Top-Down Processing of Color Information
How do we perceive color? Color is the product of responses in wavelength-specific cells in our retina. Expectations and context can also influence color perception, which will be powerfully demonstrated in this lab activity.
Bottom-Up vs. Top-Down
Bottom-up theories of color perception focus on activation of wavelength-specific cells in the retina along with opponent processes among ganglion cells that project to subcortical and cortical brain regions. Top-down processing, where expectations and knowledge influence color perception, also can be demonstrated to have powerful effects.
Contextual Cues
This lab activity begins with a simple color-sorting task in which you will move red-, green-, or blue-colored circles to separate boxes. This activity determines whether you are able to distinguish among the three colors, or trichromats, that map onto the three different color-sensitive cones in the retina. After this, you will view four different images in which you will judge whether one color in a subregion of the image is lighter or darker than the color of another subregion. Feedback will be provided later in the activity showing you which region you picked with an additional demonstration showing how the two subregions were actually identical. These activities show that the interpretation of color of the objects in the subregions is based on the surroundings instead of purely on the wavelength of light the object is reflecting. In general, colors surrounded by a dark context are perceived as being lighter than colors surrounded by a lighter context. Colors that appear to be in the shadows are perceived as lighter than colors that appear to be exposed to direct light. Your knowledge of how light operates has a strong influence on color perception.
Daylight and Dim Light Effects
You will also be presented with an example of a dress that became famous a few years ago when it was reported that a substantial proportion of people would see it as black and blue the first time they viewed it, while a smaller but still large proportion interpreted the same dress as white and gold. This dress picture presents ambiguous light cues about how light hits the surroundings. Bright light in the background provides a daylight cue, while shadows on the dress provide another cue indicating dim light. We adjust perception of color differently depending on whether we believe the object is in daylight or whether it is in dim light. On the one hand, daylight effects occur because objects in daylight reflect more orange light and so people use top-down processing to reduce orange elements of the image resulting in perception of the dress appearing blue and black. On the other hand, dim light effects refer to the perception that in darker settings objects tend to take on blue hues so that people tend to ignore blue light, which makes the dress appear white and gold.
Color Constancy
In the final part of the lab activity, you will see how objects in the same scene can appear different in different lighting conditions. In the scene on the left, the second piece of paper from the left that the man is holding seems to be a stronger shade of pink compared to the same piece of paper in the scene on the right. The scene on the left is a photo with blue tint, while the scene on the right has a reddish tint. These tints provide context that changes the piece of paper in each case. The term color constancy ensures that the perceived color of objects remains relatively constant despite varying illumination. We know that a banana is the same banana at midday when the main illumination is white sunlight and also at sunset when the main illumination is red. Color constancy is another top-down mechanism driven by knowledge that helps us identify objects.