An optical stimulator for studying the topography of electrical and magnetic visual evoked responses

Visual processing areas cover more than 50% of the cortex in primates, but in humans only about half of this area is projected on gyral crests and thus readily accessible to investigation by evoked potential mapping. However, neuromagnetometry does not reflect the activity of these radial dipoles, but instead it strongly represents tangential dipoles, which may arise from activity within the sulci. It follows that the full exploration of the visual areas can only be achieved by combining these complementary techniques. Software-controlled video stimulators are almost universally used for evoked potential recording, but they generate troublesome interference during neural magnetometry. A simple optical stimulator was therefore designed to project a stimulus into magnetically shielded rooms from a remote situation. The system is capable of providing a wide variety of visual stimuli, including pattern reversal, onset, movement and progressive adjustment of chromatic and achromatic contrast. Evoked responses to complex images and isoluminant colored gratings were studied to demonstrate the performance of this type of system.