To understand neural circuit mechanisms of signal processing across cortical areas, we have to identify connected pairs of cells (or cortical sites) between cortical areas and then to analyze difference and similarity in response property of the pairs. This strategy, however, is usually not available since it is difficult to find the pairs of cells (or sites) in large area of cortices in vivo. To achieve this goal, we establish a technique to identify unknown cortico-cortical projection pattern in macaque by combining optogenetics and optical intrinsic signal imaging (OISI): We used optogenetics to stimulate neurons in one cortical area and OISI to detect the response in the area receiving the projections.

Previously, we created a vector (AAV9- CaMKIIa- hChR2(ETTC)- EYFP- MBD) specifically expressing channel rhodopsin 2 (ChR2) in soma and dendrite but not axon to avoid stimulating passing axonal fibers and confirmed that it works in mouse and macaque cortex. In the present study, we used interhemispheric connections at V1/V2 border region as a model system to test feasibility of our technique. Optical stimulation at V1/V2 border of one hemisphere elicited statistically significant intrinsic signals in the other hemisphere reproducibly. To validate the reliability, we also recorded multi-unit activities and obtained neural firing elicited by stimulating the same site as in OISI experiment. In addition, regular interval stimuli along V1/v2 border revealed that projection around V1/V2 border is not completely symmetric and sometimes appears at multiple sites. Therefore, our technique is promising to identify unknown cortico-cortical projection patterns in monkey.