John Podd's Theses
Submitted for the degree of Master of Arts in Psychology
Victoria University of Wellington, New Zealand
A major aim of this research was to investigate two assumptions often made in
the psychological literature. First, that human observers, in a binary choice
situation, use a decision axis at least monotonic with likelihood ratio. Second,
that their ability to discriminate among stimulus alternatives is equivalent to
their ability to discriminate among response alternatives.
A single-interval, forced-choice experiment was run. The observers had to
decide which of two possible events had occurred, and rate their certainty that
their decision was correct. The results indicate that, when an observer is given
insufficient information to decide with certainty, he uses a decision axis
similar to stimulus magnitude. When, however, he is give knowledge of results
(KR) about his performance, he may change his decision axis to one similar to
likelihood ratio. The implications of these results were discussed with respect
to the common practice of providing KR without treating it as a major
The theory of Type II ROC curve analysis was developed and used in the
assessment of the observers' abilities to evaluate their own decisions. It was
shown, both theoretically and empirically, that an observer's ability to
discriminate between his own correct and incorrect decisions is not equivalent
to his ability to discriminate between stimulus events. The more often an
observer was informed by KR that he was making correct stimulus-contingent
decisions, the better he was able to evaluate his own performance. Reasons for
this improvement were discussed in terms of a reduction in criterion instability
for the response-contingent decisions.
The Aural Discrimination of Simultaneous Changes in Amplitude and Frequency
Submitted for the degree of Doctor of Philosophy in Psychology
Victoria University of Wellington, New Zealand
The present research had two aims. First, to investigate the ability of human
observers to discriminate simultaneous changes in amplitude and frequency.
Second, to find out whether this ability was a function of the interaural noise
Two experiments were run using a two-alternative, forced-choice procedure
with the tonal signals presented binaurally in phase. There were five possible
increments in amplitude, and five in frequency. In the first experiment there
were four separately run conditions. There was an amplitude discrimination task
(frequency held constant), and a frequency discrimination task (amplitude held
constant). Signals where only one of the signal cues was varied were called
single-cue signals. The other two conditions used different subsets of
combined-cue signals, where amplitude and frequency were simultaneously
incremented. Observers always knew which signal condition was being used.
Signals were presented in a continuous noise masker where the interaural
noise correlation was one of +1.00 (N0S0), 0.00 (NuS0), or -1.00 (NpiS0). Each
interaural condition was used with each of the four signal conditions giving 12
experimental conditions in all.
The results showed that once the masking level differences created by the
NpiS0 and NuS0 conditions were compensated for, there were no other effects of
varying the interaural noise correlation on the discrimination of simultaneous
changes in amplitude and frequency.
The results also showed that performance levels for the combined-cue signals
were frequently lower than those obtained for the best comparable single-cue
signals. One factor associated with performance level was the size of the
absolute difference between the discrimination probabilities of the two signal
cues. The larger this difference, the less likely was there to be any gain in
The results of the first experiment were surprising in that performance
decrements occurred for so many combined-cue signals. A second experiment was
run partially to verify this result, and also to ascertain whether the type of
signal ensemble use in the first study had affected the outcome. All 25 possible
combined-cue signals were presented randomly over a 10,000 trial sequence, with
the continuous masker in phase.
A comparison of the results from the two experiments showed that signal
subset size had no effect on performance levels, and that decrements in
performance again occurred for many of the combined-cue signals.
The data from both experiments provided an opportunity to test the
predictions of the integration model and the decision threshold model, two
decision models that have been used to predict performance for the
discrimination of simultaneous changes in amplitude and frequency. Reasons for
the failure of both models to predict the present results were discussed.
The overall pattern of results was discussed in terms of a noisy decision
process. The size of the absolute difference between the discrimination
probabilities of the two signal cues, and the asymmetry in these discrimination
probabilities in favour of the amplitude cue were identified as two major
factors contributing to the decision noise.
08 Nov 2009 04:37 PM