ASD Geometric Responders: Toward a Biologically Meaningful Subgroup

Background: Autism (ASD) is a heterogeneous disorder both biologically and clinically and the search for biologically meaningful subgroups is essential. Using eye tracking technology, we previously identified a subgroup of children with autism (i.e., 39% of the ASD sample) that preferred to visually fixate on dynamic geometric images rather than dynamic social images. This subgroup of “geometric responders” (GEO), was not identified in DD or TD contrast groups who each preferred social images (SOC).  This unique visual attention preference should, in theory, be mediated by an equally unique feature of the ASD brain. The cerebellum, long known to be abnormal in autism (Courchesne 1987) has been shown to display overgrowth during the first years of life (Webb 2009) followed by reduced volumes during later development (Hallahan 2009).  As a structure highly involved in attention, it is ideally suited to examine relationships with performance on the “geometric preference test.”

Objectives: This study had 3 primary objectives: 1) to examine similarities and differences in cerebellar volumes between ASD GEO and ASD SOC subgroups; 2) to examine the degree that cerebellar gray matter (GM), white (WM), or overall (GM+WM) volume is associated with visual preference for geometric images and 3) to compare cerebellar volumes between ASD GEO and TD.

Methods: Ninety seven toddlers (63 ASD, 34 TD), ranging in age between 21-48 months participated in both the geometric preference test as well as a sleep MRI scan. Based on fixation times recorded with a Tobii T120, 27 ASD toddlers were classified as GEO (i.e., fixation times > 50% viewing geometric images), whereas the remaining 36 ASD toddlers and all TD toddlers were classified as SOC (i.e., fixation times > 50% viewing social images). Structural MRIs were acquired using a standard T1-weighted 3D protocol on a GE 1.5 T scanner. FSL was used to calculate brain volumes.

Results:  Directly comparing the two ASD subgroups revealed significantly larger left and right cerebellar GM as well as overall cerebellar volume (p=0.001, p=0.05, and p=0.004) in ASD GEO toddlers relative to ASD SOC toddlers.  Moreover, there was a non-significant trend for ASD GEO toddlers to have a greater total brain volume (p=0.05).  The degree of cerebellar enlaragement was also positively correlated with percent fixation time towards geometric images (r=.30). Moreover, there was a trend towards right cerebellar enlargement in GEO relative to TD (p=0.08).  In contrast, ASD SOC toddlers, had significantly smaller left and right cerebellar GM (p=0.05, p=0.03) and a significantly smaller overall cerebellum volume (p=0.03) relative to TD.

Conclusions:  Children with autism often delight in examining everyday visual repetition, such as the spinning of a fan or a car wheel.  Here we present data suggesting that a subgroup of children who display a preference for geometric repetition have distinctly different neuroanatomical profiles, namely increased cerebellar gray matter, than ASD toddlers who do not display this preference. Our data shows, for the first time, that the cerebellum may be centrally involved in mediating a visual preference for repetition in a clearly definable subgroup of ASD toddlers.