Spatial Navigation wk7

Question 1

what do eye movements (EMs) allow?

Answer 1

-allow us to fixate and track objects
-offers a relatively simple example of neural control and understanding of cortical-subcortical interactions without gravity compared to limbs that always need to take gravity into account

Question 2

what do problems with EM underlie?

Answer 2

• diplopia (double vision),
• drift (nystagmus)
• vital for reading, contributions to dyslexia

Question 3

reasons for moving the eyes

Answer 3

1. moving across the visual scene allows relevant parts of the image to fall onto high-resolution parts of the retina via fixating
2. to converge the eyes at different distances in a co ordinated manner
3. stabilizes the visual image on the retina despite eye (reafferent) or scene motion (afferent)

Question 4

gaze paths

Answer 4

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

Question 5

gaze stabilisation (reading)

Answer 5

EMs jump between locations instead of constantly moving left to right, for a stable image to fall on the retina

Question 6

why can reading be both automatic and context dependent?

Answer 6

• the ability to skip common words suggests EMs are governed by some top-down control mechanisms
• make predictions based on context and common words of respective language
• ensures relevant words are projected sharply rather than blurry

Question 7

gaze stabilisation (viewing visual screen )

Answer 7

• displays how EMs and image-processing is entirely dependent on context and goals, prioritizing only relevant info
• meaning EMs can be both automatic and volitional/deliberate
• e.g. when we ask to guess a persons age, we look at their face

Question 8

what are the different eye muscle controls?

Answer 8

• intra-ocular muscles control pupil diameter based on brightness, allowing for optimization
• extra-ocular muscles move eyeball within the socket and are innervated by specific cranial nerves
• rectus muscles move the eye along the horizontal and vertical axes
• oblique muscles contribute to rotational movements

Question 9

functional types of eye movement

Answer 9

1. gaze stabilizing mechanisms ( evolutionary system to make image sharp)
   - optokinetic-reflex (OKR) = driven by visual system
   - Vestibular-ocular reflex (VOR) = driven by head motion system
2. gaze shifting mechanisms (new system to scan and track objects)
   - vergence= shifting focus
   - smooth pursuit= fixation of moving objects
   - saccade = rapid movement between fixation
3. gaze fixation (eyes must actively be held stationary between movements)

Question 10

what is the optokinetic reflex (OKR)?

Answer 10

mechanism to stabilise gaze position by detecting motion across the retina, to minimise the ‘slip’ of images along the retina when tracking moving objects.

activated during situations when we perceive the world as stationary but observe motion around us. (retinal slip)

Question 11

Why is the optokinetic reflex (OKR) slow?

Answer 11

• Slow as it relies on a big cascade of complex series of signals that pass through various visual processing regions, e.g., eyes to the LGN and beyond, before integration can occur.

Question 12

stages of the optokinetic reflex (OKR)

Answer 12

brain triggers the OKR upon recognizing visual motion to ensure the scene remains clear and smooth on the retina.

Question 13

optokinetic nystagmus (OKN)

Answer 13

involves alternation of slow drift EM followed by rapid saccades (rapid movement) . Slow process of integrating vision (LGN, V1) and motion (V5) with brainstem.

this is an adaptive mechanism to stabilise retinal images when the world drifts past your eyes.

Question 14

When does the optokinetic nystagmus (OKN) trigger

Answer 14

prolonged optokinetic relfex can lead to the trigger of this.

Question 15

what does abnormal behaviour in the optokinetic nystagmus (OKN) indicate

Answer 15

absence of this reflex indicates damage to the cerebellum or brain stem, responsible for positional control of eyes

Question 16

vestibular ocular reflex (VOR)

Answer 16

rapid mechanism to maintain gaze stability despite head-movement.

process is fast (~14ms) since only 3 neurons in the brainstem are involved, not needing to go past the cortex

Question 17

stages of the verbal ocular reflex (VOR)

Answer 17

1. head movements are detected by the semi-circular canals within the vestibular system, and signals of this movement are relayed to the vestibular nucleus., crossing info to the other side of the body
2. innervates ocular-motor neurons (OMNs) which directly control the extraocular muscles responsible for moving the eyes.
3. so, as signal crossed over , when moving head to right, the VOR induces compensatory EM to the left to create stability

Question 18

PPH neurons in the vestibular ocular reflex circuit

Answer 18

• mechanism that allows for fixation after we moved head, to continue fixation of same location
• also receive info about head motion, connecting to ocular motor neurons to control rectus muscles
• PPH neurons can stimulate themselves via recurrent connections , allowing for maintaining of fixation after motion has stopped
• if weren’t actively excited, fixation would be at natural position (fowards) losing object fixation

Question 19

mechanisms of gaze shifting

Answer 19

vergence
smooth pursuit
saccades

Question 20

vergences

Answer 20

describes the simultaneous movement of both eyes in opposite directions to maintain focus, and accommodate different viewing distances (objects moving towards and away from us)

Question 21

smooth pursuit

Answer 21

• evolutionary new system that allows for slow simultaneous movement of both eyes to fixate on slow-moving objects against a stationary background (e.g. plane)
• slow mechanism that requires voluntary movement and motivation unlike reflexes

requires suppression of OKR (involuntary mechanism that will be induced due to retinal slip) and involves ‘feedback’ driven by visual motion signals from MT/MST.

Question 22

saccade movements

Answer 22

rapid eye movements between fixation to allow for gaze shifts at 600/s velocity.

needs to be fast since it allows us to adapt to rapid changes + vision is degraded during movement making it useless
- as visual system is unstable, brain needs to fill in gaps

Question 23

how are saccades produced?

