question about addressing matrix elements

In the 4th line in the code below, I am having difficulty understanding what the -1 is doing. Is that a -1 step size?

Thanks, Chris

G0=zeros(m/2,n);
G0(:,1:2:n)=bay(1:2:m,1:2:n); %bay is a 192x128 double matrix
hh1 = [0 0.5 1 0.5 0];
xx1 = [G0(:,3:-1:2), G0, G0(:,n-1:-1:n-B)]; 
f1 = conv2(1,hh1,xx1,'valid');

Answer 1

[James Tursa]

Yes. The colon operator when used with three values is

   starting_value:step:ending_value

e.g.,

5:-1:2
ans =
     5     4     3     2

So, the expression

G0(:,3:-1:2)

means "all the rows" and "columns 3 and 2 in that order". I.e., the 3rd and 2nd columns of G0.