1. An optical information processor comprising a first spatial light modulator (3, 23) for containing interconnection weight data, a second spatial light modulator (1, 21) for containing input data, and a third spatial light modulator (5, 25), the first spatial light modulator (3, 23) being optically addressable, the first, second, and third spatial light modulators (1, 3, 5, 21, 23, 25) defining an optical update path for updating of weight data from the third spatial light modulator (5, 25) via the second spatial light modulator (1, 21) to the first spatial light modulator (3, 23), characterised in that the third spatial light modulator (5, 25) further comprises optoelectric transducer means co-operating with the first and second spatial light modulators (1, 3, 21, 23) to define an optical processing path from the first spatial light modulator (3, 23) via the second spatial light modulator (1, 21) to the optoelectric transducer means, processing means for processing output data from the optoelectric transducer means and predetermined target output data to form error data, and control means for controlling the third spatial light modulator to contain the error data during weight updating.