This was the first known study to systematically assess the distinct contributions of visual and proprioceptive feedback on precision visual-motor control in persons with ASD. Two key findings were identified. First, we found that inaccurate proprioceptive feedback (applying tendon vibration) during visually-guided gripping led to significant increases in force variability for TD individuals only, suggesting individuals with ASD show reduced involvement of proprioceptive (secondary) feedback to guide precision manual motor control. Second, force variability decreased with age in individuals with ASD only, indicating delayed maturation of visual feedback mechanisms of precision manual control. Similarly, age-associated increases in force irregularity (SampEn) were stronger in individuals with ASD than TD controls suggesting protracted development of motor control processes involved in integrating multisensory inputs that operate on different time scales.
Sensory feedback processing during motor behavior in ASD
Our findings that only TD controls showed changes in force control during proprioceptive feedback interference suggests that the ability to integrate sensory feedback information from multiple sensory modalities is deficient in ASD. Multisensory feedback integration during motor behavior involves modulating the weighting of feedback from separate sensory modalities to optimize motor output [51]. Vision is dominant for visually-guided upper limb and precision motor behaviors [29–31], though secondary sources also contribute to the refinement of behavioral output [52, 53], consistent with our finding that TD controls showed increased force variability when proprioception was inaccurate. Individuals with ASD and TD controls showed similar changes in force variability when visual feedback was manipulated demonstrating that both groups used the primary feedback source during precision gripping. Our previous studies of a similar precision gripping test indicated that individuals with ASD show more severe deteriorations in their ability to limit variability of force output when visual feedback is perturbed, further supporting the hypothesis that they are highly reliant on visual input (i.e., the dominant source of sensory feedback) for precision gripping [14, 54]. In the present study, individuals with ASD did not show elevations in force variability in ASD that varied as a function of visual gain, perhaps reflecting the narrower range of visual gains and ages studied here relative to our prior work [14].
Our findings of decreased integration of non-primary sensory feedback processes in ASD is consistent with prior studies of separate sensorimotor behaviors. For example, a study of postural control in ASD documented an over-reliance on proprioceptive feedback, which is the dominant sensory input for maintaining postural stability [55]. Specifically, Morris et al. [13] showed that disrupting proprioceptive feedback resulted in increased center of pressure (COP) variability in individuals with ASD regardless of whether visual feedback was available; however, TD controls only showed increased COP variability when both visual and proprioceptive feedback were disrupted. These results suggest that TD controls were able to compensate for disrupted proprioceptive feedback by up-weighting secondary sources of feedback (e.g., visual), whereas individuals with ASD continued to rely on the primary source of feedback (proprioceptive) even though it was unreliable. Combined with our findings, these results indicate that, individually, visual and proprioceptive feedback mechanisms are relatively intact in ASD, but the ability to integrate and optimally weight feedback across multiple sensory modalities during motor behavior is compromised.
Motor learning studies also have demonstrated that persons with ASD are better at adapting to induced proprioceptive errors than TD controls during upper limb reaching, but they were less effective at adapting to visually induced errors [6, 16, 17]. On the surface, these studies seemingly contradict our finding that participants with ASD were under-reliant on proprioceptive feedback. However, the prior motor learning studies assessed adaptation in response to external sensory perturbations, which is a fundamentally different behavioral process than monitoring and adjusting ongoing behavior during precision grip force and likely requires a different weighting of sensory feedback inputs. These studies provide evidence that deficits across diverse sensorimotor behaviors in persons with ASD reflect atypical weighting of sensory inputs and an inability to integrate multiple sources of feedback.
Development of sensorimotor control in ASD
We found that individuals with ASD show stronger age-associated gains in precision force control (decreased variability, increased entropy) relative to TD peers across all visual gain and tendon vibrator conditions. These results indicate that the development of precision sensorimotor control is delayed in ASD, and that sensorimotor deficits (increased SD, reduced entropy) may represent important markers of neurodevelopmental dysfunction in childhood. Our findings are consistent with considerable evidence from infant sibling and early childhood studies that show sensorimotor deficits are some of the earliest indicators of ASD [56, 57] and may be most severe during the first years of life. While our data suggest sensorimotor impairments may be attenuated or even normalize by adolescence/early adulthood in ASD, their disruption early in life likely interferes with the maturation of cognitive, social, and language processes that are known to rely on early ontological progression of reaching and grasping behaviors [58–61]. Tracking the early childhood development of precision manual variability and regularity will be an important next step in characterizing key behavioral indicators of ASD, and in defining neurodevelopmental mechanisms contributing to the range of clinical issues associated with ASD.
We also found that differences between individuals with ASD and TD peers in age-associated gains in force control varied across sensory feedback conditions suggesting distinct timing of separate sensory feedback control mechanisms. More specifically, age-related gains in motor variability (decreases) and irregularity (increases) were stronger in the ASD group during conditions in which visual feedback was most precise (higher gains). These findings are consistent with prior studies of normative development showing that while motor variability decreases and entropy increases with age, the rates and timing of these changes are dependent on the quality and nature of sensory feedback [62–64]. For example, no age-associated differences are seen in precision grip force variability and entropy across childhood and into adulthood (ages 6–22 years) when visual feedback is occluded, suggesting the ability to dynamically and precisely adjust motor behavior in response to sensory feedback improves with age due, at least in part, to a greater capacity to integrate multiple sensory inputs [62–64]. The stronger age-related improvements in force control that we observed in the ASD group relative to the control group suggest delayed maturation of sensory feedback processing for refining motor output. Unlike controls, age-related decreases in force regularity in the ASD group were similar across proprioceptive feedback conditions indicating age-related improvements in the ASD group were dependent on the ability to utilize the dominant (visual) source of sensory feedback rather than the integration of multiple sensory modalities.
