I am trying to do GLM analysis for cortical thickness with a behavioral score. I wanted to explore square fit in the GLM. Basically wanted to see weather the behaviour is related in a quadratic function instead of linear one. 

  Couldn't find any option for this is GLM fit. 

   I was thinking of feeding a squareroot values of the behavioural parameter in the GLM as an alternative. But not sure weather that will work. Please any one can help me with this. 

Thanks, 

Abhinav