Ablation of the cerebellum does not abolish locomotion in mammals (Ioffe 2013); it merely induces atonia: body movements become clumsy with postural and vestibular deficits, which is related to the negation of both proprioceptive and vestibular input to the cerebellum, which encodes where the body is with respect to itself and the outside world, i.e., with respect to the gravitational axis (Carriot et al. 2021; Demontis et al. 2017; Fuchs and Kornhuber 1969; Lawson et al. 2016; Miles and Lisberger 1981). Animals have difficulty crossing a balance beam following complete cerebellar damage and the righting reflex is interrupted. Consciousness, which is a declarative attribute, is not affected following cerebellar damage (D’Angelo and Casali 2013; Petrosini et al. 1998; Tononi and Edelman 1998). As with cerebellar impairment, following neocortical ablation, locomotion is not eliminated but the sequencing of movement is severely affected (Vanderwolf 2007; Vanderwolf et al. 1978). Stepping responses can be evoked in spinal animals, but with a total loss of balance and muscular coordination since both cerebellar and neocortical support is now absent (Audet et al. 2022; Grillner 2003; Sherrington 1910).

Following a stroke that affected the left mediolateral and posterior lobes of the cerebellar cortex (including the left dentate nucleus), it was found that the subject (aged 72), a (right handed) war correspondent who had been versed in seven languages, could no longer communicate in his non-primary languages (see Fig. 1, Mariën et al. 2017): French, German, Slovenian, Serbo-Croatian, Hebrew, and Dutch (in the order of having learned the languages before the age of 40). Before the stroke, the subject used Dutch, French, and English regularly. After the stroke his primary language, English, remained intact. Most significantly, the day of the stroke, all thinking in the second languages was abolished (see Footnote 1). One day following the stroke, however, the French language returned. Nevertheless, the remaining secondary languages were abnormal. Reading was better preserved than oral and written language, likely because reading is dependent mainly on scanning a page with the eyes and having an intact neocortex for word comprehension (fMRI revealed language activations in neocortex and in the intact right cerebellar hemisphere, Mariën et al. 2017). Speaking and writing, on the other hand, are more dependent on the sequencing of multiple muscle groups, a task of the cerebellum (Heck and Sultan 2002; Sultan and Heck 2003; Thatch et al. 1992). When speaking or writing in a non-primary language, English words would intrude. The naming of objects and actions verbally was impaired, and writing was severely disrupted. When high-frequency visual stimuli (objects, animals, etc.) were presented visually (1 month after the stroke), identifying an object with the correct word surpassed 80% correctness for English, French, and Dutch, whereas it remained at under 20% correctness for German, Slovenian, Serbo-Croatian, and Hebrew. Since the execution of behavior depends on loop integrity between the neocortex and cerebellum (Hasanbegović 2024), it is highly likely that damage to the cerebellum undermined this integrity such that the least overlearned routines—German, Slovenian, Serbo-Croatian, and Hebrew—were disturbed. Note that a functional left neocortex (of the right-handed subject) with a preserved right cerebellum was sufficient to execute the overlearned languages—English, French, and Dutch.

Based on our understanding of cerebellar function, if the entire cerebellum (including the subjacent nuclei) were damaged in the subject, we would expect that even English, the primary language, would be compromised, and most importantly, the learning of a new language would be rendered impossible, given the dependence of behavioral executions (and learning) on intact neocortical-cerebellar loops (Hasanbegović 2024; also see: Sendhilnathan and Goldberg 2000b; Thach et al. 1992). Thus, thinking is affected by damage to neocortical-cerebellar loops, which concurs with the behavioral findings of Hasanbegović (2024).

Footnote 1: Self-report by the patient about the day of the cerebellar stroke: “I was watching television at my apartment in Antwerp when suddenly the room seemed to spin around violently. I tried to stand but was unable to do so. I felt a need to vomit and managed to crawl to the bathroom to take a plastic bowl. My next instinct was to call the emergency service, but the leaflet I have outlining the services was in Dutch and for some reason, I was unable to think (or speak) in any language other than my native English. I have lived in Antwerp for many years and use Dutch (Flemish) on a day-to-day basis. I called my son-in-law, who speaks fluent English and he drove me to Middelheim Hospital. We normally speak English when together. I understood none of the questions asked to me in Dutch by hospital staff and they had to be translated back to me in English. My speech was slurred. I had lost some words, I was aware of that, but I cannot recall which words. I made no attempt to speak any of the other languages I know, and in the first hours of my mishap happening, I do not think I realized that I had other languages.” (Mariën et al. 2017, p. 19)

Figure 1. Human cerebellar cortex. The mediolateral and posterior lobes are indicated. The mediolateral lobe of the cerebellum (right and left) is part of the cortico-frontal-cerebellar language loop (Stoodley and Schmahmann 2009), and cerebellar grey matter density in bilingual speakers is correlated with language proficiency (Pliatsikas et al. 2014). Typically, the innervation of the left neocortical language areas is strongest to the right cerebellum in right-handed subjects (Van Overwalle et al. 2023). Illustration from figure 8 of Tehovnik, Patel, Tolias et al. (2021).