There is a portion of the brain called the pleasure center. Pornography is considered to cause this part of the brain to lose sensitivity to Dopamine. It's the same part of the brain affected by drug addiction. The drugs cause a surge of dopamine to the pleasure center, which causes the initial dosage not to be enough to stimulate the dopamine receptors over time. This leads to tolerance.  
   
 I noticed in the thread below, as well as other threads, that vibrators cause more difficulty in orgasms through natural sex. There has also been Computed Tomography testing on this area of the brain while one watches porn. This area has the same reaction to porn as it does drugs. (Levels, I don't know, but porn does stimulate it.)  
   
 I'm assuming, though I have never actually studied or heard about vibrators and this area of the brain, that the intense pleasure from a great vibrator may take on the same effect.  
   
 Fun video attached.