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The so-called ventriloquism aftereffect is a remarkable example of rapid adaptative changes in spatial localization caused by visual stimuli. After exposure to a consistent spatial disparity of auditory and visual stimuli, localization of sound sources is systematically shifted to correct for the deviation of the sound from visual positions during the previous adaptation period. In the present study, this aftereffect was induced by presenting, within 17 min, 1800 repetitive noise or pure-tone bursts in combination with synchronized, and 20° disparate flashing light spots, in total darkness. Post-adaptive sound localization, measured by a method of manual pointing, was significantly shifted 2.4° (noise), 3.1° (1 kHz tones), or 5.8° (4 kHz tones) compared with the pre-adaptation condition. There was no transfer across frequencies; that is, shifts in localization were insignificant when the frequencies used for adaptation and the post-adaptation localization test were different. It is hypothesized that these aftereffects may rely on shifts in neural representations of auditory space with respect to those of visual space, induced by intersensory spatial disparity, and may thus reflect a phenomenon of neural short-term plasticity.

Transmission and reception of high-frequency sound in the natural environment of bushcrickets (Tettigonia viridissirna L.) was studied using the activity of an identified neuron in the insect's auditory pathway as a "biological microphone". Different positions of the receiver within the habitat were simulated by systematic variation of the distance from a loudspeaker and the height above the ground. Attenuation and filtering properties of the habitat were investigated with pure-tone frequencies between 5 and 40 kHz. Sound attenuation in excess of the attenuation due to geometrical spreading alone increased with increasing frequency, distance between sender and receiver, and decreasing height within the vegetation (Figs. 2-4). The data also confirm the existence of two kinds of excess attenuation. The amount of amplitude fluctuations in the sound signals was investigated by analysing the variability of the neuronal responses at a given receiver position. Variability increased with decreasing bandwidth of a noise signal at some distance from the loadspeaker. The variability in the responses to pure tones increased with both increasing frequency and distance from the source (Fig. 7). In the selected habitat, the temporal pattern of the natural calling song of male T. viridissima was very reliably reflected in the activity of the recorded neuron up to a distance of 30 m at the top of the vegetation, and 15-20 m near ground level (Figs. 5, 8). The maximum hearing distance in response to the calling song was about 40 m. Environmental constraints on long-range acoustic communication in the habitat are discussed in relation to possible adaptations of both the signal structure and the behavior of the insects.

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