A scatter diagram for displaying the response of sensory neurons to sinusoidal stimulation.

The quantitative study of the activity of single neurons in sensory systems has been a subject of increasing interest in recent years. As a result there are a number of statistical tools suitable for analyzing the patterns of neuronal activity from both receptors and higher-order cells 1. This has contributed to a greater understanding of the detailed mechanisms of sensory transduction and coding in these peripheral and central processes. Since the response of sensory neurons is usually characterized by the average frequency of discharge, a number of good quantitative techniques have found application in the analysis of tonic responses. However, very little quantitative analysis of phasic neurons has been carried out. Most investigators simply note their existence. This communication describes a convenient display for characterizing the 'frequency response' of a phasic sensory cell. The technique developed out of the use of a swept frequency technique for extracting transfer functions of neural systems 6. Briefly the technique utilizes a swept frequency sine wave stimulus that is then correlated with the neural output. If a describing function analysis is used, the system transfer function can be found regardless of whether the response is tonic or phasic, i.e. whether or not there is a carrier frequency. In addition to the transfer function of the sensory system, it is often of interest to display the occurrence of pulses relative to the phase of the input signal. To do this the swept frequency data can be processed in a manner that is similar to the standard cycle histograms 4 often employed with discrete sine wave inputs. A program was written to implement the display on a small laboratory computer (Hewlett-Packard 2115a). Phase angle is the abscissa and input modulation frequency is the ordinate. The beam of an oscilloscope is driven by the D/A converter and follows the instantaneous frequency and phase of the stimulus. The beam is brightened only when a spike occurs. The display is essentially a binary matrix: the columns correspond to the intervals of phase (in our case, 5 °) from 0 ° to 360 ° and each row is one period of the