An electronic simulated target apparatus was used in a two-experiment study to compare the target detection performance of an echolocating bottlenose dolphin with an optimal receiver. Random Gaussian noise with a relatively flat spectrum from 20 to 160 kHz was used as a masking source. Experiment I was conducted to establish a technique for estimating the echo energy-to-noise ratio,E e /N, at the dolphin's threshold of detection. Dolphins typically vary the amplitude of their emitted signal over a large range making it difficult to estimateE e /N. In the first part of experiment I, the simulated echo was a double click, the pulses separated by 200 s, with each pulse being a replica of the dolphin's transmitted signal. A staircase psychophysical procedure was used to obtain the detection threshold, and the echo energy-to-noise ratio based on the highest amplitude click emitted per trial, (E e /N)max, was determined at each reversal point. The second echo type consisted of one of the animal's echolocation clicks, previously measured, digitized and stored in an erasable programmable read-only memory (EPROM). The electronic target simulator was modified so that every time the dolphin emitted an echolocation signal, the EPROM was triggered to produce two pulses separated by 200 s. On any trial, the EPROM signal was played back at a fixed amplitude, regardless of the amplitude of the dolphin's emitted signal. TheE e /N obtained with the EPROM signal at threshold was found to be 2.9 dB lower than (E e /N)max obtained with the normal phantom target. Therefore an estimate ofE e /N can be obtained by subtracting 2.9 dB from (E e /N)max.
Experiment II was conducted to obtain isosensitivity data that could be plotted in an ROC (receiver operating characteristic) format. The response bias of the dolphin was manipulated by varying the food reinforcement payoff matrix. In terms of the ratio of correct detections to correct rejections, the payoff matrix was varied over four values: 11, 41, 81, and 14. A modified method of constants procedure was used to obtain the dolphin target detection performance data. Each session consisted of two 20-trial blocks in which a strong echo was used in the first block and a weak echo in the second block. The energy-to-noise ratio required by an optimal detector to approximate the dolphin's performance was obtained by determining the appropriate detection sensitivity,d, that best fitted the dolphin's data plotted in an ROC format. The results of experiment II indicated that the dolphin required approximately 7.4 dB higherE e /N than an optimal detector to detect the phantom target.
Abbreviations response bias - d detection sensitivity - E e echo energy flux density - EPROM erasable programmable read-only memory - N noise spectral density - p(t) instantaneous acoustic pressure - P(Y/SN) probability of detection - P(Y/N) probability of false alarm - ROC receiver-operating-characteristics - SE source energy flux density
| ||ISSN: 0340-7594 (Print) 1432-1351 (Online) |
1989///A comparison of signal detection between an echolocating dolphin and an optimal receiverJOUR