How to blur some part of image without affecting rest of the part?

Suppose I have an image (Cameraman.tif) and I want to select some part of the input image then I Want to apply imfilter() operation only for the selected part not for the rest part. Lastly, when i will showing the blurred image then It has the same size which is equal to input image.

Answer 1

[Chunru]

I = imread("peppers.png");
imshow(I)

LEN = 31; THETA = 11;
PSF = fspecial('motion', LEN, THETA);

% Blur part of image
I(100:300, 100:300, :) = imfilter(I(100:300, 100:300, :), PSF);

figure
imshow(I)