A method of operating a hearing instrument for processing an input sound and to provide an output stimulus according to a users particular needs, and related system, computer readable medium and data processing system. An object is to provide an improved customization of a hearing instrument. The method includes the steps a) providing an estimate of the present cognitive load of the user b) providing processing of an input signal originating from the input sound according to a users particular needs and c) adapting the processing in dependence of the estimate the present cognitive load of the user. The estimate of the present cognitive load of a user is produced by in-situ direct measures of cognitive load (e.g. based on EEG-measurements, body temperature, etc.) or by an on-line cognitive model in the hearing aid system whose parameters have been preferably adjusted to fit to the individual user.
