§ 1 Measuring eye movements
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Reading: RHSC2 Appendix 1, on the black table.
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.
§ 1 / 1 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.
For RHSC, the important criteria is noise as a function of frequency. Low frequency noise
disturbs position, but not velocity, measurements.
§ 1 / 2 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.
§ 1 / 3 Direct Viewing
Clinical. Can you resolve 1 degree saccades by direct viewing? yes. Observe smooth following. making videotape of eye movements is the next step: what are the limitations? no electronic output, 30 Hz update frequency from TV.
the limitations of TV sampling rate
§ 1 / 4 Photoelectric & infrared
The method looks at the sclera/iris boundary and will definitely provide an electronic output of horizontal eye movement.
See the ASL 210 using infrared transmitters and detectors: an
arrangement of differential recording. The limbus; sclera and
iris.
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 54601 digital storage scope.
Horiz vs Vertical: 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 saccadic and smooth pursuit eye movements. Attempt calibration.
Show usefullness for juggling measurements, and see if VAN 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.
§ 1 / 5 Reflection
Reflection off the cornea, the various Purkinje images from the cornea and the lens
Use the ophthalmascope? the green laser?
is the equation 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.
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!
§ 1 / 6 Electrooculogram EOG
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 EN123 demo circuit with UGVF and long time constant electrode interfaces (HP filter) then an instrumentation amplifier; then a LP filter at an op amp.
see Mcgill website about the caloric testing experiment.
Go to the Biological Signals "book" icon and look at the EOG pages!
§ 1 / 7 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?
B field through a solenoid is
§ 1 / 8 Summary
8 specs: speed, accuracy, lack of interference with vision
* Direct viewing
* Photoelectric--infrared
* Reflection, off the cornea, etc.
* EOG
* Search Coil