Interpreting a negative d' ("Landsberger, David" )


Subject: Interpreting a negative d'
From:    "Landsberger, David"  <DLandsberger@xxxxxxxx>
Date:    Thu, 12 Mar 2009 11:04:18 -0700
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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. Thanks, David


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