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Session: Sensors and pain
Session starts: Friday 25 January, 10:30
Presentation starts: 11:15
Room: Lecture room 558

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John van Opstal (Radboud University, Dept. Biophysics)
Bahadir Kasap (Radboud University, Dept. Biophysics)

Abstract:
The midbrain superior colliculus (SC) generates saccadic eye movements to sensory stimuli through a population of cells in its topographically organized motor map. Supra-threshold electrical microstimulation in the SC reveals that each stimulation site produces a normometric saccade vector, with little effect of the stimulation parameters. Interestingly, the kinematics of electrically evoked saccades (E-saccades) are indistinguishable from natural, visual-evoked saccades (V-saccades). These findings support models, in which the saccade vector is determined by a center-of-gravity computation of activated neurons, while its trajectory and kinematics arise from downstream feedback circuits. Recent single- unit recordings from our lab, however, indicated that the SC population also specifies instantaneous kinematics. These results support an alternative model, in which the desired saccade trajectory, including its kinematics, follows from instantaneous summation of movement effects of all SC spike trains. But how to reconcile this model with microstimulation results? Although it is thought that microstimulation activates a large population of SC neurons, the mechanism through which it arises is unknown. We developed a spiking neural network model of the SC, in which microstimulation directly activates a relatively small set of neurons around the electrode tip, which subsequently sets up a large population response through lateral synaptic interactions. We show that through this mechanism the population drives an E-saccade with normal kinematics that are largely independent of the stimulation parameters. Only at very low stimulus intensities the network recruits a small cell population with low firing rates, resulting in small saccades with abnormally slow kinematics, as observed in experiments.