Muscle Tone, Postural Stability, and Motor Control

Although one study by Jones and Prior (1985) found no significant differences in muscle tone between older children with autism and mental-age-matched typical children, other researchers have reported such differences in children with autism spectrum disorders (Rapin, 1996b). Children with autism were posturally more unstable than typical children, and they were less sensitive to visually perceived environmental motion. They also displayed unusual reactions to vestibular tasks (Gepner et al., 1995; Kohen-Raz et al., 1992). Children with autism relied on proprioceptive feedback over visual feedback to modulate goal-directed motor actions, including reaching and placing objects under conditions that required adaptation to the displacement of a visual field by prisms. This finding might be indicative of a perceptual deficit resulting in poor visual control and visual sequential processing (Masterton and Biederman, 1983). Although vestibular mechanisms may be generally intact and postural responses adequate under some conditions, postural mechanisms may be more compromised in children with autism when integration of visual-proprioceptive, vestibular functions, and motor skills is required.

Prevalence of Atypical Sensory Responses and Motor Stereotypies

Standardized behavioral examinations demonstrated that the overwhelming majority of children with autistic spectrum disorders displayed atypical sensorimotor behaviors at some point during the toddler or preschool years, including both heightened sensitivities or reduced responsiveness across sensory modalities, and motility disturbances such as stereotypies (Ermer and Dunn, 1998; Kientz and Dunn, 1997; Rapin, 1996b). Unusual sensory and motor behaviors included but were not limited to failing to respond to sounds (81%), heightened sensitivity to loud noises (53%), watching hands or fingers (62%), and arm flapping (52%) (Volkmar et al., 1986). Hand-finger mannerisms, whole body mannerisms other than rocking, and unusual sensory interests, as recorded on the Autism Diagnostic Interview, discriminated children with autism from those with other developmental delays (Le Couteur et al., 1989; Lord et al., 1994). A pattern of atypical sensory modulation and motor behaviors, including rubbing surfaces, finger flicking, body rocking, and absence of responses to stimuli, was present in almost 60 percent of one cohort (in 15% to a severe degree) (Rapin, 1996b). This pattern similarly distinguished children with autistic spectrum disorders from children with other developmental disorders, even those with very low developmental levels (Adrien et al., 1987; Rapin, 1996b).

Some studies report pronounced individual differences and suggest subtypes based on patterns that include unusual sensory or motor behaviors, in addition to social and communicative differences (Eaves et al.,

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