1. A neural network comprising: an input layer having a plurality of electrically addressed spatial light modulator units for outputting light information by transforming input electric signal information; an optical mask for receiving and modulating siad light information from said input layer; at least one intermediate layer having a plurality of spatial light modulator units for receiving said modulated light information from said optical mask; a further optical mask for receiving and modulating light information from said at least one intermediate layer; and an output layer having a plurality of spatial light modulator units for receiving said modulated light information from said further optical mask; wherein each spatial light modulator unit of said at least one intermediate layer and said output layer includes: a photoconductive layer that generates a photocurrent responsive to an intensity of the light incident thereto, said photoconductive layer having a photoconductivity .sigma..sub.ph with the relationship of log (.sigma..sub.ph)=a.multidot.log (I.sub.ph), wherein I.sub.ph is the intensity of light incident thereon and at least within the range of 5 to 50 .mu.W/cm.sup.2 with the wavelength between 500 and 600 nm and "a" is a value within a range from 0.95 to 1.0; a light modulation layer electrically contacting said photoconductive layer, which varies a light transmittance in accordance with a voltage applied thereto; and a pair of electrically conductive electrodes which sandwich said photoconductive layer and said light modulation layer for applying a voltage to the sandwiched layers; whereby light information incident to said photoconductive layer are summed as photocurrents induced therein, and wherein said voltage applied to said light modulation layer is varied in accordance with the sum of said photocurrents to yield a light transmittance dependent on the total of said incident light information.