The first 5 values in this partial model fit vector correspond to
condition A and the amplitude of the first 5 values in the partial
model fit vector is a*EV1_(1-5) + b*EV2_(1-5). Both EV1 and EV2 are
set to 1 for the first 5 values, therefore the modelled height in the
partial model fit is a+b, assuming that we have normalised EV1 and
EV2 to unit amplitude prior to model fitting. During condition B the
modelled amplitude is a*EV1_(6-10) + b*EV2_(6-10) and therefore the
amplitude we estimated during the model fitting is -a etc.