selecting subsets from mvnpdf

Hi all. Matlab is still growing on me, so I might be asking a very simple question. I have a multivariate normal distribution created using mvnpdf. See below.

Sigma1 = [1 0; 0 .5]; mu1=[0 0];

x1 = -4:.1:4; x2 = -4:.1:4;

[X1,X2] = meshgrid(x1,x2);

F1 = mvnpdf([X1(:) X2(:)],mu1,Sigma1);

F2 = reshape(F1,length(x2),length(x1));

Now what I want to do is set a certain range of values to (F(x:x,y:y)=0) and do a convolution with another mvnpdf. Integrating two mvnpdf results is easy since they have the same format, but I'm not sure who to select the range I want since it the output is all rows. Conversely, I can set those values to zero in F2 (after reshape) but then I can't do a convolution with a separate mvnpdf. Clearly, I'm not understanding the output of mvnpdf.

Any help/explanation is greatly appreciated.