Electrocorticogram (ECoG), obtained by low-pass filtering the brain signal recorded from a macroelectrode placed on the cortex, is extensively used to find the seizure focus in drug-resistant epilepsy and is of growing importance in cognitive and brain-machineinterfacing studies. To accurately estimate the epileptogenic cortex or to make inferences about cognitive processes, it is important to determine the "spatial spread" of ECoG (i.e., the extent of cortical tissue that contributes to its activity). However, the ECoG spread is currently unknown; even the spread of local field potential (LFP) obtained from microelectrodes is debated, with estimates ranging from a few hundred micrometers to several millimeters. Spatial spread can be estimated by measuring the receptive field (RF) and multiplying by the cortical magnification factor, but this method overestimates the spread because RF size gets inflated due to several factors. This issue can be partially addressed using a model that compares the RFs of two measures, such as LFP and multi-unit activity (MUA). To use this approach for ECoG, we designed a customized array containing both microelectrodes and ECoG electrodes to simultaneously map MUA, LFP, and ECoG RFs from the primary visual cortex of awake monkeys (three female Macaca radiata). The spatial spread of ECoG was surprisingly local (diameter ϳ3 mm), only 3 times that of the LFP. Similar results were obtained using a model to simulate ECoG as a sum of LFPs of varying electrode sizes. Our results further validate the use of ECoG in clinical and basic cognitive research.
Local field potential (LFP) has become a promising candidate for neural prosthesis, but how different frequencies of the LFP spread has not been studied experimentally, with theoretical models predicting either an “all-pass” (all frequencies spread equally) or “low-pass” (lower frequencies spread farther than higher frequencies) behavior. Our findings suggest that the LFP spread is “band-pass,” with frequencies in the high-gamma (60–150 Hz) range spreading significantly more than both lower (20–40 Hz) and higher (>250 Hz) frequencies.
Objective. Brain functions such as perception, motor control, learning, and memory arise from the coordinated activity of neuronal assemblies distributed across multiple brain regions. While major progress has been made in understanding the function of individual neurons, circuit interactions remain poorly understood. A fundamental obstacle to deciphering circuit interactions is the limited availability of research tools to observe and manipulate the activity of large, distributed neuronal populations in humans. Here we describe the development, validation, and dissemination of flexible, high-resolution, thin-film (TF) electrodes for recording neural activity in animals and humans. Approach. We leveraged standard flexible printed-circuit manufacturing processes to build high-resolution TF electrode arrays. We used biocompatible materials to form the substrate (liquid crystal polymer; LCP), metals (Au, PtIr, and Pd), molding (medical-grade silicone), and 3D-printed housing (nylon). We designed a custom, miniaturized, digitizing headstage to reduce the number of cables required to connect to the acquisition system and reduce the distance between the electrodes and the amplifiers. A custom mechanical system enabled the electrodes and headstages to be pre-assembled prior to sterilization, minimizing the setup time required in the operating room. PtIr electrode coatings lowered impedance and enabled stimulation. High-volume, commercial manufacturing enables cost-effective production of LCP-TF electrodes in large quantities. Main Results. Our LCP-TF arrays achieve 25× higher electrode density, 20× higher channel count, and 11× reduced stiffness than conventional clinical electrodes. We validated our LCP-TF electrodes in multiple human intraoperative recording sessions and have disseminated this technology to >10 research groups. Using these arrays, we have observed high-frequency neural activity with sub-millimeter resolution. Significance. Our LCP-TF electrodes will advance human neuroscience research and improve clinical care by enabling broad access to transformative, high-resolution electrode arrays.
20Electrocorticogram (ECoG), obtained from macroelectrodes placed on the cortex, is typically 21 used in drug-resistant epilepsy patients, and is increasingly being used to study cognition in 22 humans. These studies often use power in gamma or high-gamma (>80 Hz) ranges 23 to make inferences about neural processing. However, while the stimulus tuning properties of 24 gamma/high-gamma power have been well characterized in local field potential (LFP; obtained 25 from microelectrodes), analogous characterization has not been done for ECoG. Using a hybrid 26 array containing both micro and ECoG electrodes implanted in the primary visual cortex of 27 two female macaques, we compared the stimulus tuning preferences of gamma/high-gamma 28 power in LFP versus ECoG and found them to be surprisingly similar. High-gamma power, 29 thought to index the average firing rate around the electrode, was highest for the smallest 30 stimulus (0.3 radius), and decreased with increasing size in both LFP and ECoG, suggesting 31 local origins of both signals. Further, gamma oscillations were similarly tuned in LFP and 32ECoG to stimulus orientation, contrast and spatial frequency. This tuning was significantly 33 weaker in electroencephalogram (EEG), suggesting that ECoG is more like LFP than EEG. 34Overall, our results validate the use of ECoG in clinical and basic cognitive research. 35 Electrocorticography (ECoG), also known as intracranial electroencephalography (iEEG), is 37 obtained from macroelectrodes placed subdurally on the pial surface of cortex and is widely 38 used in drug-resistant epilepsy patients. The patients are often monitored for weeks for 39 localization of the seizure focus, allowing (with patient's consent) researchers to conduct 40 cognitive and neuroscience studies 1-9 . 41 42 These studies often use power in gamma (30-70 Hz) and high-gamma (>80 Hz) ranges to make 43 inferences about the underlying neural processing 10 . High-gamma activity (>80 Hz) refers to 44 power over a broad range of frequencies above the gamma band that, in ECoG, is modulated 45 by stimulus presentation as well as the behavioral state 4,5,10-13 . High-gamma activity is also 46 observed in local field potential (LFP) obtained by inserting microelectrodes in the cortex of 47 animals, where it is tightly correlated with the spiking activity of neurons in the vicinity of the 48 microelectrode [13][14][15][16][17] . 49 50 Gamma rhythm (30-70 Hz), which is different from high-gamma activity 17 , has been 51 extensively studied in electroencephalogram (EEG) in humans and LFP in animals, and has 52 been associated with high level cognitive functions such as attention, memory and 53 perception 18-24 . Further, gamma is known to be strongly induced by stimuli such as 54 bars/gratings and depends on stimulus properties such as size, orientation, spatial frequency, 55 contrast and temporal frequency 16,17,[25][26][27][28][29] . Stimulus dependence of gamma has also been 56 characterized in EEG/MEG studies [30][31][32][33][34][35] . However, only a...
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