Human eye fields in the frontal lobe as studied by epicortical recording of movement-related cortical potentials

We studied the generator location of premovement subcomponents of movement-related cortical potentials (MRCPs) [Bereitschaftspotential (BP), negative slope (NS') and motor potential (MP)] associated with voluntary, self-paced horizontal saccade in the human frontal lobe. Self-paced horizontal saccade, wrist (or middle finger) extension and foot dorsiflexion were employed in 10 patients (lateral surface of the frontal lobe in seven and mesial in three) as part of the presurgical evaluation, and data of five patients (lateral in four and mesial in three) were used in the final analysis. On the lateral frontal lobe, the maximum BP, NS' or MP with horizontal saccade was seen at or 1-2 cm rostral to the hand, arm or face area of the primary motor cortex (MI) in all four subjects investigated. This area exactly corresponded to the frontal eye field (FEF) identified by electrical stimulation. The amplitude of MRCPs with saccade was smaller than that with hand movements. On the mesial surface, within the supplementary motor area (SMA) proper, BP and/or NS' for horizontal saccade was located 1-2 cm rostral to that for hand and foot movements. BP and/or NS' delineated the supplementary eye field (SEF) at the rostral part of the SMA proper, and SEF partly overlapped with the hand and foot areas of the SMA proper. At the area just rostral to the vertical anterior commissure line and/or the pre-SMA defined by electrical stimulation, BP and/or NS' was seen invariably, regardless of the sites of movements, and in contrast with the SMA proper, there was no somatotopic representation. No clear MPs were elicited by eye movements on the mesial surface. In one of the two subjects whose MRCPs with horizontal saccade were recorded simultaneously from the lateral and mesial surfaces of the frontal lobe, BP from the SEF and pre-SMA preceded that from the FEF. It is concluded that MRCPs with horizontal saccade are useful for defining the FEF, SEF and pre-SMA, and that the SEF and pre-SMA become active in preparation for horizontal saccade earlier than the FEF.