While the link between amyloid β (Aβ) accumulation and synaptic degradation in Alzheimer's disease (AD) is known, the consequences of this pathology on population coding remain unknown. We found that the entropy, a measure of the diversity of network firing patterns, was lower in the dorsal CA1 region in the APP/PS1 mouse model of Aβ pathology, relative to controls, thereby reducing the population's coding capacity. Our results reveal a network level signature of the deficits Aβ accumulation causes to the computations performed by neural circuits. Alzheimer's disease (AD) is a progressive neurodegenerative disorder associated with cognitive decline that is thought to arise in part from the pathological accumulation of amyloid β (Aβ) plaques 1 throughout the neocortex and hippocampus. Plaques cause a constellation of changes in neural circuits including, but not limited to, degradation of dendritic spines 2 , reductions in synapse density 2,3 , and increases in the intrinsic excitability of neurons 3. Aβ pathology has been linked to various behavioral and cognitive changes 4,5 ; for example in mouse models, plaque burden correlates with degradation of place fields in the dorsal CA1 (dCA1) subfield of the hippocampus, and is associated with poor performance on spatial memory tasks 4. Such behaviors require the orchestration of activity across large groups of neurons, or ensembles, whose dynamics are governed by the structure of neural circuits 6. However, although Aβ pathology disrupts multiple features of these circuits 2,7,8 , the net effect of these changes on the structure of population activity and the resulting disruptions in neural computation remains unknown. To address this question, we performed electrophysiological recordings in the hippocampus of awake APP/ PS1 mice (model of Aβ pathology 9), where amyloid plaques can be seen at 12 months of age 10 and plaque burden corresponds to poor performance on spatial cognition and memory tasks, such as the T-maze alternation task 4,5. We found a reduction in the pair-wise correlations between neurons in APP/PS1 animals as compared to controls. Additionally, we identified a reduction in the entropy of population activity across a large array of ensemble sizes, suggesting that the coding vocabulary of populations of neurons in the dCA1 region of hippocampus is compromised in this mouse model of amyloid pathology. Materials and Methods Animals. All protocols and procedures were approved by the University Committee on Animal Resources (UCAR) at the University of Rochester and were performed in accordance with the guidelines of the Institutional Animal Care and Use Committee (IACUC) at the University of Rochester. 4 APP/PS1 double transgenic mice and 4 littermate control mice were used in this study. The APP/PS1 mice expressed chimeric mouse/human amyloid precursor protein (Mo/HuApp695swe) 11 and mutant human presenilin-1 (PS1-dE9) 12 under the control of the neuron-specific prion protein promotor element 9. All mice were males aged 11 to 13 months. By this age, amyl...