Abstract
Electrical Impedance Tomography (EIT) is an emerging technique which has been used to image evoked activity during whisker displacement in the cortex of an anaesthetised rat with a spatiotemporal resolution of 200 μm and 2 ms. The aim of this work was to extend EIT to image not only from the cortex but also from deeper structures active in somatosensory processing, specifically the ventral posterolateral (VPL) nucleus of the thalamus. The direct response in the cortex and VPL following 2 Hz forepaw stimulation were quantified using a 57-channel epicortical electrode array and a 16-channel depth electrode. Impedance changes of −0.16 ± 0.08% at 12.9 ± 1.4 ms and −0.41 ± 0.14% at 8.8±1.9 ms were recorded from the cortex and VPL respectively. For imaging purposes, two 57-channel epicortical electrode arrays were used with one placed on each hemisphere of the rat brain. Despite using parameters optimised toward measuring thalamic activity and undertaking extensive averaging, reconstructed activity was constrained to the cortical somatosensory forepaw region and no significant activity at a depth greater than 1.6 mm below the surface of the cortex could be reconstructed. An evaluation of the depth sensitivity of EIT was investigated in simulations using estimates of the conductivity change and noise levels derived from experiments. These indicate that EIT imaging with epicortical electrodes is limited to activity occurring 2.5 mm below the surface of the cortex. This depth includes the hippocampus and so EIT has the potential to image activity, such as epilepsy, originating from this structure. To image deeper activity, however, alternative methods such as the additional implementation of depth electrodes will be required to gain the necessary depth resolution.
Highlights
A plethora of techniques have been developed to record and image neuronal networks in the brain to gain a better understanding of the underlying mechanisms and interactions that drive complex cognitive and behavioral tasks
No significant difference in the latency between the evoked potential and impedance change was observed in the ventral posterolateral (VPL) nor the cortex (p < 0.01)
When the forepaw was stimulated at 20 Hz, the mean amplitude of the evoked potential measured in the VPL decreased to 168 Æ 51 μV (n 1⁄4 12 in N 1⁄4 3) and occurred at 6.1 Æ 1.5 ms
Summary
A plethora of techniques have been developed to record and image neuronal networks in the brain to gain a better understanding of the underlying mechanisms and interactions that drive complex cognitive and behavioral tasks. Populated micro-electrode arrays can be used to record local field potentials (LFP) and multi-unit activity (MUA) from neurons (Berenyi et al, 2014; Buzsaki, 2004; Raducanu et al, 2017). EEG inverse source localisation can image direct neural activity over large regions of the brain. It has a low spatial resolution (Burle et al, 2015; Nunez et al, 1994) and is blind to dipole sources oriented tangentially to recording electrodes (Ahlfors et al, 2010; Hunold et al, 2016). No satisfactory method exists to record direct neural activity occurring over large regions of the brain
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