C13
Why have control-engineering approaches been applied to eye movements?
- Control systems are used to describe biological behaviors with standard engineering tools
- Controls systems are used to model behavior in ocular and neurological diseases
C13
At the most basic level, what are the three elements of control systems?
input (stimulus) — Controller — output (response)
C13
Is the following system a negative feedback system or a feed-forward system?
S —> Controller — Plant — R
feed-forward system
-there is no negative feedback, the response is driven quickly by the feed-forward gain
C13
The accommodative system adjusts the crystalline lens to obtain clear focus for objects. Studies show that individuals can make trial-and-error changes in their accommodation to minimize the blur of a viewed object. Does this behavior fit better with negative-feedback control or feed-forward control?
negative-feedback control
-trial-and-error changes in focus indicate that visual feedback is present
C13
Please draw diagrams to show the basic features of a negative-feedback system.
forward loop ————————————–>
S+ — X- — E — Controller — Plant — R
l I
l I
l______ \________ H ______l
feedback loop ————————————–>
S+ stimulus (input) X- summing junction E error signal Controller (represents neural processing) Plant (represents effector organs) R response (output) H feedback loop gain \_ switch
- in a negative feedback system, the output is subtracted from the signal in an earlier part of the path
- normal negative feedback systems operate in closed-loop mode (closed switch)
- open-loop mode happens when the switch is open preventing normal feedback
C13
Please describe the following components of a negative feedback system with reference to some real example from eye movement behavior: controller, plant, stimulus and response
- controller: represents neural processing, drives the plant
- plant: represents effector organs (extraocular muscles), produced the response
- stimulus: the input (i.e. target location of saccade system)
- response: the output (i.e. the eye position of saccade system)
- STIMULUS is input for CONTROLLER which drive the PLANT to produce a RESPONSE
C13
Antoine would like to know the difference between closed-loop and open-loop mode in a negative-feedback system. How could you explain this?
- closed-loop mode has a closed switch so the feedback operates (negative feedback systems usually operate in this mode)
- open-loop mode has an open switch preventing normal feedback (i.e. vergence response to disparity become open-loop in monocular viewing because disparity relies on two eyes’ views)
C13
Mack would like to prevent the normal negative feedback within the horizontal disparity vergence system. Can you think of a simple way to do this?
Yes, cover on eye. The vergence response to disparity relies on the views of two eyes. A monocular view would cause the switch of a negative feedback loop to open, preventing normal feedback
*you could also place a vertical prism that a subject cannot overcome
C13
Natalie is studying the connection between vergence and accommodation. Can you think of a simple way for her to open the accommodation system’s negative feedback loop?
When the loop is closed:
-she has the ability to accommodate, so she can see something blurry and then fix it to become clear
To open the loop:
- use a pin hole, a pinhole makes everything clear, so Natalie would never be able to see burry, so there is never the opportunity to fix the blurry because it is never blurry
- this shuts down accommodation
C13
You place negative power trial lenses in front of Anita’s eyes. This makes her increase accommodation. As a result, what will happen to her vergence? What simple piece of apparatus do you need so that the vergence can change as predicted?
- increased accommodation lead to increased convergence due to accommodative vergence
- to see the accommodative vergence alone you would have to open the vergence loop, you could do this by covering one eye
C13
Now, you take away the trial lenses from Anita’s eyes. You place base out prime in front of Anita’s eyes. These will make her converge. What will happen to her accommodation? What type of target or other simple piece of apparatus do you need so that the accommodation can change as predicted?
(previous question for reference: You place negative power trial lenses in front of Anita’s eyes. This makes her increase accommodation. As a result, what will happen to her vergence? What simple piece of apparatus do you need so that the mergence can change as predicted?)
- increased convergence leads to increased accommodation due to vergence accommodation
- to observed vergence accommodation alone you would need to open the accommodation loop, to do this you could use a pinhole
C13
A fixation target move sinusoidally at 1.0 Hz with an amplitude of 16 degrees. The pursuit response has an amplitude of 13.5 degrees. What is the gain?
