Saccade Modulation by Optical and Electrical Stimulation in the Macaque Frontal Eye Field

Abstract

Recent studies have demonstrated that strong neural modulations can be evoked with optogenetic stimulation in macaque motor cortex without observing any evoked movements (Han et al., 2009, 2011; Diester et al., 2011). It remains unclear why such perturbations do not generate movements and if conditions exist under which they may evoke movements. In this study, we examine the effects of five optogenetic constructs in the macaque frontal eye field and use electrical microstimulation to assess whether optical perturbation of the local network leads to observable motor changes during optical, electrical, and combined stimulation. We report a significant increase in the probability of evoking saccadic eye movements when low current electrical stimulation is coupled to optical stimulation compared with when electrical stimulation is used alone. Experiments combining channelrhodopsin 2 (ChR2) and electrical stimulation with simultaneous fMRI revealed no discernible fMRI activity at the electrode tip with optical stimulation but strong activity with electrical stimulation. Our findings suggest that stimulation with current ChR2 optogenetic constructs generates subthreshold activity that contributes to the initiation of movements but, in most cases, is not sufficient to evoke a motor response.

FEF inactivation with improved optogenetic methods

“…First, although earlier primate optogenetics studies reported significant firing rate changes in only 38-68% of neurons, all within 400 μm to 1 mm of the light source (14,18,19), we found inactivation (>80% firing reduction) over a volume (>10 mm 3 ) comparable to the inactivated tissue volumes in cooling (1) and pharmacological inactivation studies (2,3,7,72), which silence 80-100% of neurons to <80% of baseline over 4.2-14 mm 3 . Second, in contrast with previous studies of inhibitory opsins in primate cortex (14,18,19), which report a subpopulation (∼10-25% of the total cells) that increases its firing rate during illumination (14,18,19) and potentially cancels the effects of inhibition on behavior, not a single neuron in this study increased its firing rate in response to illumination. Optogenetics studies of excitatory opsins have reported similar heterogeneity (17).…”

“…The maximum total light power density applied in this study was 100 mW/mm 2 , similar to what was used in the first electrophysiological demonstrations of optogenetics in the primate (19,61), as well as to what has been used for optogenetic modulation of superficial cortical neurons through a window on the brain (62,63). However, it is much lower than subsequent primate studies that reported behavioral changes using maximal light power densities ranging from several hundred milliwatts per square millimeter to >10 W/mm 2 (11)(12)(13)(14)(15)49).…”

“…The size of the primate brain has limited the efficacy of primate optogenetics up to this point because most primate neuroscience research, and eventual human translation, requires inactivation of large brain volumes. For example, Ohayon et al (14) expressed the green-light-sensitive, inhibitory opsin ArchT in the FEF of monkeys and reported no behavior change, no bias of saccadic eye movements, no significant change in saccade direction during combined electrical and optical stimulation, and no change in saccade latency despite 68% of neurons being modulated <1 mm from the light source. Here, the key difference seems to be inactivation of a larger tissue volume (i.e., ∼10 mm, based on our electrophysiological recordings and modeling).…”

“…In contrast, FEF pharmacological inactivation studies report inactivation of volumes of ∼10 mm 3 (2, 3). Further, optogenetically driven neuronal effects in primates are often heterogeneous, decreasing the activity of some neurons and increasing the activity of others (14,(17)(18)(19). No primate optogenetics study has reported inactivation levels near the levels of FEF pharmacological inactivation studies, namely, >80% reduction in firing rate relative to baseline reported in >80% of neurons (1-3).…”

“…Unfortunately, although optogenetics is widely used to study functional physiology and disease models in rodents and invertebrates, technical challenges have limited the use of optogenetics in the nonhuman primate brain, which has a volume ∼100-fold larger than the rodent brain (10). Pioneering studies in monkeys reported small behavioral effects with excitatory (11)(12)(13)(14) and inhibitory opsins (15)(16)(17). These studies used large light power densities (several hundreds of milliwatts per square meter to 20 W/mm 2 ) but illuminated only small volumes, at most 1 mm 3 , due to both the chosen wavelength and light-delivery method.…”