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Summary. Doppler shift compensation behaviour in horseshoe bats, Rhinolophus rouxi, was used to test the interference of pure tones and narrow band noise with compensation performance. The distortions in Doppler shift compensation to sinusoidally frequency shifted echoes (modulation frequency: 0.1 Hz, maximum frequency shift: 3 kHz) consisted of a reduced compensation amplitude and/or a shift of the emitted frequency to lower frequencies (Fig. 1).Pure tones at frequencies between 200 and 900 Hz above the bat's resting frequency (RF) disturbed the Doppler shift compensation, with a maximum of interference between 400 and 550 Hz (Fig. 2). Minimum duration of pure tones for interference was 20 ms and durations above 40 ms were most effective (Fig. 3). Interfering pure tones arriving later than about 10 ms after the onset of the echolocation call showed markedly reduced interference (Fig. 4). Doppler shift compensation was affected by pure tones at the optimum interfering frequency with sound pressure levels down to -48 dB rel the intensity level of the emitted call (Figs. 5, 6).Narrow bandwidth noise (bandwidth from + 100 Hz to _ 800 Hz) disturbed Doppler shift compensation at carrier frequencies between -250 Hz below and 800 Hz above RF with a maximum of interference between 250 and 500 Hz above resting frequency (Fig. 7). The duration and delay of the noise had similar influences on interference with Doppler shift compensation as did pure tones (Figs. 8, 9). Intensity dependence for noise interference was more variable than for pure tones (-32 dB to --45 dB rel emitted sound pressure level, Fig. 10).The temporal and spectral gating in Doppler shift compensation behaviour is discussed as an effective mechanism for clutter rejection by improving the processing of frequency and amplitude transients in the echoes of horseshoe bats.Abbreviations: CF constant frequency; FM frequency modulation; RF resting frequency; SPL sound pressure level * To whom offprint requests should be sent

This paper is concerned with the revision of beliefs in the face of new and possibly contradicting information. In the Logic of Theory Change developed by Alchourron, Gärdenfors and Makinson this nonmonotonic process consists of a contraction and an expansion of a set of formulas. To achieve minimal change they formulated widely accepted postulates that rational contractions have to fulfill.Contractions as defined by Alchourron, Gärdenfors and Makinson only operate on deductively closed sets of formulas. Therefore they cannot be used in practical applications, eg. knowledge representation, where only finitely representable sets can be handled.We present a semantical characterization of rational finite contractions (the class of rational contractions maintaining finite representability) which provides an insight into the true nature of these operations. This characterization shows all possibilities to define concrete functions possessing these properties.When regarding concrete contractions known from literature in the light of our characterization we have found that they are all defined according to the same semantical strategy of minimal semantical change. As this strategy does not correspond to the goal of keeping as many important formulas as possible in the contracted set, we suggest a finite contraction defined according to the new strategy of maximal maintenance.

The aim of this paper is to find criteria for the evaluation of oral business discourse produced in the language laboratory by students of business German. I felt the lack of an instrument that measures more precisely, not only linguistic criteria like lexis and grammar, but also the pragmatic background and the functions that discourse types within 'language for specific purposes' (LSP) have to fulfil. By evaluation I mean the assessment of individual progress, but indirectly I evaluate a teaching concept. A good teaching programme should facilitate progress.I am going to describe a teaching concept called Übertragung 'text transfer', focusing on the project that provided the data material for the present study. I will then present the model I have chosen as a basis for the study. In the study itself I concentrate on 'text organisation' and 'content structure'.

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