Effects of switching between leftward and rightward pro- and antisaccades

Saccade Latency and Metrics in the Interleaved Pro- and Anti-Saccade Task in Open Skill Sports Athletes

Evidence has demonstrated that athletes exhibit superior cognitive performance associated with executive control. In the oculomotor system, this function has been examined using the interleaved pro-saccade and anti-saccade task (IPAST), wherein participants, prior to target appearance, are instructed to either automatically look at the peripheral target (pro-saccade) or suppress the automatic response and voluntarily look in the opposite direction (anti-saccade). While the IPAST has provided much insight into sensorimotor and inhibitory processing, it has yet to be performed in athletes. Moreover, limited research has examined saccade metrics in athletes. Here, we examined saccade latency and movement kinematics in the IPAST among athletes (N = 40) and nonathletes (NON) (N = 40). Higher direction error rates were obtained in the anti-saccade compared to the pro-saccade condition, with no differences between athletes and NON noted. Significantly faster saccade latencies were observed in athletes compared to NON in both conditions, in addition to faster pro-saccades compared to anti-saccades. Furthermore, athletes showed significantly higher frequencies and faster latencies of express saccades compared to NON in correct pro-saccades. Additionally, athletes exhibited significantly faster latencies of express saccades compared to NON in erroneous anti-saccades. Differences in saccade metrics between athletes and NON were not seen. Overall, these findings demonstrate that athletes display altered saccade performance likely associated with sensorimotor and preparatory processing, highlighting the potential of using IPAST to objectively investigate sensorimotor and cognitive functions in athletes.

Interleaved Pro/Anti-saccade Behavior Across the Lifespan

The capacity for inhibitory control is an important cognitive process that undergoes dynamic changes over the course of the lifespan. Robust characterization of this trajectory, considering age continuously and using flexible modeling techniques, is critical to advance our understanding of the neural mechanisms that differ in healthy aging and neurological disease. The interleaved pro/anti-saccade task (IPAST), in which pro- and anti-saccade trials are randomly interleaved within a block, provides a simple and sensitive means of assessing the neural circuitry underlying inhibitory control. We utilized IPAST data collected from a large cross-sectional cohort of normative participants (n = 604, 5-93 years of age), standardized pre-processing protocols, generalized additive modeling, and change point analysis to investigate the effect of age on saccade behavior and identify significant periods of change throughout the lifespan. Maturation of IPAST measures occurred throughout adolescence, while subsequent decline began as early as the mid-20s and continued into old age. Considering pro-saccade correct responses and anti-saccade direction errors made at express (short) and regular (long) latencies was crucial in differentiating developmental and aging processes. We additionally characterized the effect of age on voluntary override time, a novel measure describing the time at which voluntary processes begin to overcome automated processes on anti-saccade trials. Drawing on converging animal neurophysiology, human neuroimaging, and computational modeling literature, we propose potential frontal-parietal and frontal-striatal mechanisms that may mediate the behavioral changes revealed in our analysis. We liken the models presented here to "cognitive growth curves" which have important implications for improved detection of neurological disease states that emerge during vulnerable windows of developing and aging.

Switch costs in inhibitory control and voluntary behaviour: A computational study of the antisaccade task

An integral aspect of human cognition is the ability to inhibit stimulus-driven, habitual responses, in favour of complex, voluntary actions. In addition, humans can also alternate between different tasks. This comes at the cost of degraded performance when compared to repeating the same task, a phenomenon called the "task-switch cost." While task switching and inhibitory control have been studied extensively, the interaction between them has received relatively little attention. Here, we used the SERIA model, a computational model of antisaccade behaviour, to draw a bridge between them. We investigated task switching in two versions of the mixed antisaccade task, in which participants are cued to saccade either in the same or in the opposite direction to a peripheral stimulus. SERIA revealed that stopping a habitual action leads to increased inhibitory control that persists onto the next trial, independently of the upcoming trial type. Moreover, switching between tasks induces slower and less accurate voluntary responses compared to repeat trials. However, this only occurs when participants lack the time to prepare the correct response. Altogether, SERIA demonstrates that there is a reconfiguration cost associated with switching between voluntary actions. In addition, the enhanced inhibition that follows antisaccade but not prosaccade trials explains asymmetric switch costs. In conclusion, SERIA offers a novel model of task switching that unifies previous theoretical accounts by distinguishing between inhibitory control and voluntary action generation and could help explain similar phenomena in paradigms beyond the antisaccade task.

