Widely used to treat cognitive, affective, psychiatric, and neurological disorders, electroencephalographic neurofeedback (EEG-NF) provides individuals with a real-time feedback of their EEG activity to modify brain function. However, the mechanisms behind the EEG changes targeted by EEG-NF remain unclear. The present study addresses this gap by examining methodological issues in the assessment of spontaneous EEG changes during EEG-NF sessions. Over multiple trials, healthy young adults observed a grey circle that either remained constant (control condition) or was continuously modified in size at different frequency rates PASSIVE EEG MODULATION (1, 5 and 10 Hz). We investigated whether EEG frequency bands classically targeted by EEG-NF: (i) change spontaneously over time, (ii) are influenced by a continuously modified visual stimulus and by (iii) the frequency at which this stimulus is modified. Results revealed: (i) a spontaneous increase in alpha power throughout the entire task, (ii) an increase in theta power when exposing participants to a continuous modification of the visual stimulus (relative to perceiving the same unmodified stimulus), and (iii) an absence of changes in the EEG frequency bands studied when manipulating the frequency of stimulus modification. These findings suggest that the EEG changes observed during EEG-NF are influenced by the task environment itself rather than successful EEG self-modulation. It is therefore crucial to carefully design EEG-NF protocols to account for non-specific effects and ensure that observed EEG changes are due to the hypothesised mechanisms. Further research is needed to delineate the mechanisms underlying EEG modulation in EEG-NF and to refine protocols prior to clinical application.