Oculormotor control

We have shown previously that pursuit eye movements are predictive of occluded object trajectories, and that concurrent upper limb movement can facilitate this predictive process. More recently, we have shown that prefrontal cortex networks reorganise depending on the availability of retinal and extra-retinal input during a pursuit task.

However, the effect of performing concurrent upper limb movement was less than expected, which could in part be due to the use of discrete, short duration externally-generated object motion.

In addition, our imaging was limited to prefrontal cortex, and thus did not consider the contribution of posterior parietal cortex and frontal eye fields, both of which are active during smooth pursuit, and play an important role in eye-hand coordination.

These regions are part of the dorsal attention network (DAN), which is involved in overt and covert spatial attention, and thus linked with the fronto-parietal network (FPN), which is involved in cognitive processes such as working memory during goal-directed tasks.

Our current work investigates cortical activity and network organisation within a large number of cortical areas known to be involved during pursuit eye movements, as well as being involved in eye-hand coordination.

For this purpose, we are using a functional near-infrared spectroscopy system from NIRx, which comprises a 24x24 optode array. This results in a total of 50 long distance channels, enabling us to image 7 regions of interests in each hemisphere; prefrontal (MPFC, DLPFC), frontal (FEF) and parietal (IPL, SPL) and visual cortex (VC).

In addition, we have included 8 short distance channels, allowing us to regress out superficial physiological noise and isolate true cortical activation. It is expected that insights from our work with neurotypical young adults will inform future research with populations (e.g., normal ageing; autism, peripheral neuropathy) who have different sensorimotor processes underpinning eye-hand coordination.

Smooth pursuit eye movements allow us to track moving objects, but when the object is briefly occluded, the brain must rely on prediction rather than visual input.

Earlier work showed that when participants generate the movement themselves (self‑generated motion), additional sensorimotor signals help maintain pursuit during occlusion.

Our project examined whether moving the arm in sync with an externally generated moving target (i.e., the arm does not cause the motion) could similarly support predictive pursuit.

Key findings were that concurrent upper‑limb movement improved smooth pursuit during occlusion, both for constant‑velocity and accelerating targets, but could not overcome the loss of drive of retinal input.

These results suggest that the brain integrates extra-retinal signals from the arm to enhance smooth pursuit during occlusion, supporting the idea that more complex predictive processes are required to account for target extrapolation in the absence of retinal input.

The ineffectiveness of efference copy to eye gaze control can be clearly demonstrated by measuring pursuit of a single object that appears with regular, predictable timing and velocity, or undergoes a transient occlusion after steady-state pursuit has been achieved.

In the latter case, smooth pursuit decays rapidly following object occlusion and is then maintained at a reduced gain if the object is expected to reappear. We have shown in multiple experiments that when the reappearance characteristics are known in advance, both anticipatory and predictive smooth pursuit is observed during occlusion.

Notably, there is also a saccadic response that works in combination with smooth pursuit in order to locate the eyes close to the unseen position of the occluded object.

To explain anticipatory and predictive gaze control, we proposed a model involving a direct and indirect pathway.

The model simulated well the ocular response during short-duration transient occlusion by incorporating short-term (i.e., within-trial) and long-term (between-trial) predictive influences in the form of a direct (e.g., efference copy) loop that operates during random-order trials, and an indirect (i.e., internal memory structure) loop that provides more persistent input during blocked-order trials.

The model was subsequently extended by other researchers to account for pursuit tasks that place an even greater demand on cognitive processes, and how these change as a function of normal ageing and neurodegenerative disease.

Tracking a moving object with the eyes seems like a simple task but involves areas of prefrontal cortex (PFC) associated with attention, working memory and prediction.

Increasing the demand on these processes with secondary tasks can affect eye movements and/or perceptual judgments, and is particularly evident in chronic or acute neurological conditions such as Alzheimer’s disease or mild traumatic brain injury.

Using combined near‑infrared spectroscopy and video‑oculography, we examined the effects of concurrent upper limb movement, which provides additional afferent and efferent signals, on tracking of a moving object in a novel dual-task pursuit protocol.

We confirmed the expected effects on judgement accuracy in the primary and secondary tasks, as well as a reduction in eye velocity when the moving object was occluded. There was limited evidence of oculo-manual facilitation on behavioural measures, but concurrent upper limb movement did result in lower activity in left medial PFC, as well as a change in PFC network organisation.

These findings extend previous work by demonstrating how PFC networks reorganise to support eye-hand coordination under task demands that more closely resemble everyday behaviour.

Read more about Prefrontal cortex activity

The study examined whether concurrent arm tracking enhances smooth pursuit during brief occlusion of externally generated target motion.

Across two experiments using constant‑velocity (Experiment 1) and accelerating (Experiment 2) targets, we manipulated predictability by presenting the two velocity profiles and occlusion durations in either random or blocked order.

As expected, smooth pursuit velocity declined during occlusion, but eye velocity was consistently higher in blocked‑order trials, particularly for positively accelerating motion, indicating a stronger contribution of predictive mechanisms.

For fast, constant‑velocity targets, simultaneous arm movement further facilitated pursuit during occlusion. However, even with increased predictability and the availability of sensorimotor signals from the arm, ocular pursuit never fully matched target velocity during occlusion. In contrast, manual tracking closely approximated target velocity in both blocked and random conditions.

The findings are interpreted within contemporary models of pursuit that incorporate short‑ and long‑term predictive processes to support target extrapolation during transient loss of visual input.

Read more about the Facilitation of ocular pursuit