Abstracts

Simple and complex movements can be elicited by electrical stimulation of the cortical surface. These are positive motor effects. Another type of stimulation-elicited effect is the cessation of an ongoing motor activity, including speaking, movements of the tongue, or finger/toe tapping. This effect has been demonstrated in the lateral and mesial frontal cortex in areas called the primary and secondary negative motor areas. This study futher localizes the function of cortex surrounding negative motor areas identified by subdural electrode stimulation. 8 patients undergoing subdural electrode placement and cortical stimulation as part of pre-surgical epilepsy workup had one or more negative motor areas identified by cortical stimulation during video EEG monitoring. Standard stimulation parameters were used. Median and facial nerve evoked potentials were used to identify the central sulcus with respect to the subdural electrode grid.
Hemisphere, handedness, lesion characteristics, number of negative motor electrodes, and function of surrounding electrodes was recorded. The location of these areas with respect to sulcal and gyral landmarks was ascertained based on co-registered MRI and electrode imaging.
Findings are described for each patient and compared among patients. Dominant and non-dominant hemispheres were equally represented. 75% were right handed. Prior or indicated surgeries included temporal, frontal, and orbitofrontal lobectomies. The average number of electrodes at which a negative motor response was found was 4.5, with a range of 2 (one patient) to 8 (one patient).
The primary negative motor area in the lateral frontal cortex is adjacent to face (ventral group) and hand (rostrodorsal group) areas in the primary motor area. Negative motor effect was always found anterior to the central sulcus but ranged from 1mm (crown of the precentral gyrus) to 4cm, and was found in both inferior and superior lateral frontal gyri.
The secondary negative motor area is usually in the mesial frontal lobe, but may be displaced dorsally to appear on the most dorsal aspect of the frontal lobe. Negative motor effects are robust and elicited in many patients undergoing stimulation of cortical areas anterior to the primary and supplementary motor areas. Several functions of these areas have been proposed but mechanisms or interaction with primary motor and more frontal cortex suggest and important mediating role between volition and action.
In the left hemisphere, overlap of the negative motor area with Broca[apos]s area may reflect evolution of speech function in a higher-order negative motor area involved with organization of speech.
Both mesial and lateral areas may be altered directly by pathology or indirectly by a changed distribution of cortical function. Areas with no stimulation induced function are also common more anteriorly and new stimulation models and analysis techniques are needed to further explore these areas and their relationship to adjacent cortex.