Human optical signals and Visual Evoked Potentials (VEPS) from different cone systems

Purpose. Fast changes in the optical properties (absorption and scattering) of cortical tissue following activation can be recorded noninvasively in humans by measuring delays in the migration of near infrared photons through the head (Event-Related Optical Signals, EROS).^1,2 The latencies of optical signals were compared to those of VEPs to examine the human cortical response elicited by stimuli chosen to activate the S-cone and L/M-cone systems. Methods. Two subjects viewed stimuli in Maxwellian view. Stimuli were red (3.2 log td) or blue (2.1 log td) flashes (duration 20 ms) subtending 4.2° of visual angle on top of a steady yellow background (3.5 log td). The blue flashes and yellow background were selected to activate the S-cone system, while the red flashes were selected to activate the L- and M-cone systems. Photon delay was measured every 20 ms from 28 scalp locations over area 17, as verified by MRI. VEPs were recorded from 22 locations spread out over the back half of the head. Results. The first peak of the optical response occurred at a latency of 50-60 ms for red flashes and 70-80 ms for blue flashes. The first peak of the VEP occurred at a latency of 110-120 ms for red flashes and a latency of 180-200 ms for blue flashes. In fact, there was no significant VEP response to the blue flashes within the first 150 ms at any of the 22 electrode locations. Conclusions. The optical response occurs earlier than the VEP response for both the red and blue flashes. In the case of the blue flash stimuli, the optical response can occur more than 100 ms earlier than the VEP response. It appears that the EROS technique may detect early neuronal activity which differs from the activity detected by traditional VEP recording. This activity can be mapped spatially to show the time course of cortical activation at a specific location, providing a measure combining high temporal and spatial resolution.