Eye movements are often made over a stationary background, which means that the retinal image contains information about eye velocity. Reference signal models therefore combine extra-retinal, retinal and vestibular information to generate a single compound signal. Since retinal motion generating extra-retinal information is somewhat contradictory, the extra-retinal signal of the classical model is replaced by a reference signal (Wertheim, 1994).
Figure 2.3 presents a schematic for a simple reference signal model, which does not include vestibular information. Vestibular feedback is excluded since the use of a chin rest and forehead bar in the current experiments limited head movement. The nature of the pursuit speed estimate differs between the classical model and the reference signal model. In figure 2.3 it can be seen that the pursuit estimate is a function of both extra-retinal and retinal information. Retinal motion provides information for its own interpretation, and corresponds to a feed-forward signal that forms a referential circularity within the perceptual system (Wertheim, 1994).
Figure 2.3 The Reference Signal Model. To estimate world motion, an estimates of both retinal motion and smooth pursuit eye movement are combined. The purely extra-retinal signal of the classical model is replaced by a reference signal. The reference signal is a function of both retinal motion and the pursuit eye movement.
It should also be noted that the term reference signal is often used interchangeably with a purely extra-retinal signal (e.g. Li, Brenner, Cornelissen and Kim, 2002), with the classical model sometimes referred to as a reference signal in its simplest form (Dash et al, 2009). The two models do not differ when retinal, extra-retinal and vestibular in formation are considered as being processed in parallel - a reference signal, as defined here, differs from the classical model when the separate sources of information are combined into a single signal. The reference signal thereby attains additional properties to those of a purely extra-retinal signal. The magnitude of a reference signal is modulated by constant retinal and vestibular feedback.
The whole-field flow of retinal motion during an eye movement provides a source of information about the eye’s velocity (Crowell & Anderson, 2001). Reference signal experiments often manipulate the background motion during an eye movement to investigate the related changes to a pursuit estimate (Brenner and van den Berg, 1994; Haarmeier & Their, 1996, Haarmeier, Bunjes, Linder, Berret and Thier, 2001). Brenner and van den Berg (1994) investigated the situations under which reliable eye velocity information could be obtained from retinal flow. Under the majority of circumstances they reported no change in perceived velocity whilst the relative motion between a background and pursuit was maintained. When the background motion during an eye movement was manipulated to the...