CATEGORICAL DISCRIMINATION, NOT STATISTICAL LEARNING, DRIVES NEURAL RESPONSES IN AN ODDBALL FAST PERIODIC VISUAL STIMULATION PARADIGM

Oddball fast periodic visual stimulation uses steady-state evoked potentials to study visual categorization. Periodic visual stimulation at a fixed frequency elicits neural responses at the same frequency. When rare “deviant” stimuli from a different category are periodically embedded within a stream of base stimuli, an additional response emerges at the deviant’s presentation frequency (subharmonic of base stimulation frequency), which is considered as a neural marker of discriminability. There is a hypothesis that discrimination response may occur in the absence of categorical differences and reflect statistical learning. Participants (N=17, mean age=25.3) were presented with a sequence of visual stimuli at 6Hz frequency, while 160-channel EEG was recorded. In the Discrimination condition, base stimuli (letters) alternated with deviants (pseudoletters), appearing every fifth item. In the Repetition condition, both base and deviant stimuli were pseudoletters, differing in presentation probability (base stimuli four times more frequently). In the Control condition, all stimuli were pseudoletters presented with equal probability. Significant responses (z>3.1) at the deviant presentation frequency (1.2Hz) and its harmonics were observed across all conditions. A permutation T-test showed a significantly higher response in the Discrimination condition compared to Repetition and Control conditions, but showed no differences between the Repetition and Control conditions. These results provide no evidence for statistical learning and suggest that general physiological mechanisms contribute to steady-state responses at subharmonics. At the same time, categorical processing is clearly catching up with this paradigm, as evidenced by the larger response in the Discrimination condition.

Figure1. A) Experimental Paradigm; B) SNR spectra