Jeffrey Weiler

Vector inversion diminishes the online control of antisaccades.

Antisaccades require the suppression of a stimulus-driven response (i.e., response suppression) and the computation of a movement plan mirror-symmetrical to the location of a target (i.e., vector inversion). The goal of the present study was to determine whether response suppression, vector inversion or both contribute to previously reported differences in the online control of pro- and antisaccades (Heath in Exp Brain Res 203:743–752, 2010a). Pro- and antisaccades were completed in separate blocks (i.e., blocked schedule) and a block wherein the spatial relation between stimulus and response was provided at response cuing (i.e., random schedule). Notably, the random schedule provides a relative means for equating response suppression across pro- and antisaccades. To examine online trajectory amendments, we computed the proportion of variance (R2 values) explained by the spatial location of the eye at early, middle and late stages of saccade trajectories relative to the saccade’s ultimate endpoint. The basis for this analysis is that between-task differences in R2 magnitudes reflect differences in the use of feedback for online trajectory amendments: small R2 values represent a trajectory supported via online control whereas larger R2 values reflect a reduction in online control. Results show that antisaccades yielded larger R2 values than prosaccades from early to late stages of saccade trajectories, and this finding was observed regardless of whether or not tasks were equated for response suppression. Thus, we propose that the intentional nature of vector inversion disrupts the normally online control of saccades and renders a mode of control that is not optimized to support error-reducing trajectory amendments.

Task-switching in oculomotor control: Unidirectional switch-cost when alternating between pro- and antisaccades

The antisaccade task requires the suppression of a reflexive prosaccade (i.e., response suppression) and the remapping of a target location to mirror-symmetrical space (i.e., vector inversion). Moreover, antisaccades are associated with increased activation of cortical oculomotor networks: a finding attributed to the top-down requirements of response suppression and vector inversion. The goal of the present study was to determine if the increased cortical activity associated with antisaccades elicits a residual inhibition of oculomotor planning networks. To that end, each trial in this investigation entailed the onset of a single and exogenously presented target (i.e., archetypical antisaccade task) and participants were instructed to alternate between pro- and antisaccades in blocked and random task-switching schedules. In the blocked schedule, the saccade tasks (i.e., pro- and antisaccades) alternated on every second trial (AABB paradigm) whereas in the random schedule the saccade tasks were pseudo-randomly interleaved on a trial-by-trial basis. Reaction times for task-switch prosaccades were longer and more variable than their task-repetition counterparts, whereas antisaccades did not vary as a function of task-switch and task-repetition trials: a finding that was consistent across blocked and random presentation schedules. In other words, results demonstrate a unidirectional switch-cost for prosaccades. As such, we propose that the top-down processes required to complete an antisaccade results in residual inhibition of oculomotor networks supporting a subsequent prosaccade.

Oculomotor task switching: alternating from a nonstandard to a standard response yields the unidirectional prosaccade switch-cost

The completion of an antisaccade (i.e., a nonstandard task) lengthens the reaction time (RT) of a subsequent prosaccade: a behavioral phenomenon termed the unidirectional prosaccade switch-cost. One explanation for the unidirectional prosaccade switch-cost is suppressing a stimulus-driven prosaccade during the preceding antisaccade trial engenders a residual inhibition of the oculomotor networks that support prosaccade planning (i.e., the oculomotor inhibition hypothesis). Alternatively, the unidirectional prosaccade switch-cost may reflect the persistent activation of the antisaccades nonstandard task rules (i.e., task set), which delays the planning of the next prosaccade (i.e., task-set inertia hypothesis). To determine which hypothesis provides the most parsimonious account for the unidirectional prosaccade switch-cost, participants alternated between pro- and antisaccades wherein task instructions (i.e., pro- and antisaccade) were provided before (i.e., classic cuing) or concurrent (i.e., delayed cuing) with response cuing. Importantly, pro- and antisaccades elicited via the delayed cuing condition required the suppression of a stimulus-driven prosaccade at response cuing (i.e., response suppression) to discern the appropriate to-be-performed task. Results showed that classic and delayed antisaccades, but not delayed prosaccades, lengthened the RT of subsequent prosaccades. That delayed prosaccades, which require response suppression for their successful execution, did not lengthen the RT of subsequent prosaccades indicates that the oculomotor inhibition hypothesis does not account for the unidirectional prosaccade switch-cost. Instead, the current findings are in line with the assertion that the task set associated with a nonstandard antisaccade persists inertially and delays the planning of a subsequent prosaccade (i.e., task-set inertia hypothesis).

