where is the magnitude of the physical stimulus , is the psychophysical function capturing sensation ( the subjective size of the stimulus ) , is an exponent that depends on the type of stimulation and is a proportionality constant that depends on the type of stimulation and the units used . where is the magnitude of the physical stimulus , Ï ( I ) is the psychophysical function relating to the subjective magnitude of the sensation evoked by the stimulus , a is an exponent that depends on the type of stimulation and k is a proportionality constant that depends on the type of stimulation and the units used .