Problems filtering image with prewitt I'm trying to filter an image vertically to get the edges, in MATLAB, but I get very different results from convultion and correlation.

CODE:

vf = fspecial('prewitt');

iyconv = imfilter(im, vf, 'replicate', 'conv');
iycorr = imfilter(im, vf, 'replicate', 'corr');


result in the image below,



why does the convultion only outputs the lower part of the mouth, and on contrary the correlation teh upper?
Prewitt kernel is rotated 180 degree between correlation and convolution. 

You can check this link: http://www.equalis.com/blogpost/731635/129540/Convolution-and-Correlation-in-Image-Processing