Cognitive Functions of Cerebellum Research Paper

In the 1950s, however, a broader concept of cerebellar function was suggested, with a possible cerebellar involvement in the control of autonomic and limbic activity. Clinical observations made an important contribution to this broader concept (for a summary see Daum and Ackermann 1995). Congenital cerebellar malformations, for example, were found to be associated not only with ataxia, balance problems, and other motor deficits, but also with mental retardation. In addition, disorders such as schizophrenia and autism were related to neuropathological abnormalities of the cerebellum. The hypothesis of a cerebellar involvement in the control of emotions was further supported by findings of a modulation of fear and aggression by cerebellar stimulation for the control of epileptic seizures.

More recent concepts of cerebellar function were influenced by reports of two-way connections between the cerebellum and the cerebral cortex, findings of cognitive deficits in patients with cerebellar dysfunction, and neuroimaging reports of cerebellar activation during a range of cognitive tasks (Schmahmann 1997).

1. Anatomy Of The Cerebellum

Like the cerebrum, the cerebellum consists of two hemispheres. Three functional regions can be distinguished: the centrally located vermis (Latin ‘worm’) and the lateral and intermediate zones in each hemisphere. Mossy fibers are the major afferents to the cerebellum, receiving their input from brain stem nuclei and spinal chord neurons. The climbing fibers, which are the second excitatory input to the cerebellum, originate from a single site in the medulla and the inferior olivary nucleus. Efferent projections are mediated by the Purkinje cells and the deep nuclei.

There are several reciprocal pathways between the cerebellum and the cerebral cortex. The cerebellum receives input via pontine nuclei from the parietal cortex, the prefrontal cortex, and the superior temporal sulcus. The cerebellum projects back to the same regions via the thalamus (Schmahmann 1997). These afferent and efferent projections imply a possible role of the cerebellum in the modification of information which is projected from the cortex to the cerebellum and sent back to the cortex.

2. Motor Learning And Motor Imagery

It is well known that the cerebellum plays a critical role in motor control, with the lateral regions of the cerebellum mediating movement planning and programing, while the medial regions are involved in the execution of movement (Dichgans and Diener 1984).

Imaging studies using positron emission tomography have also demonstrated a cerebellar role during motor learning of sequential finger movements as well as in trajectorial learning. In addition, the cerebellum was found to contribute to the monitoring and optimizing of movements by using sensory (proprioceptive) feedback (Jueptner and Weiler 1998).

Motor skill learning describes the qualitative improvement of performance through practice which ensures that movements can be performed fast, accurately, and with little attentional control. Electrophysiological and lesion studies in nonhuman primates with cerebellar hemispherectomies have demonstrated the critical contribution of the cerebellum at a stage of motor learning when performance becomes fast and accurate. Similar results were observed in patients with cerebellar dysfunction, who had problems in learning the skillful execution of serial movements (Doyon 1997).

The most frequently used motor learning paradigm is classical conditioning of the eyeblink response. An acoustic stimulus, the conditioned stimulus (CS), is paired with a corneal airpuff, the unconditioned stimulus (US), which evokes an eyeblink. After repeated pairing of the CS and the US, the eyeblink occurs to the CS but before onset of the airpuff and thereby forms a conditioned response (CR). The essential neuronal circuitry involves the convergence of CS and US information in the cerebellum and an efferent projection from the cerebellum to motor nuclei in the brain stem, which control eyeblink responses (see Thompson 1991). Patients with cerebellar dysfunction are severely impaired at acquiring eyeblink CRs, although the reflex blink to the US is unaffected. Conditioning of simultaneously recorded nonmotor autonomic and electrocortical responses are also intact (Daum et al. 1993a). The critical involvement of the cerebellum in the conditioning of motor responses has now been documented by a large number of clinical investigations, functional neuroimaging studies, and studies of physiological manipulations of cerebellar functions in normal subjects (for a summary see Schugens et al. 2000).

In motor imagery, a motor program that is stored elsewhere in the CNS is activated without any overt movement. There is some evidence that the cerebellum becomes active during motor imagery, in tasks such as silent counting and imagination of tennis training movements (Decety et al. 1990).

3. Timing

The notion that the cerebellum computes timing requirements for motor performance is supported by investigations in animals, as well as by clinical data (Keele and Ivry 1991). Keele and Ivry have argued that the lateral regions of the cerebellum are critically involved in the internal timing of motor and nonmotor behavior which requires temporal computation. They attributed the deficits in classical conditioning of cerebellar patients to problems in timing the initiation of the CR. This hypothesis is supported by the finding of an inappropriately timed CR reported by Daum et al. (1993a) and by Topka et al. (1993). Key symptoms of cerebellar symptoms, such as dysmetria (problems with precise movements) or dysdiadochokinesia (problems with fluent alternating movements), can also be interpreted within the context of deficient cerebellar timing functions. Further support for this idea stems from impairments in rhythmic tapping in cerebellar patients (Ivry et al. 1988) as well as from deficits in speech production, which reflects a decline in temporal coordination of neuromuscular interaction needed for articulation (Ackermann and Ziegler 1992). Impairments of speech perception are also consistent with the idea of a timing deficit in the nonmotor domain (Ackermann et al. 1999).

