subsubsection_Triggered_averaging_rat_somatosensory__.tex

\subsubsection{Triggered averaging - rat somatosensory cortex}
\label{MTrig}
The ScouseTom system was utilised in measurements of evoked activity using epicortical electrode arrays in anesthetised rats, repeating methodology from previous studies \cite{Aristovich_2016} \cite{Vongerichten_2016}. Arrays of platinised stainless-steel electrodes embedded in silicon, with 0.6 mm diameter contacts spaced 1.2 mm apart were placed directly onto the surface of the brain, targeting the somatosensory cortex. Whilst previous experiments had employed a single grid of 30 electrode contacts on a single side of the brain, in this study two grids of 57 electrodes were used, one on each hemisphere, for a total of 114. The activity was induced via electrical stimuli to the forepaw, delivered in 10 mA pulses of 1 ms duration at a rate of 2 Hz, triggered by the ScouseTom controller. Current was injected at 1.7 kHz with 50 $\mu$A amplitude, for 30 seconds across a pair of electrodes for a total of 60 stimuli, with voltages on 114 electrodes recorded in parallel using the ActiChamp system with 25 kHz sampling rate. This process was repeated for c. 50 pairs of injection electrodes over the course of 25 minutes to produce a complete data set of c. 7000 voltages. The signals were demodulated with 1 kHz bandwidth, yielding 2 ms time resolution, and coherent averaging was performed on 60 500 ms trials centered around the time of stimulation. EIT images were reconstructed at each 2ms time point, using 10 ms prior to stimulation as the baseline. To allow for a comparison to previous studies with previous systems, the noise was also calculated after demodulation using a reduced bandwidth of 250 Hz \cite{Oh2011}.