Inhibition failures and late errors in the antisaccade task: influence of cue delay

In the antisaccade task participants are required to saccade in the opposite direction of a peripheral visual cue (PVC). This paradigm is often used to investigate inhibition of reflexive responses as well as voluntary response generation. However, it is not clear to what extent different versions of this task probe the same underlying processes. Here, we explored with the Stochastic Early Reaction, Inhibition, and late Action (SERIA) model how the delay between task cue and PVC affects reaction time (RT) and error rate (ER) when pro- and antisaccade trials are randomly interleaved. Specifically, we contrasted a condition in which the task cue was presented before the PVC with a condition in which the PVC served also as task cue. Summary statistics indicate that ERs and RTs are reduced and contextual effects largely removed when the task is signaled before the PVC appears. The SERIA model accounts for RT and ER in both conditions and better so than other candidate models. Modeling demonstrates that voluntary pro- and antisaccades are frequent in both conditions. Moreover, early task cue presentation results in better control of reflexive saccades, leading to fewer fast antisaccade errors and more rapid correct prosaccades. Finally, high-latency errors are shown to be prevalent in both conditions. In summary, SERIA provides an explanation for the differences in the delayed and nondelayed antisaccade task. NEW & NOTEWORTHY In this article, we use a computational model to study the mixed antisaccade task. We contrast two conditions in which the task cue is presented either before or concurrently with the saccadic target. Modeling provides a highly accurate account of participants' behavior and demonstrates that a significant number of prosaccades are voluntary actions. Moreover, we provide a detailed quantitative analysis of the types of error that occur in pro- and antisaccade trials.

Working memory capacity and the antisaccade task: A microanalytic-macroanalytic investigation of individual differences in goal activation and maintenance

The association between working memory capacity (WMC) and the antisaccade task, which requires subjects to move their eyes and attention away from a strong visual cue, supports the claim that WMC is partially an attentional construct (Kane, Bleckley, Conway, & Engle, 2001; Unsworth, Schrock, & Engle, 2004). Specifically, the WMC-antisaccade relation suggests that WMC helps maintain and execute task goals despite interference from habitual actions. Related work has recently shown that mind wandering (McVay & Kane, 2009, 2012a, 2012b) and reaction time (RT) variability (Unsworth, 2015) are also related to WMC and they partially explain WMC's prediction of cognitive abilities. Here, we tested whether mind-wandering propensity and intraindividual RT variation account for WMC's associations with 2 antisaccade-cued choice RT tasks. In addition, we asked whether any influences of WMC, mind wandering, or intraindividual RT variation on antisaccade are moderated by (a) the temporal gap between fixation and the flashing location cue, and (b) whether targets switch sides on consecutive trials. Our quasi-experimental study reexamined a published dataset (Kane et al., 2016) comprising 472 subjects who completed 6 WMC tasks, 5 attentional tasks with mind-wandering probes, 5 tasks from which we measured intraindividual RT variation, and 2 antisaccade tasks with varying fixation-cue gap durations. The WMC-antisaccade association was not accounted for by mind wandering or intraindividual RT variation. WMC's effects on antisaccade performance were greater with longer fixation-to-cue intervals, suggesting that goal activation processes-beyond the ability to control mind wandering and RT variability-are partially responsible for the WMC-antisaccade relation. (PsycINFO Database Record

Decomposing mechanisms of abnormal saccade generation in schizophrenia patients: Contributions of volitional initiation, motor preparation, and fixation release

