Effects of cerebellar lesions on monkey jaw-force control: implications for understanding ataxic dysarthria

Eight macaques were trained on one of two isometric force regulation tasks. Six animals were trained to maintain a static force for a 2-sec period, and two animals were trained to make a rapid series of bites on a force transducer four times in succession. Electromyographic (EMG) electrodes were chronically implanted into temporalis and masseter muscles. Lesions were made in the cerebellum in seven monkeys and a control surgery with no neural damage was performed on one monkey. Following cerebellar lesions, the abilities of the monkeys to maintain a static isometric force for 2 sec were impaired. Animals overshot the correct force band and force tremors increased in amplitude following the lesions. Cerebellar lesions were followed also by a reduction in biting frequency from about 3 Hz to 2 Hz. Measurements of the temporalis EMG indicated that the reduced biting rate was associated primarily with a prolongation of the inter-EMG interval and secondarily with a prolongation of the EMG duration. Changes in jaw-force regulation following the lesions appear to be similar to the types of changes observed in arm control in monkeys, and in the speech of persons with ataxic dysarthria.