Saccadometry

16 February 2022
10 - 30 mins
Reading
The Saccadometry optional protocols in VisualEyes allow for more advanced saccade test types to be performed. The two most common tests are the Prosaccade and Antisaccade. These are advanced ocular motor tests that are used to diagnose central vestibular injuries including traumatic brain injury, concussion and neurodegenerative diseases.

Prosaccade description

Prosaccade protocols allow easy and quick collection of saccadic responses within the shortest physiologically possible time (100 saccades in 5 min) with minimum influence of subject fatigue. The eye movement measurement is automated and synchronized with stimuli presentation. The user’s task is only to give instructions for the patient to keep their head still and only move their eyes back and forth to follow the target and then return to the center dot each time.

The calibration of the eye movement is performed using the standard VNG calibration for VisualEyes. The default protocol is 1 center dot at the very beginning of the test which is red on a black background and then a black dot randomly to the left and right at 10 degrees from center, 100 times. If the patient fatigues, you can stop the test early.

 

Prosaccade instructions

Relax, look straight ahead, don’t move your head only move your eyes while tracking the targets on the screen in front of you. You will start by looking at the middle dot, then another red dot will appear to the right or the left. Move your eyes to the new dot and when it disappears move your eyes quickly back to middle.

Accuracy and speed are both important so please do your best to follow the target and move your eyes as soon as the target jumps, keeping your eyes on the dot until it moves again.

Figure 1 – The TV screen is black. A red center dot appears first, then a second center dot will flash to either the right or left side of the center dot.

 

Prosaccade data analysis

During Prosaccade data collection you will see the conventional VNG saccade graphs for latency, velocity and accuracy.

Prosaccade data, including eye position, latency, velocity, and accuracy.

Figure 2 – Pro Saccade Data Collection

After the test is completed a summary screen will appear which shows several new graphical displays that are key to interpreting the new protocols. The process of data analysis is fully automated so the user does not have to be an expert in eye movement research and analysis.

Prosaccade summary screen, including graphs position, velocity, latency, phase, and directional error rate. Correct experiments are visible by solid lines, red for left and green for right. Incorrect experiments are visible by dotted lines, red for left and green for right. All experiments are correct in this overview. The position graph has eye position as a function of time, with time values ranging from 0 to 140 ms. The velocity graph has eye velocity as a function of time. The latency graph has eye position as a function of time, with time values ranging from 150 to 350 ms. The phase graph has eye velocity as a function of eye position. The directional error rate is 0%.

Figure 3 – Prosaccade Summary Screen

 

Antisaccade description

The default protocol for anti-saccades is similar to that of the Prosaccade with red dots on a black screen. The instructions to the patient are slightly different. The Antisaccade involves a cognitive task where the patient has to suppress the desire to look towards the new target and then make a decision to look away from the target.

 

Antisaccade instructions

Relax, look straight ahead, don’t move your head only move your eyes. You will start by looking at the middle dot, then another red dot will appear to the right or the left. THIS TIME move your eyes to the equal and opposite side of the new dot and then quickly back to middle. Accuracy and speed are both important so please do your best to move your eyes quickly to the position the dot would have been if it were on the opposite side.

 

Antisaccade data analysis

For the Antisaccades there are no suggested threshold ranges at this time since the data has not yet been published. For the Antisaccades, you can see that that the eyes (red or blue tracings) are moving opposite to the dot/stimulus tracing (yellow) because the patient is instructed to look to the opposite side of the target.

Figure 4 – Eye movement data for the antisaccade showing the eyes moving in the opposite direction of the saccade.

 

Summary graphs for prosaccades and antisaccades

The following summary graphs are available for both Prosaccades and Antisaccades

 

1. Position

This is a distribution of the eye position over time. The Position graph tells us if the eye reaches the target and how long it took it to get there.

A normal position graph, with only one incorrect experiment to the right.

Figure 5 – A normal Position Graph

Eye movements to the left are shown in red and eye movements to the right are shown in green. Dotted lines represented errors where the eye has moved into the wrong direction. When the eyes move to right, it is the left side of the brain that generates the saccade and vice versa.

