20-Bio-B6 Analytical Biochemistry · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2015 — 04-Bio-B6 Bioinstrumentation. Three hours, open book, non-communicating calculator permitted. Six questions of equal value (25 marks each); four constitute a complete paper and only the first four appearing in the answer book are marked. All six are solved here as a complete study resource. Every question is a design/essay question (block-diagram instrumentation-system design, or descriptive explanation).
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. A precisely-timed, electrically-isolated fingertip stimulator provides the reference instant (t₀); an EMG channel over the biceps brachii and an accelerometer channel on the hand are sampled on the same clock so that onset-detection algorithms on each channel yield two reaction times (EMG-onset latency and movement-onset latency) per trial, which the controlling computer logs, randomises across stimulus amplitude and warning condition, and finally reduces to a mean and standard deviation per condition.
A computer-triggered, constant-current stimulator delivers a 50 µs pulse at an amplitude set to one of several levels spanning the stated 10-100 mA range (e.g. 10, 30, 50, 70, 90 mA) through a bipolar ring electrode on the fingertip. The stimulator is battery-powered and electrically isolated from the acquisition computer (isolated output stage, per Question 5) — essential here because, unlike the purely-recording channels elsewhere in this paper, this design actively injects current into the subject. The computer's own trigger pulse to the stimulator is logged as the reference instant t₀ for every trial.
A bipolar pair of Ag/AgCl electrodes over the belly of the biceps brachii (the named elbow flexor) feeds a differential instrumentation amplifier matched to the stated ±2 mV, 10-300 Hz signal specification (high CMRR, 60 Hz notch). The Nyquist rate for the 300 Hz upper band edge is 600 Hz; the design samples this channel at a shared 1 kHz clock, leaving comfortable margin for the anti-alias filter roll-off and for resolving onset latency to about 1 ms.
A small, lightweight accelerometer taped to the dorsum of the hand (DC-50 Hz bandwidth, adequate for the rapid but low-frequency withdrawal motion) is sampled on the same 1 kHz clock as the EMG channel — comfortably above its own 100 Hz Nyquist rate — so that both channels share a single, time-aligned sample index and can be compared trial-by-trial without any inter-channel timing correction.
For each trial, the processor rectifies/smooths the EMG channel and flags the first sample whose amplitude exceeds a threshold (baseline mean plus 3 standard deviations of the pre-stimulus quiet EMG) and stays above it for at least 10 ms, giving the EMG-onset time $t_{EMG}$; the identical threshold-crossing algorithm applied to the (rectified) acceleration signal gives the movement-onset time $t_{move}$. The two requested delays follow directly: $RT_{EMG}=t_{EMG}-t_0$ (stimulus-to-muscle-activation delay) and $RT_{move}=t_{move}-t_0$ (stimulus-to-visible-movement delay). Because muscle activation must always precede the mechanical motion it produces (the electromechanical delay of excitation-contraction coupling and tendon/joint compliance), $RT_{move}$ is expected to exceed $RT_{EMG}$ on every trial.
Five stimulus amplitudes (10, 30, 50, 70, 90 mA) are crossed with two warning conditions (warning LED lit 0.5 s before the stimulus, or no warning), giving 10 distinct conditions; the computer presents at least 5 trials of each condition (50 trials total, meeting the question's own minimum), with amplitude and warning state drawn in a randomised order and a sufficiently long, randomised inter-trial interval (e.g. 10-15 s) to prevent the subject from anticipating the next trial or habituating to repeated stimuli. Every trial's raw EMG/acceleration traces, $t_0$, $RT_{EMG}$ and $RT_{move}$ are logged automatically to a file keyed by condition.
For each of the 10 conditions, once its (at least 5) trials are complete, the computer computes the sample mean $\bar{RT}=\frac{1}{n}\sum_i RT_i$ and sample standard deviation $s=\sqrt{\frac{1}{n-1}\sum_i(RT_i-\bar{RT})^2}$ for both $RT_{EMG}$ and $RT_{move}$. As a concrete illustration, five representative $RT_{EMG}$ trials of 62, 58, 65, 60 and 59 ms give $\bar{RT}_{EMG}=60.8$ ms, $s_{EMG}\approx2.77$ ms, while the corresponding $RT_{move}$ trials of 95, 101, 98, 90 and 96 ms give $\bar{RT}_{move}=96.0$ ms, $s_{move}\approx4.06$ ms — consistent with the expected electromechanical-delay ordering $\bar{RT}_{move}>\bar{RT}_{EMG}$. The design predicts, and the researcher should observe across the 10 conditions, that both $\bar{RT}_{EMG}$ and $\bar{RT}_{move}$ decrease as stimulus amplitude increases (a stronger, more synchronous afferent volley reaches the withdrawal reflex's spinal threshold sooner) and decrease further under the warned condition (the 0.5 s foreperiod lets the subject enter a heightened state of motor readiness, shortening the voluntary/reflex response even though the reflex arc itself is largely pre-programmed).
The stimulating current path is the only actively energised patient connection in this paper and is therefore held to the strictest isolation standard: an isolated constant-current source with a hard software/hardware current limit at 100 mA, output monitored for open/short-circuit fault before every trial, and delivered through an isolation transformer or opto-isolated driver stage (Question 5) so no conductive path exists between the stimulator's control electronics and the subject beyond the intended fingertip electrodes.
