As a newbie to psychophysics experiment, I have a small project to do visual field testing on healthy, adult, naive subjects. The experiment stimulus is a small sine grating (0.3 degree, 6 cycles/degree) embedded in one of 25 possible locations in a noise background picture (18 degree). The orientation of the grating, and the contrast of background noise is fixed, and I need to measure the sensitivity (d') at 25 locations given a series of (probably 3) grating contrasts, and a series of stimulus exposure time (50,100,200,300,400 ms).
In a single trial, subject knows where the grating will be located, and maintains fixation at the center of noise background. Then the stimulus will be briefly presented (once in Yes/No and 2AFC, twice in 2IFC) for one of the stimulus exposure time, and subject need to judge whether the grating is present (Yes/No), or is located in which stimulus (2IFC and 2AFC). For a each unique combination of grating location, grating contrast and stimulus exposure time, I plan to do 16-20 repetitions to measure d'. So there are LOTS of trials.
I think this should be a mature experiment, but I am stuck on choosing between the following 3 experiment paradigms. The major problem is the time required to complete the experiment. I have run pilot experiments on all of them.
1-interval yes/no task. This task runs fast, but suffers from changing response bias. Is the response bias going to negatively affect d'? In signal detection theory, d' is insensitive to bias only when the variance of response to signal and signal+noise is equal. However, I don't know whether there is firm experimental evidence to support this.
2AFC task. This task comes from a suggestion in textbook. Instead of presenting one cue of target location on each trial, I can present two cues that is symmetric around fixation point, and subject need to judge which cue location has the grating. The task assumes that human visibility map is central symmetric, and effectively cut the number of locations to half, so it runs even faster than the Yes/No task. However, it is rarely used in literature about visual field testing, what might be the potential drawback?
2IFC task. I see many literatures doing this, but it is the slowest one (more than double of the time for Yes/No). What's worse, because the stimulus is brief presented, and the interval between them is short (400 ms), sometimes subjects report that they "see the stimulus but forget which interval has it". So in some of our pilot data, the sensitivity at fovea is lower than in near-peripheral, because subjects had made an error. How do psychophysical researchers overcome the problem of subject error, and the problem of long, boring experiment?
Thanks for any help.