I’m not sure what you want to do. Your data are also not easy for me to interperet, because they lack a time base.
See if something like this works:
d = load('Heath Robinson spks.mat');
spk = d.spk;
for k1 = 1:size(spk,2)
thrsh = mean(spk{k1}(1:5)) + 5*std(spk{k1}(1:5)); % Define Threshold
dep(k1) = find(spk{k1} >= thrsh, 1, 'first'); % Initial Value Exceeding Threshold
end
figure(1)
plot([1:size(spk{1},2)], spk{1}, dep(1),spk{1}(dep(1)), 'r^')
hold on
plot([1:size(spk{2},2)], spk{2}, dep(2),spk{2}(dep(2)), 'r^')
plot([1:size(spk{3},2)], spk{3}, dep(3),spk{3}(dep(3)), 'r^')
hold off
grid