Clinical and theoretical models suggest deficient volitional initiation of action in schizophrenia patients. Recent research provided an experimental model of testing this assumption using saccade tasks. However, inconsistent findings necessitate a specification of conditions on which the deficit may occur. The present study sought to detect mechanisms that may contribute to poor performance. Sixteen schizophrenia patients and 16 healthy control participants performed visually guided and two types of volitional saccade tasks. All tasks varied as to whether the initial fixation stimulus disappeared (fixation stimulus offset) or continued during saccade initiation, and whether a direction cue allowed motor preparation of the specific saccade. Saccade latencies of the two groups were differentially affected by task type, fixation stimulus offset, and cueing, suggesting abnormal volitional saccade generation, fixation release, and motor preparation in schizophrenia. However, substantial performance deficits may only occur if all affected processes are required in a task.

Alternating between pro- and antisaccades: switch-costs manifest via decoupling the spatial relations between stimulus and response

Antisaccades are a nonstandard task requiring a response mirror-symmetrical to the location of a target. The completion of an antisaccade has been shown to delay the reaction time (RT) of a subsequent prosaccade, whereas the converse switch elicits a null RT cost (i.e., the unidirectional prosaccade switch-cost). The present study sought to determine whether the prosaccade switch-cost arises from low-level interference specific to the sensory features of a target (i.e., modality-dependent) or manifests via the high-level demands of dissociating the spatial relations between stimulus and response (i.e., modality-independent). Participants alternated between pro- and antisaccades wherein the target associated with the response alternated between visual and auditory modalities. Thus, the present design involved task-switch (i.e., switching from a pro- to antisaccade and vice versa) and modality-switch (i.e., switching from a visual to auditory target and vice versa) trials as well as their task- and modality-repetition counterparts. RTs were longer for modality-switch than modality-repetition trials. Notably, however, modality-switch trials did not nullify or lessen the unidirectional prosaccade switch-cost; that is, the magnitude of the RT cost for task-switch prosaccades was equivalent across modality-switch and modality-repetition trials. Thus, competitive interference within a sensory modality does not contribute to the unidirectional prosaccade switch-cost. Instead, the modality-independent findings evince that dissociating the spatial relations between stimulus and response instantiates a high-level and inertially persistent nonstandard task-set that impedes the planning of a subsequent prosaccade.

Trial-type probability and task-switching effects on behavioral response characteristics in a mixed saccade task

Eye movement circuitry involved in saccade production offers a model for studying cognitive control: visually guided prosaccades are stimulus-directed responses, while goal-driven antisaccades rely upon more complex control processes to inhibit the prepotent tendency to look toward a cue, transform its spatial location, and generate a volitional saccade in the opposite direction. By manipulating the relative probability of these saccade types, we measured participants' behavioral responses to different levels of implicit trial-type probability and task-switching demands in conditions with relatively long inter-trial fixation and trial-type cue lengths. Results indicated that when prosaccades were less probable in a run, more prosaccade errors were generated; however, for antisaccades, trial-type probability had no effect on the percent of correct responses. For reaction times, specifically in runs with a larger probability of antisaccade trials, latencies increased for both anti- and pro-saccades. Furthermore, task switching resulted in a lower percentage of correct responses on switched trials, but a prior antisaccade trial led to slower reaction times for both trial types (i.e., a task switch cost for prosaccades and switch benefit for antisaccades). These findings indicate that cognitive control demands and residual inhibition from antisaccades alter performance relative to trial-type probability and task switching within a run, with the prosaccade task showing greater susceptibility to the influence of a large probability of cognitively complex antisaccades.

