Imbalanced metabolism between right and left temporoparietal junction predicts spatial bias in individuals with very early Alzheimer's disease

Deficits of spatial attention are present in early stages of Alzheimer's disease (AD). This raises the question of the neural correlates of spatial bias in early AD. In particular, we wanted to know to what extent the principle of contra-lesionality can explain this spatial biasing. To address this question, we used a paradigm derived from Bundesen's Theory of Visual Attention (TVA) and resting-state 18FDG-PET in a sample of 35 patients with mild AD (n=7) or mild cognitive impairment (n=28), to look for correlations between spatial bias and regional glucose metabolism. The patient's degree of spatial bias varied across a wide spectrum, allowing for a parametric analysis of brain metabolism. Hypometabolism across all patients was slightly increased in the left hemisphere. As expected, metabolism in the left temporoparietal junction (TPJ) predicted the magnitude of spatial bias when confounding factors such as processing speed were accounted for. Furthermore, based on the biased competition framework of attention, spatial neglect has been described as the result of an inter-hemispheric imbalance. We found that the relative hypo-activity in the left TPJ as compared to the right TPJ was also significantly correlated with the degree of spatial bias, lending further support to the idea that AD-induced, slightly asymmetric changes of both hemispheres cause a metabolic imbalance across the hemispheres, which in turn leads to the increasing development of deficits in spatial attention in AD.