1.4 Combining Afferent and Efferent Signals: Models of head centred motion perception.
Sensory signals relating information about our physical movements, as well as information regarding external object motion, are required in order to preserve a stable and accurate view of the world, and estimate external motion. Space constancy is the visual system’s ability to maintain a view of the outside world that does not jump about and move with an eye movement (Deubel, Bridgeman, & Schneider, 1998; Stark & Bridgeman, 1983). A simple way of achieving this is to add the velocity estimates that are derived from afferent and efferent motion signals. The sum of these estimates would result in head-centred motion. For instance, the image on the retina of stationary objects in the world would gain a motion opposite and equal to any eye movement. As suggested above, reafferent retinal motion should provide a velocity estimate of similar magnitude to the efferent estimates of eye movement. If these two estimates are equal to one another, but have opposite sign, then their sum would correctly suggest null motion.
Figure 1.2 shows a simple schematic detailing the combination or retinal information and eye velocity estimates to generate a perceived motion direction. This diagram is analogous to figure 1.1, showing the integration of signals from a lower ‘detector’ level, at the eye movement and retinal velocity estimate level, and later at an integrator stage, after which the motions have been transformed into the perceived direction. This diagram also illustrates the focus of chapters 2, 3 and 4. Chapter 2 investigates the motion aftereffect (see adaptation section) which follows simultaneous retinal motion and repetitive smooth pursuit. Chapter 2 determines the level at which adaptation of retinal and extra-retinal signals occur when both signals are adapted simultaneously. Chapter two provided good evidence for adaptation at a lower level of motion processing, before the signals are integrated to provide a perceived direction. Chapters 3 and 4 investigate the nature of the eye velocity estimate during pursuit. The aim of these chapters is to examine the viability of two separate models, the classical model and the reference signal model. These models respectively suggest that the pursuit estimate is either based on a purely extra-retinal signal or on an extra-retinal signal mediated by retinal feedback.
Figure 1.2: Perceived motion can be derived from internal and external sources of information. Extra-retinal efferent information can detail the physical properties of self motion, as initiated by the central nervous system, whilst our sensory system introduces afferent motion signals.
Whilst defined as two separate models in this thesis, Wertheim (1994) classified classical and reference signal models as one approach, and termed them inferential perception theories. Inferential perception models calculate object and background...