Pupil diameter tracked during motor adaptation in humans.

Pupil diameter, under constant illumination, is known to reflect individuals' internal states, such as surprise about observation and environmental uncertainty. Despite the growing use of pupillometry in cognitive learning studies as an additional measure for examining internal states, few studies have used pupillometry in human motor learning studies. Here, we provide the first detailed characterization of pupil diameter changes in a short-term reach adaptation paradigm. We measured pupil changes in 121 human participants while they adapted to abrupt, gradual, or switching force field conditions. Sudden increases in movement error caused by the introduction/reversal of the force field resulted in strong phasic pupil dilation during movement accompanied by a transient increase in tonic premovement baseline pupil diameter in subsequent trials. In contrast, pupil responses were reduced when the force field was gradually introduced, indicating that large, unexpected errors drove the changes in pupil responses. Interestingly, however, error-induced pupil responses gradually became insensitive after experiencing multiple force field reversals. We also found an association between baseline pupil diameter and incidental knowledge of the gradually introduced perturbation. Finally, in all experiments, we found a strong co-occurrence of larger baseline pupil diameter with slower reaction and movement times after each rest break. Collectively, these results suggest that tonic baseline pupil diameter reflects one's belief about environmental uncertainty, whereas phasic pupil dilation during movement reflects surprise about a sensory outcome (i.e., movement error), and both effects are modulated by novelty. Our results provide a new approach for nonverbally assessing participants' internal states during motor learning.NEW & NOTEWORTHY Pupil diameter is known as a noninvasive window into individuals' internal states. Despite the growing use of pupillometry in cognitive learning studies, it receives little attention in motor learning studies. Here, we characterized the pupil responses in a short-term reach adaptation paradigm by measuring pupil diameter of human participants while they adapted to abrupt, gradual, or switching force field conditions. Our results demonstrate how surprise and uncertainty reflected in pupil diameter develop during motor adaptation.

Spinal stretch reflexes support efficient control of reaching.

Efficiently controlling the movement of our hand requires coordinating the motion of multiple joints of the arm. Although it is widely assumed that this type of efficient control is implemented by processing that occurs in the cerebral cortex and brainstem, recent work has shown that spinal circuits can generate efficient motor output that supports keeping the hand in a static location. Here, we show that a spinal pathway can also efficiently control the hand during reaching. In our first experiment, we applied multijoint mechanical perturbations to participants' elbow and wrist as they began reaching toward a target. We found that spinal stretch reflexes evoked in elbow muscles were not proportional to how much the elbow muscles were stretched but instead were dependent on the hand's location relative to the target. In our second experiment, we applied the same elbow and wrist perturbations but had participants change how they grasped the manipulandum, diametrically altering how the same wrist perturbation moved the hand relative to the reach target. We found that changing the arm's orientation diametrically altered how spinal reflexes in the elbow muscles were evoked, and in such a way that were again dependent on the hand's location relative to the target. These findings demonstrate that spinal circuits can help efficiently control the hand during dynamic reaching actions and show that efficient and flexible motor control is not exclusively dependent on processing that occurs within supraspinal regions of the nervous system.NEW & NOTEWORTHY We have previously shown that spinal circuits can rapidly generate reflex responses that efficiently engage multiple joints to support postural hand control of the upper limb. Here, we show that spinal circuits can also rapidly generate such efficient responses during reaching actions.

Spinal stretch reflexes support efficient hand control.

Motor behaviour is most efficiently controlled by correcting only disturbances that influence task success. It is currently thought that such control is computed within a transcortical feedback pathway. Here we show that, for postural hand control, even the fastest spinal feedback pathway can produce efficient corrective responses, forcing a re-evaluation of how the nervous system derives the control laws that support motor behavior.

Rapid feedback responses are flexibly coordinated across arm muscles to support goal-directed reaching.

