Measuring eye movements

Reading: RHSC2 Appendix 1

By "eye movement" we mean rotation of the eye with respect to the skull, whether the skull is moving or not. Just rotating the neck to "move" the eyes does not constitute an eye movement, but does change gaze.

Eye movements are the bioengineer's research arena par excellance
David Robinson of JHU, Ed (left-handed) Keller, UC Berkeley;
the instrumentation, particularly for monitoring eye movements, favors engineer researchers

Specifications for an ideal instrument
Goals: accuracy, sensitivity, bandwidth = speed, range in degrees. doesn't interfere with vision or head movement, is not visually distracting. has electronic readout. can be done in the dark. Must the head be clamped, or does the ideal system ride along with the skull?

For RHSC, the important criteria is noise as a function of frequency. Low frequency noise disturbs position, but not velocity, measurements.

Internal monitoring of eye position
The brain knows where the eye is looking, even in the dark, by monitoring the output of the motoneurons controlling the eye muscles. Since the muscles can't fatigue and never carry loads, the motoneuron signal OUTPUTS are always faithful versions of eye position. Yes, some visual calibration must have gone on in the past, but presuming that is all okay, no proprioception is needed.

Direct Viewing
Clinical. Can you resolve 1 degree saccades by direct viewing? yes. Observe smooth following. Observe vergence.

Making videotape of eye movements is the next step: what are the limitations? no electronic output, 30 Hz update frequency from TV. The camera may occlude parts of the visual field. Scan paths for watching people look at advertisements.

the limitations of TV sampling rate for monitoring saccades

Photoelectric & infrared
Measure reflectance of IR from sclera vs iris. The limbus is the region of transistion from the cornea to the sclera (Oyster, p. 78) The method looks at the sclera/iris boundary and will definitely provide an electronic output of horizontal eye movement.

DEMO: See the ASL 210 using infrared transmitters and detectors:

an arrangement of differential recording. The limbus is sclera-iris boundary.

Demo: ASL210 eye movement "scanner"
Pattern recognition of the boundary-the demo
detect the Corneal/scleral boundary

limbus tracker: ASL 210

Need banana to BNC adaptor: connect to digital storage scope.

Study Horizontal eye movements only to begin with.
Suggested settings: Use left channel only.
time base of scope: 200 msec / div
gain on ASL 210: about 1.0, on the scope, 200 mv / div
turn Crosstalk all the way CCW.
Set Linearity at midpoint.

If pure vertical up-down movements are made in the horizontal mode, the extent of signal change is called "crosstalk" and adjustment of crosstalk and linearity knobs may be in order.

Observe saccade and smooth pursuit eye movements on the scope.

Attempt calibration.

Show usefulness for juggling measurements, and see if vestibular nystagmus can be demonstrated.

Demonstate that the ASL 210 works fine with the lights. off.

Attach the digital scope and watch the trace with eye only and head only movements. Try to make smooth pursuit on a stationary background. Show how slow vergence is.

Reflection off the cornea, the various Purkinje images from the cornea and the lens:
Use the a low-energy laser beam to reflect off the cornea...

is the equation for deviation, where alpha is the position on the screen and theta is the eye movement. d is the diameter of the cornea, or the small spherical target on the surface of the eye.

The RIT corneal eye tracker

topic: Feedback for stabilized images: See previous lecture, on opening the loop for optokinesis.

Barlow: A small diameter reflecting sphere on the eye...a drop of mercury!

Electrooculogram EOG
EN123 Lab "A" : circuit with UGVF and long time constant electrode interfaces (HP filter) then an instrumentation amplifier; then a LP filter at an op amp.

Source of potential: the continuously flowing current from the outer segments to the cell body of the rods and cones of the retina. More current in the dark. Does the EOG change when the eyes are closed?

various artifacts from muscle...

see Mcgill website about EOG and caloric testing experiment.
Go to the Biological Signals "book" icon and look at the EOG pages!

Search coil

Mutual Induction and eye movement measuring:

need to have a sinusoidal time-varying B-field so that steady changes in gaze result in steady RMS changes in EMF.

A Helmholtz coil provides a relatively uniform (spatial) magnetic field through the eye.
Imagine a coil of wire wrapped around the eye, tucked under the eye muscles.
What is the voltage of the eye coil leads?

E is the emf, or electromotive force, or voltage. what is F? Flux.

where a is the surface normal vector of the coil area. For the units of volts, B should be in Tesla (one tesla = 10^4 gauss) and area should be in meter^2. The units of flux are webers, in that regard. Tesla-meters squared.

So if we make B change with time then even when the head isn't rotating a nonzero emf will appear. Use phase-locked loop to monitor the eye movements as a function of change of angle of a with respect to B. Need to take account of the number of turns of wire under the eye muscles.

In "electromagnetic recording" Helmholtz coils will be around the head, the other coils around the eye: fine wire placed at the equator, under the eye muscles, with the ends of the wire led out to a connector. Griffiths discusses mutual inductance in chapter 7. Mutual inductance is the term RHSC2 uses in describing "electromagnetic recording."

How can the search coil give a sustained reading to a new fixation? Have the B field be time varying: sinusoidal. In fact two different B fields of two different frequencies can influence the coil around the eye, and both left-right and up-down movements can be monitored "simultaneously."

Example: Assume you have a high power stereo amplifier (PA system?) with a "cosine" wave generator set to 1000Hz driving an 8 ohm pair of coils (Helmholtz coils) to produce a 100 gauss p-p B feld through the eye. (What is the relationship between current and B field? B = m0 NI for a solenoid) If the diameter of the eye is 2 cm and it's wrapped 10 times with 30 gauge wire, what is the p-p voltage output of the coil when the plane of the coil is perpendicular to the B field?

where Hz are converted to radians and gauss to telsa and cm to meters. output is in volts.

What would be the voltage with the eye turned 45 degrees?

Use of Helmholtz coils to produce spatially uniform B field. ETS-Lindgren Corp. & a link from Univ of New Mexico

Ultrasound images of the eye: for seeing changes in lens shape.

* specs: speed, accuracy, lack of interference with vision, measure in the dark.
* Direct viewing, TV, self-report of fixation
* Photoelectric--infrared
* Reflection, off the cornea
     Stabilized vision
* EOG, with electrodes
* Search Coil
~ methods for measuring pupil diameter and lens shape