Training neural network in matlab

i have multiple .mat files. in each file i have saved multiple features of an image (like intensity, hue etc). i want to train a neural network to categorize these images. i already have excel file containing the output categories of each image. now question is that i have to load all the .mat files and fed it as input to neural network or i have to take only single file each time and its corresponding output category. can anyone please clarify this. Also how can i give multiple .mat files containing multiple variables as input to a neural network. Thanks in advance