Distinct "driving" versus "modulatory" influences of different visual corticothalamic pathways

Area of Science:

• Neuroscience
• Visual System
• Corticothalamic Interactions

Background:

• Higher-order thalamic nuclei extensively interact with the cerebral cortex, supporting functions like sensation and attention.
• Corticothalamic (CT) projections to higher-order nuclei originate from cortical layers 5 and 6, unlike first-order nuclei receiving only layer 6 input.

Purpose of the Study:

• To investigate the distinct functional roles of layer 5 and layer 6 CT pathways in the visual thalamus.
• To test the proposed functional divergence of these two CT projection populations in vivo.

Main Methods:

• Optogenetic inactivation of specific CT populations in the primary visual cortex (V1) of awake mice.
• Recording single-unit activity in V1, dLGN, and pulvinar.
• Inactivation of the superior colliculus to assess subcortical contributions.

Main Results:

• Layer 5 CT projections, but not layer 6, were found to drive visual responses in the pulvinar.
• Both layer 5 and layer 6 CT pathways provide retinotopic, baseline excitation to their thalamic targets.
• Inactivation of the superior colliculus also reduced pulvinar visual responses, indicating combined cortical and subcortical input.

Conclusions:

• A functional division exists between "driver" (layer 5) and "modulator" (layer 6) CT pathways to the visual thalamus.
• Both cortical layer 5 and subcortical inputs contribute to higher-order visual thalamic activity.
• These findings elucidate the distinct roles of different inputs in shaping thalamic processing of visual information.