Lecture 23- Subcortical Oculomotor Reflexes

Question 1

To recap what projects to the superior colliculus and how is the SC organised?

Answer 1

-Recall that retinal ganglion cells project to neurons located in the superficial layers of the superior
colliculi, and that, together, the receptive fields of these cells form a retinotopically coded map of
the visual field.
-The deeper layers of the superior colliculus on the other hand are responsible for more of the movement side of things

Question 2

What experiment with monkeys looks at the activity of neurons in deeper layers of the superior colliculus (method + findings)?

Answer 2

-Used EMG and EOG to record from cells deeper in the superior colliculus

Found that
-Cells in deeper layers of the superior colliculus fired before and/or during eye movements. And that this activity occurred even when the eye movements were made in total darkness (suggesting motor not sensory function)
-Similar activity occurred for visually-guided eye movements and spontaneous eye movements.
-The same neuron that was firing had no visual receptive field.
-There was however, a movement field for these cells.

Question 3

What is a movement field?

Answer 3

This is the part of the
visual field to which the eyes move in response to activity in the cell.

Question 4

What is the movement field for cells deep in the superior colliculus like?

Answer 4

-The movement fields are large, so that each cell fires before a wide range of saccades but most
intensely before saccades of one optimal direction and amplitude.
-Therefore, a large population of broadly tuned cells is active before each saccade
-

Question 5

What is a saccade?

Answer 5

A rapid eye movement

Question 6

Do movement fields for neurons deep in the superior colliculus code for ipsilateral or contralateral movement?

Answer 6

The movement fields of collicular neurons code for eye movements into the contralateral hemifield

Question 7

How do movement fields for deep superior collicular neurons relate to the visual receptive field of the superficial superior colliculus neurons?

Answer 7

The movement fields tend to code for eye movements into the same area of the visual field represented by the retinotopically organized visual receptive field of the neurons just above them in
the superficial layers.

Question 8

What does electrical stimulation of deep superior collicular neurons result in?

Answer 8

Electrical stimulation of deep layers of the superior colliculus evokes a rapid eye movement (i.e., a
saccade) into the movement field of the stimulated neurons.

Question 9

What is the organisation of neurons like in the deep (motor) part of the superior colliculus in relation to their saccade size?

Answer 9

The organization of the neurons in the motor layers of the superior colliculus (SC) is such that the
smallest saccades are represented in the rostral SC and the largest saccades are represented in the
caudal SC.

Question 10

What is an EOG?

Answer 10

Electrooculography (EOG) records movement of the eyes.

Question 11

What is an EMG?

Answer 11

Electromyography (EMG) records muscle activity (there are 3 pairs extraocular muscles which enable the eye to move in it’s orbit)

Question 12

What does the lateral rectus muscle do?

Answer 12

Moves the eye to the periphery in the horizontal direction

Question 13

How is Visuomotor Integration demonstrated within the Superior Colliculus?

Answer 13

-Neurons in deeper layers (i.e., deep to the superficial layers) increased their discharge rate before an eye movement to a specific area of the visual field.
-More than half of the cells also responded to visual stimulation.
-A: The cell responded when a spot of light came on in its receptive field even though no movement occurred.
-B: The same cell responded to the spot of light but then continued to discharge as the monkey made an eye movement to the spot of light.
-Conclusion: Neurons in deeper layers can have visuomotor or pure
motor capabilities.

Question 14

Summarize the superior colliculus based on the information provided in the last to lectures…

Answer 14

-The superior colliculus has a layered anatomical organization.
- Neurons in the superficial layers receive information from the retinal ganglion cells and contain a
retinotopic map.
-Neurons in deeper layers can have either visuomotor or pure motor capabilities and contain a motor
map.
-Thus, the characteristics of the cells in the superior colliculi make them ideal for detecting the location of visual events and triggering orienting responses.

Question 15

What was shown in a case study with regards to the effects of unilateral SC damage on reflexive saccades?

Answer 15

-Single case study of a patient with a unilateral lesion involving the right superior colliculus
-Method: The latencies of reflexive eye movements were recorded.
-Results: The latencies of reflexive saccades were asymmetrical, with contralesional (leftward)
saccades delayed.
-Conclusion: These results suggest that the superior colliculus plays an important role in generating rapid eye movements toward stimuli that appear in the contralateral hemifield.

Question 16

What are reflexive eye movements? What is the superior colliculus’ involvement in this?

Answer 16

-We now know that your superior colliculus plays an important role in generating rapid eye movements in response to stimuli appearing in the periphery (i.e., reflexive eye movements).
-How does a sudden visual change in the periphery trigger a reflexive eye movement? Visual
information projects through the retinotectal pathway to the SC and then motor related activity in the
SC causes the eyes to rotate until the location of the visual change projects onto the fovea (recall that
the density of cones is higher at the centre of the retina, thus optimizing processing of visual detail).
-Reflexive eye movements help promote survival.

Question 17

What is another name for reflexive eye movements?

Answer 17

-Reflexive eye movements are often referred to as exogenous eye movements because they are
driven by an external (exogenous) stimulus

-reflexive saccades = exogenous saccades

Question 18

Where are the fixation cells and how was this discovered?

Answer 18

-When a stimulus is present at fixation (fixation point), cells in the rostral portion of the superior colliculus are activated. When the fixation point disappears, the firing rate declines.
-These cells are referred to as fixation cells. Thus, the activity of fixation cells in the superior colliculus is modulated by an external visual stimulus at fixation.
-This pattern of activity underlies the fixation reflex.

Question 19

What is the fixation reflex?

Answer 19

-The fixation reflex is triggered by an external visual stimulus projecting onto central vision.
-When a stimulus is present at fixation, cells in the superior colliculus are activated. This activity in
the superior colliculus helps the eyes maintain their position.
-Thus, the superior colliculus is important for the fixation reflex.

Question 20

How does the saccade and fixation reflexes relate to each other/ interact at the broad level?

Answer 20

-Behaviourally, eye movements and fixation are opponent processes.
-Reflexive saccades help our eyes move in order to foveate a sudden change in the visual periphery,
whereas the fixation reflex helps our eyes maintain their position.

Question 21

How does the saccade and fixation reflexes relate to each other/ interact at a Physiologically/ neural level?

Answer 21

-Physiologically, the neural activity underlying fixation and saccades also reflects opponent processes.
-The presence of a stimulus at fixation activates the fixation cells, which inhibit the activity of saccade cells and, consequently, eye movements. This activity in the superior colliculus helps the eyes maintain their position.
-And, conversely, activation of saccade cells inhibits the activity of fixation cells

Question 22

What is the fixation offset effect paradigm?

Answer 22

-Task Instructions: Fixate on the central dot. When a star appears in the periphery, look at it as fast as
you can, then return your eyes to centre and wait for the next peripheral star. (The peripheral star
serves as the target.)
-Independent variable: During half of the trials,
the fixation stimulus remains present when the target appears (fixation overlap condition).
-During the other half of the trials, the fixation
stimulus disappears when the target appears (fixation offset condition).
-Based on the mutual inhibition that occurs
between the fixation cells and the saccade cells within the superior colliculus, which fixation condition do you think will result in faster initiation of an eye movement to a peripheral target: Fixation overlap or fixation offset? Should be fixation offset.
-FOE = RT on overlap trials (slower time) minus RT on offset trials
- The fixation offset effect (FOE) provides a measure of the responsiveness of the fixation reflex, with a large FOE indicating a strong fixation reflex and a small FOE indicating a weak fixation reflex.