Troxler's Fading

Background:

First described by Ignanz Paul Vital Troxler in 1804, Troxler's fading involves the perceptual disappearance of stimuli in the periphery while fixating (staring at) a centrally located stimulus.

The visual system responds to change. An image that is stabilized (not changing) on the retina will perceptually disappear. This is attributed to neural adaptation -- the general observation that neurons will reduce their rate of neural activity with prolonged exposure to the same stimulus. When you fixate, or stare at an object while minimizing the amount of voluntary eye movements, you are somewhat stabilizing the retinal image of stimuli.

Why don't we notice this in everyday life? The eyes are constantly making very small movements even when you try to keep them absolutely still. These motions are called microsaccades. Because the eyes are constantly moving, the visual image is not stabilized and does not perceptually fade.

If we are constantly making microsaccades, why does Troxler's fading occur? The fading occurs in the visual periphery where the receptive fields are larger. Thus, even though the eyes are making microsaccades, the stimuli in the periphery may remain relatively stabilized -- the image does not move off of the relatively large receptive field. Part of the effect likely occurs cortically, and the receptive fields of some cortical cells is even larger than their retinal counterparts. Second, the fading occurs more readily when the stimuli lack definite boundaries -- no lines or edges. Since the visual system is tuned to lines and edges, stimuli lacking these are more rapidly experience neural adaptation. Because they don't strongly excite the neurons in the first place, they don't have to adapt as much before they cross the threshold for perceptual awareness.

The Activity:

The following example of Troxler's fading is based on an illusion known as the lilac chaser. Fixate (stare at) the cross in the center of the circles. When the circles perceptually disappear, you are experiencing Troxler's fading.

One way of seeing how much each of these variables influence Troxler's fading is to measure how long you must fixate before all of the colored circles perceptually fade away -- that is, how long it takes for you to see no colored circles. For each of these manipulations, try to predict what should happen before trying the manipulation.

• Change the animation rate from slow to fast. Should a slow or fast animation rate lead to more neural adaptation?
• Turn off the animation by un-checking the animate box. Should a static image lead to more or less neural adaptation than a changing image?
• The colored circles have a fuzzy edge. The fuzzy edge is technically called a Gaussian blur. You can change how blurry the edge is. Sliding the "amount of blur" slider all the way to the left will give the circles a crisp edge. Sliding the slider to the right will make the edges less and less crisp. Which amount of blur should lead to the least amount of fading? Which should lead to the most fading?
• Change how close the colored circles are to the fixation point with the Circle centrality slider. Should you experience more or less fading when the circles are more centrally located? Why?
• Change the color of the circles. Remember to think about color psychologically. Remember that lines and edges are defined as a change in contrast -- a difference in the brightness of two adjacent areas. To maximize fading, how should the color be adjusted relative to the background? For reference, the background's color is H = 0, S = 0, V = 0.75.

Animation rate:
Animate?:
Animation order:	Random Clockwise
Amount of blur:
Circle radius:
Circle centrality:
Color of circles:
H: 300
S: 0.33
V: 0.87
RGB:	#dd93dd