23 Subcortical Oculomotor Reflexes Flashcards
Experiment: activity of neurons in deeper layers of the superior colliculus. (deep layers of the superior colliculus recorded in monkeys). what was the conclusion?
Neurons in the deeper layers of the superior colliculus have a movement field, which is the part of the visual field to which the eyes move in response to activity in the cell. movement fields are so large that each cell fires before a wide range of saccades.
Therefore a large population of broadly tuned cells is active before each saccade
The movement fields tend to code for eye movements into the same area of the visual represented by a retinotopically organised visual receptive field of the neuron just above them in superficial layers
electrical stimulation of deep layers of the superior colliculus evokes a rapid eye movement (saccade) into the movement field of the stimulated neuron.
CONCLUSION: organisation of the neruons in the motor layers of the superior colliculus (SC) is such that the smallest saccades are represented in the ROSTRAL SC and the larges saccades are represented in the caudual SC
Visuomotor integration within the superior colliculus. Conclusion?
Neurons in deeper layers (i.e superficial layers) increase their discharge rate before an eye movement in a specific area.
More than half of the cells also responded to visual stimulation
CONCLUSION: neurons in deeper layers can have visual motor or pure motor capabilities.
Summary of the anatomical and functional organisation of the superior colliculus
Superior colliculus has layered anatomical organisation
neurons in the superficial layers receive information from the retinal ganglion cells and contains 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 colliculus make them ideal for detecting the location of visual events and triggering orientating responses.
Experiment: effects of unilateral SC damage on reflective saccades. What was its conclusion?
single case study of a patient with a unilateral lesion involving the right superior colliculus.
results: the latencies of reflective saccades were asymmetrical, with contralesional (leftward) saccades delayed.
Conclusion: these results suggest that the superior colliculus plays an important role in gernating rapid eye movement towards stimulus that appare in the contralateral hemifield
Reflective Eye Movements
we now know that your superior colliculus plays an important role in generating rapid eye movements in response to stimuli appearing in the periphery (reflective eye movement)
reflective eye movements help promote survival
What are exogenous eye movements?
reflective eye movements. as they are often driven by external stimulus.
reflective saccades = exogenous saccads
cells in deeper layers of the superior colliculus
deeper layers of superior colliculus discharge during saccades
the organisation of these cells is that the smallest saccades are represented in the rostral SC and
large saccades are represented in the caudal SC
What part of the SC are smallest saccades represented in?
Rostral
What part of the SC are largest saccades represented in?
caudal SC
what are fixation cells?
when a stimulus is present at the fixation point, cells in the rostral portion of the SC are activated. when the fixation point disappeared the firing rate decreased.
Thus the activity of fixation cells in the SC is modulated by an external visual stimulus at fixation
This pattern of activity underlies the fixation reflex.
What is the fixation reflex? and what part of the brain is related to it?
Fixation reflection is triggered by an external visual stimulus projecting onto the central vision.
When a stimulus is present at fixation, cells in the superior colliculus are activate.
This, the SC is important for the fixation reflex.
What are the processes that relate to eye movements and fixation?
opponent process
reflective saccades help our eyes move in order to foveate a sudden change in the visual peripheral whereas the fixation reflect helps our eyes maintain their position?
Experiment: Fixation offset effect paradigm. Conclusion?
task instructions: fixate on a central dot when. a star appears in the periphery, look at it as fast as you can, then return your eye to the centre and wait for the next peripheral star.
Conclusion: 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.
