[visionlist] signal detection query

[Joseph Brooks]

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