Processing of auditory information in insects

Insects exhibit an astonishing diversity in the design of their ears and the subsequent processing of information within their auditory pathways. The aim of this review is to summarize and compare the present concepts of auditory processing by relating behavioral performance to known neuronal mechanisms. We focus on three general aspects, that is frequency, directional, and temporal processing. The first part compares the capacity (in some insects high) for frequency analysis in the ear with the rather low specificity of tuning in interneurons by looking at Q10dB values and frequency dependent inhibition of interneurons. Since sharpening of frequency does not seem to be the prime task of a set of differently tuned receptors, alternative hypotheses are discussed. Moreover, the physiological correspondence between tonotopic projections of receptors and dendritic organization of interneurons is not in all cases strong. The second part is concerned with directional hearing and thus with the ability for angular resolution of insects. The present concepts, as derived from behavioral performances, for angular resolution versus lateralization and serial versus parallel processing of directional and pattern information can be traced to the thoracic level of neuronal processing. Contralateral inhibition, a mechanism for enhancing directional tuning, appears to be most effective in parallel pathways, whereas in serial processing it may have detrimental effects on pattern processing. The third part, after some considerations of signal analysis in the temporal domain, demonstrates that closely related species often use different combinations of temporal parameters in their recognition systems. On the thoracic level, analysis of temporal modulation functions and effects of inhibition on spiking patterns reveals relatively simple processing, whereas brain neurons may exhibit more complex properties.