The cerebellum, sensitive periods, and autism

Abstract

Cerebellar research has focused principally on adult motor function. However, the cerebellum also maintains abundant connections with nonmotor brain regions throughout postnatal life. Here we review evidence that the cerebellum may guide the maturation of remote nonmotor neural circuitry and influence cognitive development, with a focus on its relationship with autism. Specific cerebellar zones influence neocortical substrates for social interaction, and we propose that sensitive-period disruption of such internal brain communication can account for autism's key features.

Figure 1. The cerebellum as a mediator of ASD risk

(a) Patterns of ASD gene co-expression show specific expression in cerebellum during early postnatal years (image adapted from (Menashe et al., 2013); see also (Willsey et al., 2013). (b) Risk ratios for ASD for a variety of probable genetic (light blue) and environmental (dark blue) factors. Risk ratios were taken directly from the literature except for the largest four risks, which were calculated relative to the US general-population risk. At 36X, cerebellar injury carries the largest single non-heritable risk. For explanation of other risks see text.

Figure 2. A developmental diaschisis model for neurodevelopmental disorders

(left) A diagram of activity-dependent influences on neural circuit refinement in primary sensory neocortex during sensitive periods of development, as articulated by Hubel and Wiesel. (right) A proposed generalization for the influence of cerebellar processing of multisensory information on neocortical areas essential for social and cognitive processing.

Figure 3. Upstream influences and downstream targets in autism spectrum disorder (ASD)

Categorization of regions showing abnormalities in ASD brains according to whether they result in lasting ASD-like social deficits when injured neonatally (upstream), result in lasting ASD-like social deficits when injured in adulthood (downstream), or can be fully or partially compensated after an injury, regardless of age (compensatable).

Figure 4. The cerebellum and forebrain are bidirectionally linked in an orderly mapping

(a) The general structure of cerebello-thalamo-cortical loops. Each projection indicates a monosynaptic pathway. The pontine-cerebellar and deep nuclear-thalamic projections cross the midline to the contralateral side. (b) Regions are mapped precisely to form closed loops as demonstrated using classical and viral tracing methods in rodents and nonhuman primates. Adapted from (Strick et al., 2009). Loop-specific connectivity through the thalamus and pons connects the anterior cerebellum, cerebellar crus II/lobules VII-IX, and cerebellar lobules VI/VII with motor cortex (M1), dorsolateral prefrontal cortex (DLPFC) (Kelly and Strick, 2003), and areas of the neocortex (NEO) (Suzuki et al., 2012), respectively. An ascending pathway projects monosynaptically from the cerebellar nuclei to the ventral tegmental area (VTA) (Snider and Maiti, 1976). A descending pathway joins the subthalamic nuclei (STN) with cerebellar-cortical hemispheric lobule VII and crus II while an ascending pathway joins the cerebellar nuclei with the striatum and globus pallidus (Hoshi et al., 2005; Bostan and Strick, 2010). (c) In human brains, spontaneous waking activity measured using functional magnetic resonance imaging reveals a parcellated relationship of covarying activity between corresponding zones of cerebellum and neocortex. The olored maps at left indicate 7 zones in which a single color denotes regions of neocortex and cerebellar cortex with strongly covarying activity. The plot at right indicates the fraction of neocortex in each zone of a 17-zone map, plotted against the fraction of cerebellar cortex in the corresponding zone. This plot indicates that representation in the neocortical and cerebellar cortical sheets is approximately proportional (Buckner et al., 2011).

Figure 5. Circuitry for instructed learning in the cerebellar cortex

Purkinje cells (black) receive the two major excitatory streams of input to the cerebellum: the mossy fibers (red), which synapse onto granule cells (green); and climbing fibers (blue). Mossy fibers and climbing fibers also send collaterals to the cerebellar deep nuclei. Cerebellar granule cells represent approximately half the neurons of the rodent or primate brain, and convey sensory, motor efference, and other information to1 the cerebellar cortex. They give rise to parallel fibers (green) which then converge massively onto Purkinje cells. Climbing fibers act as an instructive signal that can drive plasticity at recently active parallel fiber synapses. In this way, learning in the cerebellar cortex can integrate multiple sensory modalities with precise timing in the subsecond range. The sole output of the cerebellar cortex is Purkinje cell inhibition to the cerebellar deep nuclei, which in turn project to thalamus and many other brain regions.