107B Section 8                                              TA: Flavia Filimon

VOR, OKN, SUPERIOR COLLICULUS, COORDINATE TRANSFORMATIONS

Stabilizing eye movements:

· VOR: Vestibulo-ocular reflex:

- e.g. when moving head while fixating an object – compensates for head movement and counter-rotates the eyes so you keep focusing on the same object

- VOR cancels out head rotations

· OKN: optokinetic nystagmus:

- tracking moving visual world for a while, then saccading back and tracking again, etc. E.g.: tracking train cars of a moving train

- pigeon OKN: head remains stable for a period of time during walking, then a head saccade is made to keep up with the moving body à allows pigeon to get more focused snapshots of the world rather than a continuously blurry image

- OKN cancels the average movement of the world.

· Smooth pursuit:

- tracking a small object in the world – the background is constantly moving on the retina

- completely cancels out the OKN system (the moving world)

Saccadic eye movements & the Superior Colliculus

Saccade = rapid eye movement (20-40 ms) towards something new

Superior colliculus:

· has superficial, intermediate, and deep layers;

· superficial layer: retinotopic map of the visual field

· deep layer: motor map of saccades (specifies vector of eye movement)

· the receptive field of neurons in the superficial layer is lined up with the spatial location towards which a saccade will be made if you stimulate the neurons directly underneath each respective visual neuron

· stimulating a particular location in the deep layer will lead to a saccade with a particular vector length and angle in the visual field

· superior colliculus and the FEF (frontal eye fields) are functionally equivalent: if you stimulate either, you get an eye movement

· if you stimulate twice: you get the vector sum of the two saccades

Saccades to multiple targets:

- after saccading to the first target, the superior colliculus updates the visual coordinates of the second target by moving it parallel relative to the first target

(see figure)

- the intermediate layers store the representation of targets that are no longer visible

- both the superficial and the deep layers display the “correct” (updated) activity corresponding to where the second target would be after a saccade to the first target

- similar updating going on in LIP (lateral intraparietal area)

Auditory and visual coordinates in the superior colliculus:

(see figure)

- a light and a speaker that are aligned (in the same spatial location) in the (e.g.) right visual field while the monkey is fixating ahead are represented in the same part of the superficial and middle layers of the SC, respectively

- when the eyes fixate elsewhere, with the light and speaker’s position remaining unchanged, the new visual representation now corresponds to the new visual location of the light; the auditory activity representing the speaker is remapped to match the new visual coordinates, despite the fact that the ears haven’t moved (whereas the eyes have)

- in other words: eye position remaps auditory location representation in the superior colliculus

Coordinate transformations in VIP (ventral intraparietal area):

- VIP neurons have both somatosensory (on the face) and visual receptive fields

- The somatosensory and visual RFs are lined up (e.g. if somatosensory RF is on the chin, the visual RF will be just below the mouth, in front of the face.

- If the animal changes eye position by e.g. looking up, the visual RF does not move with the eyes – instead, it is shifted/remapped to still line up with the somatosensory RF (i.e. it is still in front of the face, below the mouth)

- à VIP updates coordinates in face-based coordinates (visual map is moved to match the somatosensory map) – disregards eye position; only cares about face/head-relative position

- contrast this with the SC, where the auditory map is moved to match the visual map, i.e. the visual RF stays the same (relative to eye position).

Remaining MOTOR SYSTEM

- mammals have finer control of movement than reptiles

- reticular formation and vestibular nuclei: have control over “extensors” (antigravity muscles) – e.g. standing up on feet

- red nucleus pathway: controls “flexor” muscles: e.g. bringing in limbs (non-antigravity). In primates, however, extensors (antigravity muscles) in the arms actually correspond to bringing in the arms, because lifting oneself up is an antigravity movement.