Method for determining a biological response of a target (41; 41, 42) to a soluble candidate substance, the method comprising the following steps: - introducing a soluble candidate substance into a laminar flow of a buffer liquid flowing through a dispersion channel to form a candidate substance solute in the buffer liquid having an initial concentration profile; - by the laminar flow of the buffer liquid through the dispersion channel dispersing the initial concentration profile of the candidate substance solute in the buffer liquid to form a dispersed concentration profile of the candidate substance solute in the buffer liquid; - directing the laminar flow of the buffer liquid containing the candidate substance solute having the dispersed concentration profile into a detection channel to form a final symmetrical concentration profile of the candidate substance solute in the buffer liquid in the detection channel; - introducing a target into the detection channel (41; 41, 42) in a manner so as to obtain a combined concentration profile in the buffer liquid, the combined concentration profile comprising a constant target concentration profile overlying the final symmetrical concentration profile of the candidate substance solute; - holding in the detection channel at least one half of the combined concentration profile contained in the buffer liquid; and - optically scanning the at least one half of the combined concentration profile contained in the buffer liquid held in the detection channel to detect at the various concentrations of the candidate substance solute of the combined concentration profile an optical signal which is representative of the biological response of the target (41; 41, 42) to the soluble candidate substance. - introducing a soluble candidate substance into a laminar flow of a buffer liquid flowing through a dispersion channel to form a candidate substance solute in the buffer liquid having an initial concentration profile; - by the laminar flow of the buffer liquid through the dispersion channel dispersing the initial concentration profile of the candidate substance solute in the buffer liquid to form a dispersed concentration profile of the candidate substance solute in the buffer liquid; - directing the laminar flow of the buffer liquid containing the candidate substance solute having the dispersed concentration profile into a detection channel to form a final symmetrical concentration profile of the candidate substance solute in the buffer liquid in the detection channel; - introducing a target into the detection channel (41; 41, 42) in a manner so as to obtain a combined concentration profile in the buffer liquid, the combined concentration profile comprising a constant target concentration profile overlying the final symmetrical concentration profile of the candidate substance solute; - holding in the detection channel at least one half of the combined concentration profile contained in the buffer liquid; and - optically scanning the at least one half of the combined concentration profile contained in the buffer liquid held in the detection channel to detect at the various concentrations of the candidate substance solute of the combined concentration profile an optical signal which is representative of the biological response of the target (41; 41, 42) to the soluble candidate substance.