A transcortical pathway helps support goal-directed reaching by processing somatosensory information to produce rapid feedback responses across multiple joints and muscles. Here, we tested whether such feedback responses can account for changes in arm configuration and for arbitrary visuomotor transformations-two manipulations that alter how muscles at the elbow and wrist need to be coordinated to achieve task success. Participants used a planar three degree-of-freedom exoskeleton robot to move a cursor to a target following a mechanical perturbation that flexed the elbow. In our first experiment, the cursor was mapped to the veridical position of the robot handle, but participants grasped the handle with two different hand orientations (thumb pointing upward or thumb pointing downward). We found that large rapid feedback responses were evoked in wrist extensor muscles when wrist extension helped move the cursor to the target (i.e., thumb upward), and in wrist flexor muscles when wrist flexion helped move the cursor to the target (i.e., thumb downward). In our second experiment, participants grasped the robot handle with their thumb pointing upward, but the cursor's movement was either veridical or was mirrored such that flexing the wrist moved the cursor as if the participant extended their wrist, and vice versa. After extensive practice, we found that rapid feedback responses were appropriately tuned to the wrist muscles that supported moving the cursor to the target when the cursor was mapped to the mirrored movement of the wrist, but were not tuned to the appropriate wrist muscles when the cursor was remapped to the wrist's veridical movement. NEW & NOTEWORTHY We show that rapid feedback responses were evoked in different wrist muscles depending on the arm's orientation, and this muscle activity was appropriate to generate the wrist motion that supported a reaching action. Notably, we also show that these rapid feedback responses can be evoked in wrist muscles that are detrimental to a reaching action if a nonveridical mapping between wrist and hand motion is extensively learned.

Coordinating long-latency stretch responses across the shoulder, elbow, and wrist during goal-directed reaching.

The long-latency stretch response (muscle activity 50-100 ms after a mechanical perturbation) can be coordinated across multiple joints to support goal-directed actions. Here we assessed the flexibility of such coordination and whether it serves to counteract intersegmental dynamics and exploit kinematic redundancy. In three experiments, participants made planar reaches to visual targets after elbow perturbations and we assessed the coordination of long-latency stretch responses across shoulder, elbow, and wrist muscles. Importantly, targets were placed such that elbow and wrist (but not shoulder) rotations could help transport the hand to the target-a simple form of kinematic redundancy. In experiment 1 we applied perturbations of different magnitudes to the elbow and found that long-latency stretch responses in shoulder, elbow, and wrist muscles scaled with perturbation magnitude. In experiment 2 we examined the trial-by-trial relationship between long-latency stretch responses at adjacent joints and found that the magnitudes of the responses in shoulder and elbow muscles, as well as elbow and wrist muscles, were positively correlated. In experiment 3 we explicitly instructed participants how to use their wrist to move their hand to the target after the perturbation. We found that long-latency stretch responses in wrist muscles were not sensitive to our instructions, despite the fact that participants incorporated these instructions into their voluntary behavior. Taken together, our results indicate that, during reaching, the coordination of long-latency stretch responses across multiple joints counteracts intersegmental dynamics but may not be able to exploit kinematic redundancy.

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 Alzheimer's 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.

Goal-dependent modulation of the long-latency stretch response at the shoulder, elbow, and wrist.

Many studies have demonstrated that muscle activity 50-100 ms after a mechanical perturbation (i.e., the long-latency stretch response) can be modulated in a manner that reflects voluntary motor control. These previous studies typically assessed modulation of the long-latency stretch response from individual muscles rather than how this response is concurrently modulated across multiple muscles. Here we investigated such concurrent modulation by having participants execute goal-directed reaches to visual targets after mechanical perturbations of the shoulder, elbow, or wrist while measuring activity from six muscles that articulate these joints. We found that shoulder, elbow, and wrist muscles displayed goal-dependent modulation of the long-latency stretch response, that the relative magnitude of participants' goal-dependent activity was similar across muscles, that the temporal onset of goal-dependent muscle activity was not reliably different across the three joints, and that shoulder muscles displayed goal-dependent activity appropriate for counteracting intersegmental dynamics. We also observed that the long-latency stretch response of wrist muscles displayed goal-dependent modulation after elbow perturbations and that the long-latency stretch response of elbow muscles displayed goal-dependent modulation after wrist perturbations. This pattern likely arises because motion at either joint could bring the hand to the visual target and suggests that the nervous system rapidly exploits such simple kinematic redundancy when processing sensory feedback to support goal-directed actions.

The antisaccade task: Vector inversion contributes to a statistical summary representation of target eccentricities.

Antisaccades require the top-down suppression of a stimulus-driven prosaccade (i.e., response suppression) and the inversion of a target's spatial location to mirror-symmetrical space (i.e., vector inversion). Moreover, recent work has shown that antisaccade amplitudes are characterized by a statistical summary representation (SSR) of the target eccentricities included in a stimulus-set--a result suggesting that antisaccades are supported via the same relative visual information as perceptions. The present investigation sought to determine whether response suppression and the disruption of real-time control or vector inversion contribute to a SSR in oculomotor control. Participants completed pro- and antisaccades (target eccentricities of 10.5°, 15.5°, and 20.5°) in blocks of trials that differed with regard to the frequency that individual target eccentricities were presented. The manipulation of target eccentricity frequency was used to determine whether the most frequently presented target within a stimulus-set (i.e., the SSR) influences saccade amplitudes. Moreover, we disrupted the real-time control of prosaccades by requiring participants to suppress their response for a brief visual delay (i.e., 2000 ms: so-called delay prosaccade). As expected, antisaccades and delay prosaccades produced equivalent reaction times. In turn, amplitudes for delay prosaccades were refractory to the manipulation of target eccentricity frequency, whereas antisaccades were biased in the direction of the most frequently presented target within a stimulus-set. Accordingly, we propose that vector inversion contributes to the mediation of target eccentricities via a SSR and that such a phenomenon provides convergent evidence that a relative visual percept mediates antisaccades.

