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. 2003 Oct 29;23(30):9742–9751. doi: 10.1523/JNEUROSCI.23-30-09742.2003

Figure 6.

Figure 6.

Feedforward model of the VOR. A, Schematic of the classical feedforward realization for the VOR adapted from the original proposal of Skavenski and Robinson (1973). The circles denote summing junctions that are used to represent particular cell populations, whereas the boxes are dynamic elements. Parameters associated with signal projections (a–d) are gain elements that indicate the strength or weight of the projection. Angular head velocity, Ḣ, sensed by the semicircular canals, C(s) = Tcs/(Tcs + 1), is conveyed to motor neurons (MN) that drive the eye plant, EP(s)=1/(Tps+1), via two parallel pathways: (1) a direct pathway via vestibular neurons (VO and EM); and (2) an indirect pathway via a leaky neural integrator [1/(TIs + 1), where TI is a very long time constant of ≈20 sec] (Robinson, 1981). Vestibular neurons include vestibular-only cells (VO) and EM, including FTN cells that are key interneurons in the shortest-latency disynaptic pathways. EM cells sum both sensory head velocity signal inputs and an internal estimate of eye position, E*, provided at the output of the neural integrator. Under normal-gain conditions, the model parameters are: a = 0.12; b = 0.1; c = 17.38; d = 1; TI = 20; Tp = 0.25; Tc = 6. The bode plots in B show VOR gain and phase as a function of frequency for the normal-gain condition (solid curves) and for simulated adapted gain states when the only site for plasticity was assumed to be associated with the sensitivity of cell EM to primary afferent inputs (i.e., weight a). The dotted and dashed traces show the reflex frequency response when weight a was modified to achieve either a doubling (a = 0.34) or halving (a = 0.01) of the high-frequency VOR gain. C, Modifications of both a and c to simulate plasticity in both the direct and indirect pathways give rise to a doubling (a = 0.34; c = 34.76) or halving (a = 0.01; c = 8.69) of the VOR gain over a broad frequency range. D, Feedforward model schematic extended to incorporate interconnections between eye-movement-sensitive brainstem neurons (EM1, EM2) and cerebellar neurons, represented by Purkinje cells (P). Downward head and eye deviations are considered positive. X, Postulated sites for plasticity (see Results).