Procedure to determine the perception capacity or perception quality of a subject (1) by means of a brain-computer interface (20), where a) at least two different types of applicable stimuli (SA, SB) are predefined a subject (1), in particular acoustic, mechanical, electrical or optical stimuli, subject (1) being instructed, in particular, to perform, in the presence of a certain stimulus (SA, SB), a certain cognitive activity, preferably counting , b) where multiple stimuli (SA, SB) are applied one after another on the subject (1), and the stimuli (SA, SB) applied on the subject (1) are grouped into blocks (B1, B2 , B3, B4), where - the blocks (B1, B2, B3, B4) comprise a number of stimuli (SA, SB) that follow one after another applied to the subject (1), in which in each case a stimulus (SA) of a first type is arranged at a predefined position (p) of the block (B1, B2, B3, B4), as well as stimuli (SB) of one or more additional types are arranged in the other positions of the block (B1, B2, B3, B4), - after each of the stimuli (SA, SB) the reaction of the subject (1) is established by determining the EEG data ( U1, U2, U3, U4) of the subject (1) caused by the stimulus (SA, SB) or temporarily related to it, and these EEG data (U1, U2, U3, U4) are associated with the respective stimulus (SA , SB) thus with the respective block in the position (p) in question, c) where, from the EEG data (U) thus determined, calibration data are produced by gathering a number of EEG data (U1, U2, U3, U4) determined and stimuli (SA, SB) associated with these EEG data (U1, U2, U3, U4) in calibration blocks (KB1, KB2, KB3, KB4), where in each calibration block (KB1, KB2, KB3, KB4) individual EEG data (U1, U2, U3, U4) of the subject (1) are associated in each case when applying a first stimulus (SA, SB) and in the other points the EEG data (U1, U2, U3, U4) of the subject (1) by applying an additional stimulus (SA, SB), - where, by means of classification analysis, based on certain calibrat
