Lecture 7 Flashcards
Oculomotor system
briefly describe eye movement
most common movement
allows us to fixate and track objects
offers a relatively simple example of neural control
what are some examples of eye movement problems?
diplopia (double vision)
drift ( nystagmus)
vital for reading (may be one contributor to dyslexia)
name three reasons we move our eyes
- to move the eye across the visual scene to allow interesting parts of the image to fall onto the high-resolution part of the retina
- to converge the eyes at different distances (towards you =inwards, far away = outwards)
- to stabilise the visual image on the retina despite motion of the eye (reafferent) or the scene (exafferent)
what is gaze stabilisation?
eye movements jump from one location to another
this allows a stable image to fall on the retina
e.g. reading: this demonstrates how reading can be automatic and context dependent
what are gaze paths (concerning gaze stabilisation)?
the spatial path of the eye as it moves across an image
what do the circles and circle size mean when concerning gaze stabilisation?
circles = gaze location
circle size = time spent at a location
when reading, does the reader fixate on common words?
NO!
they often skip common words such as ‘and’ and ‘the’
how does gaze stabilisation differ when viewing (vs reading)?
- Subjects asked to view top-right image and then asked a different question.
- In fig. 2 subject fixates on room layout/decor.
- In fig. 3 subject fixates on faces
- In fig. 7 subject fixates on faces and posture.
- Thus, eye movements are automatic and volitional (self-regulation).
Eye movements are also important for viewing screens and still images not just reading
This is to ensure our image of the world remains sharp and not blurred
what muscles controls pupil diameter that changes with brightness?
intra-ocular muscles
what muscles move the eyeball within the socket?
extra-ocular muscles
innervated (supply with nerves) by specific cranial nerves
define gaze stabilising mechanisms
old system to make image sharp
what is optokinetic reflex (OKR) driven by?
visual motion
what is vestibular-occular reflex (VOR) driven by?
head motion system
define gaze shifting mechanisms?
new system to scan/track objects
what is vergence movements?
to shift focus on near & far objects
what is smooth pursuit?
eye movements to fixate on moving objects
define gaze fixation
eyes must be held stationary between movements
(an active process is required to keep them still)
what is the optokinetic reflex (the slow way)?
- A powerful reflex to maintain gaze position driven by motion across the whole retina (i.e., whole field visual motion) - assumes the world is stationary
- Minimises “visual or retinal slip” of the image on the retina to allow us to track moving objects
- Retinal slip stimulates the optokinetic reflex
For example when you are siting on the train, look out the window, and realise the train next to you is starting to move
what is optokinetic nystagmus?
Prolonged optokinetic effects can lead to optokinetic nystagmus (OKN)
This means that there is an alternation of this slow eye movement that follows the motion of the world and the quick eye movement in the opposite direction (saccades)
what does a normal vs abnormal response look like in OKN?
Normal response that stabilises the retinal image when the world drifts past your eyes (e.g., when looking out a train window).
Abnormal response when caused by lesions that abolish positional control of the eyes, or by abnormal vestibular inputs.
a slow mechanism because it relies on integrating vision (LGN + V1) and motion (V5) with brainstem compensatory mechanism
what is vestibular ocular reflex (the fast way)?
A powerful reflex to maintain gaze position despite head movement.
Fast (~14 ms) because only 3 neurons in the brainstem are involved.
how does the vestibular ocular reflex work?
- Head movement detected by semi-circular canals (vestibular/balance system).
- Balance system passes head movement signals to the vestibular nucleus.
- Movement signals cross the midline and innervate the ocular-motor neurons (OMNs), which drive the extra-ocular muscles.
- Movement to the right abducts the left lateral rectus muscle and adducts the right medial rectus muscle.
what does the indirect pathway do during vestibular ocular reflex?
the indirect pathway through the nucleus Prepositus Hypoglossi (PPH) converts the phasic (instant) vestibular input into a tonic signal via a reverberating neural circuit
(direct pathway skips PPH and goes straight to motor neurones)
define vergence
simultaneous movements of both eyes in opposite directions to maintain focus
accommodates different viewing distances
define smooth pursuit
Slow simultaneous movements of both eyes in the same direction.
Evolutionarily new system to track slow-moving objects against a stationary background.
Requires suppression of the optokinetic reflex (OKR).
Is a slow visual “feedback” mechanism because driven by visual motion signals from areas MT & MST (middle temporal & middle superior temporal cortex).
explain saccades thoroughly
simultaneous movements of both eyes in the same direction
allow for very rapid gaze shifts
constant velocity (up to 600/ sec)
need to be fast because vision is degraded during movement
thus, saccades minimise the amount of time the eyes are in motion
how do we make a saccade?
- Saccades are produced using parts of the gaze stabilisation circuitry, with an extra part that tells the eyes where to look.
- Gaze stabilisation circuit controls HOW to move the eyes.
- Saccade generation circuit controls WHERE to look.
- Burst neurons in the PPRF (Paramedian Pontine Reticular Formation) control saccades.
- Saccades can be voluntary (reading) or reflexive (in response to a stimulus).
Reflexive or involuntary saccades are controlled by the Superior Colliculus.
what is the superior colliculus and how does it work?
tiny structure in the midbrain
- Contains a map of the visual scene from the point of the retina (i.e., retinotopic map).
- Performs a sensory-motor transformation i.e., maps visual & auditory inputs to motor outputs.
- Orients the head and eyes toward something seen or heard.
- Can generate eye movements without cortical involvement (fast reflective system via signals from LGN).
- Turns off “omnipause” neurons, releasing the oculomotor system (PPRF) from inhibition (i.e., releases the eyes from fixation).
why are mental maps important?
mental maps provide us with the information from a particular perspective
briefly explain spacial processing
spatial cognition involves two important and different systems
posterior parietal cortex (self-to-object)
hippocampus (object-to-object navigation)
hippocampus is key to for allocentric navigation
how does hippocampal activity relate to navigation?
- London taxi drivers: describe a route – strong activation of right hippocampus (Maguire et al., 1997).
- Greater spatial cognition & navigation lead to increased posterior hippocampal volume.
- Storage and use of “mental maps”.
- Neurogenesis in hippocampus persists into adulthood.
what is neurogenesis in the human hippocampus?
about one third of adult cells are renewed/renewable
~ 700 new neurons per day, for an annual turnover rate of 1.75%
there is also gradual loss of cells that are in a non-renewing majority
they examined the makeup of hippocampal cells in post-mortem slices
the further you go on in your lifetime, the more cell loss outweighs the neurogenesis
what are place cells and how do they relate to hippocampal cells?
- Hippocampal cells demonstrate “place fields” which fire when an animal visits a specific region of its environment.
- When the rat stops, the cells rapidly fire in reverse order.
- Reverse replay suggests a functional role for spontaneous (resting) activity in learning and memory consolidation.
- Place fields provide an internal representation of space or ‘cognitive map’.
