THE 3-LOOP MODEL - A NEURAL-NETWORK FOR THE GENERATION OF SACCADIC REACTION-TIMES

This paper presents a computer simulation of the three-loop model for the temporal aspects of the generation of visually guided saccadic eye movements. The intention is to reproduce complex experimental reactio n time distributions by a simple neural network. The operating element s are artificial but realistic neurones. Foul modules are constructed, each consisting of 16 neural elements. Within each module, the elemen ts are connected in an all-to-all manner. The modules are working para llel and serial according to the anatomically and physiologically iden tified visuomotor pathways including the superior colliculus, the fron tal eye fields, and the parietal cortex. Two transient-sustained input lines drive the network: one represents the visual activity produced by the onset of the saccade target, the other represents a central act ivity controlling the preparation of saccades, e.g. the end of active fixation. The model works completely deterministically; its stochastic output is a consequence of the stochastic properties of the input onl y. Simulations show how multimodal distributions of saccadic reaction times are produced as a natural consequence of the model structure. Th e gap effect on saccadic reaction times is correctly produced by the m odel: depending only on the gap duration (all model parameters unchang ed) express, fast-regular, and slow-regular saccades are obtained in d ifferent numbers. In agreement with the experiments, bi- or trimodal d istributions are produced only for medium gap durations (around 200 ms ), while for shorter or longer gaps the express mode disappears and th e distributions turn bi- or even unimodal. The effect of varying the s trength of the transient-sustained components and the ongoing activity driving the hierarchically highest module are considered to account f or the interindividual variability of the latency distributions obtain ed from different subjects, effects of different instructions to the s ame subject, and the observation of express makers (subjects who produ ce exclusively express saccades), How the model can be extended to des cribe the spatial aspects of the saccade system will be discussed as w ell as the effects of training and/or rapid adaptation to experimental conditions.