is with a u201c silicon retina u201d type of circuit fig6 shows a circuit corresponding to that disclosed in the aforementioned u s pat no 6 194 695 b1 the circuit includes a first set of operational amplifiers 26 1 26 2 26 3 26 n which function as buffers a further set of operational amplifiers 28 1 28 2 28 3 28 n which function as differential amplifiers and a like plurality of resistors r 1 r 2 r 3 r n are connected as shown the inputs to buffer amplifiers 26 1 26 2 26 3 26 n are connected to receive photoreceptor signals while the outputs of differential amplifiers 28 1 28 2 28 3 28 n form the outputs of the circuit a common node 29 contains the average of the voltages the circuit of fig6 computes the mean of the input signals and computes for each photoreceptor the difference between the photoreceptor signal and the mean this effectively recovers the lost contrast caused by using the negative iris 18 other silicon retinas circuits or variations useful for enhancing contrast known to those skilled in the art are also within the scope of the present invention alternatively if the output signals from photoreceptor circuit 22 are sent to feature detectors not shown such as disclosed in the above mentioned u s pat no 6 020 953 the lost contrast can be compensated for.