How does this happen?

(Illustration taken from Goldstein, 2007. Sensation and perception.)

Figure 2 shows a picture of centre and surround disks, facilitated by photoreceptors and
horizontal cells. The centre is excitatory and the surround inhibitory. They compete with each other to activate the bipolar cells in the retina.


Figure 3 shows a process of lateral inhibition, D gets more light in the inhibitory surround, thus the excitatory centre will respond at a slower rate. B and C receives same intensity of light, but C receives less light in it’s inhibitory surround, more light in the excitatory, having a faster response than B. The contour effect (concave and convex curves) is perceived when in low intensity areas, the amount of inhibition is constant, resulting in darkening and whereas in high intensity areas, there is less inhibition, resulting in a lightening.