4. Cognitive Functions

The results of developmental dysfunction, such as cerebellar agenesis (absence of the cerebellum) or cerebellar hypoplasia (prenatal developmental deficits which result in loss or incomplete cerebellar development), varies from congenital apraxia to normal motor abilities. Similarly, cognitive development can range from profound mental retardation to normal status. Intellectual deficits after delayed motor development may be due to a close coupling of motor and intellectual functions in early life.

Performance of patients with cerebellar dysfunctions on standard intelligence tests is generally in the normal range. Shortand long-term declarative memory as well as priming effects are also largely unaffected in patients with cerebellar lesions (for a summary see Daum and Ackermann 1997).

With respect to skill learning, it has been argued that the neocortex may be primarily concerned with the generation processing of specific operations, while the cerebellum serves to modulate and optimize the functions in question (Ito 1993). In support of this idea, cerebellar damage was associated with deficits in the automatization of visuomotor sequences and visuomotor skill learning (Doyon et al. 1998). As far as nonmotor skill learning is concerned, performance of patients with cerebellar dysfunction on standard perceptual and cognitive skill acquisition was largely unimpaired (Daum et al. 1993a, Helmuth et al. 1997). The acquisition and performance of language skills may be more difficult for such patients (Fiez et al. 1992); and cerebellar activations during such tasks may be related to verbal response search (Desmond et al. 1998).

The anatomical as well as functional relationship between the cerebellum and the prefrontal cortex (Kim et al. 1994) led to the investigation of cognitive functions that are thought to be associated with prefrontal or executive function. Performance on anticipatory planning tasks was found to be impaired in patients with cerebellar atrophy in some studies (Grafman et al. 1992, Hallett and Grafman 1997). Verbal fluency abilities are also associated with executive processing. In such tasks, subjects are asked to name as many items as possible of a certain semantic category or starting with a certain letter within a specific time limit. Patients with cerebellar damage may occasionally show problems with word generation tasks of this kind (Fiez et al. 1992). Such problems may, however, be influenced by slowing of speech (‘dysarthria’) which is a frequent symptom of cerebellar damage. This motor speech slowing may interfere with the execution of verbal fluency tasks, and lead to poorer performance in some cases.

While cerebellar activation is observed in functional neuroimaging during performance of the Wisconsin Card Sorting Test, a standard test of concept formation, perseverative tendencies do not usually occur in cerebellar patients (Daum and Ackermann 1997, Hallett and Grafman 1997). By contrast, cerebellar lesion patients had problems in attentional shifting between modalities (Akshoomhoff et al. 1992). This pattern might be explained by impaired cerebellar– prefrontal interaction where ‘prefrontal’ activation, which would be associated with changing attentional behavior, is deficient due to cerebellar dysfunction.

Two-way cerebellar-parietal projections led to the investigation of visuospatial abilities that are mediated by the parietal cortex. The studies carried out so far yielded no clear evidence of a general impairment of visuospatial functions in patients with cerebellar dysfunction. Findings of difficulties of such patients with the mental manipulation of three-dimensional objects in space offers some evidence of a visuospatial processing deficit consistent with possible dysfunction of cerebellar–parietal circuits (Wallesch and Horn 1984).

5. Conclusion

In summary, the cerebellum makes an important contribution to the control of voluntary movement and movement coordination as well as to the control of balance, gait, and posture. Motor learning abilities are also largely dependent upon the functional integrity of the cerebellum. There is also strong evidence for a cerebellar role as an ‘internal clock’ which comes into play during the control of movement as well as during perceptual processing.

The exact nature of the cerebellar involvement in cognitive processes is so far less well understood. Possible contributions to prefrontal or executive functions and visuospatial processing remain to be specified by studies using patients with selective cerebellar lesions, adequately clinical and nonclinical matched control groups, and the use of a wide range of tests assessing different aspects of the abilities in question. Functional neuroimaging techniques also provide a good tool to study the cerebellar contribution to different cognitive abilities. A problem of imaging techniques is, however, that it is difficult to determine which brain area is critically involved in which aspects of cognitive processing, since essential and correlated activity cannot be easily distinguished. A combination of imaging techniques and transcranial magnetic stimulation, which elicits a transient lesion, may be a promising approach in this regard.

