[visionlist] signal detection query

Bosco Tjan btjan at usc.edu
Wed Jul 21 00:13:13 GMT 2010

Everything being equal, d' should not be affect by the prior  
probability of signal present, as Todd pointed out; the response  
criterion should.


On Jul 20, 2010, at 4:20 PM, Bahador Bahrami wrote:

> 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
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