Directional interactions between current and prior saccades

One way to explore how prior sensory and motor events impact eye movements is to ask someone to look to targets located about a central point, returning gaze to the central point after each eye movement. Concerned about the contribution of this return to center movement, Anderson et al. (2008) used a sequential saccade paradigm in which participants made a continuous series of saccades to peripheral targets that appeared to the left or right of the currently fixated location in a random sequence (the next eye movement began from the last target location). Examining the effects of previous saccades (n−x) on current saccade latency (n), they found that saccadic reaction times (RT) were reduced when the direction of the current saccade matched that of a preceding saccade (e.g., two left saccades), even when the two saccades in question were separated by multiple saccades in any direction. We examined if this pattern extends to conditions in which targets appear inside continuously marked locations that provide stable visual features (i.e., target “placeholders”) and when saccades are prompted by central arrows. Participants completed 3 conditions: peripheral targets (PT; continuous, sequential saccades to peripherally presented targets) without placeholders; PT with placeholders; and centrally presented arrows (CA; left or right pointing arrows at the currently fixated location instructing participants to saccade to the left or right). We found reduced saccadic RT when the immediately preceding saccade (n−1) was in the same (vs. opposite) direction in the PT without placeholders and CA conditions. This effect varied when considering the effect of the previous 2–5 (n−x) saccades on current saccade latency (n). The effects of previous eye movements on current saccade latency may be determined by multiple, time-varying mechanisms related to sensory (i.e., retinotopic location), motor (i.e., saccade direction), and environmental (i.e., persistent visual objects) factors.

Eye movements in ephedrone-induced parkinsonism

Patients with ephedrone parkinsonism (EP) show a complex, rapidly progressive, irreversible, and levodopa non-responsive parkinsonian and dystonic syndrome due to manganese intoxication. Eye movements may help to differentiate parkinsonian syndromes providing insights into which brain networks are affected in the underlying disease, but they have never been systematically studied in EP. Horizontal and vertical eye movements were recorded in 28 EP and compared to 21 Parkinson's disease (PD) patients, and 27 age- and gender-matched healthy subjects using standardized oculomotor tasks with infrared videooculography. EP patients showed slow and hypometric horizontal saccades, an increased occurrence of square wave jerks, long latencies of vertical antisaccades, a high error rate in the horizontal antisaccade task, and made more errors than controls when pro- and antisaccades were mixed. Based on oculomotor performance, a direct differentiation between EP and PD was possible only by the velocity of horizontal saccades. All remaining metrics were similar between both patient groups. EP patients present extensive oculomotor disturbances probably due to manganese-induced damage to the basal ganglia, reflecting their role in oculomotor system.

Influence of Stimulus Type on Effects of Flanker, Flanker Position, and Trial Sequence in a Saccadic Eye Movement Task

Using the flanker paradigm in a task requiring eye movement responses, we examined how stimulus type (arrows vs. letters) modulated effects of flanker and flanker position. Further, we examined trial sequence effects and the impact of stimulus type on these effects. Participants responded to a central target with a left- or rightward saccade. We reasoned that arrows, being overlearned symbols of direction, are processed with less effort and are therefore linked more easily to a direction and a required response than are letters. The main findings demonstrate that (a) flanker effects were stronger for arrows than for letters, (b) flanker position more strongly modulated the flanker effect for letters than for arrows, and (c) trial sequence effects partly differed between the two stimulus types. We discuss these findings in the context of a more automatic and effortless processing of arrow relative to letter stimuli.

Horizontal and vertical eye movement metrics: what is important?

OBJECTIVE

To assist other eye movement investigators in the design and analysis of their studies.

METHODS

We examined basic saccadic eye movements and smooth pursuit in the horizontal and vertical directions with video-oculography in a group of 145 healthy subjects between 19 and 82 years of age.

RESULTS

Gender and education level did not influence eye movement metrics. With age, the latency of leftward and vertical pro- and antisaccades increased (p<0.001), velocity of upward prosaccades decreased (p<0.001), gain of rightward and upward prosaccades diminished (p<0.001), and the error rate of antisaccades increased (p<0.001). Prosaccades and antisaccades were influenced by the direction of the target, resulting in a right/left and up/down asymmetry. The skewness of the saccade velocity profile was stable throughout the lifespan, and within the range of saccades analyzed in the present study, correlated with amplitude and duration only for antisaccades (p<0.001).

CONCLUSIONS

Some eye movement metrics must be separated by the direction of movement, others according to subject age, while others may be pooled.

SIGNIFICANCE

This study provides important information for new oculomotor laboratories concerning the constitution of subject groups and the analysis of eye movement metrics.

