In contrast, initial exploratory evaluation of a certain brain area frequently requires larger regions of inactivation, particularly when the relation of the area to behavior is tenuous. Small changes in behavior in such cases may go undetected, so a larger inactivation would probably be necessary to reveal the contribution of the structure to behavior. The limited size of the inactivation might well have contributed to the lack of any behavioral effect of channel rhodopsin injections made in monkey motor cortex (Diester et al., 2011). Optogenetic inactivation differs from chemical inactivation in having greater precision and added flexibility. With chemical inactivation, the effect on behavior is

dependent on the overlap of the area of neurons active in

generating the behavior (in our case a saccade) and the area of neurons inactivated by the chemical. With the optogenetic approach, however, there is selleckchem a third gradient: the illumination from the optrode on targeted neurons. Essentially, the outcomes and interpretations of each optogenetic experiment are governed by the interaction of these three gradients. The effect of these gradients on behavior was interesting: size of the effect depended on the saccade’s distance from the optrode, and we would expect this factor to govern behavioral effects in any brain area. In addition, we found a systematic change in the direction of this shift that depended on the location of the injection. EPZ-6438 supplier below Saccade related neurons are mapped on the intermediate layers of the SC as vectors pointing to different regions of the visual field. Activity during a saccade is the result of a large population of such neurons (Munoz and Wurtz, 1995), the average of whose vectors determines the generated saccade (Lee et al., 1988). In fact, the precision of the optogenetic method provides the most convincing evidence so far (Figure 3A) that shifts in saccade endpoints can be predicted if one knows the shift in the vector average resulting from inactivation. In our experiments, the shifts are most easily interpreted as the action of the injection gradient

and the light gradient acting on the SC neuronal vector gradient, as indicated by the analysis in Figures 3C–3E. Although chemical inactivation might well have a place in studying the brain, optogenetic techniques allow a new set of strategies with remarkable temporal and spatial precision, some of the principles of which we have illustrated here. Three adult male monkeys (OZ, OM, RO; Macaca mulatta) provided data for different aspects of these experiments. Monkeys weighed between 8 and 11 kg and had implanted scleral search coils for measuring eye position, had recording cylinders for accessing SC neurons, and had posts for immobilizing the head during experiments as described previously ( Sommer and Wurtz, 2000).