Problem: you don't know whether the effect is happening in the perceptual analysis of the CVs or whether it is modulating the decision process.
This worry led to the following experiment, spearheaded by Jon Venezia in my lab. Using fMRI we modulated response bias while keeping perceptual discrimination constant in a CV same-different discrimination task. Response bias was manipulated by changing the ratio of same to different trials. The stimuli were otherwise identical. We then looked for the neural correlates of response bias changes.
Behavioral results confirmed the intended effect: response bias (measured using the c, or criterion, statistic) was significantly modulated as a function of the ratio of same to different trials (p < .001) whereas d' (perceptual discrimination) was unaffected (p = .384). The figure below shows the clear effect on bias (top panel) and the somewhat wobbly effect on proportion correct (p = .061).
So what brain regions correlate with the modulation of response bias? Our favorite motor speech areas (among other non-auditory areas):
Turns out these regions line up quite well with those regions that, when stimulated with TMS, yield effects on "speech perception".
This is not conclusive evidence that all previous effects of motor modulation on speech perception are a result of response bias alone, but it does seriously question the validity of these reports and makes a strong case for the view that we need to take bias into account when measuring "speech perception."
Venezia, J. H., Saberi, K., Chubb, C., & Hickok, G. (2012). Response bias modulates the speech motor system during syllable discrimination. Frontiers in Psychology, 3.