Response selection in prosaccades, antisaccades, and other volitional saccades

Saccades made to the opposite side of a visual stimulus (antisaccades) and to central cues (simple volitional saccades) both require active response selection but whether the mechanisms of response selection differ between these tasks is unclear. Response selection can be assessed by increasing the number of response alternatives: this leads to increased reaction times when response selection is more demanding. We compared the reaction times of prosaccades, antisaccades, saccades cued by a central arrow, and saccades cued by a central number, in blocks of either two or six possible responses. In the two-response blocks, reaction times were fastest for prosaccades and antisaccades, and slowest for arrow-cued and number-cued saccades. Increasing response alternatives from two to six caused a paradoxical reduction in reaction times of prosaccades, had no effect on arrow-cued saccades, and led to a large increase in reaction times of number-cued saccades. For antisaccade reaction times, the effect of increasing response alternatives was intermediate, greater than that for arrow-cued saccades but less than that for number-cued saccades. We suggest that this pattern of results may reflect two components of saccadic processing: (a) response triggering, which is more rapid with a peripheral stimulus as in the prosaccade and antisaccade tasks and (b) response selection, which is more demanding for the antisaccade and number-cued saccade tasks, and more automatic when there is direct stimulus-response mapping as with prosaccades, or over-learned symbols as with arrow-cued saccades.

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.

Functional Connectivity Separates Switching Operations in the Posterior Lateral Frontal Cortex

Task representations consist of different aspects such as the representations of the relevant stimuli, the abstract rules to be applied, and the actions to be performed. To be flexible in our daily lives, we frequently need to switch between some or all aspects of a task. In the present study, we examined whether switching between abstract task rules and switching between response hands is associated with overlapping regions in the posterior lateral frontal cortex and whether switching between these two aspects of a task representation is neurally implemented by distinct functional brain networks. Subjects performed a cue-based task-switching paradigm where the location of the task cue additionally specified the response hand to be used. Overlapping activity for switching between abstract rules versus response hands was present in the inferior frontal junction area of the posterolateral frontal cortex. This region, however, showed very distinct patterns of functional connectivity depending on the content of the switch: Increased functional connectivity with anterior prefrontal, superior frontal, and hippocampal regions was present for abstract rule switching, whereas response hand switching led to increased coupling with motor regions surrounding the central sulcus. These results reveal that a rather general involvement of the posterior lateral frontal cortex in different switching contexts can be further characterized by highly specific functional interactions with other task-relevant regions, depending on the content of the switch.

Effect of nicotine on saccadic eye movement latencies in non-smokers

OBJECTIVE

Recently, saccadic eye movement tasks have been used to assess the effects of nicotine on higher cognitive processes, including inhibitory control. Saccadic task switching methods suggest that there is prolonged inhibition of the saccadic eye movement system following antisaccade trials. The objective of this research was to examine effects of nicotine on inhibition using saccadic task switching paradigms.

METHODS

Nicotine and placebo lozenges were administered on separate days to 40 non-smokers who performed prosaccade and antisaccade tasks. In addition, participants performed a series of trials in which prosaccade and antisaccade tasks were switched. Eye movement latencies were recorded.

RESULTS

Participants responded significantly faster for the nicotine condition than for the placebo condition. A switch benefit was observed for only placebo antisaccade trials, in that latencies of repetition trials were significantly longer than those of switch trials. In addition, an analysis of the repetition trials showed an interaction between saccade type and sequence position for the placebo condition, but not the nicotine condition.

CONCLUSION

Inhibition persists after antisaccade trials in a switching paradigm, but that the duration of this inhibition is reduced by nicotine.

