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Assessing psychophysical abilities in brushtail possums.

In Experiment 1, 6 brushtail possums were trained, using food, to perform a conditional discrimination between flickering (5.00 Hz) and steady lights. A block-wise method of limits procedure was then used to increase flicker speed over five steps to 50.00 Hz and percentages correct decreased to near or below 75 %. In Experiment 2 further flicker speeds were presented but percentages correct did not decrease as flicker speed increased. In Experiment 3 flicker speeds ranging from 16.67 Hz to 71.42 Hz were presented in either 5 or 10 steps. Percentages correct remained high at all flicker speeds regardless of the speed of stimulus change, suggesting that an extraneous variable had come to control behaviour. In Experiment 4, auditory cues, background lighting and relative luminance cues were examined using the 10 step stimulus change series. Once the relative luminance of the lights were equalised percentage correct dropped as flicker speed increased beyond 20.00 Hz. Flicker speeds between 20.00 and 25.00 Hz (equal luminance) were presented in Experiment 5 and gave all-or-none functions which fell abruptly at 24.00 Hz. In Experiment 6 a simultaneous stimulus discrimination task was trained and CFF was tested over several ranges. The threshold functions were very similar to those from the conditional discrimination experiments. These experiments suggest that the CFF of possums is 24.00Hz (at an average luminance of 2.1 log cd/m²). The implications of this for possums' visual abilities are discussed. In Experiment 7 six new possums learned a conditional discrimination between the presence and absence of a tone (at 880 Hz) using methodology similar to Experiment 1. Possums learnt the task quickly. Tone intensity (dBA) was reduced from 80 dBA in 8 dBA steps until percentages correct fell below 50 %. Threshold functions did not resemble those found in Experiments 5 and 6. There were response biases here but not in the previous flicker experiments and possible implications of this on threshold measures are discussed. In Experiment 9 six possums were trained on a conditional discrimination using dim and bright stimuli. Difficulties with producing appropriate stimuli meant only 2 possums learned the task. During threshold sessions the luminance of the bright stimulus was reduced in five steps using a blockwise method of limits. The threshold functions found were gradual resembling those of Experiment 7, suggesting that the step function found in the flicker experiments may be unique to possums and CFF determinations. These experiments show that it is possible to train possums to perform visually-based and auditory-based conditional discriminations, but suggest that caution is needed when comparing threshold estimates across experiments.
Type of thesis
Signal, T. D. (2002). Assessing psychophysical abilities in brushtail possums. (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14086
The University of Waikato
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