Anthony S. Drew

The allocation of attention during smooth pursuit eye movements.

The spatial-temporal allocation of attention during smooth pursuit eye movements is poorly understood. In this chapter we review evidence showing that attention contributes to both saccades and smooth pursuit. We then discuss results from our own recent studies using a dual-task paradigm in which subjects pursued a moving stimulus and pressed a button when targets appeared in the periphery. The results from these studies are consistent with the hypothesis that the allocation of attention is biased to a location just in front of the pursuit stimulus and that this bias can be altered by pursuit velocity.

Attentional deficits in concussion

Primary objective: The purpose of the present study was to examine deficits in the alerting, orienting and executive components of attention in individuals who have recently suffered a concussion. Research design: A group design was used in which the performance by individuals with concussion was compared to control subjects matched for age, height, weight and activity level. Methods and procedures: Participants completed the Attentional Network Test (ANT) that breaks down attention into alerting, orienting and executive components. Reaction time and response accuracy were the dependent variables. Main outcomes and results: It was found that only the orienting and executive components of attention were affected by concussion, whereas the alerting component was normal. Furthermore, participants with concussion required a significantly longer time than controls to initiate correct responses. Conclusions: These results suggest that the orienting and executive components of attention are most susceptible to the effects of concussion.

Attentional disengagement dysfunction following mTBI assessed with the gap saccade task

Concussion, or mild traumatic brain injury (mTBI), leads to a number of cognitive, attentional, and sensorimotor deficits that can last a surprisingly long time after the initial injury. We have previously shown that the ability to orient visuospatial attention is deficient in participants with mTBI within 2 days of their injury, but then recovers to normal levels within a week. Orienting attention requires disengagement from the point of fixation, movement of attention to the location of interest, and re-engagement at that location. Deficits in any or all of these processes could lead to the difficulties with orienting attention that we have observed in mTBI. To address this issue, we tested participants with mTBI using a gap saccade task. Because this task manipulates the temporal gap between the offset of the fixation target and the appearance of the peripheral saccade target, it isolates the contribution of the disengagement process to saccadic reaction time. We found that participants with mTBI had significantly longer saccadic reaction times than controls when the temporal gap was short but not when it was long. This gap-dependent difference in saccadic reaction time was present within 2 days of the injury and resolved within 1 week. This pattern of results suggests that as the contribution of the disengagement process is reduced, so too is the extent of the reaction time deficit in the participants with mTBI. Taken together, this is consistent with the idea that the deficits in orienting visuospatial attention in participants with mTBI are fully accounted for by difficulties with the initial disengagement process.

Cancelling planned actions following mild traumatic brain injury

Mild traumatic brain injury (mTBI) leads to a variety of attentional, cognitive, and sensorimotor deficits. An important aspect of behavior that intersects each of these functions is the ability to cancel a planned action. Thus, the purpose of this study was to determine the effects of mTBI on the ability to perform a countermanding saccade task. In this task, participants were asked to generate a saccade to a target appearing in peripheral vision, but to inhibit saccade execution if an auditory stop signal was presented. The delay between the appearance of the peripheral target and the presentation of the auditory stop signal was varied between 0 and 125ms. We found that the change in the probability of cancelling the saccade as a function of this delay was no different between participants with mTBI tested within 2 days of their injury and matched controls. However, saccadic reaction times and the stop signal reaction time were unexpectedly faster in the participants with mTBI and, furthermore, they inaccurately inhibited saccades during 15% of the trials with no stop signal. Taken together, this data suggests that the ability to cancel planned actions is subtly yet adversely affected by mTBI.

Eye-hand interactions differ in the human premotor and parietal cortices.

In order to successfully look at and reach for a visual target the central nervous system must perform a complex sensorimotor transformation. How this transformation is mapped onto relevant brain structures has become the subject of much recent investigation. In the present paper we examined the contribution of the human premotor cortex (PMC) to this transformation process during a task requiring coordinated eye and hand movements. For this purpose, we made use of single-pulse transcranial magnetic stimulation (TMS) to temporarily disrupt the processing occurring in the PMC during task performance. Subjects made open-loop pointing movements accompanied by saccades of the same size or two or three times larger. Under normal circumstances without TMS, the pointing movement amplitude increased with saccade amplitude. When TMS was applied over the PMC 100-200 ms after target presentation, the influence of saccade amplitude on the pointing movement amplitude was increased. This is the opposite effect to that observed in a previous study [Journal of Neurophysiology 84 (200) 1677-1680] when TMS was applied over the posterior parietal cortex (PPC) during the same task. We suggest that this pattern of results is consistent with the coding of the reach plan in eye-centered coordinates in the PPC and limb-centered coordinates in the PMC.

Cortical frames of reference for eye-hand coordination.

To reach for an object the brain must transform visual input from the eye into motor output of the arm. Recent neurophysiological experiments have shown that this transformation maps onto a network of brain areas including the posterior parietal (PPC) and premotor (PMC) cortices. In this chapter, we review evidence from our own experiments which demonstrate that this network can only partially complete the transformation when the eye and limb movement amplitudes are dissociated. We also discuss the effects of disrupting either the PPC or PMC using transcranial magnetic stimulation (TMS) on the ability to carry out the transformation successfully.

The contribution of the human PPC to the orienting of visuospatial attention during smooth pursuit

Smooth pursuit eye movements function to stabilize the retinal image of small moving targets. In order for those targets to be foveated, however, they must first be “captured” by an attentional mechanism which then interacts with the oculomotor system. Cortical sites involved with producing smooth pursuit overlap with areas known to be involved in directing visuospatial attention, particularly the posterior parietal cortex (PPC). The goal of the current study was to characterize the contributions made by the left and right posterior parietal cortices (lPPC and rPPC) to the interaction between visuospatial attention and the generation of smooth pursuit eye movements. Transcranial magnetic stimulation (TMS) was used to temporarily disrupt each area at different times around target motion onset in a pursuit task that explicitly manipulated the covert orienting of attention. TMS over the lPPC, rPPC and a control site (the vertex) evoked a similar pattern of results, in that the earlier TMS delivery times caused a reduced pursuit latency compared to baseline measures, while TMS immediately prior to target motion onset resulted in latencies slower than baseline. In addition, however, TMS over the lPPC and rPPC (but not the vertex) preferentially influenced the generation of contralateral pursuit, with the lPPC doing so in a relatively time-independent manner, and the rPPC doing so in a time-dependent manner. This pattern of results implies that both the left and right PPC are directly involved in the interaction between attention and smooth pursuit preparation.