Implicit and explicit timing in oculomotor control

Time for Memories

The ability to store information about the past to dynamically predict and prepare for the future is among the most fundamental tasks the brain performs. To date, the problems of understanding how the brain stores and organizes information about the past (memory) and how the brain represents and processes temporal information for adaptive behavior have generally been studied as distinct cognitive functions. This Symposium explores the inherent link between memory and temporal cognition, as well as the potential shared neural mechanisms between them. We suggest that working memory and implicit timing are interconnected and may share overlapping neural mechanisms. Additionally, we explore how temporal structure is encoded in associative and episodic memory and, conversely, the influences of episodic memory on subsequent temporal anticipation and the perception of time. We suggest that neural sequences provide a general computational motif that contributes to timing and working memory, as well as the spatiotemporal coding and recall of episodes.

How Does Temporal Blurring Alter Movement Timing?

Subjective uncertainty arises because the estimation of the timing of an event into the future is error prone. This impact of stimulus-bound uncertainty on movement preparation has often been investigated using reaction time tasks where a warning stimulus (WS) predicts the occurrence of a "go" signal. The timing of the "go" signal can be chosen from a particular probability distribution with a given variance or uncertainty. It has been repeatedly shown that reaction times covary with the shape of the used "go" signal distribution. This is interpreted as evidence for temporal preparation. Moreover, the variance of the response time should always increase with the duration of the delay between the WS and the "go" signal. This increasing variance has been interpreted as a consequence of the temporal "blurring" of future events (scalar expectancy). The present paper tested the validity of the temporal "blurring" hypothesis in humans with a simple oculomotor reaction time task where subjective and stimulus-bound uncertainties were increased. Subjective uncertainty about the timing of a "go" signal was increased by lengthening the delay between the WS and the "go" signal. Objective uncertainty was altered by increasing the variance of "go" signal timing. Contrary to temporal blurring hypotheses, the study has shown that increasing the delay between events did not significantly increase movement timing variability. These results suggest that temporal blurring could not be a property of movement timing in an implicit timing context.

Modality-specific sensory and decisional carryover effects in duration perception

BACKGROUND

The brain uses recent history when forming perceptual decisions. This results in carryover effects in perception. Although separate sensory and decisional carryover effects have been shown in many perceptual tasks, their existence and nature in temporal processing are unclear. Here, we investigated whether and how previous stimuli and previous choices affect subsequent duration perception, in vision and audition.

RESULTS

In a series of three experiments, participants were asked to classify visual or auditory stimuli into "shorter" or "longer" duration categories. In experiment 1, visual and auditory stimuli were presented in separate blocks. Results showed that current duration estimates were repelled away from the previous trial's stimulus duration, but attracted towards the previous choice, in both vision and audition. In experiment 2, visual and auditory stimuli were pseudorandomly presented in one block. We found that sensory and decisional carryover effects occurred only when previous and current stimuli were from the same modality. Experiment 3 further investigated the stimulus dependence of carryover effects within each modality. In this experiment, visual stimuli with different shape topologies (or auditory stimuli with different audio frequencies) were pseudorandomly presented in one visual (or auditory) block. Results demonstrated sensory carryover (within each modality) despite task-irrelevant differences in visual shape topology or audio frequency. By contrast, decisional carryover was reduced (but still present) across different visual topologies and completely absent across different audio frequencies.

CONCLUSIONS

These results suggest that serial dependence in duration perception is modality-specific. Moreover, repulsive sensory carryover effects generalize within each modality, whereas attractive decisional carryover effects are contingent on contextual details.

From anticipation to impulsivity in Parkinson's disease

Anticipatory actions require to keep track of elapsed time and inhibitory control. These cognitive functions could be impacted in Parkinson's disease (iPD). To test this hypothesis, a saccadic reaction time task was used where a visual warning stimulus (WS) predicted the occurrence of an imperative one (IS) appearing after a short delay. In the implicit condition, subjects were not informed about the duration of the delay, disfavoring anticipatory behavior but leaving inhibitory control unaltered. In the explicit condition, delay duration was cued. This should favor anticipatory behavior and perhaps alter inhibitory control. This hypothesis was tested in controls (N = 18) and age-matched iPD patients (N = 20; ON and OFF L-DOPA). We found that the latency distribution of saccades before the IS was bimodal. The 1^st mode weakly depended on temporal information and was more prominent in iPD. Saccades in this mode were premature and could result of a lack of inhibition. The 2^nd mode covaried with cued duration suggesting that these movements were genuine anticipatory saccades. The explicit condition increased the probability of anticipatory saccades before the IS in controls and iPD_ON but not iPD_OFF patients. Furthermore, in iPD patients the probability of sequences of 1^st mode premature responses increased. In conclusion, the triggering of a premature saccade or the initiation of a controlled anticipatory one could be conceptualized as the output of two independent stochastic processes. Altered time perception and increased motor impulsivity could alter the balance between these two processes in favor of the latter in iPD, particularly OFF L-Dopa.

