First-order analysis.

<p>(<b>a</b>) For each key-press the preceding stimulus stream was analysed to find which stimuli were present. The example shown here is for one subject in the motion experiment. The graph provides the probability density of motion direction for each scene prior to a key-press. The densities peak at the target direction for times around 400 ms before the key-press; this interval indicates the reaction time. Densities are close to flat for shorter and longer times. (<b>b</b>) Reaction time was quantified by calculating the chi-squared statistic for each density in part a. Chi-squared is shown here as a function of time prior to a key-press. Data from the orientation and motion experiments are shown on the left and right, respectively, and are fitted with a Gaussian function of time. The subject represented at right is the same as for part a of the figure. (<b>c</b>) Chi-squared values for the detector model are shown in the same format as for part b of the figure. The peaks are arbitrarily shifted so that reaction times are a little larger than 400 ms.