Behavioral Evidence of Hemispheric Disconnectivity in Autism Spectrum Disorders

Behavioral Evidence of Hemispheric Disconnectivity in Autism Spectrum Disorders

Corinne Jung and Jeffrey J. Hutsler, Program in Neuroscience, Psychology Department,

University of Nevada, Reno

Background: Long-range underconnectivity has been implicated as an underlying cause of cognitive and behavioral deficits associated with autism spectrum disorders (ASD).  In ASD, long-range underconnectivity has been shown to affect the dynamics of brain synchronization during complex cognitive tasks, such as coordinating motor activity and achieving a cohesive experience of the sensory world (Frith 2003; Just et al. 2004). Interhemispheric transfer is one route of long range communication within the brain, and patient populations characterized by diminished interhemispheric connectivity have demonstrated distinct patterns of reaction time and accuracy in stimulus-response tasks. Individuals with agenesis of the corpus callosum and callosotomy benefit more from the presentation of bilateral stimuli compared to unilateral stimuli (Roser and Corballis 2003).  This redundancy gain results in faster reaction times to bilateral stimuli in these disorders, but not in neurotypical subjects. The integrity of interhemispheric transfer can also be assessed by presenting conflicting stimuli to each visual field and determining processing performance.  Neurotypical groups struggle to process competing information due to task interference, while patients without a corpus callosum are not affected by this increased perceptual load (Holtzman and Gazzaniga 1985).  If impaired long-range connectivity is a characteristic of ASD, interhemispheric transfer abnormalities, such as enhanced redundancy gain and resilience to increasing perceptual load, may be observed in this population.    

Objectives: To understand the impact of long-range underconnectivity on behavioral performance in ASD, we employed two experimental paradigms that have been used to evaluate other disconnectivity syndromes. In the first, we examined redundancy gain, and in the second, we tested the effects of increased perceptual load on task performance.

Methods: 12 high-functioning ASD boys between the ages of 6-17 and 13 age-matched neurotypical (NT) controls were instructed to indicate with a key press whenever they saw a unilateral or bilateral stimulus appear. Reaction times were evaluated by trial type and subject group.  Participants then observed simultaneously presented animated patterns in both visual fields.  Patterns were either the same (redundant) or different (mixed) and, after a delay, subjects had to indicate whether a third lateralized pattern matched that which was presented in the same visual field. 

Results: As predicted, NT subjects’ reaction times did not differ when exposed to simple unilateral and bilateral stimuli.  In contrast, ASD subjects were faster to respond to bilateral stimuli relative to unilateral stimuli.  In the second study, neurotypicals were less accurate at correctly identifying the mixed patterns relative to the redundant patterns.  In contrast, the performance of ASD subjects was not affected by the increased perceptual load required for evaluating mixed patterns.

Conclusions: ASD subjects demonstrate patterns of behavioral performance in both tasks that are essentially equivalent to those found in patients with corpus callosum deficiencies.  The greater redundancy gain exhibited in the ASD group implies compromised processing efficiency for lateralized information, while their performance on tasks with increasing perceptual load demonstrates greater isolation between the hemispheres