Fig. 4.

Discrete sampling depends on focal attention. (a) We added a rapid stream of randomly rotated letters to our unbalanced counterphase gratings (here, with a contrast balance of 56–44%). Most letters were Ts, but an L was presented occasionally. (b) Notations as in Fig. 3b. Subjects (n = 5) were told to follow the dominant direction of motion under two conditions: either while ignoring the central letter stream (“motion attended” condition) or while monitoring the letter stream to report occurrences of the letter L (“motion unattended” condition). The selective impairment of motion direction judgments around 10 Hz predicted by the discrete sampling model was replicated here in the motion attended condition (open circles). However, this impairment was still visible but much decreased when motion was unattended (filled squares). At 10 Hz, motion direction judgments were more accurate in the absence of focal attention.