Spare the rod and spoil the icon

Short-term visual storage was investigated with a successive field paradigm, so that correct performance depended upon combining visual information from two targets that were never on simultaneously. In the first two experiments, the stimuli consisted of two slides, each containing a 10' red dot on a gray surround, and separated by an interstimulus interval (ISI) from 20 to 400 msec. Subjects had to determine if the dots were vertically or horizontally aligned. In Experiment 1, the stimuli had either no contrast for the rods or no luminance contrast for the cones or high contrast. At short ISIs the cone contrast determined performance, whereas at long ISIs the rod contrast determined performance. In fact, when the dots were invisible to the rods, the task was impossible for long ISIs. In Experiment 2, performance was compared for zero log rod contrast and for small departures from zero. Even a small departure from zero log rod contrast resulted in above-chance performance. In Experiment 3, the stimuli were luminous rectangles and the task was to decide whether or not a 4' spatial gap was present between the two successively presented rectangles. Wavelength, luminance, and ISI were varied under both photopic and scotopic adapting conditions. The result was that the rods performed the task for ISIs of 150 msec or longer under scotopic conditions and under the photopic conditions that we were able to test. The results of the experiments taken together are consistent with the hypothesis that the cone icon is short, whereas the rod icon is robust and long lasting.