Retinal Input to Macaque Superior Colliculus Derives from Branching Axons Projecting to the Lateral Geniculate...

Area of Science:

• Neuroscience
• Vision Science
• Primate Retinal Circuitry

Background:

• Retinal ganglion cells (RGCs) project to various brain targets, including the superior colliculus (SC) and lateral geniculate nucleus (LGN).
• The extent to which individual RGCs project to both the SC and LGN in primates remains largely uncharacterized.

Purpose of the Study:

• To investigate whether the same RGCs that project to the macaque SC also project to the LGN.
• To determine the proportion of RGCs with divergent projections to both targets.

Main Methods:

• A double-labeling retrograde tracing experiment was conducted in two male macaques.
• Cholera toxin subunit B (CTB) conjugated to different fluorescent markers (Alexa Fluor 488 and 555) was injected into the SC and LGN at matching retinotopic locations.
• RGCs in retinal flatmounts were examined for dual labeling.

Main Results:

• A significant percentage (76-98%) of RGCs projecting to the SC were also labeled by injections into the LGN, indicating collateral projections.
• RGCs projecting to the SC represented approximately 4.0-6.7% of the total labeled RGC population.
• No RGCs were found to project exclusively to the SC, with all SC-projecting cells also projecting to the LGN.

Conclusions:

• Retinal input to the macaque SC originates from collateral axonal branches of RGCs that primarily project to the LGN.
• This suggests a significant overlap in visual information processing pathways to the SC and LGN in primates, mediated by a subset of RGCs.