An adaptive optics vision simulator was used to measure the role of ocular aberrations in the photopic foveal spatial summation curve for three young subjects. After the correction of the ocular aberrations over a 6 mm pupil, the increment luminance threshold was lowered for small stimuli and the estimated area of complete summation (Ricco's area) was reduced by a factor of 2.6 on average. These results show that the image spread due to natural ocular aberrations partly accounts for the spatial summation observed in the fovea in the photopic light regime.