Written by Travis M. Moore
Last edited 22-Jun-2020
After prepping the skin, placing the electrodes, and using the chain of equipment to record and digitize the voltage at the scalp we finally get to see it! The computer screen shows a jagged, wriggly, unfriendly-looking line on a graph. Figure 1 shows the output from a normal auditory brainstem response (ABR). That's what all our hard work accomplished? Did something go wrong?
Unfortunately, the resulting waveforms can look rather anticlimactic at first glance. There's not much intuitive happening, so you'll need to learn how to quantify these biological signals. The first thing to know is that the mountain-range-like morphology (i.e., shape) of the waveform drives most clinical decisions. Common terminology for the upward-pointing regions includes peak, maximum, positivity and, positive/upward deflection. Terminology for downward-pointing regions includes trough, valley, minimum, negativity, and negative/downward deflection. The entire recording is referred to as a waveform or tracing.
Either a peak or trough can be referred to as a wave. You'll be able to tell what is being referred to by context. For example, wave V of the ABR refers to the fifth peak of the response because we really only ever refer to positive waves when interpreting the ABR. There is another naming convention that helps differentiate whether wave refers to a peak or a trough. For example, the N1 wave refers to a trough because "N" stands for negative. Recall the voltage will dip below zero (i.e., downward) for negative numbers. Can you guess whether the P2 wave refers to a peak or trough? As you probably guessed, "P" stands for positive (i.e., upward deflections) so we know the term wave here refers to a peak.
But what about those numbers following N and P? The numbers can signify two things. First, they can refer to the order of the wave. In other words, if there are 7 peaks and you're dealing with P2, you know it's the third positive deflection in the response. N1 would mean the first negative deflection. More usefully, the second thing the numbers can refer to is when to expect to see the wave. P300 tells us to expect a positive deflection around 300 ms. N23 tells us to expect a negative deflection around 23 ms. The following section discusses the timing of waves in more detail.
Like most signals we've dealt with so far in these modules, important information can be decoded by plotting the waveform in the time domain. If you don't remember what this means, review the Fourier Transform lesson in the Acoustics module. In brief, examining a signal in the time domain means the plot will have 'amplitude' on the y-axis and 'time' on the x-axis. Take another look at Figure 1 to confirm that is how the sample plot is organized.
The two measures of interest in quantifying waveforms from electrophysiological tests are latency and amplitude. Latency refers to how long something took. In electrophysiology, we care very much about how long it takes a certain peak or trough to appear after we present a stimulus. Pay careful attention to the previous sentence: after we present a stimulus. All of our latency measures begin right when the stimulus is presented. In other words, the clock starts at 0 ms when the stimulus is triggered in the software. You will hear this setup referred to as a stimulus-locked paradigm (i.e., the timing is locked to the stimulus). So then the waveforms we see are showing the changes in voltage that occur at different times after stimulus presentation. In Figure 1, the x-axis displays the time, so in order to measure latency, you find the peak or trough of interest, and measure where it falls on the x-axis.
There are two other types of latency measures you will use in the clinic. The first is called interpeak latency, and it just refers to the difference in latencies between two peaks. For example, the ABR waveform has several peaks and troughs. One useful measurement is the timing difference between the first peak and the fifth peak (wave 1 and wave V). We'll talk more about why you might want to use this method in the ABR lesson.
The second type of latency measure is interaural latency. This just refers to the difference in latency of the same peak (or trough) across ears. For example, a common measure of the ABR is the interaural latency between right and left wave V.
One way to measure amplitude is to take the peak amplitude. This measurement is simply the y-axis value at a peak or trough (see Figure 3). This measurement typically assumes the 0 μV line is a good starting point, but that isn't always the case. It is also common to begin recording before the stimulus is even presented, which gives you some quiet time before you expect to see a change in voltage related to the stimulus. If all of those quiet periods are averaged, they should cancel out to a baseline of neural activity when no stimuli are presented. This is called a pre-stimulus baseline, and can serve as the 0 point in a peak amplitude measure.
Another way to measure amplitude is called peak-to-trough amplitude. This is pretty much what it sounds like. Let's assume we are measuring a peak. First, take the peak amplitude measure of that peak. Next, take the peak amplitude measure of the following trough. The peak-to-trough amplitude is the difference between these two numbers (see Figure 3). If you start with a trough, then measure the next peak.