Relationship between the positions of the upward and downward envelopes. A, In the V1 population, the two positions were highly correlated. The points lay close to the identity line (dashed), in broad agreement with the energy model. The positions of the envelopes were not normalized by their widths. The filled symbols represent cells with significant offsets as in Figure 6 (Bootstrap resampling, p<0.05). B, In the V2 population, the two positions were not correlated, indicating much larger deviations from the energy model.