Illusion can be generally defined as the distortion of perception of senses. It is possible to deceive every human sense with illusion, but most common among such illusions is visual or optical illusion. Illusion occurs on the basis of brains organization and interpretations of sensory stimulation. It is not necessary that experience of illusion is same for all the persons viewing it, but visual illusions are mostly experienced in same way by various persons. As reality is usually distorted by illusion most people share same illusion.
Visual illusion is given more emphasis due to domination of vision over other senses of human being. For example, sense of hearing is deceived by a ventriloquist while you are watching ventriloquist, but you are aware of the fact that voice is from dummy and hence there is a view of dummy uttering words. There are certain illusions that are based on brains general assumptions during a perception. Such perceptions are based on ability of an individual to depth perception, constancy of perception and perception of motion. Illusions also occur due to sensory structures in a human body and are biological or external conditions like physical environment.
Illusion in psychiatry is referred to sensory distortion in a particular form. It is different from hallucination, which is a kind of sensory experience under situations when stimulus is absent. Illusion is a perceptions distortion and is hence interpreted and understood in a different manner. For example if some one hears a voice irrespective of environment and this is a hallucination, while voice heard by a person from an auditory source like running water will cause an illusion. One kind of most common optical illusion is grid illusion, illusion caused by particular kind of grids that are capable of deceiving the vision of a person. Most common type of grid illusion is scintillating grid illusion and Hermann grid illusion.
Hermann grid illusion is one of the most common types of optical illusion and was reported in 1870 by Ludimar Hermann. In its classical form this grid is formed by vertical and horizontal white bars intersecting each other on a black background forming grey blobs that are ghost like appearances at intersections of these bars. Grey blobs are not formed or do not appear at the intersections of white bars that are viewed directly. Grey blobs are viewed due to processing of lateral inhibition. Attempts were made by several persons to explain the process leading to this illusion. According to Baumgartner (1960) lateral inhibition circuitry that is inside retina is the cause of this illusion. He states that when on-center receptive fields lateral inhibition increased its lead to dark smudges. When an on-center receptive field coincides with the intersection, inhibition from surroundings will be more than during a coincidence with a bar. Brightness that is observed at the intersection will be less in second case in comparison to first case. Spillmann and Levine (1971) found that this is the same for a white background with black bars. But in this case it is the off center cells that display lateral inhibition and not on-center cells. But the theory of Baumgartner was proved to have some problem (Spillmann, 1971). He found that illusion diminished to a great extent when the grid was rotated by 450. If the theory (retinal lateral inhibition) proposed by Baumgartner was correct illusion would have remained constant at all states of rotation. It is proved qualitatively in recent works (de Lafuente and Ruiz, 2004) that when grid is rotated further from its vertical/horizontal position than 450 illusion is further diminishing. According to them a cortical mechanism is necessary to have the illusion that is perceived. One of the possibilities is oblique effect. Some of the visual perception tasks threshold like contrast detection and acuity increases due to oblique effect if you are presenting targets obliquely (Campbell, Kulikowski, and Levinson, 1966).
If target alignment is horizontal or vertical indicating the role of orientation selective cells in illusion lower thresholds will be demonstrated by subjects. Hubel and Wiesel (1968) states that, orientation selective neurons appears as complex and simple cells, in striate cortex in visual pathway. One common observation is that color of illusionary spots is same as that of background and this was observed while experimenting with different colors. This was in contradiction to Baumgartner’s (1960) hypothesis of center- surrounded receptive field. According to McCarter (1979) illusion was the result of double opponent cells. Double opponent cells are not present in primate retina but is in visual cortex and is having receptive fields that are circular symmetric.
Most recently Schiller and Carvey (2005) tried to explain Hermann grid illusion. Through several examples they proved that Baumgartner’s retinal ganglion cell theory cannot be explained in full. For this purpose grid was manipulated in different ways and this was without altering retinal ganglion cells center/surround activation and this showed that illusion is reducing. It was also proved that adding intersecting bars through intersections in a diagonal manner will not help illusion to gain magnitude even if we increase center/surround antagonism. These changes were explained in paper for readers to understand and were not proved experimentally.
In the theory of Schiller and Carvey (2005) simple cells of S1 type in visual cortex is involved. There is a subfield for this S1 cell and it is either ON system (excitatory for increment of light) or OFF system (excitatory for decrement of light). Thus light edges corresponding to its receptive field excites ON S1 cells and dark edges excite OFF S1 cells.
