What is Mach Band

Figure 1
(Illustration taken from http://www.yorku.ca/eye/machband1/htm)

Mach Band is a perceptual phenomenon. When the human eye looks at two bands of colours, one light and one dark, side-by-side, a the eye perceives a narrow strip of gradient light to dark light, in the middle separating the two solid bands. However, this is not the actual image.

In figure 1 below, we see the two bands as gradient however the colours are solid colours. Each solid colour reflects different amount of light with the darker band reflecting lesser than the white band . The borders appear ‘blurred’ due to focus of receptive fields on to the edge. This is caused by competing reaction in the receptor cells in the retina known as lateral inhibition.

The bottom black line shows the physical intensity of light distributed across whilst the red line indicates our perception of light and dark Mach Band field with the dip indicating the dark Mach Band and the protrusion indicating the light Mach Band.