That the Purkinje circuitry establishes the efference-copy code is believed to hold true from fishes to primates (Bell et al. 1997; De Zeeuw 2021; Gallistel et al. 2022; Giovannucci et al. 2017; Loyola et al. 2019; Shadmehr 2020; Tehovnik et al. 2021; Wang et al. 2023). The consolidation of declarative memory occurs during a period of spindle generation at the level of the neocortex/hippocampus, especially at the time of non-REM sleep (Canto, De Zeeuw et al. 2017; Logothetis et al. 2012; Ólafsdóttir et al. 2017; Wilson and McNaughton 1994; Yu et al. 2017), an activity that causally consolidates declarative memories (Girardeau et al. 2009; Logothetis et al. 2012; Vorster and Born 2015).
Canto et al. (2023) recorded EMGs from the neck muscles of head-fixed mice during wakefulness and sleep. During NREM sleep the muscle twitches were associated with the generation of complex spikes within the cerebellum. The onset of complex spike activity started just before the onset of a muscle twitch, but the activity continued to increase after the twitch (Fig. 1 from Fig. 4 of Canto et al. 2023). The authors speculated that a command to twitch is sent from the neocortex to the cerebellum (the efference copy) and to the muscles to initiate a twitch. After twitch onset, the proprioceptors send a feedback signal back to the cerebellum thereby further increasing the cerebellar complex-spike response (Fig. 1 see Fig. 4 of Canto et al. 2023). This merging of feedforward and feedback signals at the Purkinje cells reinforces the efference-copy code. The window of reinforcement occurs over a period of at least one second, starting 500 ms before a twitch and ending 500 ms after a twitch. The twitch period correlates with the period of spindle generation at the neocortex during NREM. Canto et al. (2023) suggest that the cerebellar spindle activity is initiated by the neocortex, which stores all declarative memories, a process that is synchronized with the cerebellar storage of information (via cerebellar spindle formation) as executable code for converting declarative information into a motor response (James 1890; Tehovnik, Hasabegović, Chen 2023).
It is noteworthy that during NREM, Canto et al. (2023) observed both decreases and increases in simple spike firing by Purkinje neurons. By focusing only on decreases in simple spike firing (i.e., caused by the increases in complex spike firing) the authors have missed an opportunity to understand the entire process of consolidation at the level of the cerebellum. We believe that motor routines through efference-copy development depend on both increases and decreases in complex spike activity to shape the entire motor response, which depends on changes in muscle excitation as well as inhibition (Bell et al. 1997; De Zeeuw 2021; Gallistel et al. 2022; Giovannucci et al. 2017; Loyola et al. 2019; Shadmehr 2020; Tehovnik et al. 2021; Wang et al. 2023). Future studies on this topic need to include both the excitation and the inhibition of the muscles conditioned for a behavior through the memory of learning.
Figure 1: Shown are calcium transients that represent complex spike activity in the cerebellum of head-fixed mice that were monitored during wakefulness and sleep. Twitch activity from the neck muscles was recorded while measuring the calcium transients that started before a muscle twitch and that continued to increase in magnitude after the muscle twitch during NREM 1, 2, and 3. EMG activity and triggered transients is plotted with respect to the time before and after a neck muscle twitch. From Canto et al. (2023).
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