Bibliography:

1. Ackermann H, Graber S, Hertrich I, Daum I 1999 Cerebellar contributions to the perception of temporal cues within the speech and nonspeech. Brain Lang. 67(3): 228–41
2. Ackermann H, Ziegler W 1992 Die cerebellare Dysarthrie: Eine Literaturubersicht. Fortschr. Neurol. Psychiat. 60: 28–40
3. Akshoomhoff N, Courchesne E, Press G, Iragui V 1992 Contribution of the cerebellum to neuropsychological functioning: Evidence from a case of cerebellar degenerative disorder. Neuropsychologia 30: 315–28
4. Daum I, Ackermann H 1995 Cerebellar contributions to cognition. Behavioural Brain Research 67: 201–10
5. Daum I, Ackermann H 1997 Neuropsychological abnormalities in cerebellar syndromes—fact or fiction? In: Schmahmann J D (ed.) The Cerebellum and Cognition. Academic Press, San Diego, CA
6. Daum I, Ackermann H, Schugens M M, Reimold C, Dichgans J, Birbaumer N 1993a The cerebellum and cognitive functions in humans. Behavioral Neuroscience 107: 411–9
7. Daum I, Schugens M M, Ackermann H, Lutzenberger W, Dichgans J, Birbaumer N 1993b Classical conditioning after cerebellar lesions in humans. Behavioral Neuroscience 107: 748–56
8. Decety J, Sjoholm H, Ryding E, Sternberg G, Ingvar D H 1990 The cerebellum participates in mental activity: Tomographic measurements of regional cerebral blood flow. Brain Research 535: 313–7
9. Desmond J E, Gabrieli J D E, Glover G H 1998 Dissociation of frontal and cerebellar activity in a cognitive task: Evidence for a distinction between selection and search. Neuroimage 7: 368–76
10. Dichgans J, Diener H C 1984 Clinical evidence for functional compartmentalisation of the cerebellum. In: Bloedel J, Dichgans J, Precht W (eds.) Cerebellar Functions. Springer, Berlin, pp. 126–47
11. Doyon J 1997 Skill learning. Schmahmann J D (ed.) The Cerebellum and Cognition. International Review of Neurobiology 41: 273–96
12. Doyon J, Laforce R Jr, Bouchard G, Gaudreau D, Roy J, Poirier M, Bedard P J, Bedard F, Bouchard J P 1998 Role of the striatum, cerebellum, and frontal lobes in the automatization of a repeated visuomotor sequence of movements. Neuropsychologia 36: 625–41
13. Fiez J A, Petersen S E, Cheney M K, Raichle M E 1992 Impaired nonmotor learning and error detection associated with cerebellar damage. Brain 115: 155–73
14. Grafman J, Litvan I, Massaquoi S, Stewart M, Sirigu A, Hallet M 1992 Cognitive planning deficits in patients with cerebellar atrophy. Neurology 42: 1493–96
15. Hallet M, Grafman J 1997 The cerebellum and cognition: Executive function and motor skill learning. International Review of Neurobiology 41: 297–323
16. Helmuth L L, Ivry R B, Shimizu N 1997 Preserved performance by cerebellar patients on tests of word generation, discrimination learning, and attention. Learning and Memory 3(6): 456–74
17. Ito M 1993 Movement and thought: Identical control mechanisms by the cerebellum. Trends in Neurosciences 16: 448–50
18. Ivry R B, Keele S W, Diener H C 1988 Dissociation of the lateral and medial cerebellum in movement timing and movement execution. Experimental Brain Research 73: 167–80
19. Jueptner M, Weiler C 1998 A review of differences between basal ganglia and cerebellar control of movements as revealed by functional imaging studies. Brain 121(8): 1437–49
20. Keele S W, Ivry R 1991 Does the cerebellum provide a common computation for diverse tasks. A timing hypothesis. Annals of New York Academy of Sciences 608: 179–211
21. Kim S G, Ugurbil K, Strick P L 1994 Activation of a cerebellar output nucleus during cognitive processing. Science 265: 949–51
22. Schugens M M, Topka H R, Daum I 2000 Eyeblink conditioning in neurological patients with motor impairments. In: Woodruff-Pak D S, Steinmetz J E (eds.) Eyeblink Classical Conditioning: Vol. I Applications in Humans. Kluwer Academic Publishers, Boston
23. Schmahmann J D (ed.) 1997 The Cerebellum and Cognition. International Review of Neurobiology Vol. 41
24. Stewart T G, Holmes G 1904 Symptomatology of cerebellar tumors: A study of forty cases. Brain 27: 522–91
25. Thompson R F 1991 Are memory traces localized or distributed? Neuropsychologia 29: 571–82
26. Topka H, Valls-Sole J, Massaquoi S G, Hallet M 1993 Deficit in classical conditioning in patients with cerebellar degeneration. Brain 116: 961–9
27. Wallesch C W, Horn A 1984 Long-term effects of cerebellar pathology on cognitive functions. Brain and Cognition 14: 19–25