The difference between the receptive fields and the characteristics of simple and complex cells is the hierarchical convergent nature of visual processing.
A simple addition of receptive fields would result in complex cells manifesting observable, separate excitatory/inhibitory regions, which is not the case.
The results from the above experiment determined that simple fields have clear excitatory and inhibitory divisions, where light shone on an excitatory region increases the firing of a cell and light shone on an inhibitory region decreased firing of a cell.
For simple cells, it would be expected that as long as the slit covers the excitatory field, the orientation should not matter.
Rather, it was noted that these cells perform nonlinear operations, which suggested that they have linear receptive fields, but instead sum a distorted output of subunits.
This model stated that each subunits could respond differently, but the converted responses would be offset in time, so it would sum to a constant value.
Complex cells appeared to match the subunit model, but still lacked the restriction that the receptive fields are linear.
This was also tested by measuring the response of a cell when the stimulus contains two bars, which would help show the properties of the receptive field subunit.
What they found was that by knowing these properties of the subunits, it was possible to predict spatial frequency selectivity, as was the case for simple cells.
Other computational models of complex cells have been proposed by Adelson and Bergen,[14] Heeger,[15] Serre and Riesenhuber,[16] Einhäuser et al.,[17] Kording et al.,[18] Merolla and Boahen,[19] Berkes and Wiscott,[20] Carandini,[13] Hansard and Horaud[21] and Lindeberg.