An instrument for the evaluation of cerebral hemisphere memory using the visual half-field (VHF) technique is described. The object was to improve the existing VHF-techniques by means of an infra-red eye movement detector to control visual fixation, and to automatically control the presentation of visual stimuli. The instrument consists of four functional parts; visual presentation; optical eye movement control; electronic programming; and reaction time measuring. The instrument has been used on normal controls and in neuropsychological pre- and post-operative investigations on patients with drug resistant partial epilepsy. It has proved easy to use and reveals valuable information about hemisphere memory function. The use of an optical eye movement detector together with electronic logic has improved the accuracy and reliability of the VHF-test. A schematic outline of the instrument and a brief description of electronic circuitry are given.