B. Brandauer

Size-weight illusion, anticipation and adaptation of fingertip forces in patients with cerebellar degeneration

The smaller of two equally weighted objects is judged to be heavier when lifted (size-weight illusion [SWI]). In contrast, fingertip forces show an initial size effect but adapt to the true object weights within a few trials. The aim of this study was to investigate possible contributions of the cerebellum to SWI, force anticipation, and adaptation based on object size and weight. Eighteen participants with isolated cerebellar degeneration and 18 age- and gender-matched controls alternately lifted objects of equal weight but different size in 40 trials. All participants perceived the small object to be heavier after lifting (perceptive SWI). Fingertip forces were significantly higher during the first lift of the large object compared with the small object in the control and cerebellar groups. For the load-force rate and lifting acceleration, effects of anticipation were significantly less in the cerebellar compared with the control group. Grip and load forces were adapted to object weight during repeated lifts in both groups. Preserved perceptive SWI in cerebellar patients supports the hypothesis that perceptive SWI depends on the function of the ventral visual path that receives no or few efferents from the cerebellum. The findings of preserved anticipation and adaptation of grip forces in cerebellar patients, however, were unexpected. Reduced anticipation of load forces suggests that the neural presentation of predictive grip- and load-force control may be different. Findings show that representation and adaptation of internal models of object characteristics are not exclusively located in the cerebellum.

Influences of Load Characteristics on Impaired Control of Grip Forces in Patients With Cerebellar Damage

Various studies showed a clear impairment of cerebellar patients to modulate grip force in anticipation of the loads resulting from movements with a grasped object. This failure corroborated the theory of internal feedforward models in the cerebellum. Cerebellar damage also impairs the coordination of multiple-joint movements and this has been related to deficient prediction and compensation of movement-induced torques. To study the effects of disturbed torque control on feedforward grip-force control, two self-generated load conditions with different demands on torque control-one with movement-induced and the other with isometrically generated load changes-were directly compared in patients with cerebellar degeneration. Furthermore the cerebellum is thought to be more involved in grip-force adjustment to self-generated loads than to externally generated loads. Consequently, an additional condition with externally generated loads was introduced to further test this hypothesis. Analysis of 23 patients with degenerative cerebellar damage revealed clear impairments in predictive feedforward mechanisms in the control of both self-generated load types. Besides feedforward control, the cerebellar damage also affected more reactive responses when the externally generated load destabilized the grip, although this impairment may vary with the type of load as suggested by control experiments. The present findings provide further support that the cerebellum plays a major role in predictive control mechanisms. However, this impact of the cerebellum does not strongly depend on the nature of the load and the specific internal forward model. Contributions to reactive (grip force) control are not negligible, but seem to be dependent on the physical characteristics of an externally generated load.

Impaired prehension is associated with lesions of the superior and inferior hand representation within the human cerebellum

Impairment of patients with cerebellar disease in prehension is well recognized. So far specific localizations within the human cerebellum associated with the impairment have rarely been assessed. To address this question we performed voxel-based lesion symptom mapping (VLSM) in patients with chronic focal cerebellar lesions in relation to specific deficits in prehensile movements. Patients with stroke within the posterior inferior cerebellar artery territory (n = 13) or the superior cerebellar artery (SCA) territory (n = 7) and corresponding control subjects were included in the study. Participants reached out, grasped, and lifted an object with either the left or right hand and with fast or normal movement speed. Both kinematic and grip-force parameters were recorded. Magnetic resonance imaging anatomical scans of the cerebellum were acquired, and lesions were marked as regions of interest. For VLSM analysis, a nonparametric test (Brunner-Munzel) was applied. Cerebellar patients showed clear abnormalities in hand transport (impaired movement speed and straightness) and, to a lesser degree, in hand shaping (increased finger touch latencies) while grip function was preserved. Deficits were most prominent in patients with SCA lesions and for ipsilesional, fast movements. Disorders in hand transport may be more difficult to compensate than deficits in hand shaping and grip-force control in chronic focal lesions of the cerebellum because of higher demands on predictive control of interaction torques. Lesions of the superior cerebellar cortex (lobules IV, V, VI) were associated with slower hand transport, whereas lesions of both superior (lobules VI, V, VI) and inferior cerebellar cortex (lobules VII, VIII) were associated with impaired movement straightness. These findings show that both the superior and inferior hand representations within the cerebellum contribute to hand transport during prehensile movements; however, they may have a different functional role.

