The Negative Afterimage Simulation

Introduction

When rods are exposed to light, some of their rhodopsin molecules become “bleached” (i.e., converted into the inactive trans- form). If rods are exposed to light of sufficient intensity for a long enough period of time, bleaching of their rhodopsin can become so extensive that the rod is rendered incapable of effective transduction of more photons. This, of course, underlies the phenomenon that you’ve all experienced of being unable to see well for a short period after entering a dark room from outdoors or when entering a darkened movie theater from a brightly lit lobby. Only after a period of time sufficient for the 11-retinal to be ‘recycled’ from the bleached trans- form back into the photoactive cis- form, do we become fully dark adapted. A diet deficient in Vitamin A (the substrate for biosynthesis of retinal) can result in night blindness, a condition characterized by dramatically reduced ability to see in low-light condition.

Because the chemistry of phototransduction in cones is basically the same as in rods, cones can also become bleached if exposed to intense light of the proper wavelength for a sufficient period. Experimentally, this is accomplished by staring for a while (15 – 30 s is usually sufficient) at a colored image. However, bleaching of the cones does not result in the across-the-visible-spectrum loss of sensitivity to light that accompanies bleaching of the rods’ pigment. Instead, for reasons that are still not well understood , if you look away from the image and at a blank background, you see what is referred to as a negative afterimage of the original image.

This negative afterimage, as the name implies, looks similar to the original image, but differs in color. To a first approximation, negative afterimages can be understood in the context of fatigue or bleaching of the cones of a particular color class. (click here to read a brief discussion of cones and their response to light) Thus, the negative afterimage of a blue image would be expected to exhibit a color resulting from the combined output of the green and red cones on which the image had been projected…..in other words, one would probably predict that the negative afterimage of a blue object should be yellow (you’ll get a chance to test this prediction during the exercises below).

However, many features of negative afterimages cannot be attributed to simple fatigue of the cones of a particular color class. The most current explanations for the phenomenon also incorporate (i) peripheral mechanisms involving complex interactions between horizontal and amacrine cells and the bipolar cells with which they synapse, and (ii) central mechanisms that depend on multiple pathways between the retina and the visual cortex, and between the visual cortex and the higher brain centers that are involved in producing the conscious sensation of vision. Truth be told, however, our level of understanding about the negative afterimage phenomenon is still very incomplete, and much remains to be worked out.

What Does The Simulation Allow You To Do?

The Negative Afterimage simulation is a pretty basic simulation that is intended only to let you investigate the phenomenon of its namesake, the negative afterimage. You’ve no doubt done this before.

However, this simulation allows you more latitude with respect to the appearance of the stimulating image and its background than most textbook presentations of the phenomenon. This allows you to learn something about color vision as well as investigate the negative afterimage phenomenon a little more thoroughly than you’ve probably done previously. In fact, you can use this simulation to conduct your own independent research project into the phenomenon of negative afterimages.

Controls are provided that allow you to:

1. Vary the color of the stimulus image (the image you stare at to generate the negative afterimages).

2. Vary the color of the background of the stimulus image.

3. Duplicate the color of the afterimage you’re viewing.

What Does The Simulation’s Interface Look Like?

When you start the simulation you will see a display that looks similar to this (depending on your computer’s monitor size and operating system, and on the browser you’re using):

The display of this simulation is divided into two halves by a horizontal black band. Above the band is a blank white area at which you will gaze to visualize the negative afterimages. In the upper right corner of this area is the outline of a star whose color can be changed when you wish to match the color of the afterimage. The three sliders to the right of this star control the red, green, and blue (RGB) values of the pixels in that part of the monitor's display, and can therefore be used to change the star's color to match the color of your afterimages. If you prefer, you may also use the textfield to the right of each slider to change the color of the star, but in that case you'll have to click the "Go" button to implement the changes you've made

The lower half of the display contains a stimulus image of 9 stars arranged in three rows. These stars are what you will use to generate your negative afterimages. Superimposed on the central star is a small cross to use as a focus point when staring at the stars to generate the negative afterimage. This cross will be white or black, depending on the colors you’ve selected for the stars.

The default display consists of black stars against a white background. Using controls provided, you can independently change the color of the stars and of their background. There are two ways for you to do this. First, you can click on any of the 7 “Stars” color selections and/or the 7 “Background” color selections and, when you click the “Go” button, the stars and background will appear in the colors you have selected.

The simulation provides you with a second option for setting the color of the stars. Immediately below the Color Settings radio boxes are three sliders (“RED”, “GREEN”, and “BLUE”) and corresponding text fields. These allow you to specify the color of the stars by independently varying the intensity of the red, green, and blue color used by the simulation to paint the stars. You activate this option by clicking the “USE SLIDERS” radio button located below the column of Stars Color Settings radio buttons.

In accordance with the standards for computer displays, the values for the color-control sliders are limited to a range from 0 (no color) to 255 (maximum color). You can change the value for each color by clicking on the slider with the left mouse button (if you’re not familiar with this technique, click here for instructions), or by typing in a numerical value between 0 and 255 in the text field to the right of the appropriate slider. If you use the text field option to change a slider’s numerical value, you must click the “Go” button to cause the changes you’ve made to take effect.

To the left of each slider is a small rectangle which will display the color corresponding to the slider’s numerical value. In the default display, these rectangles will be black since each slider’s value is set to 0.

Below the sliders is a “Go” button, which you will use to run the simulation and cause it to change the color of the stars and/or their background to the values you’ve specified with the radio boxes or the sliders. Below the Go button is a “Reset” button, which will reset the value of each slider to 0, and thereby regenerate the default display of black stars against a white background.