Mach Band Illusion

Ernst Mach (1838-1916)
(Illustration taken from http://econc10.bu.edu/Czech_rep/Mach_Ernst.htm )

Ernst Mach, a famous physicist published many papers such as the principles of supersonics, famously known as Mach 1 which represents the speed that an object is travelling at the speed of sound. However, interestingly, he published many papers relating to the neurophysiological theories behind the phenomenon on visual effect which is named after him, Mach Band. Mach’s theories were researched on only after World War II by Floyd Ratliff and thereafter, became the basis of many other research studies. One major contribution on Mach Band theory is to radiology (Papageorges, et al., 1989).

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.

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.

Lateral inhibition

In figure 4, you will see that lateral inhibition process causes the eye to perceive the Mach Band effect at the edge of each strip of the different shades from light to dark grey and the contouring effect when you scan your eyes across the figure. However, if you were to block of the surrounding strips and focus on one strip of colour, you will notice that each strip is just one solid colour by itself.

Shadows

We see Mach Bands everywhere on opening our eyes. Look closely at the shadows cast on the ground, there are mach bands of light and dark strips near the edge of each shadow. In Discovering Biological Psychology, the biological processes are elegantly explained.

It begins with the eye

Figure 4.
(Illustration taken from Freberg, 2006. Discovering Biological Psychology.)


Figure 4 above shows light entering the eye through the cornea and lenses and projected on to the retina, at the back of the eye. 5 types of neurons; the rods and cones, bipolar cells, ganglion cells, amacrine cells and horizontal cells form the retinal layers. The receptors made up of rods and cons, bipolar cells and ganglion cells transmit the signals vertically through the retina, whilst the horizontal and amacrine cells transmit signals horizontally across the retina.

Light is changed to energy in the receptive fields as electrical signals and sent to the visual cortex neurons of the brain via the LGN (lateral geniculate nucleus) through the firing of the ganglion cells when light passes through the receptive field. The ganglion cells respond to the antagonistic receptive photoreceptors. The receptive field consists of on-centre, off-surround ganglion cells and off-centre, on-centre ganglion cells as in figure 5.

Receptive field of the bipolar cells

Figure 5.
(Illustration taken from Freberg, 2006. Discovering Biological Psychology.)


The amacrine and horizontal cells sends signals horizontally across the receptive fields and convergence takes place in the retina. As there are approximately 120 million rods and 6 million cones in the retina, more rods have to come together to deliver their signals to one ganglion cell as opposed to one single cone having access to one ganglion cell.
This process of pulling together allows the rods to be more sensitive to changes in the visual field whilst the cones “zooms” into focus.

On-centre, off-surround and off-centre, on surround receptive field

Figure 5a
(Photographs taken from Discovering Biological Psychology on-line video)

Figure 5a shows light passing through the on-centre and off-surround receptive field. The firing rate is moderate as light passes through the off-surround receptive field and the firing rates is faster when light reaches the on-centre receptive field.

On-centre, off-surround and off-centre, on-surround receptive field

Figure 5b
(Photographs taken from Discovering Biological Psychology on-line video)

Figure 5b shows that when light passes through the middle off-centre, on-surround receptive filed, the firing rate is faster at the on-surround and is moderately fired at the off-centre, and again at a faster firing rate at the on-surround receptive field.

On-centre, off-surround and off-centre, on-surround receptive field

Figure 6a
(Photograph taken from Discovering Biological Psychology on-line video)

The eyes see objects and defines boundaries and edges through the firing of the ganglion cells in the receptive fields. In figure 6a, the on-centre, off-surround array of receptor cells is activated when bright light is shone of the on-centre, the firing rate is rapid and when the surround is lighted, the the ganglion cells fire at a moderate rate. Our visual system responds to edges in objects in this say. The LGN generates a signal to the visual cortex and sees the edge at a specific angle.

Lateral Inhibition and Simultaneous Contrast

Figure 7 shows an example of lateral inhibition and simultaneous contrast. There is same amount of light is reflected on these two squares, however, the grey box in the right square looks darker than the grey box in the black square. Both grey boxes are the same colour. Simultaneous contrast occur when when the electrical signals received from the receptors in surrounding black box is less intense than the signals received in the surrounding white box. There is more inhibition in the surround of the white box causing less firing in the grey square, thus the grey look darker as compared to the grey square in the black box.

Figure 7. Von Bersky, G. (1967) Mach Band Type Lateral Inhibition in difference sense organs.


Lateral Inhibition and Mach Band experiments can be done even on the skin. By just using your nail to press into skin of your hand, you will form an obvious imprint caused by the pressure of your nail. The pressure caused a depression followed by a protusion on your skin. There is also an inhibitory effect in the stimulus and sensation effect. However, figure 7 below shows a side view of the imprint on the skin, indicating the light intensity effect causing the dark and white Mach Band on your skin.