Interaction between two letters in visual perception

Letters were exposed by tachistoscope for 17 ms to adult subjects. In Experiment I, single letters were exposed. The proportion of errors differed strongly for particular letters. In Experiment II pairs of letters were exposed. The first letter of a pair was recognized more easily than when exposed alone in Experiment I. In Experiment III pairs of letters were exposed but the subject's task was to recognize only one letter of the pair. Both, first and second letters of the pair were then recognized better than in Experiment I. The results were interpreted in terms of facilitatory and inhibitory effects occurring on different levels of the visual system.