Implicit Versus Explicit Timing-Separate or Shared Mechanisms?

Time implicitly shapes cognition, but time is also explicitly represented, for instance, in the form of durations. Parsimoniously, the brain could use the same mechanisms for implicit and explicit timing. Yet, the evidence has been equivocal, revealing both joint versus separate signatures of timing. Here, we directly compared implicit and explicit timing using magnetoencephalography, whose temporal resolution allows investigating the different stages of the timing processes. Implicit temporal predictability was induced in an auditory paradigm by a manipulation of the foreperiod. Participants received two consecutive task instructions: discriminate pitch (indirect measure of implicit timing) or duration (direct measure of explicit timing). The results show that the human brain efficiently extracts implicit temporal statistics of sensory environments, to enhance the behavioral and neural responses to auditory stimuli, but that those temporal predictions did not improve explicit timing. In both tasks, attentional orienting in time during predictive foreperiods was indexed by an increase in alpha power over visual and parietal areas. Furthermore, pretarget induced beta power in sensorimotor and parietal areas increased during implicit compared to explicit timing, in line with the suggested role for beta oscillations in temporal prediction. Interestingly, no distinct neural dynamics emerged when participants explicitly paid attention to time, compared to implicit timing. Our work thus indicates that implicit timing shapes the behavioral and sensory response in an automatic way and is reflected in oscillatory neural dynamics, whereas the translation of implicit temporal statistics to explicit durations remains somewhat inconclusive, possibly because of the more abstract nature of this task.

The orbitofrontal cortex in temporal cognition

One of the most important factors in decision-making is estimating the value of available options. Subregions of the prefrontal cortex, including the orbitofrontal cortex (OFC), have been deemed essential for this process. Value computations require a complex integration across numerous dimensions, including, reward magnitude, effort, internal state, and time. The importance of the temporal dimension is well illustrated by temporal discounting tasks, in which subjects select between smaller-sooner versus larger-later rewards. The specific role of OFC in telling time and integrating temporal information into decision-making remains unclear. Based on the current literature, in this review we reevaluate current theories of OFC function, accounting for the influence of time. Incorporating temporal information into value estimation and decision-making requires distinct, yet interrelated, forms of temporal information including the ability to tell time, represent time, create temporal expectations, and the ability to use this information for optimal decision-making in a wide range of tasks, including temporal discounting and wagering. We use the term "temporal cognition" to refer to the integrated use of these different aspects of temporal information. We suggest that the OFC may be a critical site for the integration of reward magnitude and delay, and thus important for temporal cognition. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Ketamine reduces temporal expectation in the rhesus monkey

RATIONALE

Ketamine, a well-known general dissociative anesthetic agent that is a non-competitive antagonist of the N-methyl-D-aspartate receptor, perturbs the perception of elapsed time and the expectation of upcoming events.

OBJECTIVE

The objective of this study was to determine the influence of ketamine on temporal expectation in the rhesus monkey.

METHODS

Two rhesus monkeys were trained to make a saccade between a central warning stimulus and an eccentric visual target that served as imperative stimulus. The delay between the warning and the imperative stimulus could take one of four different values randomly with the same probability (variable foreperiod paradigm). During experimental sessions, a subanesthetic low dose of ketamine (0.25-0.35 mg/kg) was injected i.m. and the influence of the drug on movement latency was measured.

RESULTS

We found that in the control conditions, saccadic latencies strongly decreased with elapsed time before the appearance of the visual target showing that temporal expectation built up during the delay period between the warning and the imperative stimulus. However, after ketamine injection, temporal expectation was significantly reduced in both subjects. In addition, ketamine also increased average movement latency but this effect could be dissociated from the reduction of temporal expectation.

CONCLUSION

In conclusion, a subanesthetic dose of ketamine could have two independent effects: increasing reaction time and decreasing temporal expectation. This alteration of temporal expectation could explain cognitive deficits observed during ketamine use.

