Assessing spontaneous categorical processing of visual shapes via frequency-tagging EEG

Abstract

Categorization is an essential cognitive and perceptual process, which happens spontaneously. However, earlier research often neglected the spontaneous nature of this process by mainly adopting explicit tasks in behavioural or neuroimaging paradigms. Here, we use frequency-tagging (FT) during electroencephalography (EEG) in 22 healthy human participants (both male and female) as a direct approach to pinpoint spontaneous visual categorical processing. Starting from schematic natural visual stimuli, we created morph sequences comprising 11 equal steps. Mirroring a behavioural categorical perceptiondiscriminationparadigm, we administered an FT-EEG oddball paradigm, assessing neural sensitivity for equally sized differences within and between stimulus categories. Likewise, mirroring a behavioural categoryclassificationparadigm, we administered a sweep FT-EEG oddball paradigm, sweeping from one end of the morph sequence to the other, thereby allowing us to objectively pinpoint the neural category boundary. We found that FT-EEG can implicitly measure categorical processing and discrimination. More specifically, we could derive an objective neural index of the required level to differentiate between the two categories, and this neural index showed the typical marker of categorical perception (i.e., stronger discrimination across as compared to within categories). The neural findings of the implicit paradigms were also validated using an explicit behavioural task. These results provide evidence that FT-EEG can be used as an objective tool to measure discrimination and categorization and that the human brain inherently and spontaneously (without any conscious or decisional processes) uses higher-level meaningful categorization information to interpret ambiguous (morph) shapes.Significance StatementEvery time when we encounter a new image or object, we will automatically relate it to previously stored categories. This categorization process allows us to efficiently react on new information and it influences our perception. The behavioural hallmark of categorical perception entails that we perceive differences between categories more distinctly than within a category. Previous research has mainly investigated categorical processing using explicit tasks. Here, we use carefully controlled morphs with implicit, neural electroencephalography measures to assess spontaneous categorical processing. We found higher neural amplitudes for “between”- than “within”-category morph pairs, establishing a neural correlate of categorical perception. This provides evidence that the brain inherently and automatically uses higher-level meaningful categorization information to interpret ambiguous shapes.