Mach bands and Hermann grid: what do they tell us about the way perceptual system is organized?

Mach bands and Hermann grid: what do they tell us about the way perceptual system is organized?

When looking at the Hermann Grid two questions arise:
1) Why are they are irridesent gray spots located at the intersections between black squares?
2) Why do those spots disappear when you look directly at them?

The answer to those questions depends on where certain receptive fields are located and the amount of light being detected by the Photoreceptors within those fields.Using ON-Center receptive fields as an example, when your eyes are focused on the grid, except at an intersection, the dark surrounding photoreceptors counteract the activity of the light center photoreceptors.
Inhibitory Reaction (Lateral Inhibition), light detected by the dark portion of a receptive field reduces the receptive field’s activity and because there are more dark photoreceptors detecting light, they are counteracting the signals of the light photoreceptors and are, therefore, making the area appear gray.Receptive fields vary in size according to their location on the retina and gradually become smaller toward the center, or fovea, of the eye. So, when you look directly at an intersection, the receptive fields (e.g., Field B to the left) are small enough to completely fit between the black squares allowing the light photoreceptors to become active without the dark photoreceptors interfering and thus eliminateing the gray dots. The gray dots only exist on the outside portions of your eye where your receptive fields are larger.

When looking at the Mach Bands it appears as if the color bands curve inward or that each band is a gradient, however, each band is a solid color. Each band reflects different amounts of light with the darker bands reflection positioned completely within the first darker band, however its dark photoreceptor surround is partially within the second lighter band. As a result, the surround generates an Inhibitory Reaction making certain fields lighter than the receptive field.

The illusion for both the Mach band and the Hermann grid is a byproduct of the edge-detection device that plays a part in perception (of contours in the case of the Mach band). This perceptual phenomenon is predicted by responses of ganglion cell neurons. The center-surround organization of ganglion cells is very well designed to simply pick out edges in the visual environment.

While looking at the Herman Grid illusion it is important to remember the centre surround organisation of the ganglion cells reacting to light stimuli differently accordingly to the place of the receptive field which is stimulated by light. The on centre off surround cell reacts in a depolarisation when light falls onto the cente and in a hyperpolarisation when it is on the surround. If the light is falling on both the centre and surround the response caused by the light falling on the centre is inhibited by the surround and therefore the brain recieves only a mild response meaning that no contrast is present. The same relation may be seen in the case of the off centre, on surround cell; however here it is the centre that inhibits the response created by the surround. Lateral inhibition is therefore a competetive interaction between the centre and the surround.

When thingking of the Hermann grid we can imagine that a part of the surround receptive field is covered - due to the presence of the dark squares of the herman grid. Both the centre and the surround is stimulated when looking at an intersection of the white bars. "At the intersection of the white bands the inhibitory surround of an on-center/off-surround receptive field receives more inhibition than does the excitatory center of the field". This is what makes one see the grey dots at the intersections of the bars.

Simillarly in the case of the Mach bands lateral inhibition is used to detect edges in the image.

Not only may these two cases be considered as an example showing how important the organisation of ganglion cell's receptive field is for edge detection and contrast but also how perception can be influenced or even cheated by the stimuli in the environment.