TY - JOUR T1 - Cellular Mechanisms Contributing to Response Variability of Cortical Neurons <em>In Vivo</em> JF - The Journal of Neuroscience JO - J. Neurosci. SP - 2209 LP - 2223 DO - 10.1523/JNEUROSCI.19-06-02209.1999 VL - 19 IS - 6 AU - Rony Azouz AU - Charles M. Gray Y1 - 1999/03/15 UR - http://www.jneurosci.org/content/19/6/2209.abstract N2 - Cortical neurons recorded in vivo exhibit highly variable responses to the repeated presentation of the same stimulus. To further understand the cellular mechanisms underlying this phenomenon, we performed intracellular recordings from neurons in cat striate cortex in vivo and examined the relationships between spontaneous activity and visually evoked responses. Activity was assessed on a trial-by-trial basis by measuring the membrane potential (Vm) fluctuations and spike activity during brief epochs immediately before and after the onset of an evoked response. We found that the response magnitude, expressed as a change in Vm relative to baseline, was linearly correlated with the preceding spontaneousVm. This correlation was enhanced when the cells were hyperpolarized to reduce the activation of voltage-gated conductances. The output of the cells, expressed as spike counts and latencies, was only moderately correlated with fluctuations in the preceding spontaneous Vm. Spike-triggered averaging of Vm revealed that visually evoked action potentials arise from transient depolarizations having a rise time of ∼10 msec. Consistent with this, evoked spike count was found to be linearly correlated with the magnitude ofVm fluctuations in the γ (20–70 Hz) frequency band. We also found that the threshold of visually evoked action potentials varied over a range of ∼10 mV. Examination of simultaneously recorded intracellular and extracellular activity revealed a correlation between Vmdepolarization and spike discharges in adjacent cells. Together these results demonstrate that response variability is attributable largely to coherent fluctuations in cortical activity preceding the onset of a stimulus, but also to variations in action potential threshold and the magnitude of high-frequency fluctuations evoked by the stimulus. ER -