n.1. the process in which a neural impulse is propagated by the cumulative effects of two or more stimuli that alone would not be sufficient to activate the neuron. See spatial summation; temporal summation. 2. the increased intensity experienced in a sensation when two stimuli are presented to a receptor in rapid succession or to adjacent areas. 3. (symbol: Σ) a mathematical operation involving the addition of numbers, quantities, or the like. For example, in a set of related values (x1, x2, x3, …xn ), a summation of all values of the set is indicated by Σi=ni=1xi , Σ n1, or Σ i Often, the limits on the summation sign are omitted when what is intended is otherwise obvious.