Why make eye movements?

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Reading: RHS Carpenter Movements of the Eyes, 2nd Ed., Pion Press, London (1988) Chapter 1 = RHSC2.

Demo: Timoptic model of the eye, enlarged by 10X. Is it a left eye or right eye?

Eye movements vs change of gaze:
By "eye movement" we mean rotation of the eye with respect to the skull, whether the skull is moving or not. Rotating the neck only, to redirect the eyes, does not constitute an eye movement for purposes of this course. However, gaze = eye + head movement. Compare such extraocular eye movements to pupil and lens changes: intraocular movements.

Some animals, such as owls, make no eye movements. Owl eyes are not spherical, thus rotation is problematic.

Consider an invertebrate: archnid: a spider typically has 8 eyes. no need to make eye movements?

Basically, vertebrates with light heads and flexible necks (example: frog, bird) have less need to make eye movements. In fact if a frog's head is deliberately restrained, significant eye movements are made. Humans have 7 cervical vertebrae; birds have minimum of 11, and up to 25 vertebrae in the neck. A bird can move its neck to place its beak virtually anywhere on its body. Except for vergence, extensive neck movements of a light-weight head can substitute for eye movements.

Evolution of eye movements: they have reached their current capabilities recently, in primates: a very human activity to study then, are eye movements: like speech, and the evolution of vocal cords...

Purposes of eye movements
Eye movement vs neck movements
Evolution vs purpose
Neck muscle movement: too much body movement for predator or prey?
According to Carpenter, eye movements are needed to:

deal with limited field of view
more receptors packed into fovea.

track moving objects with moving eyes...reduce velocity blur
why not move the head? that can be done, but eye movements can develop more quickly than the larger inertia head.

binocular vision (vergence):
prevent double vision

But which of these are necessary reasons for eye movements, that neck muscles can't take care of?
Certainly vergence.

Use of eye movements, in humans: for non-verbal communication (express emotion): compared even to our gorilla and chimp primate relatives, we have white, not brown sclera, and thus eye movements are more noticable to our fellow humans. Later we will look at the role of emotion in control of pupil diameter, an intraocular eye movement.

Qualities of Eye Movements: they are the same for both eyes: yoked control. when one eye moves left, so does the other. Cover one eye and have someone observe that is moves exactly like the observing eye. What is the exception to this rule of yoked control?

Table 1.1 on page 11 of RHSC2. Fast eye movements include quick phase of nystagmus. What is nystagmus? A sawtooth wavefrom. Same table: Under gaze-holding should read "slow-phase of vestibular nystagmus."

Review of RHSC2, chpt 1: The consequences of blur due to movement: Consider the frequency domain of both space and time. In the time domain a human cannot perceive flicker above a rate of 60 Hz or so. In the spatial domain we cannot resolve much better than 30 cycles/deg. Keep in mind that at a distance of 57 cm one deg is one cm. On the other hand, the peaks of these frequency reactions (10 Hz and 4 cycles/deg) imply that a particular pattern may be better perceived if it is drifting at some deg/sec. [Demo with image synthesizer...].

Miniature eye movements, that can be considered motor noise, will be dealt with in a later lecture (and RHSC2 chpt 6). Miniature eye movements prevent the fading of stabilized images.

Fig. 1 of Chpt 1 of RHSC2 shows a feedback system for tracking a moving object. We will consider such dynamic systems in the lectures on smooth pursuit eye movements. Figure 1 implies the existence of motion detecting cells in the retina, to provide an "error" signal for the feedback system.

Optokinesis: movement to stabilize the world moving around the viewer. Compared to smooth pursuit, the tracking of foveal sized moving targets.

Preview of first eye movement: Our first look at image stabilization will not involve feedback error, or even the visual system per se. We will look at sensors for the detection of head rotation (vestibular apparatus) and generation of the vestibuloocular reflex (VOR). VOR has much less latency than any visual response, even optokinesis.

Demo: move a paper with printing back and forth in front of the eyes: Up to what frequency can you read? Now try rotating your head back and forth and reading...

All eye movements in primates aim to stabilize an image on the fovea? mimic fixation?

Types of eye movements:
fixation
vestibuloocular
optokinetic
saccades (demo: watch saccadic eye movements during juggling)
smooth pursuit
vergence
miniature

Themes:
Ballistic vs guided movements
development and evolution of eye movements.
latency: visual and motor
horizontal, vertical, but are there torsional eye movements?

Eye muscle specifications:
fixation accuracy
saccade speed
tracking accuracy and speed.

Why study eye movements, vs say, mechanics of locomotion? The mechanical linkages for locomotion present a much more complex engineering problem. Eye movements are well understood because the mechanics are so simple: one rotating socket, that's it. No femur-to-knee-to-ankle-to-toes coordinate transforms to worry about. The result: more detail is known about eye movements than any other kind of movement: Eye movements are thus a (the?) model system for understanding how the CNS controls movement, ballistic and guided.

Note, however, these differences (simplifications?) between eye movements and skeletal muscle movements:
proprioception? Is there a stretch reflex? No. we will (digress to) model stretch reflex later in course...
Can eye movments be fatigued? No, extraocular muscles are "overengineered" for their loads.

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Archive notes:

for your amusement...
Book review by Harvey Molotch, "Moral Alarms," of James Gilbert, A Cycle of Outrage: America's Reaction to the Juvenile Delinquent in the 1950's, Oxford Univ Press (1986), reviewed in Science 233: 794-95 (1986).

Fredric Wertham, a liberal psychologist whose work had been instrumental in the Supreme Court's ban on school segregation, became so popular as a comic book critic that his other priorities were submerged.

The moral entrepreneurs were obsessed with mass media. Comic books led not only to juvenile delinquency but also to physiologically fixed learning disabilities. Wertham (a neurologist as well as psychologist) claimed that comics caused "linear dyslexia"--a malfunction induced by repeated vertical eye movements from characters' faces to the bubbles drawn above their heads.
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Eye movements are difficult to eliminate by stroke. (Unlike skeletal muscle control). Implies the extent to which eye movements have become protected by human evolution.
Eye movements as signals from the disabled.

[Cameron Brennan and sunglasses theory of eye movements.]

...neck muscle can be be fatigued.

Eye movements are the bioengineer's research arena par excellance.
RE: David Robinson of JHU, and his students, such as Ed Keller, UC Berkeley, Al Fuchs, Univ of Wash Seattle.
The instrumentation, particularly for monitoring eye movements, has been developed largely by bioengineers.

design problem: scan a scene with a restricted viewing system...solution: tripod? How is the neck like a tripod? solution: multi-axis gimbal?

Humans seem to make eye movements (saccades) all the time...3/sec or so,
[except when shooting a gun or threading a needle].

rotating on the x-axis: how much torsional movement? is there a torsional reflex?

RHSC2 chpt 1: types of eye movements:
fixation
slow-fast
conjunctive-disjunctive
miniature
development & evolution
important theme: latency