The impact of string length on neural categorization processes for letters and digits

Area of Science:

• Neuroscience
• Cognitive Psychology
• Visual Perception

Background:

• The human brain shows hemispheric specialization for processing letters (left hemisphere) and digits (right hemisphere).
• It is unclear if the brain processes single characters and character strings differently based on their category (letters vs. digits).
• This distinction is relevant as letters form words in strings, while digits have meaning individually and in strings.

Purpose of the Study:

• To investigate how the adult brain discriminates between letters and digits.
• To examine the effect of character string length (single vs. five characters) on visual processing.
• To determine if category (letters vs. digits) and string length interact in visual cortex processing.

Main Methods:

• Utilized Fast Periodic Visual Stimulation (frequency-tagging) with electroencephalography (EEG) recordings.
• Presented stimuli streams with one character category (e.g., letters) periodically embedded within another (e.g., digits) at 10 Hz.
• Analyzed brain responses at 2 Hz (10 Hz/5) to identify category- and length-specific discrimination patterns.

Main Results:

• Clear discrimination responses were observed at 2 Hz with occipito-temporal topography, stronger for strings than single characters.
• Digits elicited right-lateralized responses regardless of string length.
• Single letters showed right-lateralized responses, while letter strings demonstrated left-lateralized responses, suggesting category- and length-dependent processing.

Conclusions:

• The findings suggest that single letters are processed as visual objects, while letter strings engage linguistic processes in the left hemisphere.
• Expert readers may perceive single letters as distinct visual entities, whereas letter strings activate orthographic and phonological networks.
• Hemispheric lateralization for character processing is modulated by both character category and string length.