Abstracts

Rationale:
Our ability to visually perceive our environment relies on the activation of specific functional regions within the occipital lobe.  Patients undergoing resective brain surgery within the vicinity of the occipital lobe are often left with devastating and irreversible postsurgical deficits. Electrical cortical stimulation (ECS) and functional magnetic resonance imaging (fMRI) are two methods that, in theory, can identify functionally relevant areas. However, ECS often yields unspecific results, and it remains challenging to accurately co-register fMRI results to the brain within the operating room. The shortcoming of these methods and the looming risk of postsurgical visual deficits often prevent patients with intractable epilepsy from undergoing resective brain surgery and thus from becoming seizure-free. In this study, we aim to overcome this limitation by exploring passive functional mapping as an alternative approach to mapping functional areas involved in processing visual information.
Method:
We recruited two human subjects with intractable epilepsy who underwent surgical implantation of subdural electrocorticographic (ECoG) strips (2.4 mm exposed diameter, 6-10 mm pitch) and stereo-electroencephalographic (sEEG) electrodes (0.8 mm diameter, 2 mm width, 1.5 mm pitch) within wide their left or right hemisphere.  Both subjects had extensive coverage within their occipital lobe. We presented a series of visual stimuli, including checkerboard patterns presented within the foveal and peripheral visual field, and within the left and right, and top and bottom visual quadrants, and a battery of greyscale and color faces, objects, bodies, scenes, and letter strings. We presented these stimuli in a randomized sequence for 400 ms, followed by a random 600-800 ms interstimulus interval. We ensured subject attention by asking them to respond with a button press whenever the same image was presented consecutively (i.e., 10% of the stimuli). Throughout this, we recorded the subject's ECoG/sEEG activity and behavioral response. Our analysis determined those cortical and subcortical electrodes whose activity in the broadband gamma (70-170 Hz) range changed with respect to a pre-stimulus baseline.
Results:
Our results show highly selective responses of electrodes within the occipital cortex.  Most importantly, our study was able to identify electrodes within areas functionally related to the foveal and peripheral visual field and the individual visual quadrants.  Interestingly, we also found highly selective responses to our visual battery stimulus categories.
Conclusion:
These results demonstrate that passive functional mapping can localize a wide range of visual areas ranging from primary visual to higher visual cortex.  They also point to a more refined understanding of the spatio-temporal dynamics of higher visual cortex and the importance of preserving these cortical areas in the course of surgical resection.  With further validation within more subjects, and within the operating room, our study's results have the potential to improve our ability to preserve visual function in patients undergoing resective brain surgery in the vicinity of the occipital lobe.
Funding:
:This work was supported by the NIH/NIBIB (P41-EB018783, R01-EB026439), the NIH/NINDS (U01-NS108916 and U24-NS109103), the NIH/NIMH (P50-MH109429), the NIH/NICHD (DP1-HD091947), Fondazione Neurone.