Figure 11. Schematic model of the topographic decoding and displacement controller for multiple sites of activity within the SC.

The topographic gradients of size and direction across the SC motor map are spatially summed (Σ_s) downstream of the SC. At post-SC locations, the summed-vector activity is diminished by a parametric representation of total SC activity by a currently undetermined mechanism (1/ Σ_s, see text). With near instantaneous dynamics (excluding the response time of OPN pause), the sum of any and all output activity across the SC is dynamically conveyed to a two-dimensional controller of conjugate horizontal and vertical eye position. In the temporal domain (Σ_t) of a down-stream brainstem burst generator, with the OPN locked in mutual inhibition to the output burst neurons (BN), the BN reflect the difference between SC output and a delayed, recurrent representation of BN output (efference copy as a variable time-course “leaky” integrator). The output of the BN is sent to the eye plant, moving the eyes with high velocity. Normally, the SC burst output is declining as the building recurrent inhibition overcomes SC drive, thus ending the movement and disinhibiting the OPN. Under conditions of prolonged experimental SC activation, the recurrent inhibition still overcomes the persistent SC drive, ending the movement at a point when the current displacement matches a normalized representation of current topographic SC output. However, as the recurrent inhibition slowly declines, persistent SC drive can trigger another cycle of movement that is component-hypometric due to residual activity from the feedback inhibition. Normally, only one site of activity across the SC map is active at any one time. Due to volitional uncertainty or experimental intervention, two sites of SC activity can lead to “averaged” and/or curving trajectories of movement. This diagram captures those operations that need to be mapped onto quantitative models for testing of these experimental SC spatio-temporal transformations. Various known and postulated feedforward and feedback loops that would be over-ridden by these experimental manipulations are not represented.