Humans do not have direct access to retinal flow during walking

Area of Science:

• Perception
• Human Movement Science
• Visual Neuroscience

Background:

• Perceived visual speed is reduced during locomotion.
• This phenomenon is often explained by subtracting self-motion speed from visual speed.
• Previous research suggests this subtraction occurs after initial visual processing.

Purpose of the Study:

• To investigate whether observers access retinal flow information before self-motion speed subtraction.
• To test the "direct access hypothesis" of visual speed perception during walking.
• To explore the complexity of visual and self-motion processing interactions.

Main Methods:

• Participants performed a two-interval forced-choice (2IFC) visual speed discrimination task.
• The task was conducted while participants walked on a treadmill.
• Walking speed was kept constant or varied between the two intervals of each trial.

Main Results:

• Visual speed discrimination performance decreased significantly when walking speed varied between intervals.
• This outcome contradicts the "direct access hypothesis."
• The degree of perceived speed subtraction was influenced by the presented visual speed.

Conclusions:

• Observers do not have direct access to retinal flow before self-motion speed subtraction.
• The interaction between visual processing and self-motion perception is more intricate than previously assumed.
• These findings refine our understanding of how the brain integrates visual and motion cues for navigation.