Farnsworth-Munsell 100-Hue Test

McCollough Effect 1

The McCollough Effect and Caffeine

David Oguns

February 21, 2007

Rochester Institute of Technology

ABSTRACT: The McCollough effect is a unique visual aftereffect perceived on visual patterns with edges. It has a particularly long lasting effect and is the product of a combination of behaviors in the human visual system. We conducted an experiment looking at the effect of caffeine on the strength of the McCollough effect. Based on the results of the experiment, the McCollough effect is at least a partially cortical process affected by the brain’s arousal level.

Running head: MCCOLLOUGH EFFECT McCollough Effect 2

Introduction: The McCollough effect is a visual perception aftereffect that causes color to be perceived in a visual pattern absent of color after a period of adaptation. The adaptation pattern is a pattern with black and colored lines aligned orthogonally and alternating complimentary colors (Humphrey et al., 1994, p 86). After induction, subjects perceive the corresponding color in the same orientation pattern when the colored lines are white. Unlike most other aftereffects of the visual system, the McCollough effect can last for a much longer period of time even after a very short induction period. Where in the visual pathway and why the effect occurs is still somewhat uncertain to psychologists. Some psychologists believe the effect is a simple example of classical conditioning. Understanding this effect would lead to an increased understanding of vision as it relates to color perception in particular. According to Humphrey et al (1999) most of the perception of the McCollough effect takes place in the extrastriate cortex even though there is a significant amount of evidence that suggests that V1 needs to be in tact for the effect to occur.

The McCollough effect occurs primarily due to human color constancy related to the edges in the induction pattern (Broerse et al, 1998, p1305). MEs have been induced simply by using color on the edges of the grating. Additionally, different duty cycles changes the strength of the resulting aftereffect further supporting the theory that color constancy is related to the cause of the McCollough effect.

Methods: In this experiment 23 participants were given a 15 minute adaptation period to the stimuli inducing the McCollough effect on two separate days using alternating magenta and green patterns on horizontal and vertical striped patterns. The patterns were displayed in 7 second intervals on a projector screen. The color values were measured using a Minolta McCollough Effect 3 Chromameter CS-100. The CIE value for the magenta used was (.355, .190) with a luminance of 22.6 cd/m2 and the green was (.293, .383) with a luminance of 73.4 cd/m2. On one of the trials the participants were instructed not to consume any caffeine in a 24 hour period prior to the adaptation. On the other trial the participants were instructed to consume an elevated amount of caffeine or their regular amount prior to the adaptation.

Prior to the adaptation period the McCollough effect was measured to establish a baseline in both trials. After the adaptation period, self assessed measurements were taken after

6 minutes, 12 minutes, 20 minutes, 40 minutes, and 60 minutes to establish the strength of the effect over time.

Results: Figure 1

Caffeine on McCollough Effect

) 30 Caffeine % (

h Vertical t

g 25 n

e Caffeine r t Horizontal S 20

r o l No Caffeine o 15

C Vertical

r 10 No Caffeine o

p Horizontal e 5 R

0 Pre-test Post 6m Post 12m Post 20m Post 40m Post 60m Time

The results show that the participants who had increased levels of caffeine intake had a stronger peak aftereffect. The strength of the after effect continued to rise beyond 6 minutes after the adaptation period. The rate of decrease in the strength of the aftereffect on the McCollough Effect 4 caffeine loaded participants is faster than those without caffeine. After 60 minutes, the aftereffect has roughly the same perceived signal strength.

Discussion: Since caffeine affected the results of the McCollough effect, it is an indicator that the effect occurs in the cortex of the brain. Figure 1 shows a significant increase in the strength of the McCollough effect even after the adaptation period which suggests the brain is still working

If the cause of the McCollough effect is indeed color spreading and edges, there would be a significant amount of processes necessary to make it happen and those processes would overlap with those responsible for color constancy. Color constancy is the process of automatically eliminating the color of illuminants to determine the color of a surface. This process can make a surface that has a strong hue and saturation appear an entirely different color or absent of color. It is comparative in nature to everything else in the visual field. A judgment is made about the global illuminant and then it is factored out.

Broerse et al suggests that the McCollough effect is caused by neural correction of the chromatic aberration in the eye combined with color spreading to achieve color constancy.

Chromatic aberration in the retina is when different wavelengths of light have a different refractive index and don’t get focused in the same spot on the back of the retina. A raw image of this would result in the offset of certain colors that are most visibly apparent on the edges of objects. Because of this, the brain has to adjust for what the eye perceives so we can process a reasonably correct image. During a McCollough adaptation, the eye adjusts to the image with color between the stripes. When the color is removed, the brain believes that it is missing the color on the edges of the black due to chromatic aberration and puts it there. Then color spreading occurs to fill in the edge colors to achieve the full McCollough effect. The results of

Broerse et al’s experiments supports this theory because they observed that increasing the size McCollough Effect 5 of the white gaps or colored striped, or duty cycle, decreased the color spreading of the

McCollough effect.

The McCollough effect also reveals another important structural property of the human visual system. The aftereffect from an induced ME only occurs when the same orientation pattern is shown to an observer. This suggests that color information is important to establish orientation. However the two aren’t strictly tied together as the parts of the brain used for orientation processing aren’t always active during color perception. The logical conclusion based on these properties is that color pathways are parallel in nature such that some of the paths for color perception are used for orientation information and they may be independent from the other paths. McCollough Effect 6

REFERENCES

Broerse, J., Valdusich, T., O’Shea, R. P., (1999). Color at edges and colour spreading in McCollough effects. Vision Research, 10, 1305-1320.

Humphrey, K. G., Herbert, A. M., Lawrence S. A., Kara, S., (1994). McCollough effect to “form”: a local phenomenon. Journal of Experimental Psychology: General, 123(1), 86-90.

Humphrey, K. G., James, T. W., Gati, J. S., Menon, R. S., Melvyn, G. A., (1999). Perception of the McCollough effect collates with activity in extrastriate cortex: A functional magnetic resonance imaging study. Psychological Science, 10(5), 444-448