Spatial Navigation Flashcards
what do eye movements (EMs) allow?
allow us to fixate and track objects, offering a relatively simple example of neural control and understanding the motor system without gravity
what do problems with EM underlie?
diplopia (double vision), drift (nystagmus), and are vital for reading
reasons for moving the eyes
- moving across the visual scene allows interesting parts of the image to fall onto high-resolution parts of the retina
- to converge the eyes at different distances
- stabilises the visual image on the retina despite eye (reafferent) or scene motion (afferent)
gaze paths
the spatial path of the eye as it moves across an image
gaze stabilisation (reading)
EMs jump between locations, for a stable image to fall on the retina
why can reading be both automatic and context dependent?
the ability to skip common words suggests EMs are governed by top-down control mechanisms and can consider knowledge to shift accordingly
gaze stabilisation (viewing)
displays how EMs and image-processing is entirely dependent on context and goals (Yarbus, 1967), meaning EMs can be both automatic and volitional
what are the different eye muscle controls?
- 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
functional types of eye movement
- gaze stabilising mechanisms (to make image sharp)
- gaze shifting mechanisms (new system to scan and track objects)
- gaze fixation (eyes must actively be held stationary between movements)
what is the optokinetic reflex (OKR)?
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.
stages of the optokinetic reflex (OKR)
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.
optokinetic nystagmus (OKN)
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.
vestibular ocular reflex (VOR)
rapid mechanism to maintain gaze stability despite head-movement.
process is fast (~14ms) since only 3 neurons in the brainstem are involved.
stages of the verbal ocular reflex (VOR)
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.
mechanisms of gaze shifting
vergence
smooth pursuit
saccades
vergences
simultaneous movement of both eyes in opposite directions to maintain focus, and accommodate different viewing distances.
smooth pursuit
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.
saccade movements
rapid eye movements between fixation to allow for gaze shifts at 600/s velocity.
needs to be fast since vision is degraded during movement
how are saccades produced?
via gaze stabilisation circuits (controls eye movements) and saccade generation circuit (controls where to look)
what controls saccades?
Paramedial Pontine Reticular Formation (PPRF) controls saccades, which can be either voluntary or reflexive and controlled by the Superior Colliculus (controls volitional eye movements)
superior colliculus
is sufficient to trigger rapid reflexive eye movements without cortical involvement.
turns off omnipause neurons and release PPRF from inhibition and fixation
burst neurons
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
allocentric maps
represent our place in the world around us
hippocampus is crucial
hippocampus role in allocentric navigation
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
when do place cells activate?
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
what do place cells create?
cognitive maps which are crucial for memory consolidation and spatial navigation
changes in behavioural context influence how cells can respond:
- rate remapping
- global remapping
rate remapping
= cells maintain spatial preferences but change amplitude across contexts
global remapping
= cells entirely shift their spatial preferences between contexts
grid cells
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)
egocentric maps
represent the position of objects relative to our own body
PPC is crucial for egocentric navigation
how is PPC involved in spatial processing?
(dorsal/where) by integrating information across senses
apraxia
left parietal lobe damage, involving difficulty in coordinating muscle movements.
neglect
right parietal lobe damage, results in difficulty in perceiving and using information about objects in external space to their left
which parietal cortex codes for each VF?
right parietal cortex codes for the LVF and RVF
while the left parietal cortex only codes for RVF
subdivisions of PPC in the intraparietal sulcus
LIP
MIP
AIP
VIP
LIP
encodes planned eye movements
MIP
supports reaching, pointing, and updates body representations
AIP
controls grasping and hand movements
VIP
processes space and motion around the head
