The orientation tuning curve: Variability of preferred orientation due to methodology

Preferred or optimal orientation of a bar stimulus is a value of central tendency for a distribution of cellular responses and has been used to classify neurons in the visual cortex. The variability in estimation of preferred orientation is dependent upon both biological and methodological sources. This study covers methodology variability. Three different procedures for calculating the central tendency of tuning curves were evaluated on the same data sets. (Each data set consisted of a collection of points derived from a mathematically defined orientation function for a model cell with, in many cases, an added perturbation. The perturbations took the form of (1) spontaneous activity, (2) response variability or (3) instrumental error. The most variable method for calculating the central tendency was the method of finding the intersection of two straight lines fitted to the flanks of the tuning curve by linear regression. In general, the mode of a function fitted to the data points by the spline fit was less variable than the intersection method. A new procedure called preferred axis of movement is described and is shown to have less variability under all conditions.