MP_maps_and_groups.mat is a Matlab cell structure organised as follows:

Row 1:	81x81 image files, each storing a macular pigment map
Row 2:	number 1 - 3 which is a label of the group to which the image belongs.
	Group 1 - subjects under 50 without AMD
	Group 2 - subjects 50 and over without AMD
	Group 3 - subjects 50 and over with AMD

There are 479 sets of the above data, each belonging to an individual subject. Normally each subject will have more than one data sets included.

Paper describing how MP maps were generated:

“Extracting histological parameters from multi-spectral retinal images: a Bayesian inverse problem approach” (Yuan Shen, Antonio Calcagni, Ela Claridge, Frank Eperjesi, Hannah Bartlett, Jonathan Gibson, Andrew Palmer, Iain Styles), In Medical Image Understanding and Analysis - MIUA 2013. 17th Annual Conference, Birmingham, UK, July 18-20, 2013. Proceedings. (Ela Claridge, Andrew D. Palmer, William T.E. Pitkeathly, eds.), 2013, pp. 219-224. 
http://events.cs.bham.ac.uk/miua2013/MIUAproceedings.pdf