Direct cortical feedforward and feedback pathways differentially shape visual responses and selectively engage looped circuits

In mouse V1–PM circuitry, feedforward and feedback pathways both drive excitation and inhibition, but feedforward yields stronger additive effects while feedback produces stronger multiplicative modulation that improves visual decoding.

• Why it matters: Hierarchical cortex models depend on how direct cortico-cortical inputs shape activity, yet separating their direct effects from indirect multisynaptic influences is difficult.
• What they did: The study combined retrograde labeling, multi-area two-photon imaging, and optogenetic perturbations to test how pathway activity in V1–PM affects visual responses and decoding, with causal confirmation of pathway asymmetries.
• The result: Feedforward and feedback preferentially modulated reciprocating looped neurons: feedforward excited them, while feedback either excited or inhibited them, supporting distinct roles for each pathway within recurrent cortical loops.

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