Single unit activity from the VI and VII lobuli of the cerebellar vermis cortex was studied following acoustical stimulation with sound signals of different parameters. Cerebellar neurons, as compared to those from the auditory system, showed low selectivity to sound frequency, intensity and duration. However, about 2/3 of the neurons were selectively sensitive to interaural time and intensity differences; about 1/3 of neurons showed a specific response to signals simulating sound motion in a definite direction. Thus, cerebellar neurons seem to be mainly responsive to those sound parameters which are essential for sound localization.
Lateralization of moving fused auditory images (FAIs) was studied under dichotic stimulation, with FAI movement from the right and left ears to midline. The movement was produced by the gradual change of interaural time delay (from +/- 630 to 0 microseconds) in a binaurally presented click train in which a constant interaural intensity difference (IID) between +/- 13 dB was also imposed. The task of the subjects was to show with her/his finger the point on the head surface where the FAI trajectory's ending or starting points were perceived. With IID change within +/- 13 dB, the FAI movement trajectory shifted toward the ear receiving the more intense stimulus. The length of the movement trajectory shortened with IID increase. Functions relating the value of perceived lateral position (Y) of the movement trajectory's ending and starting points to IID value (X) were nearly linear: Y = AX + B. These functions differed in their characteristics whether the movement was to the right versus to the left of midline. They also differed from analogous functions for stationary FAI. At IID = 0 the FAI movement trajectory's endpoint was shifted from midline in the direction of movement. Equivalence ratio for IID and ITD were estimated to be 51 and 29 microseconds/dB respectively for the trajectory's starting and ending points. The IID factor could be several times as effective in moving FAI lateralization as the ITD factor.
Activity of neuronal pairs in the inferior colliculus of the rabbit was recorded with a single stainless-steel microelectrode. Seventy pairs were investigated with monaural and binaural tonal stimuli. The most common parameter of the response in neuronal pairs was the best frequency, which was similar in 100% of the pairs (n = 45). Q10 values were identical in 44% of pairs and threshold tuning curves in 27% of pairs. Units with a smaller spike amplitude usually had a shorter latency to both binaural and monaural stimuli, when measured 10-20 dB above the best frequency threshold. Most units discharged during the entire period of the 100 ms tone stimulation at their best frequency; large differences, however, were found in their firing pattern, when peristimulus histograms were compared. High correlation was found in pairs where both neurones exhibited the same type of binaural interaction. The following types of binaural interaction were found: binaural excitatory drive with occlusion, binaural excitatory drive with facilitation; monaural excitatory drive with inhibition from the other ear and pure monaural excitatory drive. In a significant number of neuronal pairs the influence of binaural stimulation was similar for both neurones. The results suggest that: (a) many adjacent neurones in the inferior colliculus convey parallel information concerning features of the auditory stimulus; (b) units with a similar type of binaural interaction may be organized in clusters within isofrequency layers.
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