Conduction velocity of intracortical axons in monkey primary visual cortex grows with distance: implications for computation

TL;DR

This study reveals that intracortical axons in monkey V1 conduct signals faster over longer distances than previously thought, enabling local contextual influences to shape visual processing.

Contribution

It demonstrates that intracortical conduction velocity increases linearly with distance, challenging the belief that only feedback from higher areas mediates contextual effects.

Findings

Intracortical axon conduction speed increases with distance.

Conduction velocity is sufficient for local contextual influence.

Challenges the idea that only feedback from higher visual areas mediates context.

Abstract

A critical visual computation is to construct global scene properties from activities of early visual cortical neurons which have small receptive fields. Such a computation is enabled by contextual influences, through which a neuron's response to visual inputs is influenced by contextual inputs outside its classical receptive fields. Accordingly, neurons can signal global properties including visual saliencies and figure-ground relationships. Many believe that intracortical axons conduct signals too slowly to bring the contextual information from receptive fields of other neurons. A popular opinion is that much of the contextual influences arise from feedback from higher visual areas whose neurons have larger receptive fields. This paper re-examines pre-existing data to reveal these unexpected findings: the conduction speed of V1 intracortical axons increases approximately linearly with the conduction distance, and is sufficiently high for conveying the contextual influences. Recognizing the importance of intracortical contribution to critical visual computations should enable fresh progress in answering long-standing questions.