What numbers do you see? A new bistable perceptual phenomenon on symbolic numbers

Perceptual recognition of numerical characters, such as Arabic numerals, is essential to our daily activities in modern society. The study of the perceptual and neural mechanisms that allow us to understand these characters is an important scientific topic.

Researchers studied a specially designed perceptual bistable character phenomenon, called the occluded digital digit, to better understand the mechanisms underlying perceptual number recognition. The research is published in the journal Vision Journal.

The digital digit is a special version of the numeral fonts designed for use in electronic products such as traffic counters, digital calculators, and many other LED broadcast signs. Sometimes, accidental partial occlusion of these characters results in an intriguing perceptual phenomenon, namely that a digital digit can be recognized as having multiple semantic meanings, usually two.

“Due to the simple design of digital digits, partial occlusion causes visual ambiguities. In other words, the visual input does not contain enough information for us to make a unique semantic interpretation of the occluded digital character,” said Dr. Junxiang Luo of the National Institute of Physiological Sciences, a researcher who led the project.

The researchers then conducted a behavioral experiment on healthy human participants using visual adaptation tasks. This task paradigm examined how exposure to specific types of visual input biases perception.

They found that after prolonged presentation of normal numerical digits to the visual system, participants tended to report that they were seeing a single number biased relative to the adapting number.

“Visual adaptation selectively reduces the neural sensitivity of a certain stage of visual processing to a certain class of visual stimuli. Thus, by assessing the extent to which interpretation deviates from the bistable state after adaptation to different types of visual information, we have a chance to learn at what level of visual processing perceptual recognition of digits may occur,” Dr. Luo said.

The researchers selected a series of candidate adapting stimuli. They demonstrated that bistability depends neither on low-level visual information such as simple visual features, nor on high-level information such as semantics, and these results suggest a contribution from mid-level visual processing that encodes complex shapes and symbolic numeric forms.

The perceptual phenomenon discovered in this psychophysical study has the potential to be extended to physiological studies in the future, in which researchers can compare brain activity between different perceptual interpretations of the same occluded digits.

“We hope to extend this study by using some physiological methods to gain a deeper understanding and more precise localization of the brain areas that generate perceptual bistability,” Dr. Luo said.