'Alternate-goal bias' in antisaccades and the influence of expectation

Saccadic performance depends on the requirements of the current trial, but also may be influenced by other trials in the same experiment. This effect of trial context has been investigated most for saccadic error rate and reaction time but seldom for the positional accuracy of saccadic landing points. We investigated whether the direction of saccades towards one goal is affected by the location of a second goal used in other trials in the same experimental block. In our first experiment, landing points ('endpoints') of antisaccades but not prosaccades were shifted towards the location of the alternate goal. This spatial bias decreased with increasing angular separation between the current and alternative goals. In a second experiment, we explored whether expectancy about the goal location was responsible for the biasing of the saccadic endpoint. For this, we used a condition where the saccadic goal randomly changed from one trial to the next between locations on, above or below the horizontal meridian. We modulated the prior probability of the alternate-goal location by showing cues prior to stimulus onset. The results showed that expectation about the possible positions of the saccadic goal is sufficient to bias saccadic endpoints and can account for at least part of this phenomenon of 'alternate-goal bias'.

Response switching in schizophrenia patients and healthy subjects: effects of the inter-response interval

Schizophrenia patients show impaired saccadic response switching, pointing to action control deficits at the level of response selection. Previous studies on healthy subjects suggested that response switch effects might decrease if the prior response is longer ago, reflecting a slow dissipation of the response program persisting from the previous trial. The present study aimed at directly investigating whether response switch effects in schizophrenia patients and healthy subjects depend on the inter-response interval (IRI). Effects of response switching on pro- and antisaccade performance were analyzed in 19 schizophrenia patients and 19 healthy controls at 3 different IRIs (2,500, 3,000, 4,000 ms). Response switch effects of healthy subjects did not vary with the IRI, suggesting that the previous response program persists as long as no contrary response program is activated. In schizophrenia, response switch deficits were replicated at an IRI of 3,000 ms, whereas at IRIs of 2,500 and 4,000 ms, effects of response switching did not significantly differ from healthy subjects. This might suggest that there is a specific IRI range particularly sensitive to response switch deficits in schizophrenia. However, effects of response switching at different IRIs remain to be consolidated.

Saccade generation and suppression in schizophrenia: effects of response switching and perseveration

Poor antisaccade performance is a reliable index of action control deficits in schizophrenia. To further elucidate the underlying cognitive impairments, the current study aimed to confirm effects of switching the response direction on saccadic performance and to investigate whether response switch effects relate to perseveration. Fourteen schizophrenia patients and 14 healthy controls performed sequences of 1 to 3 simple volitional saccades to one direction and a subsequent volitional saccade with distractor to the same or the opposite direction. Response switches increased error rates in schizophrenia if they followed 3 saccades to the opposite side, suggesting that response switching affects performance on conditions of strong persisting response programs. The increase of response switch error rates with multiple repetitions of the prior response points to a relationship between perseveration and response selection.

Schizophrenia patients show impaired response switching in saccade tasks

Action control deficits of schizophrenia patients result from frontostriatal brain abnormalities and presumably reflect an impairment of selective cognitive processes. This study aimed at dissociating two different levels of action control in saccades toward and away from visual stimuli (pro- and antisaccades). Results of previous studies suggested that task switch effects (between pro- and antisaccades) reflect the persistence of a task-specific production rule and refer to the level of task selection, whereas response switch effects (between leftward and rightward saccades) point to the persistence of a specific response program, referring to the level of response selection. In the present study, task switching and response switching were investigated in 20 schizophrenia patients and 20 control subjects. Groups did not differ concerning task switch effects. In contrast, response switching entailed a stronger enhancement of error rates in patients, suggesting a specific deficit on the level of response selection in schizophrenia. The deficit was associated with spatial working memory capacities, confirming and specifying existing hypotheses on a relationship between working memory and action control.

