An optical information processor for use as a matrix vector multiplier comprises a vector input spatial light modulator (1) and an optically addressed weight matrix spatial light modulator (3). A read beam (10) passes through the input modulator (1) and the weight modulator and onto a combined output transducer and error spatial light modulator (5). The error modulator (5) is then controlled in accordance with the difference between a target output vector and the output vector from the transducer (5), and modulates an update beam (11) which then passes through the input modulator (1) and onto the weight modulator (3). The weight modulator (3) represents a two-dimensional array of optical attenuation values which are updated in accordance with the optical radiation incident thereon during updating.