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Re: identification task...negative d' values.


I grew up on Green's and Swets' signal detection book, as introduced by the phenomenal teacher Jim Egan was. I remember an episode that made me turn my back to negative d' presentations of any data. I had just stumbled upon an interesting phenomenon: a systematic underestimation of an unfilled time interval marked by two sine bursts frequencies f1 and f2, when compared to an interval marked by bursts of f1 only. While the results were robust when reached using the method of adjustments, I felt as being on soft ground as long as I did not have a discriminability measure as well. So, I ran a 2AFC experiment (without feedback, to allow the subjects to respond to what they heard rather than to what they were told to hear) and obtained a d' psychometric function extending to values below zero, as a function of time difference. When I presented those data at an Acoustical Society meeting, Dave Green asked what I really meant by negative d' , commenting that d' was a measure of differential sensitivity which, whether positive or negative, marked discriminability, and that a negative d' only meant that the subject mislabeled the stimuli.
My interpretation of Daniel's results is similar: his subjects identified the stimuli consistently incorrectly, meaning that they could hear the difference between them but put them in the wrong bins.