Unable to perform assignment because the left and right sides have a different number of elements -- variables do have same number

I am trying to make a for loop loop through data, detrend it, then output it is a variable so that i can plot them. For some reason i am getting the error of 'Unable to perform assignment because the left and right sides have a different number of elements' when the variables do have same dimensions. Does anyone have any ideas?

subj = {'HCA1','HCA2','HCA3'};  %HCPsubj list
%% loading paths and such

D = '/Users/ll/Documents/data_analysis/HCP/seventy/';

%% indexing and putting all seventy graphs into one

a = zeros(478,1); %pre-allocate a vector to store results

for k = 1:length(subj)
    name = subj{k};
    c = load(fullfile(D,name,'ts','lcortex_run01regPA.txt'));
    v = load(fullfile(D,name,'ts','ventricle_run01regPA.txt'));
    %s = load(fullfile(D,name,'ts','spinal_run01regPA.txt'));

%     c_ts = l
%     v_ts = v
%
%% detrend ventricle and cortex

    d_v_ts = detrend(v)   %detrended time series of ventricle

    d_c_ts = detrend(c) %detrended time series of cortex

  for n=1:numel(a)
    ventricle_data = d_v_ts; %do something with that data
    a(n)=ventricle_data; %store it in the vector
  end
end

[Walter Roberson]

You load() a file. The result in v is almost certainly a vector or 2D array. detrend() applied to that would be the same size. You then loop 478 times, each time copying the entire vector or array into ventricle_data, and trying to store that entire vector or array into the single numeric location a(n) .