Brightness discrimination in budgerigars.

<p>Each data point represents the correct choice frequency for one bird and 40 choices. The three different markers (squares, filled circles and plus signs) indicate the results from each of the three individuals tested. A logistic function (Eq. 1) was fitted to the data (dashed line) from which the threshold was interpolated (filled diamond). Error bar shows the 68% confidence interval for the fitted function (obtained by using the program <i>confint</i> in Matlab). The brightness discrimination threshold at 72.5% correct choices (binomial test, <i>n</i> = 40, p<0.01) is 0.09 Michelson contrast units.