Impaired and preserved aspects of independent finger control in patients with cerebellar damage

The influence of the cerebellum on independent finger control has rarely been investigated. We examined multidigit control in 22 patients with cerebellar degeneration, 20 patients with cerebellar stroke, and 21 patients with surgical lesions after cerebellar tumor removal. In the first task, either the index finger or the middle finger was actively lifted from an object during static holding. Both controls and cerebellar patients increased the forces of the nearby digits in synchrony with lift-off to maintain the total finger force. Patients used increased finger forces but showed no significant deficits in the pattern and timing of rearrangement of finger forces. In the second task, subjects had to press and release one finger against a force-sensitive keypad with the other fingers being inactive. All patient groups showed increased force production of the noninstructed (enslaved) fingers compared with controls. Lesion-symptom mapping in the focal patients revealed that lesions of the superior hand area were related to abnormal levels of enslaving. Increased finger forces in the finger-lifting task likely reflect an unspecific safety strategy. Increased effects of enslaving in the individuated key-press task, however, may be explained by a deterioration of cerebellar contribution to feedforward commands necessary to suppress activity in noninstructed fingers or by increased spread of the motor command intended for the instructed finger. Despite the large and diverse patient sample, surprisingly few abnormalities were observed. Both holding an object and finger typing are overlearned, automatized motor tasks, which may not or little depend on the integrity of the cerebellum.

Prehension Kinematics, Grasping Forces, and Independent Finger Control in Mildly Affected Patients with Essential Tremor

Although the pathophysiology of essential tremor (ET), one of the most common movement disorders, is not fully understood, evidence increasingly points to cerebellar involvement. To confirm this connection, we assessed the everyday hand and finger movements of patients with ET, as these movements are known to be affected in cerebellar diseases. In 26 mildly affected patients with ET (compared to age- and gender-matched controls), kinematic and finger force parameters were assessed in a precision grip. In a second task, independent finger movements were recorded. The active finger had to press and release against a force-sensitive keypad while the other fingers stayed inactive. Finally, control of grip force to movement-induced, self-generated load changes was studied. Transport and shaping components during prehension were significantly impaired in patients with ET compared to controls. No significant group differences were observed in independent finger movements and grip force adjustments to self-generated load force changes. However, in the latter two tasks, more severely affected ET patients performed worse than less affected. Although observed deficits in hand and finger movement tasks were small, they are consistent with cerebellar dysfunction in ET. Findings need to be confirmed in future studies examining more severely affected ET patients.

Die unabhängige Kontrolle der Finger bei der Objektmanipulation bei Patienten mit zerebellärer Degeneration

Eine essentielle feinmotorische Fahigkeit bei der Manipulation von Objekten ist die Kontrolle der einzelnen Finger und deren Krafte. Die Kontrolle unabhangiger Fingerkrafte und -bewegungen wird eng in Zusammenhang mit dem pyramidalmotorischen System gesetzt. Aber auch Patienten mit Schadigungen des Kleinhirns berichten von einer Ungeschicklichkeit der Finger bei feinen alltaglichen Manipulationen. Welchen Einfluss das Kleinhirn spezifisch auf die unabhangige Fingerkontrolle hat, ist allerdings bisher unklar. Wenn ein Objekt ergriffen und angehoben wird, muss erstens die Kraft der vier Finger und des gegenuberliegenden Daumens hoch genug sein, damit das Objekt nicht aus der Hand rutscht, und zweitens muss die Verteilung der Krafte auf beiden Seiten so sein, dass sich Drehmomente kompensieren und das Objekt nicht kippt. Um einen moglichen Einfluss des Kleinhirns auf die unabhangige Kontrolle der Finger zu untersuchen, wurde ein Messobjekt, das mit 5 Kraftsensoren fur jeden Finger ausgestattet war, erst angehoben und anschliesend wurde wahrend des statischen Haltens entweder der Zeige- oder der Mittelfinger abgehoben. Dabei wurde die Kraftubernahme der anderen Finger in zeitlichen und „raumlichen“ Aspekten analysiert. Die Stichprobe setzte sich aus 22 Patienten mit einer isolierten Kleinhirndegeneration und 22 geschlechts- und altersentsprechenden Kontrollpersonen zusammen. Kurz vor dem eigentlichen Abheben, und auch wahrend der Phase des Abhebens wurden die Krafte der anderen Finger antizipatorisch und zeitlich synchron moduliert. Das Muster unterschied sich charakteristisch fur die jeweilige Aufgabe. Bei Patienten mit einer Kleinhirndegeneration zeigten sich eher moderate Defizite. Die Verteilung der Fingerkrafte beim Abheben eines einzelnen Fingers zeigte erst eine Beeintrachtigung in der zeitlichen Nachregulation und auch nur, wenn der Mittelfinger, nicht aber, wenn der Zeigefinger abgehoben wurde. Die Ergebnisse scheinen zu bestatigen, dass das Kleinhirn keine zentrale Rolle bei der Verteilung und Adjustierung der individuellen Fingerkrafte spielt. Eine Atrophie des Kleinhirns scheint demnach bei der Kontrolle unabhangiger Fingerkrafte eher zeitliche Aspekte zu beeintrachtigen, hingegen war die Modulation der Krafte erhalten. Den Bewegungen liegen womoglich eher hochuberlernte, automatisierte Synergismen der Finger zugrunde, als Feedforward-Prozesse zur Berechnung der Konsequenzen von Handlungen. Gefordert durch DFG TI 239/8–1 und HE 3592/4–1