ABSTRACT
Correlated firing is a common expression used in Neuroscience. It refers to two or more neurons that tend to be activated at the same time. It is used so frequently in part because there are so many timescales at which one may analyze neural activity. In a 1-58488-362-6/04/$0.00+$ 1.50
sense, correlation might appear as a trivial phenomenon. For instance, if one looks at daylong activity, practically the whole cerebral cortex fires in a correlated manner, because of the sleep-wake cycle. Similarly, whenever an object appears within the visual field, many neurons in visual cortex are expected to respond throughout the same time interval. Clearly, such correlations are to be expected. However, as the observation time window becomes smaller, explaining the presence of correlations becomes more difficult and, at the same time, potentially much more useful. Suppose the activity of two visual neurons is monitored during presentation of a visual stimulus, after its onset. Suppose also that within a short time window of, say, a few hundred milliseconds, spikes from the two neurons tend to appear at the same time. Why is this? Neither the sensory information nor the state of the subject are changing in an appreciable way, so the correlation must reflect something about the internal dynamics of the local circuitry or its connectivity. This is where correlations become interesting.
