Biomedical Engineering Reference
In-Depth Information
The Spike Rate Data Display application is used for demonstration purposes. In an actual
laboratory environment, a Windows service reads the spike counts from the USB port and stores
them in shared memory so any Windows application can process them.
7.3.1 In Vivo Testing and Results
Because invasive microwire recording technologies are targeting single-unit neuronal activity, we
will use the action potential (spike) as our standard for evaluation of data collected from the neural
probes [
48
]. The Pico system and custom amplifier have been previously tested extensively us-
ing a Bionic Neural Simulator to generate stereotypical physiological signals on the electronics
workbench. In vivo tests used adult male 250-g Sprague-Dawley rats to benchmark the record-
ing performance of the custom amplifier and Pico system. All procedures have been approved by
the University of Florida IACUC Board. A 16-electrode (2 × 8) microwire array was implanted
to a depth of 1.66 mm into the forelimb region of the primary motor cortex. The array was then
grounded by using a 1/16-in.-diameter stainless steel screw implanted in the skull.
Figure
7.10
shows a 30-sec trace of neural recordings captured with the Pico system and gives
a global perspective into the peak-to-peak amplitudes and occurrences of spiking neurons. The
average of the data over the 30 sec is expected to be about zero. Instead, there is an offset of nearly
9 µV, which has been determined to be attributed to not precisely biasing the amplifiers.
In Figure
7.11
, we zoom in on two individual action potentials where the range of the noise
floor can take values of ±25 µV with the spikes reaching −75 µV.
CED's Spike 2 analysis software was used to extract neural data from the dataset. Figure
7.12
shows the average waveshape of two neurons firing on the probe from which we sampled.
It is difficult to calculate meaningful SNRs on neurophysiological data, because the spike
signal is not continuous like those used in traditional communication's calculations [
53
]. Table
7.3
presents an attempt to derive meaningful SNR calculations from the average spike value. The RMS
voltage of the noise floor was calculated by taking the square root of the average of the square of
FIgURE 7.10:
Thirty seconds of extracellular recordings from an anesthetized rat.
