The findpeaks function has a number of name-value pair agruments that can customise its behaviour to give the desired results. One option here is to use 'MinPeakProminencce', and, 'MinPeakHeight' alone or together.
Use findpeaks twice, once on the data vector to get the positive peaks, and once on the negative of the data vector to get the negative peaks. Make the appropriate corrections to get the actual peak amplitudes of the negative peaks, if necessary.
