configuration employed. A stabilization of initial latency was frequently observed with fast sweeping FM-stimuli. 5. Threshold Signal/Noise-ratios were measured by masking the neuron's response to the signal (pure tone or FM) by bandpassed pseudorandom noise. The threshold Signal/Noise-ratios where either equal or lower for FM-stimuli than for pure tone stimuli of equal duration, i.e. more noise intensity is needed to mask the response to FM.
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