Eye movements paint us a picture of cerebral function. However, studies focused on hand and limb motor function will very often leverage visual guidance as a core of the paradigm, but do not have a protocol in place for concurrent eye movement recordings. This method enables three-dimensional recordings of simultaneous eye-hand movements, which allow us to assess eye and hand impairment in visually guided saccade to reach tasks.
Here we will use this to compare patients with chronic middle cerebral artery, or MCA, stroke to healthy controls. The objective analysis of eye movements in the characterization of motor control have been proven to assist in disease detection, monitoring, and prognosis in the setting of brain injury. Thus serving as techniques to monitor physiologic phenotype.
Here we combine the biomarker potential for a maximized effect. Demonstrating the experimental procedure, is Professor Todd Hudson and Dr.Mahya Behshti from our research laboratory. Begin by greeting the participant and escorting the participant into the testing room.
Then, briefly explain the exam and experimental task. Start the exam by asking the participant to follow the researcher's finger with their eyes while keeping their head in one position. Then, draw an imaginary H letter in front of them and make sure that the finger moves far enough out and up and down, assessing center, up, down, left, right, down-left, down-right, up-left, and up-right.
Next, assess smooth pursuit by asking the participant to follow and maintain their gaze on a pencil moving slowly back and forth, in horizontal and vertical directions through their visual field. Following that, assess saccades by asking the participant to look as fast as possible between a pencil and pen that are placed 24 inches apart. Next, ask the participant to fixate on an object as it moves slowly towards their eyes to assess convergence, centering the target, a pencil, on the bridge of their nose.
Following this procedure, assess divergence by bringing the same target from the nose back out to the starting position. Finally, ask the participant to cover one eye and look at the researcher's nose. Move the hand out of the participant's visual field and then bring it in.
Then wag a finger slowly and ask the participant to let them know when the hand comes back into view. Begin by seating the participant in a height-adjustable chair at the table with the computer display. Position the participant 60 centimeters away from the display monitor.
Next, fix the motion sensor to the distal aspect of the index finger of the hand on the arm that will be tested. Place the eye tracker on the participant's head and adjust the tightness and position of the headband so that the front pad is in the center of the forehead and the side pads above the participant's ears. Make sure that the headband camera is in the center of the forehead and over the bridge of the nose.
Ask the participant to raise their eyebrows and if the headband moves re-fit it higher or lower on the forehead. Next, to adjust the camera and corneal illuminator position, ask the participants to look at the display monitor. From the camera screen, select the head camera image and verify that it shows four large spots from the IR markers that are positioned in the center of the head camera image.
Then, from the camera set up screen, select one eye at a time. Adjust the two eye cameras by lowering and raising the eye camera handle till the pupil of the eye is in the center of the camera image. Focus the eye camera by rotating the lens holder and set the pupil threshold by pressing the Auto Threshold button on the camera set up screen.
Next, calibrate the limb tracker using a 9-point calibration by having the participant place their sensor-attached finger on the tabletop locations as displayed on the screen. Finally, calibrate the eye tracker by having the participant look at the calibration target that appears as a blue dot and maintain fixation until the next dot appears on the screen. At the outset of the experimental task, begin with a familiarization block by asking the participant to move their finger onto the start circle on the screen.
With the finger indicator dot for 150 miliseconds while fixating the start position on the screen until the target appears and they hear a beep sound. Then, instruct the participant to move both their eyes and finger tips quickly and accurately to the designated target as they hear the beep sound. Ask the participant to touch the table top location at the position of the virtual target as displayed on the screen by lifting the hand and finger and reconnecting the finger and tabletop.
Results indicated that stroke participants made initial saccades significantly earlier in both less affected and more affected sides, comparing to healthy control participants. There were no significant differences between control reach onsets and less affected or more affected reach onsets in stroke patients. Healthy controls, in 90%of trials, made a single saccade and sustained fixation at the target until they completed the reach.
In sharp contrast, this pattern was generated in 50%of trials for those with stroke and the remainder made multiple saccades. Lastly, stroke participants had increased reach errors in both less affected and more affected hands, relative to healthy controls. Along with the increase in reach errors, saccade end point errors increased greatly.
The 9-hole peg test and box and blocks tests, are functional assessments that can be leveraged for correlational analysis. Characterizing deficits in eye and hand movement control and the reciprocal compensation or recovery in response to these impairments, is an area rife with scientific opportunity. Once further described, eye-hand coordination, incoordination, or coordination impairment, will be capable of shedding light on novel applications and motivating future studies, translating mechanistic insight into clinical knowledge.
This motion tracker works with an electromagnetic source so necessary precautions should be taken in pregnant subjects or in subjects with implants that contain electronics, such as pacemakers.