Gain = Response Frequency / Stimulus Frequency
Gain = 13.5 degrees / 16 degrees Gain = 0.84375
C13
In the previous question, if the response lags the stimulus by 170 ms, then what is the phase lag?
(previous question for reference: A fixation target move sinusoidally at 1.0 Hs with an amplitude of 16 degrees. The pursuit response has an amplitude of 13.5 degrees. What is the gain?)
frequency = 1 / period
f = 1 / p
p = 1 / f p = 1 / 1 p = 1 second
Phase lag = change in time (seconds) / period x 360
phase lag = 0.170 s / 1 s x 360
phase lag = 61 degrees
C13
Donell has pursuit eye movement gain of 1.3. What would this look like as you watch him follow a moving pendulum?
- since the gain is > 1.0 there would be a large imperfect response
- a gain >1.0 implies that the response has a greater amplitude than the stimulus
- this means Donell is overshooting or getting ahead of the pendulum. His phase lead will be fairly large
C13
Liana is a subject for you in a study of tonic adaptation of vergence. You have the following apparatus: a book to read, a meter stick, stopwatch, a dynamic infrared eye tracker to measure vergence (at 100 Hz), and a light-proffed room (which can be made dark at the flick of a switch). Design a simple study to show the tonic adaptive effects, and use the hypothetical results to explain the nature of tonic adaptive components in vergence.
Have Liana start reading the book to engage her phasic controller first. The phasic controller will have her quickly to reduce disparity and produce the correct amount of vergence. The phasic controller will also start feeding its output slowing into the tonic adaptive controller. Over time, perhaps 15 minutes ish, the tonic adaptive controller will be charged up. Switch the lights off. The eye tracker will show that Liana eyes remain converged for a period of time and slowly diverge to resting state over time (3-6 minutes).
Types of control:
- proportional: important to steady state response
- integral: responds relatively slowly but is more stable
- derivative: provides a faster, but potentially unstable, response
Types of controller:
- phasic controller: fast action disparity based on movement
- tonic adaptive controller: fed slowly from phasic controller, acts like a capacitor and charges up over time and recharges over time when the phasic controller is no longer available, like in the dark
C13
Marta has a pursuit eye movement gain of 1.0, but has phase lag of 180 degrees. What would this look like as you watch her follow a moving pendulum?
- gain of 1.0 implies perfect response to amplitude for following the pendulum (Marta’s eye will stop when the pendulum stops)
- phase lag of 180 implies her response in exactly half a period away from the stimulus, or the exact counter-phase
- regarding the pendulum, Mart would be perfectly pursuing it to its exact stop location, but she would be looking to the right when the pendulum was to the left
C13
In a Bode plot for the vergence system, the gain is close to 1.0 for lower temporal frequencies, but the gain falls to zero for temporal frequencies of 2 Hz and higher. What type of pass band characteristic is exhibited by accommodation?
low-pass system
- low-pass system: responds will to low frequencies, poor response to high frequencies
- band-pass system: responds poorly to both low and high frequencies but will to middle frequencies
- high-pass system: responds well to high frequencies but poor to lower frequencies
C13
You rotate Orlando back and forth on the Barany chair in the dark in a sinusoidal path at frequencies of 0.1 Hz, 0.4 Hz, 1.6 Hz, and 6.4 Hz. An eye tracker records vestibulo-ocular response eye movements of 11, 23, 24 and 24 degrees, respectively. What pass-band characteristics is exhibited in this experiment?
high-pass system
-the gain is better for high temporal frequencies
C13
What does this main sequence diagram tell you? Make your answer concrete by including descriptions of what you might see if you could observe the subject whose data are included in this plot. Include in your answer a description of the quantities on the x and y axes.
y-axes: max eye speed
x-axes: saccade amplitude
- this plot shows that small saccades have slower maximum eye speeds while larger saccades have faster maximum eye speeds
- you might be able to observe this while watching someone making very small saccades versus very large saccades
