Spatial Navigation

Question 1

what do eye movements (EMs) allow?

Answer 1

allow us to fixate and track objects, offering a relatively simple example of neural control and understanding the motor system without gravity

Question 2

what do problems with EM underlie?

Answer 2

diplopia (double vision), drift (nystagmus), and are vital for reading

Question 3

reasons for moving the eyes

Answer 3

1. moving across the visual scene allows interesting parts of the image to fall onto high-resolution parts of the retina
2. to converge the eyes at different distances
3. stabilises 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 moves across an image

Question 5

gaze stabilisation (reading)

Answer 5

EMs jump between locations, 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 top-down control mechanisms and can consider knowledge to shift accordingly

Question 7

gaze stabilisation (viewing)

Answer 7

displays how EMs and image-processing is entirely dependent on context and goals (Yarbus, 1967), meaning EMs can be both automatic and volitional

Question 8

what are the different eye muscle controls?

Answer 8

• intra-ocular muscles control pupil diameter
• extra-ocular muscles move eyeball within the socket and are innervated by 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 stabilising mechanisms (to make image sharp)
2. gaze shifting mechanisms (new system to scan and track objects)
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.

Question 11

stages of the optokinetic reflex (OKR)

Answer 11

brain triggers the OKR upon recognising visual motion to ensure the scene remains clear on the retina.

process is relatively slow and relies on a 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

optokinetic nystagmus (OKN)

Answer 12

involves alternation of slow drift followed by rapid saccades, caused by prolonged OKR. 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 13

vestibular ocular reflex (VOR)

Answer 13

rapid mechanism to maintain gaze stability despite head-movement.

process is fast (~14ms) since only 3 neurons in the brainstem are involved.

Question 14

stages of the verbal ocular reflex (VOR)

Answer 14

head movements are detected by the semi-circular canals within the vestibular system, and signals of this movement are relayed to the vestibular nucleus.

innervates ocular-motor neurons (OMNs) which directly control the extraocular muscles responsible for moving the eyes.

Question 15

mechanisms of gaze shifting

Answer 15

vergence
smooth pursuit
saccades

Question 16

vergences

Answer 16

simultaneous movement of both eyes in opposite directions to maintain focus, and accommodate different viewing distances.

Question 17

smooth pursuit

Answer 17

slow simultaneous movement of both eyes to fixate on slow-moving objects.

requires suppression of OKR and involves ‘feedback’ driven by visual motion signals from MT/MST.

Question 18

saccade movements

Answer 18

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

needs to be fast since vision is degraded during movement

Question 19

how are saccades produced?

Answer 19

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

Question 20

what controls saccades?

Answer 20

Paramedial Pontine Reticular Formation (PPRF) controls saccades, which can be either voluntary or reflexive and controlled by the Superior Colliculus (controls volitional eye movements)

Question 21

superior colliculus

Answer 21

is sufficient to trigger rapid reflexive eye movements without cortical involvement.

turns off omnipause neurons and release PPRF from inhibition and fixation

Question 22

burst neurons

Answer 22

involved in breaking object fixation to initiate sudden eye movements.

under inhibitory control of omnipause neurons, which are inhibited during eye movements, enabling motor commands to produce rapid movement at burst neurons

Question 23

allocentric maps

Answer 23

represent our place in the world around us

hippocampus is crucial

Question 24

hippocampus role in allocentric navigation

Answer 24

seen in greater posterior volume and neurogenesis of London taxi drivers (Maguire, 1997) due to their increased use and storage of mental maps, and involvement in spatial memory

Question 25

when do place cells activate?

Answer 25

place cells (found in the Hippocampus) 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 26

what do place cells create?

Answer 26

cognitive maps which are crucial for memory consolidation and spatial navigation

Question 27

changes in behavioural context influence how cells can respond:

Answer 27

1. rate remapping
2. global remapping

Question 28

rate remapping

Answer 28

= cells maintain spatial preferences but change amplitude across contexts

Question 29

global remapping

Answer 29

= cells entirely shift their spatial preferences between contexts

Question 30

grid cells

Answer 30

place cells receive input from grid cells (found in the entorhinal cortex).

these fire in a grid-like pattern across multiple locations, and different resolutions combine to provide precise spatial tuning information to place cells

(research focused on rodent brains but similar mechanisms in humans)

Question 31

egocentric maps

Answer 31

represent the position of objects relative to our own body

PPC is crucial for egocentric navigation

Question 32

how is PPC involved in spatial processing?

Answer 32

(dorsal/where) by integrating information across senses

Question 33

apraxia

Answer 33

left parietal lobe damage, involving difficulty in coordinating muscle movements.

Question 34

neglect

Answer 34

right parietal lobe damage, results in difficulty in perceiving and using information about objects in external space to their left

Question 35

which parietal cortex codes for each VF?

Answer 35

right parietal cortex codes for the LVF and RVF

while the left parietal cortex only codes for RVF

Question 36

subdivisions of PPC in the intraparietal sulcus

Answer 36

LIP
MIP
AIP
VIP

Question 37

LIP

Answer 37

encodes planned eye movements

Question 38

MIP

Answer 38

supports reaching, pointing, and updates body representations

Question 39

AIP

Answer 39

controls grasping and hand movements

Question 40

VIP

Answer 40

processes space and motion around the head