Walter has been working at his computer for the past 2 hours and the hum that he found so annoying when he started no longer bothers him. The change in Walter’s sensitivity to the noise from the computer illustrates the principle of:

sensory adaptation            

Explanation:

Sensory adaptation is described as a phenomenon in which an individual experience a reduction in the 'sensitivity' that is being related to a specific stimulus after he or she is being exposed to it constantly. Because sensory adaptation is capable of reducing an individual's awareness related to a specific and a constant stimulus, however, it is responsible for freeing up his or her resources and attention in order to attend to some other stimuli in the surrounding environment around him or her.

The given statement represents 'sensory adaptation.

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