Signal-, set- and movement-related activity in the human brain: An event-related fMRI study

Electrophysiological studies on monkeys have been able to distinguish senso ry and motor signals close in time by pseudo-randomly delaying the cue that instructs the movement from the stimulus that triggers the movement. We ha ve used a similar experimental design in functional magnetic resonance imag ing (fMRI), scanning subjects while they performed a visuomotor conditional task with instructed delays. One of four shapes was presented briefly. Two shapes instructed the subjects to flex the index finger; the other two sha pes coded the flexion of the middle finger. The subjects were told to perfo rm the movement after a tone. We have exploited a novel use of event-relate d fMRI. By systematically varying the interval between the visual and acous tic stimuli, it has been possible to estimate the significance of the evoke d haemodynamic response (EHR) to each of the stimuli, despite their tempora l proximity in relation to the time constant of the EHR. Furthermore, by va rying the phase between events and image acquisition, we have been able to achieve high temporal resolution while scanning the whole brain. We dissoci ated sensory and motor components of the sensorimotor transformations elici ted by the task, and assessed sustained activity during the instructed dela ys. In calcarine and occipitotemporal cortex, the responses were exclusivel y associated with the visual instruction cues. In temporal auditory cortex and in primary motor cortex, they were exclusively associated with the audi tory trigger stimulus. In ventral prefrontal cortex there were movement-rel ated responses preceded by preparatory activity and by signal-related activ ity. Finally, responses associated with the instruction cue and with sustai ned activity during the delay period were observed in the dorsal premotor c ortex and in the dorsal posterior parietal cortex. Where the association be tween a visual cue and the appropriate movement is arbitrary, the underlyin g visuomotor transformations are not achieved exclusively through frontopar ietal interactions. Rather, these processes seem to rely on the ventral vis ual stream, the ventral prefrontal cortex and the anterior part of the dors al promotor cortex.