Hermann Grid

One of the coolest part of my perception class is that we get to know the real science behind all of those cool visual illusions that we’ve probably all been seeing for years. One of particular interest to me is the Hermann Grid.

If you look at the grid you’ll notice ghostly grey spots appearing at the cross sections of the squares.Why does our brain perceive these spots, instead of simply looking at a series of black boxes and white lines? If we hold our hand up to the image to just show one singular row of boxes, we can see that the grey dots are not really¬†present, but rather filled in by the mind.

The process responsible for this phenomenon is lateral inhibition. To understand this, imagine a small circle where the grey dot appears as point “A” and four other dots in the columns surrounding as points B-E. When you look at the image, each one of these five spots is receiving the same light stimulation. Each one of these points sends a signal to a bipolar cell. A bipolar cell is a sensory neuron. The four points B-E that surround “A” send inhibitory signals to the bipolar cell of “A”.

If when “A” is stimulated by light, there are 100 units generated, there is no inhibition. B-D, being equal, also receive 100 units. If we take 1/10th of the units, each responsible for ten units of lateral inhibition, we find the response of the bipolar cell stimulated by “A”.

100(A) – 10-10-10-10(BCDE)= 60

Then we repeat this calculation for a receptor that surrounds the center “A” – receptor D which is surrounded by two black lines from the boxes.

100(D) – 10 – 2 – 10 – 2(Surrounding) = 76

The response from the center point (60) is less than the response from the surrounding points (76). Because of the difference in light received to point D, we perceive a darker area in the intersections of points.

More simply put, the intersection receives more light. The other points receive less light. This lateral inhibition concept shows you a true “grey area.”

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