Direct Cortical Recording of Pattern Reversal Visual Evoked Responses Better Defines Primary Visual Cortex than Cortical Stimulation of Subdural Electrodes
Abstract number :
G.07
Submission category :
Year :
2000
Submission ID :
737
Source :
www.aesnet.org
Presentation date :
12/2/2000 12:00:00 AM
Published date :
Dec 1, 2000, 06:00 AM
Authors :
Andrew F Bleasel, Julie Birkett, Westmead Hosp and New Children's Hosp, Sydney, Australia; Westmead Hosp, Westmead, Australia.
RATIONALE: The precise definition of primary visual cortex in relation to epileptogenic zone in patients with parieto-occipital epilepsy undergoing epilepsy surgery is essential. Both cortical stimulation and visual evoked potentials (VEP) can provide localising data in patients with subdural grids. We compare the results of each. METHODS: Six consecutive patients, aged 14-39 years, with intractable partial epilepsy undergoing evaluation with subdural electrodes were studied with incremental cortical stimulation and subdural recording of pattern reversal VEPs. Each patient had between 9 and 45 electrodes stimulated in the partietal and occipital cortex. The number of electrodes with positive visual responses to cortical stimulation and the number recording reproducible VEPs were directly compared in each case. These findings were compared to the number of electrodes overlying the primary visual cortex as defined by post-implantation CT scans (2 patients) and MRI (4 patients). RESULTS: The number of electrodes producing phosphenes was greater than the number recording VEPs in each patient; between 36-80% of electrodes positive for stimulation recorded VEPs. Electrodes with both VEPs and positive stimulation results showed concordance for upper or lower quadrants of the visual fields when limited upper or lower field pattern reversal stimulation was used. Thus, both results reliably defined the position of the calcarine sulcus. The electrodes recording VEPs showed close correlation with anatomically defined primary visual cortex. Of 4 patients undergoing occipital resections, cortex positive for stimulation was resected without incident if VEP positive electrodes were avoided. CONCLUSIONS: Unformed visual responses to cortical stimulation are distributed over both primary visual cortex and visual association cortex. Occipital cortex producing VEPs is restricted to the primary visual cortex. Subdural recording of VEPs can provide valuable localising data for epilepsy surgery in the occipital cortex more specific than conventional cortical stimulation studies.