A convergence of data across multiple studies of patients with cerebellar damage indicates three primary patterns of results. First, in these types of studies, cognitive deficits have been subtle, often falling within the low-normal range (Justus et al. 2005
; Leggio et al. 2008
; Ravizza et al. 2006
). When patients have been compared to well-matched controls, however, deficits become more obvious. Second, verbal working memory is consistently, and sometimes disproportionately, affected in this population (Chiricozzi et al. 2008
; Justus et al. 2005
; Kirschen et al. 2008
; Leggio et al. 2008
; Ravizza et al. 2006
; Silveri et al. 1998
). Third, there appears to be a hemispheric dissociation of function within the inferior lobes of the cerebellum: the left side is more strongly involved in the processing of aurally presented information, and the right side is more strongly involved in the processing of visually presented information. This third point is expanded upon below.
Following a stroke localized to the right cerebellar lobule V and the left cerebellar lobule VIIIA, a 54 year old male was given a battery of cognitive tests nine days after the stroke event (Chiricozzi et al. 2008
). Performance on many tests fell within the normal range, including tests of intelligence, constructional praxia, frontal lobe function, verbal/visuospatial spans, and verbal recall when information was presented in the visual modality. However, the patient showed a specific deficit for verbal recall (immediate and delayed) when information was presented in the auditory modality. A follow-up battery of tests administered 18 months later indicated that persistent working memory deficits involved an inability to rehearse phonological information held in short-term storage, which was pronounced for information presented in the auditory modality.
A similar link between left inferior cerebellar lobule VIII and auditory working memory performance was found in a study of 12 children (ages 6–19 years) with cerebellar tumors resections (Kirschen et al. 2008
). A lobular analysis of the regions of cerebellar damage indicated, after Bonferroni correction for multiple comparisons, that damage to the left inferior cerebellar lobule VIII significantly predicted impairment of auditory digit span. An additional test of working memory was conducted using a phonological similarity paradigm. The phonological similarity paradigm measures one’s ability to hold information in mind despite interference from phonologically similar material (e.g., B, C, D, G, P, T,V) (Baddeley 1966
). The presence of interference, known as the phonological similarity effect (PSE), is a healthy effect and is manifested as better immediate recall of dissimilar (e.g., F, K, Q, R, X, W, Z) versus similar material. In this study, three patients showed particularly low performance on the phonological similarity test (more than 3 SD below the mean) for aurally presented material, and three other patients showed similarly low performance for visually presented material. A conjunction analysis of the lesions for each of these three subjects in each modality was performed. All subjects had in common damage to the inferior cerebellar lobules VIIB/VIII. However, the modality affected was specific to hemisphere: those with deficits in the auditory modality shared damage in the left inferior cerebellum, and those with deficits in the visual modality shared damage in the right inferior cerebellum. These findings of a left inferior cerebellar hemisphere bias for auditory information were consistent with the case report described above. These findings additionally implicated a right inferior cerebellar hemisphere bias for visual information in working memory.
A right-lateralized visual modality effect was further supported in the study of an 18-year-old male with a right inferior cerebellar tumor resection (Silveri et al. 1998
). Three days after surgery, the patient showed an advantage of auditory over visual presentation of stimuli in a test of phonological similarity. Thus, the PSE was present for auditory stimuli but not for visual stimuli, and therefore indicated a specific deficit in the visual modality in association with a right inferior cerebellar lesion. Follow-up testing 5 months later showed that plasticity had occurred, and the PSE was intact for both modalities.
Finally, neuropsychological studies of older patients suggest that the hemispheric asymmetry of function in the inferior cerebellum described above is reduced as a function of age. In a study by Ravizza et al. (2006)
of fifteen patients (mean age = 60) with cerebellar damage, patients were tested on several working memory paradigms, including auditory digit span. Performance on digit span was negatively correlated with damage to the inferior cerebellar lobe, consistent with the role of the inferior lobe assisting with phonological storage and working memory functions. However, there was no relation between lesion hemisphere and performance: patients with right-sided inferior cerebellar lesions were just as impaired on this auditory working memory task as were those with left-sided lesions. It should be noted that a test of visual digit span was not administered, prohibiting a direct comparison of auditory versus visual span performance. A separate study of 10 cerebellar patients (mean age = 67) by Justus et al. (2005)
tested phonological similarity, with stimuli delivered aurally and visually. They reported overall reductions in the PSE, but did not find evidence for laterality effects that were associated with stimulus modality. Laterality effects may have been diminished by several factors in this study, including variability of the lesion within the hemisphere (i.e., not specific to the inferior cerebellum), and the inclusion of only one left-hemisphere patient in the visual portion of the study. However, given that both the Ravizza et al. and Justus et al. studies tested older patients (relative to the tumor resection studies described previously), negative laterality effects in older subjects may simply mean that laterality in the cerebellum attenuates with age. A reduction in laterality in the aging cerebellum would be consistent with age-related activation shifts from the left prefrontal cortex to bilateral prefrontal cortex during verbal working memory (Cabeza 2002
; Reuter-Lorenz et al. 2000
Given that processing of language-related stimuli in the neocortex is typically lateralized on the left side, and that cortico-ponto-cerebellar projections have been shown to be predominately (though not exclusively) crossed in animal tracing studies, the dissociation of stimulus modality and hemispheric lateralization observed in verbal working memory deficits is unexpected. That is, verbal working memory impairment for visually presented letters resulting from right cerebellar damage would be predicted based on the crossed cortico-cerebellar connectivity from left-sided language centers. However, similar impairment for aurally-presented letters after left cerebellar damage would not be predicted. Further research on this puzzling pattern is required, but one possible explanation may be that encoding of aurally-presented letters involves greater demand on sensory discrimination than encoding of visually-presented letters, especially for phonologically similar letters (e.g., “P” and “T” are more similar in sound than in appearance). The importance of the cerebellum in sensory acquisition and discrimination has been emphasized in several studies, and left cerebellar hemisphere activations pertaining to sensory discrimination have been reported (Belin et al. 1998
; Bower 1997
; Gao et al. 1996
; Holcomb et al. 1998
; Parsons et al. 1997