The prior-antisaccade effect influences the planning and online control of prosaccades

The latency of a prosaccade is increased when completed following an antisaccade (the prior-antisaccade effect). This finding has been attributed to the inhibition of the oculomotor networks necessary for an antisaccade engendering a persistent response set that delays a to-be-executed prosaccade. The goal of the present investigation was to determine whether the prior-antisaccade effect influences not only the planning but also the control of an unfolding prosaccade trajectory. To accomplish that objective, we employed a task-switching paradigm wherein participants alternated between pro- and antisaccades on every second trial (i.e., AABB paradigm). Importantly, trajectory control was evaluated by computing the proportion of variance (R2 values) explained by the spatial position of the eye at decile increments of movement time relative to the response’s ultimate movement endpoint: small R2 values indicate a response that unfolds with error-reducing trajectory amendments (i.e., online control), whereas larger R2 values reflect a response that unfolds with few—if any—online corrections. As expected, results showed a prior-antisaccade effect for response planning; that is, prosaccade latencies were increased when completed after an antisaccade. Moreover, prosaccades completed after an antisaccade elicited larger R2 values and less accurate endpoints than trials wherein a prosaccade was completed after another prosaccade. These results provide first evidence of a prior-antisaccade effect for trajectory control and indicate that the persistent and inhibitory response set arising from an antisaccade diminishes the online corrections, and thus endpoint accuracy, of a subsequent prosaccade.

Stimulus-driven saccades are characterized by an invariant undershooting bias: no evidence for a range effect

Saccade endpoints are most frequently characterized by an undershooting bias. Notably, however, some evidence suggests that saccades can be made to systematically under- or overshoot a target based on the magnitude of the eccentricities within a given block of trials (i.e., the oculomotor range effect hypothesis). To address that issue, participants completed stimulus-driven saccades in separate blocks of trials (i.e., proximal vs. distal) that entailed an equal number of targets but differed with respect to the magnitude of their eccentricities. In the proximal block, target eccentricities were 3.0°, 5.5°, 8.0°, 10.5° and 13.0°, whereas in the distal block target eccentricities were 10.5°, 13.0°, 15.5°, 18.0° and 20.5°. If the range effect represents a tenable hypothesis, then the magnitude of target eccentricities within each block should selectively influence saccade endpoint bias. More specifically, the eccentricities common to the proximal and distal blocks (i.e., 10.5° and 13.0°) should elicit a systematic under- and overshooting bias, respectively. Results for the proximal and distal blocks showed a reliable undershooting bias across target eccentricities, and a direct comparison of the common eccentricities indicated that the undershooting bias was not modulated between blocks. Moreover, our results show that the presence of online target vision did not influence the undershooting bias. Thus, the present findings provide no support for an oculomotor range effect; rather, results evince the mediation of saccades via a control strategy that minimizes movement time and/or the energy requirements of the response.

Pro- and antisaccades: dissociating stimulus and response influences the online control of saccade trajectories.

ABSTRACT The authors examined whether the diminished online control of antisaccades is related to a trade-off between movement planning and control or the remapping of target properties to a mirror-symmetrical location (i.e., vector inversion). Pro- and antisaccades were examined in a standard no-delay schedule wherein target onset served as the movement imperative and a delay cuing schedule wherein responses were initiated 2,000 ms following target onset. Importantly, the delay cuing schedule was employed to equate pro- and antisaccade reaction times. Online control was evaluated by indexing the strength of trajectory amendments at normalized increments of movement time. Antisaccades exhibited fewer online corrections than prosaccades, and this result was consistent across cuing schedules. Thus, the diminished online control of antisaccades cannot be tied to a trade-off between movement planning and control. Rather, the authors propose that the intentional nature of dissociating stimulus and response (i.e., vector inversion) engenders a slow mode of cognitive control that is not optimized for fast oculomotor corrections.

The unidirectional prosaccade switch-cost: Electroencephalographic evidence of task-set inertia in oculomotor control

The execution of an antisaccade selectively increases the reaction time (RT) of a subsequent prosaccade (the unidirectional prosaccade switch-cost). To explain this finding, the task-set inertia hypothesis asserts that an antisaccade requires a cognitively mediated non-standard task-set that persists inertially and delays the planning of a subsequent prosaccade. The present study sought to directly test the theoretical tenets of the task-set inertia hypothesis by examining the concurrent behavioural and the event-related brain potential (ERP) data associated with the unidirectional prosaccade switch-cost. Participants pseudo-randomly alternated between pro- and antisaccades while electroencephalography (EEG) data were recorded. As expected, the completion of an antisaccade selectively increased the RT of a subsequent prosaccade, whereas the converse switch did not influence RTs. Thus, the behavioural results demonstrated the unidirectional prosaccade switch-cost. In terms of the ERP findings, we observed a reliable change in the amplitude of the P3 - time-locked to task-instructions - when trials were switched from a prosaccade to an antisaccade; however, no reliable change was observed when switching from an antisaccade to a prosaccade. This is a salient finding because extensive work has shown that the P3 provides a neural index of the task-set required to execute a to-be-completed response. As such, results showing that prosaccades completed after antisaccades exhibited increased RTs in combination with a P3 amplitude comparable to antisaccades provides convergent evidence that the unidirectional prosaccade switch-cost is attributed to the persistent activation of a non-standard antisaccade task-set.

