Extrastriate feedback to primary visual cortex in primates: a quantitative analysis of connectivity

Area of Science:

• Neuroscience
• Visual Cortex Research
• Primate Brain Anatomy

Background:

• Top-down influences on the primary visual cortex (V1) are mediated by feedback connections from extrastriate areas.
• Understanding the quantitative and qualitative aspects of extrastriate pathway connectivity is crucial for elucidating information flow to V1 neurons.

Purpose of the Study:

• To quantitatively analyze the feedback connectivity from extrastriate area V2 to area V1 in macaque monkeys.
• To estimate the number of V2 axons projecting to V1 and their synaptic contribution to V1 neurons.

Main Methods:

• Quantitative analysis of anatomical data.
• Estimation of axonal projections and synaptic convergence based on connectivity data.

Main Results:

• Approximately ten million or more axons project from area V2 to area V1.
• Synaptic inputs from area V2 constitute less than 6% of all excitatory inputs to upper-layer V1 pyramidal cells.
• High convergence of V2 afferents, potentially over 100 axons per V1 cell, is suggested.

Conclusions:

• Extrastriate feedback synapses in primate V1, similar to geniculocortical synapses, represent a small fraction of total excitatory input to upper-layer neurons.
• The findings align with empirical observations of axon density and extrastriate feedback proportions in primate and rodent visual cortices.