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Michael Arbib: CS564 - Brain Theory and Artificial Intelligence University of Southern California, Fall 2001

Michael Arbib: CS564 - Brain Theory and Artificial Intelligence University of Southern California, Fall 2001. Lecture 22. Saccades 2 Reading Assignments: Reprint

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Michael Arbib: CS564 - Brain Theory and Artificial Intelligence University of Southern California, Fall 2001

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  1. Michael Arbib: CS564 - Brain Theory and Artificial IntelligenceUniversity 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, 2:153-175. • 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 NeurophysiologyPeter Dominey & Michael Arbib: Cerebral Cortex, 2:153-175 • 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 oneof 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 B’ B” B A • 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.

  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

  20. Memory Saccade

  21. 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.

  22. 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.

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