Lack of oculomotor response after transcranial magnetic stimulation

Abstract

In awake humans transcranial magnetic stimulation (TMS) of the brain elicits responses in skeletal muscles by stimulating the motor cortex to produce activity in the monosynaptic corticospinal tract. It has also been demonstrated that responses can be evoked in muscles supplied by the cranial nerves V, VII and XI. Short-latency ipsilateral responses are reported to be induced by direct stimulation of the peripheral cranial nerves; bilateral responses of longer latency seem to be a consequence of face-associated motor cortex stimulation. In the present study the authors investigated whether eye movements can be evoked by TMS from frontal, precentral, posterior- and inferior-parietal, occipital and temporal positions of the stimulating coil. Their findings were negative: they evoked neither short-latency ipsilateral responses from optomotor nerves nor responses of longer latency, which might have been expected after stimulation of cortical areas concerned with voluntary eye movements, particularly the frontal eye field (FEF) and the inferior parietal lobe (IPL). The lack of response after trying to stimulate peripheral optomotor nerves can be explained by the considerably long distance between the coil and these more medially located cranial nerves. The lack of oculomotor responses after stimulation of the cortex will be discussed in the following context: (1) With low-threshold intracortical stimulation, the extension of the FEF is confined to small areas which are partly extended to the floor of sulci. TMS does not reach these structures to any sufficient extent. Furthermore, in contrast to the monosynaptic corticospinal tract, efferences 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.

Key Words:

• transcranial magnetic stimulation
• cortical organization of voluntary eye movements
• frontal eye field
• inferior parietal lobe
• optomotor cranial nerves