At least 6 weeks after the injection of the virus, we tested the effect of shining green laser light (532 nm wavelength) onto these SC neurons. In two monkeys (OZ and OM), we presented the laser light while the monkey made visually guided saccades. In the third monkey (RO), we studied changes in neuronal responses during free viewing. The light reached the SC typically via a 200 μm diameter CP-868596 in vitro optic fiber attached to a recording electrode extending 500 μm beyond the flat fiber end (the optrode). We found consistent behavioral effects in monkeys OZ and OM using laser light inactivation. Visually guided saccades showed the same triad of effects as with chemical inactivation: shift in saccadic end

point, reduced peak velocity, and increased latency. Figure 1 shows the effects of laser inactivation at an example site in monkey OZ. We located the optrode

in the SC intermediate layers during each experiment by the center of the movement fields represented by neuronal activity recorded 500 μm below the fiber tip. While the monkey fixated a central bright spot on a dark background, we presented a second spot of light and the monkey was rewarded for making a visually guided saccade to that spot once the fixation spot disappeared. Figure 1A shows the locations in the visual field of the ArchT injection site (hexagon) and the optrode (starburst). Gray points are the endpoints of normal saccades to the visual target. Green points MK-8776 mouse are saccade endpoints to the same visual target during SC inactivation. Saccade endpoints shifted on average about 1.02° down in

this example (shown by the arrow), or about 7.3% of the saccade magnitude. These distributions of saccade endpoints were significantly different with and without light (2D Kolmogorov-Smirnov L-NAME HCl [KS] test, p < 0.001). We did not methodically study the effect of laser intensity on behavior. However, we were of course able to eliminate any change in behavior by sufficiently turning down the laser from our default intensity of about 650 mW/mm2. Effects at less than 300 mW/mm2 were negligible if present at all. Also, at several stimulation sites where we tested multiple laser intensities, we could not further increase the magnitude of the saccadic shift by increasing laser illumination, even up to 1600mW/mm2. In addition to changing the endpoints of saccades, photostimulation changed saccade latency and peak velocity. Figure 1B shows the cumulative distribution of saccade latencies without (black) and with (green) laser light. The distribution was shifted to the right with light, an increase of about 7 ms in saccade latency (p = 0.020, Wilcoxon rank-sum test). Figure 1C shows that the light shifted the distribution of peak saccade velocities to the left, a significant reduction in peak velocity of about 79°/s (p < 0.001, t test). Note that subsequent p values without a specified test were obtained from a t test. Figure 1D shows neuronal activity as spike density histograms, aligned to saccade onset.