Abstract
The midbrain superior colliculus (SC) is a crucial sensorimotor interface in the generation of rapid saccadic gaze shifts. For every saccade it recruits a large population of cells in its vectorial motor map. Supra-threshold electrical microstimulation in the SC reveals that the stimulated site produces the saccade vector specified by the motor map. Electrically evoked saccades (E-saccades) have kinematic properties that strongly resemble natural, visual-evoked saccades (V-saccades), with little influence of the stimulation parameters. Moreover, synchronous stimulation at two sites yields eye movements that resemble a weighted vector average of the individual stimulation effects. Single-unit recordings have indicated that the SC population acts as a vectorial pulse generator by specifying the instantaneous gaze-kinematics through dynamic summation of the movement effects of all SC spike trains. But how to reconcile the a-specific stimulation pulses with these intricate saccade properties? We recently developed a spiking neural network model of the SC, in which microstimulation initially activates a relatively small set of (~50) neurons around the electrode tip, which subsequently sets up a large population response (~5,000 neurons) through lateral synaptic interactions. Single-site microstimulation in this network thus produces the saccade properties and firing rate profiles as seen in single-unit recording experiments. We here show that this mechanism also accounts for many results of simultaneous double stimulation at different SC sites. The resulting E-saccade trajectories resemble a weighted average of the single-site effects, in which stimulus current strength of the electrode pulses serve as weighting factors. We discuss under which conditions the network produces effects that deviate from experimental results.
Highlights
Superior ColliculusBecause high spatial resolution is limited to the central fovea, the primate visual system needs to explore the environment through rapid and precise saccadic eye movements
We show that linear dynamic ensemble-coding with lateral excitatory-inhibitory interactions in the motor map can account for most of the experimental vector-averaging results to double stimulation [9, 20, 35], without the need for additional computational nonlinearities, such as a downstream population center-of-gravity computation [20, 21, 34], or a spike-counting cut-off threshold [13, 39, 40]
Synchronous double stimulation in a spiking neural network model of the superior colliculus (SC) with Gaussian excitatory-inhibitory interactions results in saccade responses that display many of the features that have been reported in electrophysiological studies [9, 25, 34]: when the electrodes were located on an iso-direction line (v = constant) the resulting saccade amplitudes were a weighted average of the individual stimulus effects, with the current strengths acting as weighting parameters (Figures 5– 8)
Summary
Because high spatial resolution is limited to the central fovea, the primate visual system needs to explore the environment through rapid and precise saccadic eye movements. We implemented a simple spiking neural network model for the SC that can generate realistic saccades to visual targets [23] This minimalistic (one-dimensional) model with lateral excitatory-inhibitory interactions among the SC cells accounts for most of the experimentally observed firing properties of saccade-related neurons in the motor map [8, 13], and yields saccades with normal main-sequence properties. We show that linear dynamic ensemble-coding with lateral excitatory-inhibitory interactions in the motor map can account for most of the experimental vector-averaging results to double stimulation [9, 20, 35], without the need for additional computational nonlinearities, such as a downstream population center-of-gravity computation [20, 21, 34], or a spike-counting cut-off threshold [13, 39, 40]. We discuss to what extent the model’s responses deviate from experimental findings, and suggest some further refinements to the model
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
