The brain uses motion signals to predict the current position of an object, enabling us to respond promptly and accurately. But how does the brain use motion information to predict an object's position and generate visual awareness? The causal relationship between neural activity in the primary visual cortex (V1) and visual awareness has long been controversial.

A new study by Prof. ZHANG Peng's team from the Institute of Biophysics of the Chinese Academy of Sciences (CAS), together with collaborators, provides evidence that feedback signals expressed in V1 are necessary for visual awareness of an object's "predicted" current position. The researchers identified a critical role for these feedback signals in visual awareness by combining a visual position illusion with experiments involving patients with different types of visual pathway damage.

The findings were published in Cognitive Science Research on October 8.

The researchers used the flash-grab effect (FGE) to dissociate an object's physical position from its perceived position. They conducted experiments in hemianopic patients with V1 lesions, hemianopic patients with damage to the optic radiation (the white matter tract from the visual thalamic LGN to V1), and healthy participants.

In the Grab-in condition, patients with V1 lesions could not see the red bar when the illusion displaced it into their blind visual field, or scotoma. They could only see it when it was moved farther outward, beyond the scotoma boundary. However, in the Grab-out condition, they could see the bar when it appeared in their intact visual field.

In contrast, healthy participants with an artificial scotoma, as well as patients with optic radiation damage and intact V1, could see the displaced red bar within the blind field. These results suggest that when V1 is intact, participants can perceive a red bar displaced by background motion within their blind field.

The study suggests that consciously seeing a stimulus requires V1 to receive position-prediction feedback. When V1 is damaged, higher motion areas may still compute the displaced position, but the feedback cannot reach the corresponding V1 region. Thus, the red bar "disappears" within the blind field. When V1 is intact, however, these feedback signals can reach it, allowing the displaced object to remain visible.

This study of patients with brain lesions demonstrates that feedback signals from the higher cortex to V1 are necessary for visual awareness under attentional conditions. The motion-illusion paradigm used in this study has important clinical implications for assessing the specific location of damage to the visual pathway in patients with visual field defects.

Schematic of flash position shifts under Grab-in and Grab-out conditions. (Image by ZHANG Peng's group)

Loss of V1 feedback signals renders a flashed red bar invisible within the blind field. (Image by ZHANG Peng's group)