[Size as a parameter for tuning visual attention]

The ability of visual attention to tune to the stimulus size (when this size could not be described by spatial frequencies) was studies. Sinusoidal gratings with frequencies of 1.5, 3, and 6 cycle/degree were used as test stimuli. All these stimuli consisted of 3 periods, consequently, they had different sizes: 2 x 2, 1 x 1, and 0.5 x 0.5 degrees. Three reference stimuli had the same sizes but were constructed as a superposition of all the test frequencies. The reference stimulus of suprathreshold contrast was displayed for 400 ms to the left or to the right of a fixation point at a distance of 3 degrees. After that, the test stimulus of threshold contrast was for 100 ms displayed symmetrically to the fixation point on the other side. Subjects were instructed that the sizes of the reference and test stimuli were the same. It was found that the probability of test detection decreased with increase in the difference between the sizes of the reference and test stimuli. Since in our experiments the spatial frequency could not be used for tuning visual attention, the obtained results suggest that there are specialized mechanisms in the visual system for estimation of the general image size.