ira casson. >> dr., who is an expert, but an abrasive person who is contemptuous of the arguments that concussion can cause damage. >> narrator: casson had once joined pellman in attacking omalu's work. now one of casson's first moves: a public denial of omalu's conclusions. >> ira casson leads a team of nfl doctors who did a study of several hundred active players and reported that the concern over head injuries is overblown. >> is there any evidence, as far as you're concerned, that links multiple head injuries among pro football players with depression? >> no. >> dr. ira casson ends up with this very famous exchange that earns him the nickname "dr. no." >> with dementia? >> no. >> with early onset of alzheimer's? >> no. >> and ira casson was asked repeatedly, "is there any link between trauma, head trauma, and the kind of dementia we're seeing in these players?" and he says, "no, no, no, no." >> is there any evidence as of today that links multiple head injuries with any long-term problem like that?