Dissociation of motor and sensory inhibition processes in normal aging

OBJECTIVE

Age-related cognitive impairments have been attributed to deficits in inhibitory processes that mediate both motor restraint and sensory filtering. However, behavioral studies have failed to show an association between tasks that measure these distinct types of inhibition. In the present study, we hypothesized neural markers reflecting each type of inhibition may reveal a relationship across inhibitory domains in older adults.

METHODS

Electroencephalography (EEG) and behavioral measures were used to explore whether there was an across-participant correlation between sensory suppression and motor inhibition. Sixteen healthy older adult participants (65-80 years) engaged in two separate experimental paradigms: a selective attention, delayed-recognition task and a stop-signal task.

RESULTS

Findings revealed no significant relationship existed between neural markers of sensory suppression (P1 amplitude; N170 latency) and markers of motor inhibition (N2 and P3 amplitude and latency) in older adults.

CONCLUSIONS

These distinct inhibitory domains are differentially impacted in normal aging, as evidenced by previous behavioral work and the current neural findings. Thus a generalized inhibitory deficit may not be a common impairment in cognitive aging.

SIGNIFICANCE

Given that some theories of cognitive aging suggest age-related failure of inhibitory mechanisms may span different modalities, the present findings contribute to an alternative view where age-related declines within each inhibitory modality are unrelated.

A spatial explicit strategy reduces error but interferes with sensorimotor adaptation

Although sensorimotor adaptation is typically thought of as an implicit form of learning, it has been shown that participants who gain explicit awareness of the nature of the perturbation during adaptation exhibit more learning than those who do not. With rare exceptions, however, explicit awareness is typically polled at the end of the study. Here, we provided participants with either an explicit spatial strategy or no instructions before learning. Early in learning, explicit instructions greatly reduced movement errors but also resulted in increased trial-to-trial variability and longer reaction times. Late in adaptation, performance was indistinguishable between the explicit and implicit groups, but the mechanisms underlying performance improvements remained fundamentally different, as revealed by catch trials. The progression of implicit recalibration in the explicit group was modulated by the use of an explicit strategy: these participants showed a lower level of recalibration as well as decreased aftereffects. This phenomenon may be due to the reduced magnitude of errors made to the target during adaptation or inhibition of implicit learning mechanisms by explicit processing.

Failure to engage spatial working memory contributes to age-related declines in visuomotor learning

It is well documented that both cognitive and motor learning abilities decline with normative aging. Given that cognitive processes such as working memory are engaged during the early stages of motor learning [Anguera, J., Reuter-Lorenz, P., Willingham, D., & Seidler, R. Contributions of spatial working memory to visuomotor learning. Journal of Cognitive Neuroscience, 22(9), 1917-1930, 2010], age-related declines in motor learning may be due in part to reductions in cognitive ability. The present study examined whether age-related declines in spatial working memory (SWM) contribute to deficits in visuomotor adaptation. Young and older adult participants performed a visuomotor adaptation task that involved adapting manual aiming movements to a 30° rotation of the visual feedback display as well as an SWM task in an fMRI scanner. Young adults showed a steeper learning curve than older adults during the early adaptation period. The rate of early adaptation was correlated with SWM performance for the young, but not older, adults. Both groups showed similar brain activation patterns for the SWM task, including engagement of the right dorsolateral prefrontal cortex and bilateral inferior parietal lobules. However, when the SWM activation was used as a limiting mask, younger adults showed neural activation that overlapped with the early adaptation period, whereas older adults did not. A partial correlation controlling for age revealed that the rate of early adaptation correlated with the amount of activation at the right dorsolateral prefrontal cortex. These findings suggest that a failure to effectively engage SWM processes during learning contributes to age-related deficits in visuomotor adaptation.

