The effect of sequence repeat time on auditory cortex stimulation during phonetic discrimination

Acoustic noise generated by the MR scanner gradient system during fMRI stud ies of auditory function is a very significant potential confound. Despite these deleterious effects, fMRI of the auditory cortex has been successful and numerous practitioners have circumvented the problem of acoustic maskin g noise. In the context of auditory cortex fMRI, the sequence repeat time ( TR) has the effect of altering the length of time during which the scanner is quiet. Indeed, the move to whole-brain fMRI makes the problem of acousti c noise more acute and points to the need to examine the role of TR and its influence on the BOLD signal. The aim of this study was to examine the eff ect of varying the TR time on activation of auditory cortex during presenta tion and performance of a phonetic discrimination task. The results present ed here demonstrate that the influence of sequence repeat time is considera ble. For a short repeat time it is likely that the noise from the scanner i s a significant mask and hinders accurate task performance. At the other ex treme, a repeat time of 9 s is also suboptimal, probably due to attentional effects and lack of concentration and not least because of the longer over all measurement times. The results of this study point to a complicated int erplay between psychophysical factors as well as physical parameters; atten tion, acoustic noise, total duration of the experiment, consideration of th e volume of acquisition, and overall difficulty of the task have to be asse ssed and balanced. For the paradigm used here, the results suggest an optim al TR of around 6 a for a 16-slice acquisition. (C) 2000 Academic Press.