1. Lecture 22. Saccades 2 Reading Assignments: Reprint
Michael Arbib: CS564 - Brain Theory and Artificial Intelligence University of Southern California, Fall 2001
Lecture 22. Saccades 2
Reading Assignments:
Reprint
Dominey, P. F., and Arbib, M. A., 1992, A Cortico-Subcortical Model for Generation of Spatially Accurate Sequential Saccades, Cerebral Cortex, 2:
The NSL Book
The Modular Design of the Oculomotor System in Monkeys
Peter Dominey, Michael Arbib, and Amanda Alexander
Supplementary Reading in the NSL Book:
Crowley-Arbib Saccade Model
M. Crowley, E. Oztop, and S. Marmol

2. Experimental Setup

3. Three Types of Saccade

4. Filling in the Schemas: Neural Network Models Based on Monkey Neurophysiology Peter Dominey & Michael Arbib: Cerebral Cortex, 2:
2D arrays of neurons
topographic correspondence from layer to layer
external world: 27x27 array
model retina: 9x9 layer
each model neuron represents a small population of biological neurons

5. Experimental Protocols and Simulation Interfaces
An experimental protocol defines a class of experiments based on:
Preparation used
External stimuli
Experimental manipulations : electrical stimulation, drug application, etc.
Measurements and observations
The protocol can be translated into a simulation interface
for “stimulating” a simulation and displaying the results. a a a a a a

6. Experiment - Double Saccade

7. Double Saccade -- Neural Array Activity Snapshot

8. Brain Stem Saccade Burst Generator
LLBN-Long Lead Burst Neurons, MLBN-Medium Lead Burst Neurons, EBN-Excitatory Burst Neurons, PN-Omni-Pause Neurons, RI-Resettable Integrator, TN-Tonic Neurons, MN-Oculomotor Neurons.
In the present Chapter/Lecture, this is treated as an unanalyzed module (available from the NSL Library) so that attention may focus on the other brain regions, which are described next.

9. Another View of the Dominey Model
SC - Superior Colliculus: The midbrain’s path from vision (and other senses) to eye movement.
FEF - Frontal Eye Field: The cortex’s path from vision to eye movement.
Data point: A cat or monkey can still make saccades if one of SC or FEF is lesioned.
DCEP-Damped Change in Eye Position
7a/LIP-Oculomotor Region of Posterior Parietal Cortex

10. Visual Input At every iteration, eye position determines position
of 9x9 retinal window within
27x27 outside world
if eye velocity over 200deg/sec,
retinal input is reduced
(saccadic suppression)

11. Direct connection retinaSC
To superficial layer of SC (vs)
responsible for reflex saccades = short-latency saccades
to target which has not been recently fixated

12. visual pre-processing
LGN, V1, V2, V4 and MT areas
abstracted by a single layer
possible only because we have a
very coarse (9x9) retinal input
with no image noise!

13. Dynamic Remapping
Initially, targets A and B are represented on PPv; then as a saccade is made to A, the representation of B is shifted via B’ to B’’ which is related to the foveal point as B was related to A. B B’ B” A

14. quasi-visual cells in PP
Andersen et al. (1988) found
in PP cells that code for future
eye movements.
Quasi-visual because in
double-saccade task
found cells which fire at location
of second target respective
to first target, while there never
was a retinal stimulus there!
right movement field but wrong receptive field

15. Remapping Hypothesis: occurs primarily in PP
(in reality, may occur in many regions at once,
with connections between regions serving for fine-tuning).
problem: eye velocity signals have not been found in PP.
but eye position signals have 
Dominey and Arbib’s computational hypothesis: remapping is done such as to compensate for difference between current eye position, and a damped/delayed eye position signal

16. Basic structure for
Dynamic Remapping

17. frontal eye fields Bruce & Goldberg (1984): FEF contains:
visual cells (vm)
(receive input from PP)
movement cells (ms)
(fire just before saccade)
visuomovement cells (sm)
(memory: fire during delay
in memory saccade task)
postsaccadic cells
PPctr: active as long
as fixation cross present
(inhibits eye movements) FOn = fixation is on

18. superior colliculus Input from retina (reflex saccades)
Input from PPqv  SC qv layer
(yield saccades when FEF
lesioned)
Inputs from FEF
How can we choose?
WTA array: saccade to
currently strongest target

19. Basal Ganglia SNr provides tonic inhibition of SC and thalamus, unless
prevented to do so by FEF
(directly or via CD)
Goals:
prevent saccades while
a target is being fixated
memorise location of
future target in memory
saccade task
FEF can selectively control the targets for saccades, overriding collicular attempts to initiate saccades to distracting peripheral targets

23. Memory Saccade

29. Timing Diagram for Lesioning of SC Experiment
"visinP3M3" is the stimulus for the first target , "fixation" is the fixation timing, "verticalTheta" is the vertical eye movement response, and "horizontalTheta" is the horizontal eye movement response.

30. Compensatory Saccade: Stimulation of FEF with Lesioning of SC
A visual target is briefly presented and removed before a saccade can begin. Before the visual saccade can begin, an electrical stimulus is applied to the FEF. The monkey will first saccade to the stimulated location and then to the real target even though timewise the real target appeared first. This is due to the fact that the the visual signal takes time to get from the retina to the FEF.