How can i use datastore (mat-files) in map function?

I have 2000 images, and each of them has a mat-file with the following matrices(struct) as image info:

the struct for each image is ( x is variable for each images):

desc: [x * 128 double]
locs: [x * 4 double]
color_hist: {1 * x cell}

color_hist has x vectors as [1 * 11 double]

I used the following statement for creating datastore:

ds = fileDatastore(fullfile(pwd,'property'),'ReadFcn',@load,'FileExtensions','.mat')

Now, i want to compare the first image info with another images, but i don't know how can i access images info in map function, separately, for comparing images.

for example, the following map function :

function maxMapper(data, ~, intermKVStore)
  t = data.desc;
  struct(t)
end

output is:

 Error using maxMapper (line 7)
   Reference to non-existent field 'desc'.

 Error in mapreduce (line 99)
   outds = execMapReduce(mrcer, ds, mapfun, reducefun, parsedStruct);

 Error in MapReduce (line 151)
   result = mapreduce(ds, @maxMapper, @maxReducer);

Is there a nice way to do this?