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This paper focuses on how the electric ¢eld from the prey of the platypus is detected with respect to the questions of threshold determination and how the platypus might localize its prey. A new behaviour in response to electrical stimuli below the thresholds previously reported is presented. The platypus shows a voluntary exploratory behaviour that results from a temporal integration of a number of consecutive stimulus pulses. A theoretical analysis is given, which includes the threshold dependence on the number of receptors and temporal integration of consecutive stimuli pulses, the close relationships between electrical ¢eld decay across the bill, electroreceptive thresholds and directionality of the platypus bill acting as an antenna. It is shown that a lobe shape, similar to that which has been measured, can be obtained by combining responses in a speci¢c way from receptors sensing the electric ¢eld decay across the bill. Two possible methods for such combinations are discussed and analysed with respect to measurements and observed behaviour of the platypus. A number of factors are described which need to be considered when electroreceptive thresholds are to be determined. It is shown that some information about the distance to the source is theoretically available from the pattern of ¢eld decay across the platypus's bill. The paper includes a comparative analysis of radar target tracking and platypus prey localization.

Heart sounds can be considered as mechanical fingerprints of myocardial function. The third heart sound normally occurs in children but disappears with maturation. The sound can also appear in patients with heart failure. The sound is characterised by its low-amplitude and low-frequency content, which makes it difficult to identify by the traditional use of the stethoscope. A wavelet-based method has recently been developed for detection of the third heart sound. This study investigated if the third heart sound could be identified in patients with heart failure using this detection method. The method was also compared with auscultation using conventional phonocardiography and with characterisation of the patients with echocardiography. In the first study, 87% of the third heart sounds were detected using the wavelet method, 12% were missed, and 6% were false positive. In study 2, the wavelet-detection method identified 87% of the patients using the third heart sound, and regular phonocardiography identified two (25%) of the subjects.

The stability criterion of linear four-terminal networks is given in a compact form similar to the activity criterion. The reduction of the Llewellyn criterion [2] to this new form is presented. By means of the stability and activity criteria in their compact form, an activity-stability diagram is derived, which is shown to be useful in the analysis of nonreciprocal networks, indicating stability, activity and nonreciprocity, and in the field of synthesis of nonreciprocal, passive and active networks. The new insight that is gained, indicates a novel way of achieving nonreciprocal stable amplification, using reciprocal active elements such as tunnel diodes and varactor diodes.