[visionlist] signal detection query

Todd S. Horowitz toddh at search.bwh.harvard.edu
Tue Jul 20 20:03:30 GMT 2010


Hi Joe

(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/



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