In the example below you can see that the left eye (shown in red) is falling short of the target. This is an undershoot abnormality. The eye positions should be the same for jumps to the left and jumps to the right.

The cluster of left eye positions is visibly lower than the cluster of right eye positions.

Figure 6 – An abnormal position graph

 

2. Latency

The latency graph shows how fast the eyes move towards the target. This graph also shows where the eyes are in position relative to the 20 deg target. It shows how fast the saccades take off and land on target. The speed of initiation should be the same in both directions.

Latencies between 300-550 ms, evenly distributed between both eyes.

Figure 7 – A normal Latency graph

In the example below you can see that the latencies are much longer than in the healthy normal patient above. There are also fewer jumps as the unhealthy patient typically fatigues and cannot finish the test.

Long latencies especially for the left eye, between 500-700 ms.

Figure 8 – An abnormal latency graph

 

3. Phase

The phase graph shows the speed of the eye movement at different positions along the way to reaching the target. It shows acceleration and deceleration parameters as the eye moves towards the target and then stops on the target.

Evenly distributed phase, with both eyes reaching an eye position between 10-17 degrees.

Figure 9 – a normal phase graph

Below is a patient with multiple scleroris and you can see a noticeable difference between the jumps to the right and left, with the left eye movements (shown in red) falling short of the target.

Unevenly distributed phase, with the left eye reaching an eye position between 4-8 degrees, whereas the right eye is between 8-14 degrees.

Figure 10 – An abnormal phase graph

 

4. Velocity

The velocity graph shows how fast the eyes move over time as they approach the target. This tells us how fast the saccades are going and it shows both acceleration and deceleration movements as the eyes move towards and then reach the target.

Eye velocities reaching 200-300 degrees per second for both eyes.

Figure 11 – A normal velocity graph

In the example below you can see that the concussed patient has a slower velocity eye movement compared to the healthy subject.

Eye velocities only reaching 130-200 degrees for both eyes.

Figure 12 – An abnormal velocity graph

 

5. Directional error rate

The directional error rate represents how many times the patient looked in the wrong direction. For Prosaccades an error is made if they look away from the new target and in Antisaccades an error is made if they look towards the target.

Directional error rate of 1%.

Figure 13 – A normal directional error rate

In the Antisaccade example below the patient is looking away from the target as instructed and is therefore getting an appropriate low error rate.

Directional error rate of 2%.

Figure 14 – A patient with a normal antisaccade directional error rate

In the Antisaccade example below you can see the patient already starts to fatigue after just 25 seconds into the test. When they are fatigued, they either don’t look at the target at all or they look towards the target (yellow) many times resulting in a high error rate. Remember, in an Antisaccade the patient should be looking in the opposite direction of the saccade.

Directional error rate of 53%.

Figure 15 – A patient with an abnormal antisaccade directional rate

 

Summary

In summary, the Saccadometry protocols add additional assessment value to the standard saccade test. Because saccades to the left or right are controlled by a different hemisphere of the brain, measuring these movements give us insights about functional asymmetries between sides of the brain and/or deficits in the precise neural generators that promote these movements. The fatigability and increased error rates further assist in the diagnosis of central abnormalities including concussion and traumatic brain injuries.


Presenter

Michelle Petrak
Michelle Petrak, Ph.D., is the Director of Clinical Audiology and Vestibular Research for Interacoustics. Her primary role is development and clinical validation of new technologies in the vestibular and balance areas. She is located in Chicago where she is a licensed private practice clinical audiologist at Northwest Speech and Hearing (NWSPH). Dr. Petrak received her doctorates in Electrophysiology (1992) and Biomolecular Electronics (1994) from Wayne State University and her Masters in Audiology in 1989. Her special areas of expertise include vestibular and balance assessments and management of the dizzy patient. Dr. Petrak is involved with new innovative product developments, clinical evaluations of new protocols, and publishing, teaching, and training on the management of patients with dizziness. She continues to lecture extensively nationally and internationally, and she has numerous articles published in the hearing industry journals. She also participates on the committees for several doctoral students as support for the research projects.

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