Enhanced cognitive control in Tourette Syndrome during task uncertainty

Tourette Syndrome (TS) is a developmental neurological condition that is characterised by the presence of multiple motor and one or more vocal tics. Tics are highly stereotyped repetitive behaviours that fluctuate in type, complexity and severity. TS has been linked to impaired cognitive control processes, however, a recent study (Mueller et al. in Curr Biol 16:570-573, 2006) demonstrated that young people with TS, although exhibiting chronic motor and vocal tics, nevertheless performed significantly better than a group of age-matched controls on a task that required extremely high levels of cognitive control (i.e., predictably shifting between executing pro-saccade and anti-saccade responses to a visual stimulus). As predictable task sequences allow task-related cognitive processes to commence prior to the presentation of target stimuli we examined whether the superior performance of the TS group could be replicated when task sequences were varied unpredictably. Our results confirmed that both the TS group and an age-matched control group benefited, by the same extent, when the saccade task (pro-saccade vs. anti-saccade) was pre-cued. In contrast, while the control group showed a significant decrease in performance on task switch trials relative to task repetition trials-the TS group exhibited no significant 'costs' of switching task. While task performance was modulated by response and target location shifts in the control group, these factors had less impact on the TS group's performance on task switch trials. These results confirm and extend the previous demonstration that individuals with TS exhibit paradoxically greater levels of cognitive control than healthy controls.

Brain potentials indicate poor preparation for action in schizophrenia

Impaired antisaccade performance in schizophrenia (SZ) may originate from poor task preparation, suggested by low amplitudes of the contingent negative variation (CNV) before antisaccades. To dissociate components of preparation we measured the CNV in standard pro- and antisaccades and a stimulus preceding negativity (SPN) in delayed pro- and antisaccades. In healthy controls the SPN had lower amplitudes than the CNV, reflecting mere stimulus expectation in SPN and combined stimulus expectation and action readiness in CNV. SZ patients had lower CNV amplitudes than controls, but there was no reliable indication of a general SPN reduction, suggesting a particular impairment of action readiness. The CNV and SPN amplitudes of controls were larger if tasks had incongruent (anti) compared to congruent (pro) S-R mappings. This difference was absent in SZ patients, suggesting a failure to activate specific resources for incongruent S-R mappings.

Impaired action control in schizophrenia: The role of volitional saccade initiation

Schizophrenia patients show prefrontal cortex dysfunctions of neurodevelopmental origin, but the cognitive implications of these dysfunctions are not yet understood. This study used experimental variations of oculomotor tasks to evaluate the relative roles of volitional action initiation and the inhibition of reflexive behavior. Thirty schizophrenia patients and 30 control participants performed standard prosaccades (SP), standard antisaccades (SA), delayed prosaccades (DP), and delayed antisaccades (DA). The delayed tasks allowed separating the inhibition of erroneous prosaccades and the initiation of volitional saccades, which coincide in the SA task. Arrow-cued (AC) saccades were used to evaluate initiation without any inhibitory component. Erroneous prosaccades were less frequent in the delayed tasks than in the SA task. Error rates were generally larger in schizophrenia patients than in control participants, but the deficit was smaller in the delayed tasks than in the SA task. Correct saccade latencies of schizophrenia patients were normal in the SP task, but not on conditions of volitional saccade initiation (all other tasks). Volitional saccade latencies were positively correlated with error rates in the schizophrenia group. These results confirm that schizophrenia patients have a specific deficit in initiating volitional action, which may also contribute to the increased error rates.

The inter-trial effects of stimulus and saccadic direction on prosaccades and antisaccades, in controls and schizophrenia patients

We investigated the influence of the direction of preceding saccadic trials on the latency of current prosaccades and antisaccades, in healthy subjects and patients with schizophrenia. When prosaccades and antisaccades were performed in separate, single-task blocks, we found that only prosaccades were delayed if the saccade in the prior trial was in the same direction, consistent with the expected directional effect from an 'inhibition of return'-like alternation advantage. However, both types of saccades were executed more quickly when the saccade in the penultimate trial was in the same direction, consistent with previous demonstrations of directional plasticity in monkeys. In blocks of randomly mixed prosaccades and antisaccades, the directional effects in healthy subjects were greatest when a prosaccade was preceded by an antisaccade, consistent with a summation of effects of alternation advantage (from the prior stimulus) and directional plasticity (from the prior saccade). Schizophrenic patients showed an additional phenomenon, a directionally specific inhibition of upcoming saccades by preceding antisaccades. These results suggest that saccades in humans are modulated by inter-trial effects attributable to both an 'inhibition of return'-like alternation advantage and directional plasticity.