Two linear rules relate the latencies of visual responses to their critical durations.

The latency of a neural response sets a limit on its critical duration since stimulation delivered after a response has already occurred can no longer affect that response. However, this tautologic upper limit does not uniquely define the critical duration. Intracellular recordings from the lateral eye of Limulus yield two linear rules which empirically relate the critical duration of a neural response to its latency: When response magnitude (peak amplitude, spike frequency) is used to construct the temporal summation function, the critical duration is equal to the latency minus a constant. When response latency is used instead, the critical duration of the response latency is equal to the latency divided by a constant.