Figure 2.

Model of ocular pursuit. The basis of the model is a negative feedback loop in which retinal velocity error is processed by internal dynamics F(s) with variable gain K and a delay (τ_v) of ~80–100 ms. The negative visual feedback is supplemented by extra-retinal input from either a reactive or predictive loop. The input to both reactive and predictive pathways comes from sampling (for ~150 ms) and holding a copy of the reconstructed target velocity signal (T′) in module S/H. The reactive loop can thus sustain eye velocity even if visual input is withdrawn (i.e., if sw1 is opened). The predictive loop includes a more robust short-term memory (MEM), which can hold velocity information over longer periods and during fixation. Both direct and indirect pathways feed out through an expectation-modulated gain control (β < 1) and filter F”(s). In a reactive response, S/H output is fed out directly but is also temporarily stored in MEM. In predictive mode, output of MEM is fed out to form an anticipatory response with timing based on external cues or on the detection of direction changes in the reconstructed target velocity signal and held in the predictive timing store. F″(s) = F′(s) = F(s) = (1 + Te.s)⁻¹. Te = 0.12 s. Non-linear gain function approximated by: K = K₀ (1 + e/e₀)^−0.5, where e = retinal error, e₀ = 4°/s, typically, K₀ ≈ 2.4. For information on putative brain areas (MT, MST, FEF, SEF, PFC, CER, and BG) see section “Allocation of Model Functions to Specific Brain Areas.” Adapted from Barnes and Collins (2011).