  Trial-to-trial variability is an essential feature of neural responses, but
its source is a subject of active debate. Response variability (Mast and
Victor, 1991; Arieli et al., 1995 & 1996; Anderson et al., 2000 & 2001; Kenet
et al., 2003; Petersen et al., 2003a & b; Fiser, Chiu and Weliky, 2004; MacLean
et al., 2005; Yuste et al., 2005; Vincent et al., 2007) is often treated as
random noise, generated either by other brain areas, or by stochastic processes
within the circuitry being studied. We call such sources of variability
external to stress the independence of this form of noise from activity driven
by the stimulus. Variability can also be generated internally by the same
network dynamics that generates responses to a stimulus. How can we distinguish
between external and internal sources of response variability? Here we show
that internal sources of variability interact nonlinearly with stimulus-induced
activity, and this interaction yields a suppression of noise in the evoked
state. This provides a theoretical basis and potential mechanism for the
experimental observation that, in many brain areas, stimuli cause significant
suppression of neuronal variability (Werner and Mountcastle, 1963; Fortier,
Smith and Kalaska, 1993; Anderson et al., 2000; Friedrich and Laurent, 2004;
Churchland et al., 2006; Finn, Priebe and Ferster, 2007; Mitchell, Sundberg and
Reynolds, 2007; Churchland et al., 2009). The combined theoretical and
experimental results suggest that internally generated activity is a
significant contributor to response variability in neural circuits.