Orientation selectivity is also demonstrated along with elongated receptive fields that vary in length through orientation axis. They also found that some S1 type cells are sensitive to color. They are of the opinion that chromatic and achromatic illusionary effect of Hermann grid is the result of co-activation of S1 cells that are color selective.
This study aims to understand the effect of tilt and size on Hermann Grid illusion.
To understand the effect of titling on Hermann grid illusion a circular Hermann grid was used. Grid was of 12.5 centimeters in diameter. There were five intersecting lines and they intersected with five lines in an orthogonal fashion. Width of these lines was 0.65 centimeters and the gap between each line was 1.55 centimeters. Grid was viewed from a distance of 1 meter in a room that was dimly lit. There were 21 intersections in this Hermann grid and the contrast was 50%. Computer monitor on which display was given was 17 inches ion size. Grid was viewed at 6 angles 00, 50, 100, 150, 300 and 450. It was through habitual refractive perception that the grid was viewed. Experiment was conducted in a time frame of 20 minutes.
Oblique effect was also measured during experiment and for this oblique effect was controlled during experiment. For this test was conducted on two achromatic sets in addition. There were five positive, one isoluminant and five negative contrast levels in these two sets and there contrast levels were presented at 450 and 00. These control sets aimed to find whether there is a change in threshold at which it is possible to detect grid on the basis of its orientation. This is due to the fact that oblique effect in reality is a threshold phenomenon. It was viewed whether it is possible to view the grid at different contrast levels and not the illusion for the purpose of this test.
Effect of distance on Hermann grid illusion
In order to understand the effect of distance on illusion, it was decided to check the formation of illusion from various distances. With this it is also possible to understand human perspective. To conduct this test a grid with light bars and dark squares were prepared on a computer monitor. Grid was placed at the center of monitor and was viewed from various distances raging from 50cm to 1 meter. Grid was viewed at center and peripheral view was also considered. Distance from where illusion was best seen was measured and at the same distance at which illusion disappeared was also measured. Measurement of visual angle produced by the light bars at each of the five distances was taken. When illusion disappeared at a certain distance angle was measured as surround field size. With the help of this technique surround ratio at these five distances observed was measured. Perceptive center was measured at retinal eccentricities at various distances.
During observation it was also made sure that not only direct view, but peripheral view is also to be considered. In most cases it was seen that there is absence of illusion at center but at the same time in peripheral view illusion can be clearly found. Observer marked the absence or presence of illusion at fixation point which was taken as the central intersection of grid and at the peripheral view at each distance as yes or no. Grid was viewed for almost 30 seconds at each distance and results were marked only after clearly focusing at central intersection which is the fixation point. Strength of illusion at center and peripheral areas at each distance was noted as strong or weak.
Results and conclusion
Effect of tilting – It was found that contrast threshold was not affected by tilting of Hermann grid. It was also found that when grid was tilted by 50 strongest illusions was viewed and weakest illusion was viewed during 150 and 450 tilting of grid. When tilt angle increased it was found that illusions magnitude is reducing along. But there was an illusion during all these titling and this proved that illusion is present at all angles though its strength varies from one angle to another.
Graph 1. Average illusion rating
It was observed that with tilting illusions perceived strength also reduced for a chromatic Hermann grid. It is normal to assume that the reason behind this phenomenon is oblique effect. But in reality it is not so. There are two reasons for this conclusion. Though oblique effect was measured during experiment it showed no effect on illusion created by grids. Another reason is that it is only for stimuli with ≥ 8 cycles spatial frequencies for each degree that oblique effect will occur. Thus it was found that decline of magnitude in grids that are obliquely presented is caused by some other factor and not tilting.
Effect of size – As a result of experiment with distance of observation of Hermann grid it was found that a strong illusion is possible when you are viewing the grid at medium distance, not too close and not too far. Further it was also observed that illusion is more effective at peripheral view than direct view and this is why no illusion is found at the center but is found in peripheral area. It was also found that center size at fovea was small and this is only four to five minutes of arc and this keeps on increasing to reach 1.70 when the view is at 150 eccentricity and at 600 eccentricity this is 3.40. Reason behind formation of illusion at peripheral view and not in central intersectional area is the size of receptive field which is small in central field than in other areas of retina. It is due to this reason that illusion is viewed even at the central intersectional area when observer is at a certain distance. With increasing distance receptive field at center also becomes large and this results in formation of illusion even at the central focal point. But such illusion will be not that clear as illusion in peripheral view.
During this study illusionary spots that are seen in outline grid were not considered. At the same time it was found that on adding extra bars to grid illusion creating spots were found absent. These two factors can be further studies in detail.
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