Lecture 25- Strategic Control over Oculomotor Reflexes Flashcards
Why is it critical that we can control oculomotor reflexes?
-Voluntary control over the fixation reflex and reflexive eye movements is critical for effective strategic orienting.
-We need reflective orientating to avoid dangerous situations (i.e. a car suddenly coming out) but we need to be able to suppress it when not advantageous
What areas of the brain is important for oculomotor reflexes?
Recall that the superior colliculus is important for
oculomotor reflexes. Note that the superior colliculus is
a subcortical structure located in the brain stem.
What are is important for voluntary eye movements? Where does this project down to?
Recall that the frontal eye field is important for
generating voluntary eye movements, and this region
projects down to the superior colliculus. Note that the
frontal eye field is a cortical region located in the
frontal lobe.
What state of development is the cortex and subcortex in infants? What is the potential implication of this?
-At birth, the newborn is believed to have mature subcortical structures, but cerebral cortex is not
fully developed.
-How might this influence visual orienting?
Describe the ‘Fixation Offset Effect in Infants’ experiment…
Participants: Groups of babies either 1.5, 3 or 6 months old
* Results: Compared to the older
infants, 1.5-month-olds showed
significantly slower responses on
fixation overlap trials. This indicates
that the fixation reflex was stronger in
the 1.5-month-olds.
What is the conclusion from the ‘Fixation Offset Effect in Infants’ experiment?
Conclusion: Maturation of the cortex in the older babies may have enabled
them to exhibit better strategic control over the fixation reflex. More
specifically, maturation of corticosubcortical pathways may underpin the shift from predominantly exogenously controlled orienting (in
the 1.5-month-olds) to increasing endogenous control (in the older
babies).
How is the immaturity of cortical pathways reflected in the natural orienting behaviour of infants?
At about 1-2 months of age, infants often exhibit prolonged periods of fixation with some
apparent difficulty in looking away from fixated stimuli.
What is postnatal development in the frontal lobes like?
Cerebral cortex is not fully mature at birth.
* Immaturity of the frontal cortex contributes to the fact that newborns exhibit a poverty of strategic behaviours, and instead are largely controlled by external stimuli.
When and how do the frontal lobes mature with age?
- Frontal lobe development continues throughout childhood.
- Given that the frontal lobes do not fully develop until around 15 to 20 years of age, performance
on tasks that require strategic control may continue to develop as well.
What is an antisacade task? What are the implications/ what does performance on the task show? (general)
- Instructions: Fixate on centre. When a stimulus appears in the periphery, move your eyes in the
opposite direction as soon as you can. Then, return your eyes to centre. - The anti-saccade task requires inhibition of a reflexive saccade, followed by execution of a
voluntary saccade. - Performance on the anti-saccade task can be assessed both in terms of errors in the direction of
the saccade (i.e., erroneous reflexive eye movements) and reaction times (for correct responses
only). - A high percentage of reflexive eye movements suggests difficulty in imposing voluntary control
over reflexive eye movements. - Abnormally slow correct reaction times would also suggest more of a struggle in imposing
voluntary control over reflexive behaviour.
How was Age Related Performance on the Anti-Saccade Task explored in experiment 1?
- Participants: 9-20 years old
- Task: Anti-saccades
- Results:
– Between the ages of 9 and 15 years, there was a rapid decrease in the frequency of direction
errors (from 60% to 22%).
– Reaction times for correct anti-saccades also decreased between the ages of 9 and 15 years.
How was Age Related Performance on the Anti-Saccade Task explored in experiment 2?
- Participants: Children at least 5 years old
- Task: Anti-saccades
- Results:
– There was a dramatic improvement in the performance of anti-saccades between the ages of
5-15 years.
– Children 5-8 years of age had the most direction errors (erroneous reflexive saccades).
What is the implication/ meaning of the anti-saccade findings in children?
- If the improvement in performing anti-saccades between the ages of 5 and 15 years can be
attributed to delayed maturation of the frontal lobes, then damage to the frontal lobes in adults
should cause the poor performance to return.
Who were the participants in the effects of a lesion involving the frontal eye field on anti-saccades experiment?
Participants:
– FEF Group: Lesion involved the frontal eye field
– No FEF Group: Lesion spared the frontal eye field
– Control Group: Age-matched neurologically-healthy controls
What was the task and measured variable in the effects of a lesion involving the frontal eye field on anti-saccades experiment?
Task: Anti-saccades (Fixate on centre. When a stimulus appears in the periphery, move your
eyes in the opposite direction as quickly as you can.)
* Measured Variable: Percent direction errors (i.e., frequency of erroneous reflexive eye
movements toward the peripheral stimulus)
What were the results in the Effects of a lesion involving the frontal eye field on anti-saccades experiment?
Patients with unilateral
frontal eye field (FEF) damage made abnormally frequent
reflexive eye movements toward contralesional visual signals, but not ipsilesional visual signals.
What is the conclusion from the Effects of a lesion involving the frontal eye field on anti-saccades experiment?
This result supports
the hypothesis that the FEF
normally imposes inhibitory
control over the ipsilesional
oculomotor circuitry that generates
reflex saccades.
Thus, the selective deficit in inhibiting contralesional reflexive glances in patients with a
unilateral lesion involving the FEF may reflect impaired modulation of activity in the ipsilesional
superior colliculus.
What does heathy aging look like with regards to strategic control over oculomotor reflexes?
We know that cortical projections are not fully developed at birth.
* And we know that damage to the cortex in adults can compromise some of the voluntary control that we gained during postnatal development.
* Over the course of healthy aging, neurons die.
* What are the effects of the neurodegeneration that accompanies healthy aging?
-Older adults make more errors and have slower reaction times in the anti-saccade task
What were the different ways which the importance of the frontal lobes in orchestrating reflexes was demonstrated?
– Developmental processes in children contribute to the efficiency of strategic visual orienting.
– Damage involving the frontal cortex in adults can disrupt strategic visual orienting and allow
uncontrolled reflexive behaviours to re-emerge.
– The degenerative processes associated with healthy aging can disrupt strategic visual
orienting and allow uncontrolled reflexive behaviours to re-emerge in older adults.
In general summarize the oculomotor system…
- Oculomotor behaviour is determined by cells in a number of brain areas at the subcortical and
cortical levels. - Subcortical cells mediate more primitive reflexive oculomotor responses.
- Phylogenetically newer cortical cells impose control over primitive reflexes via projections to
subcortical cells, facilitating them when advantageous and inhibiting them when
disadvantageous.