The internal representation of temporal orienting: A temporal pulse-accumulation and attentional-gating-based account

Timing can be processed explicitly or implicitly. Temporal orienting is a typical implicit timing through which we can anticipate and prepare an optimized response to forthcoming events. It is, however, not yet clear whether mechanisms such as temporal-pulse accumulation and attentional gating (more attention, more accumulated temporal pulses) underly the internal representations of temporal orienting, as in explicit timing. To clarify this, a dual-task paradigm, consisting of a temporal orienting and an interference task, was adopted. Consistent with the temporal-pulse-accumulation and attentional-gating model, reaction times to the target detection of temporal orienting increased as the interference stimuli were temporally closer to the target, i.e., a location effect for temporal orienting. This effect is likely due to attention being diverted away from temporal orienting to monitor the occurrence of the interference stimuli for a longer time, resulting in greater temporal pulse loss and less accurate temporal orienting for conditions with later interference stimuli. The temporal-pulse-accumulation aspect in temporal orienting received further support by taking an explicit duration reproduction (containing a second temporal-pulse accumulation) as the interference task. On the one hand, temporal orienting became less accurate with increased temporal-pulse-accumulation overlaps between the dual tasks; on the other hand, two-way (one for temporal orienting and the other for duration reproduction), rather than one-way, location effects were observed, implying processing conflicts between the two temporal-pulse accumulations. Taken together, these results suggest that implicit and explicit timing may share common mechanisms upon internal temporal representations.

Perception of saccadic reaction time

That saccadic reaction times (SRTs) may depend on reinforcement contingencies has been repeatedly demonstrated. It follows that one must be able to discriminate one’s latencies to adequately assign credit to one’s actions, which is to connect behaviour to its consequence. To quantify the ability to perceive one’s SRT, we used an adaptive procedure to train sixteen participants in a stepping visual target saccade paradigm. Subsequently, we measured their RTs perceptual threshold at 75% in a conventional constant stimuli procedure. For each trial, observers had to saccade to a stepping target. Then, in a 2-AFC task, they had to choose one value representing the actual SRT, while the other value proportionally differed from the actual SRT. The relative difference between the two alternatives was computed by either adding or subtracting from the actual SRT a percent-difference value randomly chosen among a fixed set. Feedback signalling the correct choice was provided after each response. Overall, our results showed that the 75% SRT perceptual threshold averaged 23% (about 40 ms). The ability to discriminate small SRT differences provides support for the possibility that the credit assignment problem may be solved even for short reaction times.

Express saccades during a countermanding task

Express saccades are unusually short latency, visually guided saccadic eye movements. They are most commonly observed when the fixation spot disappears at a consistent, short interval before a target spot appears at a repeated location. The saccade countermanding task includes no fixation-target gap, variable target presentation times, and the requirement to withhold saccades on some trials. These testing conditions should discourage production of express saccades. However, two macaque monkeys performing the saccade countermanding task produced consistent, multimodal distributions of saccadic latencies. These distributions consisted of a longer mode extending from 200 ms to as much as 600 ms after target presentation and another consistently less than 100 ms after target presentation. Simulations revealed that, by varying express saccade production, monkeys could earn more reward. If express saccades were not rewarded, they were rarely produced. The distinct mechanisms producing express and longer saccade latencies were revealed further by the influence of regularities in the duration of the fixation interval preceding target presentation on saccade latency. Temporal expectancy systematically affected the latencies of regular but not of express saccades. This study highlights that cognitive control can integrate information across trials and strategically elicit intermittent very short latency saccades to acquire more reward.NEW & NOTEWORTHY A serendipitous discovery that macaque monkeys produce express saccades under conditions that should discourage them reveals how cognitive control can adapt behavior to maximize reward.

Temporal Preparation, Impulsivity and Short-Term Memory in Depression

Patient suffering of major depressive disorder (MDD) often complain that subjective time seems to "drag" with respect to physical time. This may point toward a generalized dysfunction of temporal processing in MDD. In the present study, we investigated temporal preparation in MDD. "Temporal preparation" refers to an increased readiness to act before an expected event; consequently, reaction time should be reduced. MDD patients and age-matched controls were required to make a saccadic eye movement between a central and an eccentric visual target after a variable duration preparatory period. We found that MDD patients produced a larger number of premature saccades, saccades initiated prior to the appearance of the expected stimulus. These saccades were not temporally controlled; instead, they seemed to reflect reduced inhibitory control causing oculomotor impulsivity. In contrast, the latency of visually guided saccades was strongly influenced by temporal preparation in controls; significantly less so, in MDD patients. This observed reduced temporal preparation in MDD was associated with a faster decay of short-term temporal memory. Moreover, in patients producing a lot of premature responses, temporal preparation to early imperative stimuli was increased. In conclusion, reduced temporal preparation and short-term temporal memory in the oculomotor domain supports the hypothesis that temporal processing was altered in MDD patients. Moreover, oculomotor impulsivity interacted with temporal preparation. These observed deficits could reflect other underlying aspects of abnormal time experience in MDD.

