Long-term potentiation and visual recognition memory in autism spectrum disorders

Abstract

Autism spectrum disorders (ASD) are defined based on behavioural deficits that affect social functioning, language and behaviour. However, neural abnormalities are also evident, which include abnormal connectivity and an increase in dendritic spines. An increase in dendritic spines may lead to problems in the synaptic process of long-term potentiation (LTP), which is thought to underlie learning and memory. LTP has been studied at the cellular level in animal models of autism, although these yield inconsistent results. The aim of the present study was to apply a paradigm for eliciting LTP in humans non-invasively to a group of adults with ASD compared with neurotypical (NT) controls, using sensory stimulation of the visual cortex and EEG to measure visual-evoked potentials (VEPs). As found in previous studies using this paradigm, a change in the N1 component of the VEP after visual stimulation would indicate LTP. As face processing deficits are apparent in ASD, we applied a second experiment to assess recognition memory for faces compared with objects, using repetitions of images. The aim was to see if memory at the behavioural level was related to LTP. The results of the LTP experiment showed the ASD group to have a relatively large LTP effect in comparison to the control group, which was associated with worse face and object recognition, suggesting excess plasticity can disrupt memory functions. Behaviourally, face recognition in the ASD group was impaired relative to the control group and relative to object recognition. The VEPs to faces did not reveal abnormalities in an early structural encoding component (the N170) in the ASD group, although an abnormality was found in a later component (the N400), which has been related to semantic processing, suggesting that the ASD group had an impairment in encoding semantic meaning from faces. The LTP results provide support for the “Intense World” theory, in which hyper-plasticity is thought to underlie the cognitive symptoms in ASD. This non-invasive paradigm for eliciting LTP has potential to be applied in a clinical setting where hyper-plasticity may serve as a neural marker and possible target for treatment for ASD.