Task-switching effects for visual and auditory pro- and antisaccades: evidence for a task-set inertia.

The completion of an antisaccade delays the reaction time (RT) of a subsequent prosaccade; however, the converse switch does not influence RT. In accounting for this result, the task-set inertia hypothesis contends that antisaccades engender a persistent nonstandard task-set that delays the planning of a subsequent prosaccade. In contrast, the coordinate system transformation hypothesis asserts that the transformation required to construct a mirror-symmetrical target representation persistently inhibits prosaccade planning. The authors tested the latter hypothesis by examining switch-costs for pro- and antisaccades directed to visual (i.e., the stimuli used in previous work) and auditory targets. Notably, auditory cues are specified in a head-centered frame of reference prior to their conversion into the retinocentric coordinates necessary for saccade output. Thus, if the coordinate system transformation hypothesis is correct then auditory pro- and antisaccades should elicit a bidirectional switch-cost because each requires a coordinate transformation. RTs for visual and auditory modalities showed a reliable--and equivalent magnitude--prosaccade switch-cost. Moreover, performance (e.g., movement time) and kinematic (e.g., velocity) variables indicated the switch-cost was restricted to response planning. As such, results are incompatible with the coordinate system transformation hypothesis and therefore provide convergent evidence that a task-set inertia contributes to the prosaccade switch-cost.

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.

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 antisaccade's 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).

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.

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.

The unidirectional prosaccade switch-cost: correct and error antisaccades differentially influence the planning times for subsequent prosaccades.

Antisaccades produce longer reaction times (RT) than their prosaccade counterparts and this latency increase has been linked to an oculomotor 'pre-setting' that prevents the evocation of a stimulus-driven prosaccade. Moreover, a consequence of oculomotor pre-setting is a lengthening of the RTs associated with a subsequent prosaccade. The goal of the present study was to determine whether the constituent elements associated with planning a correct antisaccade (i.e., response suppression and vector inversion) imparts a residual delay that inhibits the planning of a subsequent prosaccade. To that end, participants alternated between pro- and antisaccades in a pseudo-randomized task-switching schedule (e.g., AABBAAB…) and responses were cued via a paradigm that was designed to evoke frequent error antisaccades (i.e., a saccade initially, and incorrectly, planned to the target stimulus). Results showed that RTs for correct antisaccades were longer than error antisaccades and that prosaccades preceded by the former, but not the latter, trial-type were associated with a reliable increase in RT (i.e., prosaccade switch-cost). In other words, error antisaccades were associated with a failure to withhold a stimulus-driven prosaccade and did not delay the planning of a subsequent prosaccade. Based on these findings we propose that the prosaccade switch-cost is not related to an explicit awareness of task goals; rather, our results are consistent with the assertion that a consequence of response suppression and vector inversion is a residual inhibition of stimulus-driven oculomotor planning networks.

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.

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.

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.

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

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.

Revisiting Fitts and Peterson (1964): width and amplitude manipulations to the reaching environment elicit dissociable movement times.

The classic theorem of Fitts (1954) asserts that the combined effects of movement amplitude and target width (index of difficulty: ID) define movement times (MTs) for goal-directed reaches. Moreover, Fitts' theorem states that reaches yielding the same ID produce equivalent MTs regardless of the response's amplitude and width combination. However, most work providing direct support for Fitts' theorem has employed short movement amplitudes and small target widths. Thus, no direct evidence supports the unitary nature of MT/ID relations across a range of amplitudes and widths used in contemporary studies of goal-directed reaching. To that end, we contrasted MT/ID relations for discrete reaches equated for movement ID but differing with respect to their amplitude (15.5, 19, 25.5, and 38 cm) and width (2, 3, 4, and 5 cm) requirements. Results show that amplitude and width manipulations yielded robust linear MT/ID relations; however, the slope of the MT/ID function was markedly steeper in the former (amplitude=92 ms; width=13 ms). Such findings indicate that the constituent elements of movement ID are dissociable and that the fixed parameter nature of Fitts' theorem cannot be applied to a continuous range of veridical movement amplitudes and target widths.

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.