Biology subjects

Forrence, A. D.

Publications and source records attributed to Forrence, A. D..

4 recordsLinked to original sources

bioRxiv2022

Action inhibition revisited: Stopping is not faster than going

Much work has examined the process of canceling or inhibiting an impending action. However, the converse process of initiating a movement has, comparatively, been much less studied. Action initiation and action inhibition are generally considered to be independent and qualitatively distinct processes, while the exact relationship between these processes remains unclear. One respect in which action initiation and action inhibition are often thought to differ is in their speed; action inhibition is typically considered to be faster than action initiation, which allows for impending actions to be inhibited before they are initiated. This apparent contrast is, however, largely observed in tasks in which there is much greater urgency to inhibit an action that there is to initiate an action. This asymmetry in the urgency between action initiation and action inhibition, as well as other asymmetries in how action initiation and inhibition are cued, make it impossible to compare their relative time courses. Here, we demonstrate that, when action initiation and action inhibition are measured under conditions that are matched as closely as possible, their speed is the same. In light of this, we suggest that action initiation and action inhibition may not necessarily be qualitatively distinct processes, but may instead reflect two opposing states of a single process supporting a decision about whether to act or not. This perspective carries significant implications for computational models and presumed neural mechanisms of action control.

neuroscience↗

bioRxiv2021

Learning from the path not taken: Sensory prediction errors are sufficient for implicit adaptation of withheld movements

Prediction errors guide many forms of learning, providing teaching signals that help us improve our performance. Implicit motor adaptation, for instance, is driven by sensory prediction errors (SPEs), which occur when the expected and observed consequences of a movement differ. Traditionally, SPE computation is thought to require movement execution. However, recent work suggesting that the brain generates and accounts for sensory predictions based on motor imagery or planning alone calls this assumption into question. Here, by measuring implicit adaptation during a visuomotor task, we tested whether motor planning and well-timed sensory feedback are sufficient for SPE computation. Human participants were cued to reach to a target and were, on a subset of trials, rapidly cued to withhold these movements. Errors displayed both on trials with and without movements induced single-trial implicit learning. Learning following trials without movements persisted even when movement trials had never been paired with errors, and when the direction of movement and sensory feedback trajectories were decoupled. These observations demonstrate that the brain can compute SPEs without generating overt movements, leading to the adaptation of planned movements even when they are not performed. SIGNIFICANCE STATEMENTWe are always learning from our mistakes, because the brain is constantly generating predictions and monitoring the world for any surprises, which are also referred to as "prediction errors." Whenever a prediction error occurs, the brain learns to update future predictions and be more accurate. Here, we demonstrate that the brain predicts the consequences of movements, computes prediction errors, and updates future movements, even if we subsequently decide to withhold the movement. Thus, the brain can learn to update movements that are not performed, representing a mechanism for learning based only on movement planning and sensory expectation. These findings also provide further support for the role of prediction in motor control. SIGNIFICANCE STATEMENTOur brains control aspects of our movement without our conscious awareness - allowing many of us to effortlessly pick up a glass of water or wave "hello." Here, we demonstrate that this implicit motor system can learn to refine movements that we plan but ultimately decide not to perform. Participants planned to reach to a target, and they sometimes withheld these reaches. When reaches were withheld, an animation simulating a reach that missed the target played. Afterwards, participants reached opposite the direction of the mistake without awareness of this change in their movements, indicating that the implicit motor system had learned from the animated mistake. These findings indicate that movement is not strictly necessary for motor adaptation, and that we can learn to update our actions based only on movement planning and observation of related events in the world.

neuroscience↗

bioRxiv2021

Age-related increases in reaction time result from slower preparation, not delayed initiation

Recent work indicates that healthy younger adults can prepare accurate responses faster than their voluntary reaction times indicate, leaving a seemingly unnecessary delay of 80-100ms before responding. Here we examined how the preparation of movements, initiation of movements, and the delay between them are affected by ageing. Participants made planar reaching movements in two conditions. The Free Reaction Time condition assessed the voluntary reaction times with which participants responded to the appearance of a stimulus. The Forced Reaction Time condition assessed the minimum time actually needed to prepare accurate movements by controlling the time allowed for movement preparation. The time taken to both initiate movements in the Free Reaction Time and to prepare movements in the Forced Response condition increased with age. Notably, the time required to prepare accurate movements was significantly shorter than participants self-selected initiation times; however, the delay between movement preparation and initiation remained consistent across the lifespan (~90ms). These results indicate that the slower reaction times of healthy older adults are not due to an increased hesitancy to respond, but can instead be attributed to changes in their ability to process stimuli and prepare movements accordingly, consistent with age-related changes in brain structure and function.

neuroscience↗

bioRxiv2017

Skill Acquisition and Habit Formation as Distinct Effects of Practice

Converging evidence indicates that separate goal-directed and habitual systems compete to control behavior1. However, it has proven difficult to reliably induce habitual behavior in human participants2-4. We reasoned that habits may be present in the form of habitually prepared responses, but are overridden by goal-directed processes, preventing their overt expression. Here we show that latent habits can be unmasked by limiting the time participants have to respond to a stimulus. Participants trained for 4 days on a visuomotor association task. By continuously varying the time allowed to prepare responses, we found that the probability of expressing a learned habit followed a stereotyped time course, peaking 300-600ms after stimulus presentation. This time course was captured by a computational model of response preparation in which habitual responses are automatically prepared at short latency, but are replaced by goal-directed responses at longer latency. A more extensive period of practice (20 days) led to increased habit expression by reducing the average time of movement initiation. These findings refine our understanding of habits, and show that practice can influence habitual behavior in distinct ways: by promoting habit formation, and by modulating the likelihood of habit expression.

neuroscience↗