An introduction to Saccadometry

In this video, you will learn where the core concepts behind the Saccadometry test and the different metrics it measures. Time will be spent on how to interpret the graphs and highlight core conditions which show abnormality in these.

You can read the full transcript below.

Introduction

Hi, my name is Liz Fuemmeler. I'm a vestibular audiologist and the balance clinical product manager for Interacoustics. I'm super excited to be here today. We're going to be talking about what tests can be helpful beyond the VNG.

Especially in patients that have central disorders, we know that recently comprehensive vestibular assessment has expanded much beyond the traditional VNG. In fact, we've been able to incorporate VEMP, vHIT, the VORTEQ™ IMU head sensor, and are also incorporating advanced oculomotor and vestibular tests so that we can give a better diagnostic picture for our patients.

Like I said, today we're going to focus mainly on how we can utilize VNG to help us with those populations that have central disorders.

Random saccade test

Now, looking at this population is not new for us. We have, especially through our oculomotor testing. Our main goal when using oculomotor tests is to determine whether there is anything of central origin we should be concerned about.

Here is an example of an oculomotor test that we perform on a daily basis. It's called the random saccade test and we know a saccade is when you can change the focus of your fovea from one target to another quickly. It's a reflexive part of human nature that we can quickly look at a target or a stimulus that's moving.

And like I said, we have used this for many years to help us decide does the patient have anything centrally that I need to be concerned about? Unfortunately, there's many times that we see patients who have diagnosed central disorders and yet they have a normal random saccade test.

And so it makes us think, is this test sensitive enough in order to pick up true central dysfunction?

Saccadometry

There's been a new test that's been released in the VisualEyes™ software for a number of years called saccadometry. And although it's new to the equipment, it is not new to the literature. Neurologists have been utilizing saccadometry for many years because they know the value of the sensitivity of this test, specifically with those who have central disorders.

Saccadometry looks pretty similar to the saccade test in many ways. The patient is wearing video goggles, looking at the screen and watching dots move around the screen. However, the amount of data that we can get from this test is much more extensive than what we have in that random saccade test.

Saccadometry actually consists of two subtests. The first one is called pro-saccades. The second is called anti-saccades.

In the pro-saccade test, the patient starts looking at the center dot and a dot pops up either to the right or to the left of that center dot. Their job is to move their eyes towards the dot, which most patients do quite well.

So you'll see in our graphic a graphical example of that on the left side is a correct pro-saccade movement. The patient starts with their eyes on the center dot. The dot moves to the right and the patient also moves their eyes to the right.

Now anti-saccades, which is the second step test in saccadometry is a little bit different. So the patient's going to see some of the same things that center dot. And then there's going to be a dot that pops up to the right or to the left. However, this time you're instructing the patient instead of looking at the dot, I want your eyes to go equal and opposite direction of where that dot goes.

So you'll see in this graphical example that the dot pops up to the right and the patient correctly moves their eyes equal and opposite to the left side.

So even as I'm explaining anti-saccades, you're probably thinking that sounds a lot more difficult. That's because it is. And it's because it also involves a lot more complex processes for our brains in order to accomplish this type of eye movement.

Anti-saccades

So generating an anti-saccade eye movement really involves two main processes.

The first one is we have to inhibit triggering a reflexive saccade. So when a dot pops up, it's very normal that we would want to look at the dot. We like to look at interesting things, but the ability for us to not look at something is actually a function of our dorsolateral prefrontal cortex. For us to be able to inhibit that reflexive saccade. So that’s stage one of generating an anti-saccade movement is not looking at the dot.

The second stage is we actually have to invert this visual vector to the other hemifield and create our own imaginary target. So this is a function of the posterior parietal cortex, and it involves both sides of our brain in order to see where the dot is going out of our peripheral vision and create an imaginary target on the opposite side.

So it's really quite complex. And because it involves more areas of the brain, it gives us a lot more detailed look at how the brain is functioning.

So this is an example of the summary screen that you get, in saccadometry testing and you'll notice it's a lot more extensive than what you get in random saccade tests. And this is because it's looking at more neural networks of the brain. There are new measurement parameters that you may not be used to looking at.

So next I'm going to review how to read this data and what central disorders you may see that affect certain measurement parameters. To get you oriented to the colors in general and to the lines.

Saccadometry data summary

You're going to notice a couple of things. First of all, there are two different colors you see on the graph. There are red lines and there are also green lines. Eye movements that move to the left are shown as red lines. Eye movements that are going to the right are shown as green lines. So the color helps determine the direction of which way the eyes are moving.

There are also two different types of lines you may see. If they are a solid line, that means it was a correct eye movement. If it's a dotted line, that means it was an error.

So this is an example of a pro-saccade test. So this is when the eyes are supposed to look at the target. You'll see that again, the eye movements to the left are shown as red lines and eye movements to the right are shown as green lines. This is the same that you see in anti-saccades when the eyes are looking away from the target.

Again, correct eye movements to the left are shown in red and eye movements to the right are shown in green.

Now, especially if you come from an audiology background like me, you may be wondering why these colors. Because we are used to an audiologist red being right ear, blue being left ear. But saccadometry is a little bit different and it's not because we chose the colors. The colors have been in existence for many years.

So let's think about why the colors are the way that they are. My favorite way to think about this is thinking about what side of the brain is generating the eye movement. So the left side of the brain is actually what generates all of our eye movements to the right. So our left brain generates eye movements to the right, and we display that on our graph as green.

