What causes illusions? Is there any neural processing in the eye and low-level visual centers in the brain?
These occurrences have been explained by various mechanisms, from low-level brain processes to high-level ones that consider prior knowledge or context. Significantly, only a small portion of these phenomena can be predicted by quantitative models of color perception.
Scientists from the University of Exeter have now developed a model that suggests simple limits to neural responses – not deeper psychological processes – to explain these illusions. Numerous visual illusions are caused by limits in the way our eyes and visual neurones work – rather than more complex psychological processes, suggests a new study.
The model predicts color appearance based on the principle of efficient coding. This considers that neural bandwidth is limited and that no valuable bandwidth should be wasted when coding information across different spatial scales in a typical natural scene.
Dr. Troscianko, from the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall, said, “Our eyes send messages to the brain by making neurons fire faster or slower. However, there’s a limit to how quickly they can fire, and previous research hasn’t considered how the limit might affect the ways we see color.”
The model suggests that our vision works best when viewing natural settings and combines this “limited bandwidth” with data on how humans interpret patterns at various scales.
How the human brain and eyes can handle the high contrast is still puzzling. The test has shown that the highest contrasts humans can see at a single spatial scale is around 200:1. What’s more confusing is the neurons connecting our eyes to our brains can only handle contrasts of about 10:1.
The newly developed model has shown that neurons with such limited contrast bandwidth can combine their signals to allow us to see these enormous contrasts. Still, the information is ‘compressed’ – resulting in visual illusions.
Dr. Troscianko added, “The model shows how our neurons are precisely evolved to use every bit of capacity.”
“For example, some neurons are sensitive to tiny differences in grey levels at medium-sized scales but are easily overwhelmed by high contrasts. Meanwhile, neurons coding for contrasts at larger or smaller scales are much less sensitive but can work over a much wider range of contrasts, giving big black-and-white differences.”
“Ultimately, this shows how a system with a severely limited neural bandwidth and sensitivity can perceive contrasts larger than 10,000:1.”