Transcranial magnetic brain stimulation: lack of oculomotor response

We have investigated whether eye movements can be evoked by transcranial magnetic brain stimulation (TMS) from frontal, precentral, posterior- and inferior- parietal, occipital and temporal positions of the stimulating coil. Our findings were negative, even for structures concerned with voluntary eye movements, such as the frontal eye field (FEF) and the inferior parietal lobe (IPL). The lack of oculomotor responses after stimulation of the cortex shall be discussed in the following context: (1) Low-threshhold intracortical stimulation experiments suggest that in functional terms, the FEF is confined to small areas which are extend to the floor of sulci. TMS does not reach these structures to a sufficient extent. Furthermore efferent connections of the cortex to the paramedian pontine reticular formation (PPRF) seem to be polysynaptic. (2) The function of the cortex in rapid eye movement is to analyze and process conditions with differing functional requirements, rather than to directly generate saccades. TMS does not elicit oculomotor responses, demonstrating again that the role played by the cortex in eye movement is not analogous to the role of the somatic motor cortex in controlling skeletal movements.