Hi, June
I am not sure what you are analyzing for your measurement. What is your dependent variable? “disp” , as you wrote, is a dependent variable that means you obtained any features from surface (e.g. surface area, cortical thickness, and any your interesting value). But the dimension of your disp has to same the number of vertex (e.g. 1442 in your case).
Could you explain the “ellalign.coef”? Is it your interesting dependent variable? If so, is the dimension matching to the number of vertex?