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, 2:153-175. 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

Experimental Setup

Three Types of Saccade

Filling in the Schemas: Neural Network Models Based on Monkey Neurophysiology Peter 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

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

Experiment - Double Saccade

Double Saccade -- Neural Array Activity Snapshot

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.

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

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)

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

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!

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

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

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

Basic structure for Dynamic Remapping

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

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

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

Memory Saccade

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.

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.