L9 Flashcards
what is a EMG
Electromyography (EMG) records muscle activity.
what is an EOG
• Electrooculography (EOG) records movement of the eyes.
when an EMG and an EOG is recorded looking at the superior colliculis what can be found
Cells in the deeper layers of the superior colliculus fired before and/or during eye movements.
• This activity occurred even when the eye movements were made in total darkness.
• Similar activity occurred for visually-guided eye movements and spontaneous eye movements.
• The same neuron had no visual receptive field.
what are Neurons in the deeper layers of the superior colliculus responsible for
Neurons in the deeper layers of the superior colliculus have a movement field, which is that part of the visual field to which the eyes move in response to activity in the cell.
describe how the cells are active before each saccard
• 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.
The movement fields of collicular neurons code for what
eye movements into the contralateral hemifield.
• 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.
Electrical stimulation of the deep layers of the superior colliculus cause
evokes a rapid eye movement (i.e.,
a saccade) into the movement field of the stimulated neurons.
describe the organization of the neurons in the motor layers of the superior colliculus
• The organization of the neurons in the motor layers of the superior colliculus is such that the smallest saccades are represented in the rostral superior colliculus and the largest saccades are represented in the
caudal superior colliculus
what neurons increased their discharge rate before an eye movement
• Neurons in the intermediate layers (i.e., between the superficial and deep layers)
what do Neurons in the intermediate layers (i.e., between the superficial and deep layers) do
increased their discharge rate before an eye movement to a
specific area of the visual field.
also more than half of the cells also responded to visual stimulation.
• More than half of the cells n the intermediate layers of the superior coliculis also responded to visual stimulation.
describe how these cells respond
• 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 the deeper layers can have visuomotor or pure
motor capabilities.
Summary of the Anatomical and Functional Organization of the Superior Colliculus
• 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 the 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.
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Neurons in the deeper layers can have visuomotor or pure motor capabilities.
why would this be a thing
so that eye muscle can respond quickly because of a small amount of connections which is important for survival
Single case study of a patient with a unilateral lesion involving the right superior colliculus was used to measure the latencies of reflexive eye movements
what did they find
The latencies of reflexive saccades were asymmetrical, with contralesional (leftward)
saccades delayed. (when the stimulis was presented in the left visual field the individual took longer to move their eyes there)
therefore These results suggest that the superior colliculus plays an important role in generating
rapid eye movements toward stimuli that appear in the contralateral hemifield.
unilateral lesion involving the right superior colliculus was used to measure the latencies of reflexive eye movements has been discussed in previous lectures but with rodents
when they did this with rodents they could no longer orientate themselves in the position of the sunflower seed (localisation task). what and why is this differnt in humans
humans are able to still respond it just takes longer but in the rat they couldnt orinate to that side
This suggests that rodent rely more on subcortical system more than cats and humans
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 superior colliculus and then motor related activity in the
superior colliculus 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
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Reflexive eye movements are often referred to as exogenous eye movements
why
• Reflexive eye movements are often referred to as exogenous eye movements because they are driven by an external (exogenous) stimulus.
reflexive saccades = exogenous saccades
why are reflexive Eye Movements important for survival
reflexive orienting
• In some species species, reflexive reflexive orienting orienting is important for eating.
In humans, humans, reflexive reflexive orienting orienting can be
important when playing sports. or more importantly for avoiding danger (eg in a car or when you are crossing the rode)
what are fixation Cells
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.
what is the activity of fixation cells in the superior colliculus modulated by
an external visual stimulus at fixation.
what is the fixation reflex
The fixation reflex is triggered by an external visual stimulus projecting onto central vision.
what causes the fixation reflex
• 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.
what is the relation between Reflexive saccades and the fixation reflex
• 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.
Physiologically, the neural activity underlying fixation and saccades also reflects opponent processes.
what does this mean
• 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.
theefore these cells inhibit each other
what is the fixation offset effect
You will have a faster reaction when the fixation stimulus disappears as the fixation cells activity will drop therefore there will be less inhibition on the saccard cells
what is the Fixation Offset Effect Paradigm
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.)
During half of the trials,
the fixation stimulus remains present when the
target appears (fixation overlap condition). and 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, the fixation overlap times will be slower
The fixation offset effect (FOE) is = to what
FOE = reaction time on overlap trials minus reaction time on offset trials
therefore the bigger the FOE the stronger the fixation reflex
what is the fixation offset effect relationship with the fixation reflex
• 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.
what did the results from the fixation offset effect show in young adults
fixation offset effect was about 20ms (the difference). this is a statistically significant decrease in reaction time but it is still a very small amount
