EVIDENCE FOR A PHOTOSENSITIVE REGION IN THE CAUDAL MESENCEPHALON OF THE TURTLE BRAIN

Using an in vitro brainstem-cerebellum preparation from the turtle Chr ysemys picta, burst discharge was recorded from the abducens nerve whe n light was directed on the brainstem. This burst discharge likely rep resents a neural correlate of the eye-blink reflex. Increasing the int ensity of the light stimulus reduced the response latency from a durat ion of many seconds to approximately 1-2 s. No response was recorded w hen the light source was covered. The response was present when infrar ed light was blocked, and it could only be produced when the light spe ctra contained wavelengths below approximately 550 nm. Lesion experime nts reveal that the photosensitive area is located caudal to the troch lear nerve and rostral to the trigeminal nerve. Single-unit microelect rode recordings demonstrate that this region is tonically active in th e dark and that activity is suppressed by light. Pharmacological resul ts show that the light response is blocked by application of compounds that act as serotonergic antagonists, and that antagonists of noradre nergic receptors (alpha or beta) either have no effect or their action s are variable. Taken together, these data suggest that an isthmo-opti c-like area in the turtle brain is photosensitive to direct light and activates neural circuits that control eye movements.