Changes in Lateral Inhibition due to Alterations of the Hermann Grid

Morgan Hansen and Jennifer McCrimmon
innocent_angel83@hotmail.com
jennlynn48818@yahoo.com
Alma College

The Hermann Grid Illusion, Figure 1, is a popular illusion because there are clouds that appear within the intersections of the rows and columns. This is due to lateral inhibition, which is the process of lowering the excitement of a receptor due to it's neighboring receptors excitation. These receptors are in the retina of the eye. Due to this process, the areas of the grid that are surrounded by white (the intersections) are being inhibited; thus resulting in the grayish looking clouds in these areas. However, the question still remains if the illusion is still present when alterations are made to the Hermann Grid. The speeds at which the Hermann Grids were flashed were altered to find out if lateral inhibiton takes a required amount of time to occur. Our hypothesis was that the quicker the time the grid flashes across the screen and the smaller the grid, the less likely that cells had enough time for lateral inhibition to occur.



Figure 1: Hermann Grid

Methods:
This study was conducted by selecting 10 Alma College students. They were seated 30 inches away from a computer monitor that flashed the alternating sizes and speeds of the Hermann Grid. The original size grid used was 5.25in x 4.75in. The original grid was distorted by using an increased size of 110% and decreased sizes of 75% and 50%. The blank screen was kept at a .9 seconds and each size grid was flashed a total of three trials, which consisted of .25 seconds, .15 seconds and .05 seconds. The subjects were asked to rate the strength of the clouds within the Hermann Grid on a scale of 1-5; 1 being the clouds are not visible and 5 being the clouds are clearly visible. This scale was equally spread out between these degrees of visibility.

Results:
The results can be seen in the following two figures.


                                      Figure 2: Illustration of how alternating the size of the Hermann
                                      Grid changes the subjects' amount of illusion.

Figure 2 illustrates the differences in the strength of the clouds in the illusion as set by the subjects. There is not an effect due to size alone as can be seen by the line which is almost horizontal. The point at 50% is a little lower than the others, but this is due to a few outlying data points, not enough to be considered an effect on the illusion due to the changing size of the grid.



                                       Figure 3: Illustration of how altering the time of the Hermann
                                       Grid affects the amount of the illusion the subjects' experience.

Figure 3 illustrates that time did have an effect on the amount of illusion the subjects experienced. With the longer grid exposure time (.25 seconds), subjects were able to see the clouds clearly; but with the shortest grid exposure time (.05 seconds), the subjects were barely able to see the clouds. It can be clearly seen that as the time of the stimulus on the screen decreased the strength of the illusion decreased as well. This effect was constant through all the different sized grids.

Discussion:
The results showed that size did not have an effect on the illusion but the speed at which the grid was flashed did. When the grid got smaller and the time got faster, the subjects saw less amounts of clouds. This suggests that our visual perception has to see a stimulus or object for a certain amount of time before the stimulus or object can actually be perceived. So in this case, we would need to see the Hermann Grid longer than .05 seconds in order to see the clouds in the intersection of the grid.

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