[visionlist] signal detection query

[Bahador Bahrami]

 Hi Todd and Joe

I am not quite sure about (1). To get the d' and Criterion, the Z score of
Hit and FA are obtained from the inverse of the guassian distribution with
mean zero and unit std; but when you have unequal number of
(signal+noise) and (noise) trials, it seems reasonable to me that the
correct distribution should have a shifted, non-zero mean (and who knows
what the best assumption about std would be in this case?)

bahador


>  (1) As far as I know, the absolute numbers of trials in each cell is not
> really relevant. What might happen is that having more old pictures will
> bias the subjects to respond "old", but that will show up in the
> criterion/bias measures. I don't think that it affects the underlying
> assumptions of the model.
>
> (2) Using the same false alarm rate is commonly done in similar
> circumstances. Again, I don't think it poses a problem, as long as you are
> mixing the two types of old pictures in the same block of test trials.
>
> thanks
> Todd
>
>  On Jul 20, 2010, at 1:03 PM, Joseph Brooks wrote:
>
>  We are calculating d-prime for a visual memory experiment. We have two
> issues which are unclear to us and we are hoping that someone may be
> able to give us some guidance or direct us to the best resource.
>
> In our experiment people see a set of pictures. Then later they are
> presented with a set of pictures (the test set) and asked whether they
> saw each picture before (old/new judgment). The test set includes both
> the previously seen pictures (2/3 of the total test set) and some
> novel pictures that were not seen previously (1/3 of the total test
> set). Unfortunately we had to have this uneven number of old and novel
> pictures because of constraints on our stimulus set.
>
> (1) Does the calculation of d-prime need to be adjusted in any way
> because of greater number of signal (old pictures) than noise (novel
> pictures) trials?
>
> (2) Half of the previously seen pictures were seen during high
> intensity transcranial magnetic stimulation whereas the other half
> were seen during low intensity stimulation. We will calculate the hit
> rate separately for the high and low conditions. However, none of the
> novel stimuli (which will be used to calculate the false alarm rate)
> had any stimulation at all. Thus, the two hit rates mentioned above
> don't have two separate corresponding false alarm rates. Rather there
> is just one false alarm rate based on all of the novel trials. Any
> d-prime values that we calculate from the two hit rates and the one
> false alarm rate will share the same false alarm rate. I don't see any
> other way to do this and I'm assuming that this does NOT pose any
> problems down the line for comparing the two d-primes (given that they
> are now no longer independent). Am I wrong here?
>
> Any help with these issues will be much appreciated!
>
> Regards,
>
> Joseph Brooks
> Institute of Cognitive Neuroscience
> University College London
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>
>   Todd S. Horowitz, PhD
> Assistant Professor of Ophthalmology
> Harvard Medical School
>  Associate Director
> Visual Attention Lab
> Brigham & Women's Hospital
> 64 Sidney Street, Suite 170
> Cambridge, MA 02139
> phone:  (617) 768-8813
> fax:    (617) 768-8816
> *http://search.bwh.harvard.edu/* <http://search.bwh.harvard.edu/>
>
>
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