I'm pretty new to neural networks myself, but I'll try to help. Do you only have 1 matrix with size 7701x51? Because I'm a bit confused about your formulation. In my experience, in most cases you have an input matrix X with size samples-by-features and a target matrix T with size samples-by-classes.
% Your complete data
%
% f1 f2 f3 f4 class1 class2 class3
%D = [0.1 10 5 2.3;1 0 0;
% 0.2 12 4 1.3;0 1 0;
% 0.4 15 3 1.1;0 0 1;]
% Your input matrix
% f1 f2 f3 f4
%X = [0.1 10 5 2.3;
% 0.2 12 4 1.3;
% 0.4 15 3 1.1]
% Your target matrix
% class1 class2 class3
%T = [1 0 0;
% 0 1 0;
% 0 0 1]
Maybe it's a good idea to attach a picture (or your data) as an attachment (using the paperclip icon).
