Lecture 7

Question 1

briefly describe eye movement

Answer 1

most common movement

allows us to fixate and track objects

offers a relatively simple example of neural control

Question 2

what are some examples of eye movement problems?

Answer 2

diplopia (double vision)

drift ( nystagmus)

vital for reading (may be one contributor to dyslexia)

Question 3

name three reasons we move our eyes

Answer 3

1. 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
2. to converge the eyes at different distances (towards you =inwards, far away = outwards)
3. to stabilise the visual image on the retina despite motion of the eye (reafferent) or the scene (exafferent)

Question 4

what is gaze stabilisation?

Answer 4

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

Question 5

what are gaze paths (concerning gaze stabilisation)?

Answer 5

the spatial path of the eye as it moves across an image

Question 6

what do the circles and circle size mean when concerning gaze stabilisation?

Answer 6

circles = gaze location

circle size = time spent at a location

Question 7

when reading, does the reader fixate on common words?

Answer 7

NO!

they often skip common words such as ‘and’ and ‘the’

Question 8

how does gaze stabilisation differ when viewing (vs reading)?

Answer 8

• 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

Question 9

what muscles controls pupil diameter that changes with brightness?

Answer 9

intra-ocular muscles

Question 10

what muscles move the eyeball within the socket?

Answer 10

extra-ocular muscles
innervated (supply with nerves) by specific cranial nerves

Question 11

define gaze stabilising mechanisms

Answer 11

old system to make image sharp

Question 12

what is optokinetic reflex (OKR) driven by?

Answer 12

visual motion

Question 13

what is vestibular-occular reflex (VOR) driven by?

Answer 13

head motion system

Question 14

define gaze shifting mechanisms?

Answer 14

new system to scan/track objects

Question 15

what are vergence movements?

Answer 15

to shift focus on near & far objects

Question 16

what is smooth pursuit?

Answer 16

eye movements to fixate on moving objects

Question 17

define gaze fixation

Answer 17

eyes must be held stationary between movements

(an active process is required to keep them still)

Question 18

what is the optokinetic reflex (the slow way)?

Answer 18

• 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

Question 19

what is optokinetic nystagmus?

Answer 19

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)

Question 20

what does a normal vs abnormal response look like in OKN?

Answer 20

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

Question 21

what is vestibular ocular reflex (the fast way)?

Answer 21

A powerful reflex to maintain gaze position despite head movement.

Fast (~14 ms) because only 3 neurons in the brainstem are involved.

Question 22

how does the vestibular ocular reflex work?

Answer 22

1. Head movement detected by semi-circular canals (vestibular/balance system).
2. Balance system passes head movement signals to the vestibular nucleus.
3. Movement signals cross the midline and innervate the ocular-motor neurons (OMNs), which drive the extra-ocular muscles.
4. Movement to the right abducts the left lateral rectus muscle and adducts the right medial rectus muscle.

Question 23

what does the indirect pathway do during vestibular ocular reflex?

Answer 23

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)

Question 24

define vergence

Answer 24

simultaneous movements of both eyes in opposite directions to maintain focus

accommodates different viewing distances

Question 25

define smooth pursuit

Answer 25

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).

Question 26

explain saccades thoroughly

Answer 26

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

Question 27

how do we make a saccade?

Answer 27

* 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.

Question 28

what is the superior colliculus and how does it work?

Answer 28

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).

Question 29

why are mental maps important?

Answer 29

mental maps provide us with the information from a particular perspective

Question 30

briefly explain spacial processing

Answer 30

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

Question 31

how does hippocampal activity relate to navigation?

Answer 31

* 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.

Question 32

what is neurogenesis in the human hippocampus?

Answer 32

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

Question 33

what are place cells and how do they relate to hippocampal cells?

Answer 33

* 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’.