Private and shared variability. (a) The geometric relationship between neural activity and shared variability. Black curves denote mean responses to different stimuli. Variability for a specific stimulus (black dot) may be private (left), shared (middle), or take on the structure of differential correlations (right). The red arrow represents the tangent direction of the mean stimulus response. (b) Schematic of the types of variability that a neural population can encounter. The variability of a neural population contains both private components (e.g., synaptic vesicle release, channel noise, thermal noise) and shared components (e.g., variability of presynaptic spike trains, shared input noise). Shared variability can be induced by the variability of afferent connections (which is shared across a postsynaptic population) or inherited from the stimulus itself. Furthermore, shared variability is shaped by synaptic weighting. (c) Estimates of the private variability contributions to the total variability of neurons $(N=28)$ recorded from auditory cortex of anesthetized rats. Diagonal line indicates the identity. Figure reproduced from Deweese and Zador (2004).