Abstracts

Rationale: Among important cognitive functions to be preserved in epilepsy surgery is the naming of visible objects. It has been hypothesized that naming of a vague and ambiguous silhouette, compared to naming of an unambiguous, realistically drawn object, may require a greater utilization of cognitive resources, mediated partly by the frontal lobe. We determined whether naming of ambiguous images, compared to that of unambiguous images, would elicit augmentation of gamma activity to a larger extent in frontal regions. Methods: We studied 340 cortical sites sampled from four patients (age: 13 - 28 years) undergoing extraoperative electrocorticography (ECoG) as part of clinical management of uncontrolled focal epilepsy. A total of 120 images (60 ambiguous images of low spatial frequency and 60 unambiguous images of high spatial frequency) were presented on a screen and each patient was asked to overtly state the name of the object as it is first perceived. We determined when and where the amplitude of gamma activity at 50-120 Hz was augmented compared to that during the silent resting period. Results: The response time of naming of ambiguous images was larger than that of naming of unambiguous ones (median: 1,675 vs 976 ms; p<0.0001). Both tasks commonly elicited gamma-augmentation sequentially involving occipital, occipital-temporal, lateral-frontal, and Rolandic regions bilaterally. Naming of ambiguous images elicited gamma-augmentation specifically involving the right orbitofrontal, left inferior-frontal and left supramarginal regions following the onset of image presentation but prior to vocalization. Naming of unambiguous objects elicited gamma-augmentation more intensely in the occipital regions, compared to that of ambiguous ones, immediately following the onset of image presentation. Conclusions: Behavioral results suggest naming of ambiguous images to demand a larger workload, relative to unambiguous images. Frontal-parietal gamma-augmentation specifically elicited by naming of ambiguous images may reflect the additional cortical processing involved in generating a guess. Enhanced occipital gamma-augmentation elicited by unambiguous images can be explained by visual processing of stimuli with richer detail.