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L. Itti: CS564 - Brain Theory and Artificial Intelligence University of Southern California

L. Itti: CS564 - Brain Theory and Artificial Intelligence University of Southern California. Lecture 16. Saccades 2 Reading Assignments: The NSL Book The Modular Design of the Oculomotor System in Monkeys Peter Dominey, Michael Arbib, and Amanda Alexander Supplementary Reading:

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L. Itti: CS564 - Brain Theory and Artificial Intelligence University of Southern California

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  1. L. Itti: CS564 - Brain Theory and Artificial IntelligenceUniversity of Southern California • Lecture 16. Saccades 2 • Reading Assignments: • The NSL Book • The Modular Design of the Oculomotor System in Monkeys • Peter Dominey, Michael Arbib, and Amanda Alexander • Supplementary Reading: • Crowley-Arbib Saccade Model • M. Crowley, E. Oztop, and S. Marmol

  2. Filling in the Schemas: Neural Network Models Based on Monkey NeurophysiologyPeter Dominey & Michael Arbib: Cerebral Cortex, 2:153-175 Develop hypotheses onNeural Networksthat yield an equivalent functionality: mappingschemas (functions)to the cooperative cooperation of sets ofbrain regions (structures)

  3. Last time, we saw that… • Double-saccade experiments suggest direction/amplitude coding rather than absolute target location • Lesion/stimulation studies suggest that the overall system still works when either SC or FEF is missing (but not both!) • FEF stimulation just after presentation of a visual target (SC lesioned) elicits a saccade towards the “fake” FEF target first

  4. Experimental Findings • connection FEF  PP • FEF, PPC  SC • SC  saccade generator (SG) • FEF  BG (CD and SNr)  SC (role in disinhibition of SC for saccades) • Simple saccade: study topographic relations between sensory and motor areas • Memory saccade: study cortical and subcortical activity that sustains spatial memory • Double saccade: study dynamic remapping of target location with intervening eye movements

  5. Basic Model Element: Layer 2D array of neurons topographic correspon- dence from layer to layer external world: 27x27 array model retina: 9x9 layer; so, each model neuron represents a small population of biological neurons

  6. 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)

  7. 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

  8. 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!

  9. 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

  10. Double Saccade Experiment + time

  11. 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

  12. 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

  13. 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

  14. 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

  15. The Full Dominey Model • DCEP-Damped Change in Eye Position • 7a/LIP-Oculomotor Region of Posterior Parietal Cortex

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