Abstracts

Subdural electrodes are widely used in clinical and research studies of brain function. It is well known that these electrodes can cause local mass effect and reversible impairments in neurological function, however the effect of subdural electrodes on neurological function in the absence of clinically evident dysfunction is unknown. To determine if subdural electrodes even subtly impair the function of underlying brain, we measured perceptual performance for an orientation discrimination task both inside and outside the receptive field (RF) of a region of visual cortex covered by a subdural electrode., Local field potentials recorded from semi-chronically implanted subdural electrodes positioned on early visual cortex in four epilepsy patients were used to map receptive fields. Electrodes in early visual cortex were considered ideal for this study because a visual processing deficit restricted to a small portion of the visual field would be expected if an electrode produced a local impairment in cortical function. Subjects did a task that required them to detect an orientation change in a flashing visual stimulus that was presented either inside the mapped RF, or outside the RF in the diametrically opposite portion of the of the hemifield. The size of the orientation change was varied to span a wide range of behavioral performance. Psychometric curves were then generated by fitting behavioral responses to a logistic function. Threshold was defined as the point where the fitted function crossed 50% detection., Data were well fit by the logistic function in all four subjects for both RF and non-RF conditions. None of the subjects showed a statistically significant difference in reaction time (p [gt] 0.25, z test), slope of the psychometric function (p [gt] 0.14, direct Monte Carlo test), or detection threshold (p [gt] 0.08 with Bonferroni correction, direct Monte Carlo test) for stimuli presented inside or outside the RF., Subdural electrodes implanted for extraoperative monitoring do not impair psychophysical performance for a task based on stimuli lying within the RF for recording electrodes. This finding suggests that subdural electrodes can reliably be used for accurate assessment of neurological function., (Supported by NINDS and HHMI.)