Dissociating the Roles of the Cerebellum and Motor Cortex during Adaptive Learning: The Motor Cortex Retains What the Cerebellum Learns

Abstract

Adaptation to a novel visuomotor transformation has revealed important principles regarding learning and memory. Computational and behavioral studies have suggested that acquisition and retention of a new visuomotor transformation are distinct processes. However, this dissociation has never been clearly shown. Here, participants made fast reaching movements while unexpectedly a 30-degree visuomotor transformation was introduced. During visuomotor adaptation, subjects received cerebellar, primary motor cortex (M1) or sham anodal transcranial direct current stimulation (tDCS), a noninvasive form of brain stimulation known to increase excitability. We found that cerebellar tDCS caused faster adaptation to the visuomotor transformation, as shown by a rapid reduction of movement errors. These findings were not present with similar modulation of visual cortex excitability. In contrast, tDCS over M1 did not affect adaptation, but resulted in a marked increase in retention of the newly learnt visuomotor transformation. These results show a clear dissociation in the processes of acquisition and retention during adaptive motor learning and demonstrate that the cerebellum and primary motor cortex have distinct functional roles. Furthermore, they show that is possible to enhance cerebellar function using tDCS.

Effects of tDCS on motor learning and memory formation: A consensus and critical position paper

“…tDCS-related effects on adaptation have also been studied in young healthy adults (Avila et al, 2015;Galea et al, 2011;Herzfeld et al, 2014;Hunter, Sacco, Nitsche, & Turner, 2009;Orban de Xivry et al, 2011) (Table 3). Galea and colleagues (2011) compared the effects of anodal tDCS applied to the cerebellum versus M1 during concurrent adaptation to 30-degree rotation of visual feedback (Galea et al, 2011).…”

“…Galea and colleagues (2011) compared the effects of anodal tDCS applied to the cerebellum versus M1 during concurrent adaptation to 30-degree rotation of visual feedback (Galea et al, 2011). Here, cerebellar tDCS resulted in faster initial adaptation to the perturbed task environment, while M1 stimulation showed no effect in this regard.…”

“…tDCS has also been used as a tool to gain insight into brain-behavior interactions and to explore possible causal relationships between altered activity in relatively large regions of the brain and particular behaviors . More specifically, tDCS has been used to study effects on, and mechanisms of, motor learning (Antal et al, 2004;Galea, Vazquez, Pasricha, Orban De Xivry, & Celnik, 2011;Nitsche et al, 2003;Reis et al, 2009). In a previous consensus document, it was stated that "Improved understanding of the involvement of a brain region in a type of behavior was followed by attempts to modify activity … to secondarily influence performance, learning and memory functions" (Reis et al, 2008).…”

“…Several recommendations from that paper have been advanced in the literature. For example, many studies have utilized multi-session rather than single-session tDCS application (Hashemirad et al, 2016), greater emphasis has been placed on monitoring long-term effects of tDCS on motor learning (Hashemirad et al, 2016), evidence of dissociation of tDCS effects applied to distinct brain regions on different stages of motor learning (Galea et al, 2011;Reis et al, 2015;Wymbs, Bastian, & Celnik, 2016) has begun to emerge, and mechanisms underlying tDCS effects are starting to be elucidated (Fritsch et al, 2010; H. I. Kuo et al, 2013; M. F. Kuo et al, 2008;Lang, Nitsche, Sommer, Tergau, & Paulus, 2003;Stagg, Bachtiar, & Johansen-Berg, 2011).…”

Multiple sessions of transcranial direct current stimulation and upper extremity rehabilitation in stroke: A review and meta-analysis

“…They also enhance motor performance and can change reaction times, movement accuracy and speed (Nitsche et al, 2003b;Kobayashi et al, 2004). More importantly, in the context of possible therapeutic application, they can improve motor skill learning (Reis and Fritsch, 2011;Teo et al, 2011) or adapt already learned skills to new conditions (Galea et al, 2011). There has therefore been considerable interest in examining the potential of these interventions to augment recovery of motor function after stroke.…”