Repetitive antisaccade execution does not increase the unidirectional prosaccade switch-cost

An antisaccade is the execution of a saccade to the mirror-symmetrical location (i.e., same amplitude but opposite visual field) of a single and exogenously presented visual target. Such a response requires top-down decoupling of the normally direct spatial relations between stimulus and response and results in increased planning times and directional errors compared to their spatially compatible prosaccade counterparts. Moreover, antisaccades are associated with diffuse changes in cortical and subcortical saccade networks: a finding that has, in part, been attributed to pre-setting the oculomotor system to withhold a stimulus-driven prosaccade. Moreover, recent work has shown that a corollary cost of oculomotor pre-setting is that the planning time for a to-be-completed prosaccade is longer when preceded by an antisaccade (i.e., the unidirectional prosaccade switch-cost). Notably, this result has been attributed to antisaccades imparting a residual inhibition of the oculomotor networks that support the planning of stimulus-driven prosaccades. In the current investigation, we sought to determine if the number of antisaccades preceding a prosaccade increases this residual inhibition and thus influences the magnitude of the unidirectional prosaccade switch-cost. To that end, participants alternated between pro- and antisaccades after every second (i.e., AABB schedule) and every fourth (i.e., AAAABBBB schedule) trial. In addition, participants completed pro- and antisaccades in separate blocks of trials. Results demonstrated that task-switch prosaccades produced longer reaction times than their task-repetition and blocked condition counterparts, whereas antisaccade reaction times did not vary across task-repetition, task-switch and blocked condition trials. Most notably, the magnitude of the unidirectional prosaccade switch-cost was not modulated across the different task-switching schedules. Thus, we propose that the top-down requirements of the antisaccade task do not produce additive inhibition of stimulus-driven saccade networks.

A Six-Month Cognitive-Motor and Aerobic Exercise Program Improves Executive Function in Persons with an Objective Cognitive Impairment: A Pilot Investigation Using the Antisaccade Task

Persons with an objective cognitive impairment (OCI) are at increased risk for progression to Alzheimers disease and related dementias. The present pilot project sought to examine whether participation in a long-term exercise program involving cognitive-motor (CM) dual-task gait training and aerobic exercise training improves executive function in persons with an OCI. To accomplish our objective, individuals with an OCI (n = 12) as determined by a Montreal Cognitive Assessment (MoCA) score of less than 26 and older adults (n = 11) deemed to be cognitively healthy (i.e., control group: MoCA score ≥26) completed a six-month moderate-to-high intensity (65-85% maximum heart rate) treadmill-based CM and aerobic exercise training program wherein pre- and post-intervention executive control was examined via the antisaccade task. Notably, antisaccades require a goal-directed eye-movement mirror-symmetrical to a target and represent an ideal tool for the study of executive deficits because of its hands- and language-free nature. As well, the cortical networks mediating antisaccades represent regions associated with neuropathology in cognitive decline and dementia (e.g., dorsolateral prefrontal cortex). Results showed that antisaccade reaction times for the OCI group reliably decreased by 30 ms from pre- to post-intervention, whereas the control group did not produce a reliable pre- to post-intervention change in reaction time (i.e., 6 ms). Thus, we propose that in persons with OCI long-term CM and aerobic training improves the efficiency and effectiveness of the executive mechanisms mediating high-level oculomotor control.

Response Suppression Delays the Planning of Subsequent Stimulus-Driven Saccades

The completion of an antisaccade selectively increases the reaction time (RT) of a subsequent prosaccade: a result that has been interpreted to reflect the residual inhibition of stimulus-driven saccade networks [1], [2]. In the present investigation we sought to determine whether the increase in prosaccade RT is contingent on the constituent antisaccade planning processes of response suppression and vector inversion or is limited to response suppression. To that end, in one block participants alternated between pro- and antisaccades after every second trial (task-switching block), and in another block participants completed a series of prosaccades that were randomly (and infrequently) interspersed with no-go catch-trials (go/no-go block). Notably, such a design provides a framework for disentangling whether response suppression and/or vector inversion delays the planning of subsequent prosaccades. As expected, results for the task-switching block showed that antisaccades selectively increased the RTs of subsequent prosaccades. In turn, results for the go/no-go block showed that prosaccade RTs were increased when preceded by a no-go catch-trial. Moreover, the magnitude of the RT ‘cost’ was equivalent across the task-switching and go/no-go blocks. That prosaccades preceded by an antisaccade or a no-go catch-trial produced equivalent RT costs indicates that the conjoint processes of response suppression and vector inversion do not drive the inhibition of saccade planning mechanisms. Rather, the present findings indicate that a general consequence of response suppression is a residual inhibition of stimulus-driven saccade networks.