"... a matrix composed by struct (it is 1x91 cells, each one is a struct 1x1) ..."
What you describe sounds like a 1x91 cell array of scalar structures, not a single structure array. So you will need to use cell array indexing to access each structure within the cell array, e.g.:
matrix_of_structs{i}
% ^ ^ access the content of the cell array
"...because I loaded on Matlab 91 matrices 1000x512 and I joined them into a single struct"
But what you described is a cell array of structures, not a single structure array. If the structures all have the same fields then you can easily convert your cell array into a structure array:
nonscalar_structure = [cell_array_of_scalar_struct{:}]
I recommend doing this, as then you can use very convenient syntaxes for accessing your data, including the syntax that you show in your question:
