Abstract
Electrocorticogram (ECoG), obtained from macroelectrodes placed on the cortex, is typically used in drug-resistant epilepsy patients, and is increasingly being used to study cognition in humans. These studies often use power in gamma (30–70 Hz) or high-gamma (>80 Hz) ranges to make inferences about neural processing. However, while the stimulus tuning properties of gamma/high-gamma power have been well characterized in local field potential (LFP; obtained from microelectrodes), analogous characterization has not been done for ECoG. Using a hybrid array containing both micro and ECoG electrodes implanted in the primary visual cortex of two female macaques (for some stimulus conditions, separate ECoG and microelectrode arrays in two additional male macaques were also used), we compared the stimulus tuning preferences of gamma/high-gamma power in LFP versus ECoG in up to four monkeys, and found them to be surprisingly similar. High-gamma power, thought to index the average firing rate around the electrode, was highest for the smallest stimulus (0.3° radius), and decreased with increasing size in both LFP and ECoG, suggesting local origins of both signals. Further, gamma oscillations were similarly tuned in LFP and ECoG to stimulus orientation, contrast and spatial frequency. This tuning was significantly weaker in electroencephalogram (EEG), suggesting that ECoG is more like LFP than EEG. Overall, our results validate the use of ECoG in clinical and basic cognitive research.
Highlights
Electrocorticogram (ECoG), obtained from macroelectrodes placed on the cortex, is typically used in drug-resistant epilepsy patients, and is increasingly being used to study cognition in humans
Recording from a unique hybrid grid which consists of both micro and macro-electrodes, implanted in the primary visual cortex of the same two female macaques for which we had earlier compared local field potential (LFP) versus EEG tuning[29] and LFP versus ECoG spatial spreads[37], we compared the strength of ECoG and LFP gamma/high-gamma power for different stimulus properties such as size, orientation, spatial frequency and contrast
We simultaneously recorded LFP and ECoG signals using a special custom-made hybrid grid electrode array implanted in the left primary visual cortex (V1) of two monkeys (Monkeys 3 and 4), trained to perform a fixation task, while visual gratings that varied in size, orientation, contrast or spatial frequency were presented on a screen
Summary
Electrocorticogram (ECoG), obtained from macroelectrodes placed on the cortex, is typically used in drug-resistant epilepsy patients, and is increasingly being used to study cognition in humans. Apart from providing clues about the neural correlates of gamma/high-gamma activity in ECoG, such a comparison allows us to determine the spatial spread (the cortical area around the electrode that contributes to the signal that is recorded from that electrode) of ECoG, which we have recently shown to be very local[37] Both the firing rates and LFP high-gamma power reduce with increasing stimulus size because of larger surround suppression[17]. Recording from a unique hybrid grid which consists of both micro and macro-electrodes, implanted in the primary visual cortex of the same two female macaques for which we had earlier compared LFP versus EEG tuning[29] and LFP versus ECoG spatial spreads[37], we compared the strength of ECoG and LFP gamma/high-gamma power for different stimulus properties such as size, orientation, spatial frequency and contrast
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