Role of reafferent mechanisms in predictive interactions between head and eye movement.

Making smooth pursuit eye movements to track moving objects, such as a flying bird, is a familiar everyday activity. The process involves matching the speed of the eye to that of the target, thereby minimizing movement of the image on the retina and reducing blur. Such anticipatory movements are essential to allow us to overcome delays in visual processing, which otherwise impair tracking ability. Previously, we have shown how such anticipatory movements may be generated by using repeated presentations of identical motion stimuli. This creates a high expectancy of impending target motion and allows smooth anticipatory movements to be developed. The velocity of these anticipatory eye movements is proportional to future target velocity, implying that working memory is used to temporarily store velocity information gleaned from prior experience. Similar anticipatory tracking movements of the head have been found when tracking with head and eyes together.
All our previous work in this area has been confined to tracking simple movements along the horizontal axis. In the proposed experiments we plan to investigate how the system operates in more realistic conditions where objects move horizontally, vertically and toward or away from the observer. We will investigate how the brain develops anticipatory control of head and eye movements. We will also determine which areas of the brain are involved in the storage and generation of anticipatory movements by using functional magnetic resonance imaging (fMRI) to record brain activity as human subjects carry out predictable and non-predictable tracking tasks with the eye. In particular, we intend to investigate brain activity when tracking a sequence of such movements in which each component has different speed and direction. Sequence learning is a simple form of motor programming that allows subjects to learn repetitive sequences of movements, such as typing. We have found previously that subjects can learn to generate anticipatory movements to sequences during simple horizontal movement after only one or two presentations, but we expect that when the sequences involve both horizontal and vertical movements more repetitions will be required, because this places greater demands on working memory.
Generating anticipatory activity is fundamental to normal motor behaviour and is often disrupted in patients with motor disorders such as Parkinson?s disease or cerebellar degeneration. Findings from the proposed experiments will provide greater understanding of motor control and thus lead to improvements in diagnosis, monitoring of drug treatment and rehabilitation.

Tracking moving objects with the eye is a familiar everyday activity. It invokes generation of smooth pursuit eye movements that match target motion, thereby minimizing image movement on the retina. Pursuit relies on visual feedback for its initiation, but delays in sensory processing require predictive control to be adopted during pursuit maintenance. Previously we showed that predictive ability is dependent on short-term storage of visual motion and timing information. Earlier experiments investigated basic predictive processes but were restricted mostly to horizontal movements. We now plan to move this study forward by investigating more realistic conditions of object motion in 2D or 3D. We will also consider interactions that take place with vestibular output when tracking moving targets with combined head and eye movements and when the head undergoes linear motion in relation to stationary objects. The investigation will focus on an unresolved issue concerning the contribution of internally-driven predictive control during sustained pursuit, which is thought to be generated by reafferent feedback processes. Hitherto, it has been possible to examine prediction in terms of anticipatory activity for only a brief period before visual feedback takes over. Using a new presentation technique we show evidence that this anticipatory activity continues after visual feedback, but develops much more slowly than the visually?driven component. We will use this technique to investigate behaviour in five projects, in two of which we will also use fMRI to examine brain activity. The first will be a verification of the method and investigation of activity during head-free pursuit. Subsequently we will investigate how the visually- and internally-driven components contribute to prediction in pursuit of complex motion patterns and to the transfer of pursuit from one moving object to another. Finally, we will seek evidence of similar processes when pursuing objects that move in depth and laterally, invoking combined vergence and pursuit responses.
These investigations will be conducted on human volunteers using non-invasive techniques for eye and head movement recording. Imaging experiments will be carried out in a 3T Philips Achieva scanner and will use both event-related and blocked designs. Behavioural experiments will also be presented in appropriately randomized designs. Results of experiments will be published in high-quality scientific journals and presented at national and international conferences.
Prediction is fundamental to normal motor behaviour and often disrupted in movement disorders (e.g. Parkinson?s disease). Findings from the proposed experiments have the potential to improve understanding of such disorders.