Abstract: The present paper begins with constructing a mathematical model for cortex effect E caused by stimulus code along the afferent channel.The model E=Blog(1+kI),k=I￣(-b)/a in terms of stimulus intensity I is based on neurophysiological experiments and the practicality of receptor functions acquired in the long run of evolution.Accordingly, the criterion of difference limen for moderate stimuli has been shown to be △E=constant, which, being generaized to weak stimuli,leads to a resonable extension of Weber's law:△I/(I+aI￣b)=constant N,which is valid to weak and strong stimuli. Thus the old problem over a hundred years ago concerning extend of the validity of Weber’s law to include weak stimuli seems to be more or less settled.

Key words: Weber ratio, difference limen, mathematical model