Lateral spread of neural excitation during microstimulation in intermediate gray layer of cat's superior colliculus

Abstract

1. Synaptically mediated spread of excitation has been studied during microstimulation in the intermediate gray layer of the superior colliculus of cats anesthetized with ketamine. Antidromic activation was used to identify those neurons sending axons to or through the contralateral pontine reticular formation. 2. Current thresholds were dependent on pulse duration, train length, and distance from the cell to the stimulus site. Stimulation with four cathodal pulses, 0.5 ms in duration, 30 microA in intensity, delivered at 400 Hz, excited cells of the intermediated gray up to 2-3 mm from the stimulus site. The results suggest that at least half the output cells in a region about 3 mm in diameter were driven by this stimulus. 3. The extent of spread in the unanesthetized midpontine-pretrigeminal cat was as great as, or greater than, that in animals anesthetized with ketamine. 4. Quick eye movements were evoked in ketamine-anesthetized cats by 100-ms trains of 0.5-ms pulses delivered at 400 Hz. Current thresholds for eye movements ranged from 15 to 90 microA, with most falling below 25 microA. 5. These results suggest that intracollicular microstimulation, with stimuli commonly used in studies of electrically evoked saccades, is accompanied by widespread synaptic activation of the intermediate gray layer. Since the metrics of electrically evoked saccades seem, nonetheless, to depend primarily on the location of the stimulating electrode, information about amplitude and direction must somehow be encoded in the distribution of neuronal discharge. 6. One possible form of such a distributed coding mechanism is discussed. This model assumes that the spatial densities of cells projecting to vertical and horizontal pulse generators of the saccadic system vary systematically beneath the retinotopic collicular map. Signals to the pulse generators change in magnitude as the collicular discharge zone occupies different positions in the connectional gradients and engages the specific output systems in varying proportion.