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Interpreting a negative d'

I have conducted an experiment where I have obtained for one subject in one
condition a negative d' which I cannot explain. I was hoping that someone
here might be able to offer me some insight.

The experiment is a 3 interval forced choice task where a sound is presented
in each of the three intervals.  In two of the intervals, the sounds are
identical.  In the third interval, the sound is different. (Obviously the
order of stimuli is randomized.)  The patient's task is simply to tell me
which of the three sounds is different.

In a 2IFC task, a negative d' might indicate that the subject has
misunderstood the task.  For example, if two sounds were played and the
subject were asked to pick the sound that was higher pitched, a negative d'
would mean that the subject reliably picked the lower pitched sound as
having a higher pitched.

However, for a negative d' in a 3IFC task where two stimuli are the same,
the subject would have to reliably not pick the different one as being
different. What would that suggest about their perception?  I don't think
its a misunderstanding of the task as the same subject was able to perform
fine in the same task using a different set of stimuli.

Any advice would be greatly appreciated.