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NROC64H3 Final: Final Exam Notes

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Matthias Niemeier
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Spatial Stability and Active Vision
The 4 pathways:
1. MD pathway:
SC (intermediate laers) MD (medial dorsal thalamus) FEF (frontal cortex)
Conveys efference copy (synthetic sensory signal) from motor command
a. Used for motor control via internal feedback (NOT by MD pathway) to send signal before
the actual saccade is performed, to accurately stop the saccade on the correct target
b. Used to provide visual stability (via MD pathway) to account for spatial changes during eye
- Used orthodromic (SC MD) stimulation and antidromic (FEF MD) to determine a
functional connection between SC and FEF
- Then, observed a RF shift to a future field before the actual saccade; however, no future
field when MD pathway is inhibited
2. PI pathway:
SC (superficial layers) PI MT
Induces omnidirectional inhibition of cortex 60ms before saccade to eliminate retinal blur; eliminates
distracting vision during saccades
About 10ms after the saccade, the brain interprets the motion in the opposite direction to counteract
the blur that would have been produced if it didn't
3. PL/PDM pathway:
SC (intermediate and superficial layers) PL and PDM (lateral pulvinar) parieto-occipital cortex
Based on motor theory of attention:
Strict link between covert attention (orienting attention) and overt attention (programming
ocular movements)
Oculomotor program for moving the eyes shifts attention
Attention without moving eyes is achieved by inhibiting eye movement
Change blindness task: Make monkey focus on middle dot and change direction of surrounding
dots to see if monkey can detect the change in motion in the surrounding dots
Researchers then stimulate SC to subthreshold levels and observed that monkeys can still
detect the change
Suggests that subthreshold stimulation promotes covert attention without overt eye
4. TRN pathway:
Retina projects to LGN, which projects to TRN (then inhibiting LGN) which projects to cortex
TRN and LGN are mutually inhibitory
Attention increases activity in the LGN, but decreases activity in the TRN
* Parallels between mechanisms for corollary discharge and attention:
The burst preceding a saccade is crucial for the CD that compensates for displacement of the visual scene
caused by a saccade, whereas the presaccadic build-up activity in the SC is probably most important for visual
spatial attention because it could alter visual processing long before a saccade is generated
Contributions from pressure receptors:
To maintain an upright position, our center of gravity must be within the support area (relatively small for
Positive supporting reaction: pressure R’s i legs sese ore pressure i oe leg tha the other and
automatically extend/shorten to maintain upright position (tonic response)
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Extensor thrust: pressure R’s i legs sese ore pressure i oe leg tha the other; ad e thrust from the
direction in which we are falling to maintain balance (transient response)
Stepping reaction: we can also step in the direction that will re-adjust our support area
Vestibular contributions:
1. Static (tonic) Reactions:
Driven by otolith organs to maintain upright head position
Head righting reflex: changing tone of neck muscles in response to changing of body position to
Static vestibulo-ocular reflex: counter-rolling of the eyes (of only a few degrees) when head is tilted to
maintain normal attitude of the eyes
Tonic postural vestibular reflex: extension of front limbs and retraction of the rear when animal is facing
nose-downward (same pattern for other directions); this assists the positive supporting reflex by shifting
COG backwards
2. Dynamic reactions:
Driving by semicircular canals when the head is moving; velocity sensitive thus responding more quickly
VOR: sawtooth pattern; quick and slow phases to maintain eyes in stationary position relative to the
outside world; eyes move in opposite direction of head with exact velocity of head movement
Postural vestibular reflex: functionally equivalent to the static ones (head moving down gives front-leg
extension, and so on), but much more powerful
Visual contributions:
Static responses: visual cues about upright (from objects around you)
Dynamic responses: optic flow (responding to the movement of the retinal image across the retina)
Global motion suggests the eye moves
Illusory sense of rotation
Optokinetic nystagmus is an example
Efference copy is sent to the visual system to provide an estimate of eye position relative to the head
Eye-tapping experiment supports this
Paralysis of eye muscles support this (causes illusory movement of visual world in opposite direction)
Vestibular and visual interactions:
Vestibular lesions produce false sense of spinning; however, vision is able to overcome this with practice
(recalibration); vestibular system can use parametric feedback to be calibrated by signals from the eye
Motion sickness occurs when our vestibular system tells us e’re oig, ut our isual syste tells us e’re
stationary, and vice versa
Proprioceptors in the neck tell us position of the head relative to the body
Both vestibular and proprioceptive neck reflexes work complementary to each other to allow any head position
possible without producing inappropriate postural responses
It is exactly analogous to the mechanism described earlier that permits us to move our eyes around
without at the same time perceiving apparent shifts of the visual world
Pusher Syndrome:
Patients with one side paralyzed are observed to push themselves away from the non-paralyzed side towards
the paralyzed side
Altered sense of upright position of body in relation to gravity
Separate pathway in humans for sensing the orientation of gravity apart from the one for orientation perception
of the visual world
a. Either same sensory input sent to different graviceptive systems
b. Or different input sources
Cerebellum, BG, Frontal Cortex:
Cerebellar cortex deep cerebellar nuclei brain stem structures
Vermis: ventromedial pathways; pathways for trunk and gait
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