Contributions of Spatial Working Memory to Visuomotor Learning

Previous studies of motor learning have described the importance of cognitive processes during the early stages of learning; however, the precise nature of these processes and their neural correlates remains unclear. The present study investigated whether spatial working memory (SWM) contributes to visuomotor adaptation depending on the stage of learning. We tested the hypothesis that SWM would contribute early in the adaptation process by measuring (i) the correlation between SWM tasks and the rate of adaptation, and (ii) the overlap between the neural substrates of a SWM mental rotation task and visuomotor adaptation. Participants completed a battery of neuropsychological tests, a visuomotor adaptation task, and an SWM task involving mental rotation, with the latter two tasks performed in a 3.0-T MRI scanner. Performance on a neuropsychological test of SWM (two-dimensional mental rotation) correlated with the rate of early, but not late, visuomotor adaptation. During the early, but not late, adaptation period, participants showed overlapping brain activation with the SWM mental rotation task, in right dorsolateral prefrontal cortex and the bilateral inferior parietal lobules. These findings suggest that the early, but not late, phase of visuomotor adaptation engages SWM processes.

Plastic changes in hand proprioception following force-field motor learning

Motor neurophysiologists are placing greater emphasis on sensory feedback processing than ever before. In line with this shift, a recent article by Ostry and colleagues provided timely new evidence that force-field motor learning influences not only motor output, but also proprioceptive sense. In this Neuro Forum, the merits and limitations of Ostry and colleagues are explored in the context of recent work on proprioceptive function, including several recent studies from this journal.

Changes in performance monitoring during sensorimotor adaptation

Error detection and correction are essential components of motor skill learning. These processes have been well characterized in cognitive psychology using electroencephalography (EEG) to record an event-related potential (ERP) called error-related negativity (ERN). However, it is unclear whether this ERP component is sensitive to the magnitude of the error made in a sensorimotor adaptation task. In the present study, we tested the function of error-related activity in a visuomotor adaptation task. To examine whether error size is reflected in the ERP, two groups of participants adapted manual aiming movements to either a small (30 degrees) or large (45 degrees) rotation of the visual feedback display. Each participant's trials were sorted into large and small error trials using a median split to examine potential error magnitude waveform differences. We also examined these trial types at the early and late stages of adaptation. There were no group differences for the behavioral or neural measures; however, waveforms from large error trials were significantly different from small error trials. The waveforms also changed as a function of practice as early adaptation waveforms were larger than late adaptation waveforms. The observed ERP component reflected differences in error magnitude with the amount of activity corresponding to the size of the error. Movement monitoring potentials likely affected the frequency and time course of the waveform so that it did not resemble the typical ERN; however, error-related activity was still distinguishable. The present findings are discussed in terms of current theories of the ERN as well as skill acquisition.

Neural correlates associated with intermanual transfer of sensorimotor adaptation

Investigations of intermanual transfer of learning have demonstrated that individuals can transfer acquired motor skills from one hand to the other. The purpose of the current study was to use fMRI to investigate the potential overlap of neural regions engaged during learning and at transfer of learning from the dominant arm to the non-dominant arm during sensorimotor adaptation. Participants performed a visuomotor adaptation joystick task where they adapted manual aiming movements to a 30 degrees rotation of the visual feedback display. They performed eleven blocks (24 trials/block) of right-hand adaptation before performing the task with their left hand (transfer). Participants showed a selective transfer of learning effect: prior right-hand practice led to reduced endpoint errors but not trajectory errors for the left hand. This is consistent with work showing that the right arm is specialized for trajectory control while the left is specialized for endpoint control [Sainburg, R.L., 2005. Handedness, Differential specializations for control of trajectory and position. Exerc Sport Sci Rev 33, 206-213.]. Early adaptation processes were associated with activation in frontal and parietal regions, including bilateral dorsal premotor cortex. At transfer, activation was seen in the temporal cortex as well as the right medial frontal gyrus and the middle occipital gyrus. These regions have been observed in other studies during the late phases of sensorimotor adaptation. Integrating these data with the existing literature, we suggest that the left dorsal premotor cortex contributes to trajectory control, while the left visual and temporal cortices contribute to endpoint control.