On the right side of the brain is what generates eye movements to the left. And this is displayed in our software as red.

My favorite way to remember this is instead of red being rightward moving eye movements, this is right brain which moves eyes to the left is in red.

So if you see red in saccadometry, that means it's coming from the right brain, eye movements to the left. And if you happen to forget this because it is confusing, we also have a key available on our summary screens.

Saccadometry measurement parameters

Now let's look at those parameters. There are five measurement parameters that we use in saccadometry, and a few of them are new, especially when compared to that random saccade test that we're used to looking at.

1. Directional error rate

So the first one is called directional error rate, and this is simply the number of times that the patient moves their eyes in the wrong direction. Now you should remember that depending on the task, a directional error may be something different.

So for pro-saccades, the patient is instructed to look their eyes towards the target. So if they make a directional error, that means that they're not looking towards the target. Look, they're looking somewhere else.

An antisaccade however, a directional error rate, as we know they're instructed to look the opposite direction of where the target goes. So if they make a directional error at anti-saccades, that means that they likely followed the dot instead of going the opposite direction.

So I say this just as a reminder that think about the task before you think about what directional error actually means.

There is a second percentage rate that you're going to see in the summary screen, and that is the overall error rate. So I just described what a directional error rate is, how many times people went the wrong direction, but overall error rate, not only includes the directional error rate, but it also includes two other features.

It includes all the rejected artifacts, blanks, etc., and also all the saccades that were not performed. So your overall error rate, kind of like spectral purity in rotational chair testing, is a pulse on how valid or how noisy the response is.

So you really wouldn't want your overall error rate to have a significant difference between that directional error rate. They should be fairly close. If my directional error rate, for example, of what we have above is 3% and my overall error rate was 90%. That means there's quite a bit of artifact rejection or saccades not being performed that are being included in that. So I may want to be very careful with that interpretation.

So here's an example of a patient that has directional error issues. So post-concussion is a classic example of patients that may experience difficulty with not only latency but also directional error rate. So in the software, not only will you see an increase in that percentage, so this is from a patient that had a 53% directional error rate.

But even as you look at their eye tracings, you'll see that one, they may not have been able to even complete the test or to that stimulus which is shown in the yellow line. And then their eye movements, which are shown in those red and blue lines, you would imagine for anti-saccades that they should be going in the opposite direction. So if they're ever going in the same direction, that means they're making a directional error issue.

2. Latency

The second measurement parameter is latency, and we are used to looking at latency. This is the initiation time of the eye movement. Now, just as a note, remember that latency is again that reaction time and velocity is the speed of eye movement from point to point.

With latency, our graph shows eye position over time. We utilize a ten-degree standardized target. So this can even give you an idea of whether a patient is overshooting that ten degrees or undershooting. There are many issues that can cause prolonged latencies.

Our normal anti-saccade latency should really be around 360 milliseconds. This is what we know from normative data, which we'll talk about in just a bit. But here's an example from a patient that had around 550 milliseconds for their average latency. So this is almost 200 milliseconds longer than what we would expect in the normal population.

Prolonged latency can be seen not only in post-concussion patients, but also patients that have Alzheimer's, Parkinson's disease or other neurodegenerative disorders.

3. Position

The next measurement parameter is eye position. So this is where the eye is in degrees compared to time. Now, I know I just said this, but saccadometry defaults to that ten-degree target. So if this was a perfect eye position, we would imagine that all of those red and green lines are perfectly lined up over the ten-degree line, horizontal line that we see on this graph.

However, that's not always the case. So especially in patients that have cerebellar lesions, you may notice that they would undershoot or overshoot the target. So in this example, you'll see that again, we're used to that ten-degree eye position. The left moving targets are coming in around eight degrees. So they are definitely undershooting the target and that is also present in the accuracy percentage.

So accuracy compares how closely the eyes got to where the target is. And you'll see, especially again with leftward moving eye movements, they are undershooting the target. What's neat about saccadometry and what we know about oculomotor systems in general is that based on the direction of the issues, you can also determine which hemisphere of the brain may be more impacted by their disorder.

4. Velocity

The next measurement parameter is velocity. This is that speed of eye movement and we can see slowed saccades in a number of different central disorders.

This example is from a patient with multiple sclerosis, and you'll see on the right side is our normal data. So what you would expect normal velocity to look at around 250 milliseconds. And then on the left side you'll see is a slowed velocity. And really the average is coming in about 150 milliseconds.

So this is a patient with MS. We may also see these similar results in patients with basal ganglia lesions, cerebellar lesions, and oculomotor lesions.

5. Phase

Last is phase. So this is the speed of the eye movement in different eye positions. So you'll see this is an example of a normal phase. And again, it compares the eye velocity, the speed over the position.

Now where you see some interesting phase abnormalities is in patients that have issues with their mesencephalon. So this is actually when affected neurons are shutting down the saccade midflight and you actually end up getting this m-shaped bump in the phase because they're on their way to get to the target. But the neurons that help the eyes move actually shut down.

So they have this temporary drop in their speed and then it picks back up to get all the way to the rest of the target. So if you ever see any m-shaped phase abnormalities, that may be due to mesencephalon issues.

Conclusion

Hopefully you can see from looking at the data that although saccadometry is pretty easy to administer to the patients, it provides us some very significant detailed insight about how the brain is functioning, how the neurological circuits are operating.

And so this can give us really great information on the patient population that maybe we're missing out on, which is those with central disorders.