Physical model of coherent potentials measured with different electrode recording site sizes

Abstract

A question that still complicates interpretation of local field potentials (LFPs) is how electrode properties like impedance, size, and shape affect recorded LFPs. In addition, how any such effects should be considered when comparing LFP, electroencephalogram (EEG), or electrocorticogram (ECoG) data has not been clearly described. A generally accepted concrete physical model describes that an electrode records the spatial average of the voltage across its uninsulated tip, yet the effects of this spatial averaging on recorded coherence have never been modeled. Using simulations based on this physical model, we show here that for any effects to occur, a spatial voltage gradient on a scale smaller than an electrode's recording site must exist over the site's surface. When this occurs, larger electrodes on average report higher coherence between locations, with the effect continuously increasing as the voltage profile over the extent of the recording site is increasingly nonuniform. We quantitatively compared published coherence estimates of LFP, ECoG, and EEG data across a range of studies and found a possible modest effect of electrode size in published ECoG data only. We used the model to quantify the expected coherence for any electrode size in relation to any given spatial frequency of a voltage profile. From this and existing estimates of the spread of voltages underlying each of these data types, our simulations quantitatively agree with the published data and importantly suggest that LFP coherence will be independent of recording site size within the range of microelectrodes typically used for extracellular recordings.

Consequences of EEG electrode position error on ultimate beamformer source reconstruction performance

“…Though EEG electrodes are generally assumed to be point detectors, in reality they measure the averaged surface potential over the contact area. This may be a negligible factor for traditional EEG electrode caps (Nelson and Pouget, 2012), but could have significant consequences for active electrodes and dry electrodes, which tend to have larger surface areas than traditional electrodes, as well as for studies involving small children, in which electrodes may cover a larger proportion of the head surface (Ollikainen et al, 2000; Pursiainen et al, 2012). …”

Cellular Classes in the Human Brain Revealed In Vivo by Heartbeat-Related Modulation of the Extracellular Action Potential Waveform

“…One reason it is difficult to assign cellular identity using features of the average EAP is that the EAP does not directly reflect the intracellular action potential waveform (Anastassiou et al, 2015). In fact, the EAP waveform reflects details of the recording electrode (dimension, material;Nelson and Pouget, 2012), properties of the extracellular space (distance to neuron, conductance Logothetis et al, 2007;Anastassiou et al, 2015), the cellular morphology (e.g., Gold et al, 2006;Hunt et al, 2019), the ionic composition of the cell, and other aspects such as brain state (Buzsá ki et al, 1996). As the recording electrode moves farther away from the cell body of a neuron, the EAP width increases (Gold et al, 2007), so that the same neuron, recorded at different distances, can appear to elicit a range of narrow and broad EAP widths (Pettersen and Einevoll, 2008).…”

A Multi-Channel, Flex-Rigid ECoG Microelectrode Array for Visual Cortical Interfacing

“…Current ECoG microelectrode arrays used in research are highly flexible [ 41 ] and only several microns thick; they have also reached much higher electrode count: up to 252 [ 42 ] and 360 [ 43 ], with typical electrode diameters ranging from 300 to 1000 μm. The effect of electrode size on the coherence of ECoG signals has already been studied, but only with electrodes larger than 1mm and for inter-electrode distances greater than 1 mm [ 44 , 45 ]. Therefore we aimed to provide an electrode array with electrodes of different size in close neighbourhood that will allow for future investigations on the effects of electrode size in the sub-millimetre range.…”