NITHIT SINGTOKUM¹ (), Kanyarat Benjasupawan¹, Chattarin Poungtubtim¹, Sedthapong Chunamchai², Chaipat Chunharas²; ¹Faculty of medicine chulalongkorn university, Bangkok, Thailand, ²Cognitive Clinical and Computational Neuroscience Lab, KCMH Chula Neuroscience Center, Thai Red Cross society, Chulalongkorn University, Bangkok, Thailand

Visual imagery is the ability to reactivate and manipulate visual representation in the absence of visual stimuli. This ability, however, cannot be accomplished by people with aphantasia. Previous research has indicated that the intensity of visual imagery aligns with the activation of distinct brain regions, such as the fusiform area. However, it remains uncertain whether this process is influenced solely by voluntary attempts at imagination by aphantasics or if it can also occur involuntarily. The patient NP abruptly lost her ability to “picture things in her mind”. The results of detailed clinical neuropsychological tests were normal except that she could not identify objects from their silhouette images especially when they were presented from unusual viewpoints. To explore the involuntary nature of this process, we conducted an EEG experiment where Participants had to monitor a sequence of images on a screen and click a button when an image tilted clockwise or counterclockwise (occurring only 10% of the time). Unrelated to the task, stimuli were intentionally presented as triplets: a silhouette image (pre-silhouette), followed by the corresponding object image, and then the same silhouette image as the pre-silhouette (post-silhouette). Greater ERP differences between post-silhouette and pre-silhouette images would suggest the advantage of image preview or the absence of object presentation in the pre-silhouette condition. Our findings revealed that patient NP exhibited significantly higher average ERP changes between 300 and 800 ms after stimulus onset compared to control groups in occipital (p<0.001, t-value = 5.25), frontal (p<0.001, t-value = 5.87), and parietal (p<0.01, t-value = 3.68) areas. The increased adaptation after an image preview, even when object recognition was unrelated to the task, implies a potential impairment in the top-down signal that facilitates visual imagery in individuals with aphantasia, and notably, this impairment may occur involuntarily.