The function you are using for maximal projection is "max", this function is generally used to find a maximum element. By specifying the "dim" that is the dimension along which you want to find the maximum element.
Similar to this "max" function, we have "mean" function which calculates the average along a dimension. More information about this function can be found here:
This boilerplate code can be used to find the Average Intensity Projection:
imageSequence = rand(256, 256, 50); % 50 frames of 256x256 images
AIP = mean(imageSequence, 3);
imshow(AIP, []);
title('Average Intensity Projection');
Hope this helps you out to find out the Average Intensity Projection.
