The Cerebellum and executive functioning: evidence for a dysmetria of thought?

Recent research into the cerebellum has suggested a role in executive functions, although the exact nature of this contribution remains unclear. Previous studies reporting reduced performances on executive measures have typically failed to identify the specific cognitive processes involved, or exclude the confounding effects of lower level skill impairments. This study presents a 34-year-old male who has documented executive impairments following the discovery of an arachnoid cyst overlaying his left cerebellum. Using the verbal fluency task as an exemplar, specific measures regarding level and maintenance of output across time, number of rule-breaking errors, degree of clustering and switching, and word retrieval efficiency were investigated across five different forms of verbal fluency to examine executive functions. Contrary to expectation, little evidence of specific executive dysfunction was found. While some disorganisation in output could be inferred from his reduced phonemic clustering and cluster sizes, speed of word retrieval was an underlying factor across tasks. These results support Schmahmann’s “dysmetria of thought” theory, which proposes that the cerebellum contributes to the conditions required for optimum performance, rather than executive functioning, per se. Future studies, using a similarly detailed approach, should be conducted to confirm this peripheral role of the cerebellum in executive functions.