The age-associations observed in the present study differ from our prior precision gripping study, which found that TD individuals show greater improvements in motor regularity with age than individuals with ASD [14]. These opposing trends may be due to the age distributions in the samples. The prior study (range: 5–35 years, median: 13 years) likely captured a period of rapid maturation in TD children that also may represent an epoch of relatively slowed sensorimotor development in ASD. The present study restricted the age distribution to later childhood and early adulthood (range: 10–20 years, median 13.6 years) during a period in which typical motor development is relatively stable. The present findings, in addition to studies showing that motor deficits in ASD are more severe in early childhood and improve over the course of adolescence [65, 66], indicate that individuals with ASD experience a delayed trajectory of motor development.
Implications for understanding neurodevelopmental processes associated with ASD
Our findings of sensorimotor impairment in ASD and reduced integration of multisensory feedback implicate dysfunction of cortical-cerebellar sensorimotor networks. Posterior parietal cortex, including superior and inferior parietal lobules, integrate multiple sensory inputs during motor behavior [67–69] and innervate premotor and primary motor cortices to generate reactive motor adjustments based on feedback error information [70–72]. Parietal-cerebellar circuits also form a faster subcortical pathway for translating sensory error information into corrective motor commands relayed to motor cortex [73, 74]. During motor behavior, cerebellar circuits critically compare the expected sensory consequences of motor output (received from primary motor cortex) to the actual consequences of the behavior (processed initially by primary and association sensory cortex) to correct errors in the motor command, which are relayed to the primary motor cortex though the thalamus [75, 76]. Our findings that persons with ASD relied almost exclusively on visual feedback during precision motor control suggest deficits in parietal-cerebellar networks that are responsible for integrating feedback from multiple sources to accurately update motor commands. Additionally, stronger age-related improvements in force regularity at higher visual gains in the ASD group suggest delayed development of cortical-cerebellar circuits involved in rapid visual feedback processing and feedback error processing.
Our prior fMRI studies have found that increased motor variability and regularity in ASD during precision gripping are associated with increased activation and functional connectivity of cerebellar-parietal networks and decreased activation and functional connectivity of intra-cerebellar networks [23, 24]. These prior studies also showed that increased force variability and regularity in ASD are associated with reduced activation and functional connectivity of frontal-parietal networks involved in the executive control of movement. Specifically, persons with ASD showed increased activation of putamen and cerebellum relative to TD controls during precision gripping behavior, indicating greater reliance on subcortical sensorimotor processes [23]. Unlike controls, individuals with ASD showed no association between force variability and premotor activation, indicating that they do not modulate cortical motor planning circuits in response to sensory feedback [23]. At rest, individuals with ASD showed increased functional connectivity between cerebellum and superior occipital and parietal gyri, which are involved in visual and sensorimotor processing [24]. Persons with ASD also showed reduced resting functional connectivity relative to TD controls between cerebellum and frontal (superior and medial frontal gyri) and temporal (Heschl’s and superior temporal gyri) cortices, which are involved in cognitive and multisensory processing [24]. An independent study similarly found increased intrinsic functional connectivity between cerebellum and sensorimotor regions of cortex (superior temporal, primary somatosensory, pre/primary motor, and occipital) and reduced intrinsic functional connectivity between cerebellum and cognitive regions of cortex (prefrontal, superior frontal, anterior cingulate, medial temporal gyrus), indicating that persons with ASD rely on basic sensory processing rather than complex multisensory or executive processing for sensorimotor control [77]. The only known prior fMRI study of precision visuomotor behavior found reduced activation of cerebellum as well as parietal and frontal eye fields, but increased activation of prefrontal-striatal-thalamocortical circuits suggesting increased reliance on non-motor regions during visuomotor control [78]. These findings implicate reorganization of cortical and subcortical sensorimotor networks in persons with ASD potentially resulting from delayed maturation and specialization.
Sensorimotor behavior and clinical impairments
We found that force variability and regularity explained 9 to 15% of variability in clinically rated ASD symptom severity suggesting that sensorimotor feedback deficits may contribute to core symptoms or share common developmental pathways. For example, learning and interpreting social gestures requires early advances in sensorimotor behavior that facilitate both action representations, imitation, and reciprocal social interactions. More specifically, early developing sensorimotor processes involve integration of visual information regarding the timing and intention of others’ movement and mapping this information onto internal sensorimotor representations to estimate the expected visual and somatosensory consequences of the movement [79, 80]. Difficulties integrating visual and proprioceptive feedback for motor control in ASD may not only impact self-generated movements, including socially relevant behaviors, but also compromise the developing child’s ability to interpret and predict others’ behaviors [81]. Further, our findings that more severe force control impairments in ASD are associated with clinical measures of motor ability indicate deficits of multisensory feedback control may contribute to functional motor issues in ASD.
Limitations and Future Directions
Several limitations of the present study should be noted. First, the inclusion of younger children in future studies will be important for characterizing key epochs of sensorimotor dysmaturation in ASD. Second, while our findings of under-reliance on proprioceptive feedback for precision gripping and prior findings of over-reliance on proprioception during postural control each suggest reduced integration of non-primary sensory inputs during motor behavior in ASD, studies testing manipulations of multiple sensory inputs across multiple behaviors are needed to further clarify sensory feedback mechanisms of distinct behavioral impairments in ASD. Additionally, our study did not include a sham vibration condition (i.e., vibration on at a frequency that does not induce a proprioceptive illusion), so it is possible that our finding that persons with ASD were not affected by altered proprioception may reflect a reduced susceptibility to the proprioceptive illusion rather than reduced reliance on proprioceptive feedback.