Answer 23

via gaze stabilisation circuits (controls eye movements) and saccade generation circuit (controls where to look)

Question 24

what controls saccades?

Answer 24

• Burst neurons in the PPRF controls saccades ,initiating eye movements to desired location
• can be either voluntary or reflexive and controlled by the Superior Colliculus (controls volitional eye movements)
• superior colliculus -> PPRF burst neurons-> motor neurons that innovate rectus muscles

Question 25

superior colliculus

Answer 25

Different subregions of the superior colliculus are concerned with different positions on the retina (retinotopic map) processes sensory info into adaptive eye movements turns off omnipause neurons and release PPRF from inhibition and fixation

Question 26

why is the superior colliculus sufficient insaccades

Answer 26

because SC has a lot of different properties , it is sufficient to trigger rapid reflexive eye movements without cortical involvement

Question 27

omnipaise neurons

Answer 27

involved in object fixation - Inhibit premotor burst neurons - Always active expect when you make eye movements and releases burst neurons from inhibition to become active and initiate sudden eye movement

Question 28

maps in navigation

Answer 28

We need different forms of metal maps for effective navigation around the world - Provides us info from a particular perspective

Question 29

allocentric maps

Answer 29

represent our place in the world around us, involved with the hippocampus that contains 'place cells'

Question 30

hippocampus role in allocentric navigation

Answer 30

seen in greater posterior volume of London taxi drivers, correlated with years of experience , due to their increased use and storage of mental maps, and involvement in spatial memory

Question 31

Neurogenesis in hippocampus

Answer 31

- production of new cells within the hippocampus persists into adulthood - though, further you go on in lifetime, the greater the cell loss outweighs the neurogenesis

Question 32

when do place cells activate?

Answer 32

found in the dorsal posterior area of the hippocampus / CA1 specific place cells activate in specific regions as an animal moves through its environment when the animal pauses, they fire in reverse order – known as reverse replay

Question 33

what do place cells create?

Answer 33

cognitive maps which are crucial for memory consolidation and spatial navigation, giving accurate rep of where we are and planning routes of where we want to nav towards

Question 34

What is reverse replay

Answer 34

the brain is replaying the trajectory that the animal took in order to reach a goal state - This is linked the memory consolidation as it happens during sleep too to reach given goals

Question 35

changes in behavioural context influence how cells can respond:

Answer 35

1. rate remapping 2. global remapping

Question 36

rate remapping

Answer 36

= cells maintain spatial preferences but change amplitude across contexts, - Firing more strongly in one compared to the other even though the spatial tuning is the same

Question 37

global remapping

Answer 37

= cells entirely shift their spatial preferences between contexts - Stable spatial tuning in in context but a completely different place field in another context

Question 38

grid cells

Answer 38

place cells receive input from grid cells (found in mainly the entorhinal cortex). these fire in a grid-like pattern across multiple locations periodically to form a regularly spaced pattern across the environment -combine different spatial scales to activate place cells for unique locations (research focused on rodent brains but similar mechanisms in humans)

Question 39

egocentric maps

Answer 39

represent/encodes the position of objects relative to our own body Posterior parietal cortex (PPC) is crucial for egocentric navigation,

Question 40

how is PPC involved in spatial processing?

Answer 40

(dorsal/where) by integrating information across senses

Question 41

apraxia

Answer 41

left parietal lobe damage, involving difficulty in coordinating purposeful sequences of muscle movements + also in speech

Question 42

neglect

Answer 42

right parietal lobe damage, results in difficulty in perceiving and using information about objects in external space to their left - though they know there's a left side, attention is so heavily biased to right they will neglect

Question 43

what is damaged in neglect

Answer 43

Neglect doesn’t affect spatial memory of the square as this seems to be encoded in other parts of the brain - What's diminished is the RETREIVAL of information from an egocentric pov

Question 44

which parietal cortex codes for each VF?

Answer 44

right parietal cortex codes for the LVF and RVF while the left parietal cortex only codes for RVF (as lesions to right doesnt result in neglect)

Question 45

What does the PPC do in an egocentric POV

Answer 45

performs calculations, transforming sensory signals into sensorimotor representations of the body position to create an internal rep of our position in space, egocentrically

Question 46

subdivisions of PPC in the intraparietal sulcus from an egocentric POV

Answer 46

LIP (lateral LIP) MIP (medial) AIP (anterior) VIP (ventral)

Question 47

LIP

Answer 47

responsible for combining visual info with intentions /action plans - receptive field that fires when objects are presented in specific locations and when the animal intends on making an eye movement - silent when something is just passively presented with no action implications

Question 48

MIP

Answer 48

supports reaching, pointing, and updates body representations Damage=Difficulty in reaching for objects in periphery + disconnect between knowledge about them moving their hand and what it feels like (so having the experience their arm is still in the original position)

Question 49

AIP

Answer 49

- controls grasping and hand movements - damage= Unable to transform objects to visual system to initiate appropriate movement

Question 50

VIP

Answer 50

processes space and motion around the head

Question 51

Prospective coding in the hippocampus

Answer 51

- Found in CA1, small proportion of place cells are tuned to prospective goal location rather than goal location - spatial tuning of these cells remaps with reward location

Question 52

prospective coding in hippocampus and associated brain regions

Answer 52

correlation with univariate activity in prefrontal cortex - suggests role in nav planning

Question 53

how is prospective coding implemented in the hippocampus

Answer 53

Suggestion 1= sequential activity (place cells will become active in a sequential way) Suggestion 2= Temporal separation ( same place cell encodes both kinds of info at different time points) Suggestion 3= anatomical separation