Subject: Re: Interpreting a negative d' From: Robert Zatorre <robert.zatorre@xxxxxxxx> Date: Thu, 12 Mar 2009 21:06:08 -0400 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>There is one other interpretation, which is not related to bias at all. Rather, it is possible that the individual has indeed discriminated the stimuli and is purposefully choosing the incorrect response. One can only speculate about why someone would do this...it could be a sign of malingering, psychosis, passive agression, or just plain orneriness! Robert -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Robert J. Zatorre, Ph.D. Montreal Neurological Institute 3801 University St. Montreal, QC Canada H3A 2B4 phone: 1-514-398-8903 fax: 1-514-398-1338 e-mail: robert.zatorre@xxxxxxxx web site: www.zlab.mcgill.ca Pierre Divenyi wrote: > I remember that some 30-odd years ago I presented data at an Acoustical > Society meeting in which I showed upside-down responses in a 2IFC task > in form of negative d'. And I have never forgotten David Green's (very > kindly expressed) comment: d' always measures discriminability which is > a distance and which, by definition, can never be negative. Therefore, > every instance of a d' < 0 can only be assigned to the observer's bias. > So the question is not how to interpret a negative d' but how to > interpret the bias. > > -Pierre > > At 11:04 AM 3/12/2009, Landsberger, David wrote: >> 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