Short-term temporal memory in idiopathic and Parkin-associated Parkinson's disease

In a rapidly changing environment, we often know when to do something before we have to do it. This preparation in the temporal domain is based on a ‘perception’ of elapsed time and short-term memory of previous stimulation in a similar context. These functions could be perturbed in Parkinson’s disease. Therefore, we investigated their role in eye movement preparation in sporadic Parkinson’s disease and in a very infrequent variant affecting the Parkin gene. We used a simple oculomotor task where subjects had to orient to a visual target and movement latency was measured. We found that in spite of an increased average reaction time, the influence of elapsed time on movement preparation was similar in controls and the two groups of PD patients. However, short-term temporal memory of previous stimulation was severely affected in sporadic PD patients either ON or OFF dopaminergic therapy. We conclude that the two different contributions to temporal preparation could be dissociated. Moreover, a short-term temporal memory deficit might underlie temporal cognition deficits previously observed in PD.

Stopping smooth pursuit

If a visual object of interest suddenly starts to move, we will try to follow it with a smooth movement of the eyes. This smooth pursuit response aims to reduce image motion on the retina that could blur visual perception. In recent years, our knowledge of the neural control of smooth pursuit initiation has sharply increased. However, stopping smooth pursuit eye movements is less well understood and will be discussed in this paper. The most straightforward way to study smooth pursuit stopping is by interrupting image motion on the retina. This causes eye velocity to decay exponentially towards zero. However, smooth pursuit stopping is not a passive response, as shown by behavioural and electrophysiological evidence. Moreover, smooth pursuit stopping is particularly influenced by active prediction of the upcoming end of the target. Here, we suggest that a particular class of inhibitory neurons of the brainstem, the omnipause neurons, could play a central role in pursuit stopping. Furthermore, the role of supplementary eye fields of the frontal cortex in smooth pursuit stopping is also discussed.This article is part of the themed issue 'Movement suppression: brain mechanisms for stopping and stillness'.

Distinct developmental trajectories for explicit and implicit timing

Adults and children aged 5 and 8years were given explicit and implicit timing tasks. These tasks were based on the same temporal representation (the temporal interval between two signals), but in the explicit task participants received overt instructions to judge the duration of the interval, whereas in the implicit task they did not receive any temporal instructions and were asked only to press as quickly as possible after the second signal. In addition, participants' cognitive capacities were assessed with different neuropsychological tests. The results showed that temporal variability (i.e., the spread of performance around the reference interval) decreased as a function of age in the explicit task, being higher in the 5-year-olds than in the 8-year-olds and adults. The higher variability in the youngest children was directly linked to their limited cognitive capacity. By contrast, temporal variability in the implicit timing task remained constant across the different age groups and was unrelated to cognitive capacity. Processing of time, therefore, was independent of age in the implicit task but changed with age in the explicit task, thereby demonstrating distinct developmental trajectories for explicit and implicit timing.

A subanesthetic dose of ketamine in the Rhesus monkey reduces the occurrence of anticipatory saccades

RATIONALE

It has been shown that antagonism of the glutamatergic N-methyl-D-aspartate (NMDA) receptor with subanesthetic doses of ketamine perturbs the perception of elapsed time. Anticipatory eye movements are based on an internal representation of elapsed time. Therefore, the occurrence of anticipatory saccades could be a particularly sensitive indicator of abnormal time perception due to NMDA receptors blockade.

OBJECTIVES

The objective of this study was to determine whether the occurrence of anticipatory saccades could be selectively altered by a subanesthetic dose of ketamine.

METHODS

Three Rhesus monkeys were trained in a simple visually guided saccadic task with a variable delay. Monkeys were rewarded for making a visually guided saccade at the end of the delay. Premature anticipatory saccades to the future position of the eccentric target initiated before the end of the delay were not rewarded. A subanesthetic dose of ketamine (0.25 mg/kg) or a saline solution of the same volume was injected i.m. during the task.

RESULTS

We found that the injected dose of ketamine did not induce sedation or abnormal behavior. However, in ∼4 min, ketamine induced a strong reduction of the occurrence of anticipatory saccades but did not reduce the occurrence of visually guided saccades.

CONCLUSION

This unexpected reduction of anticipatory saccade occurrence could be interpreted as resulting from an altered use of the perception of elapsed time during the delay period induced by NMDA receptors antagonism.