The Visual Brain Colors Black and White Images

The perception and processing of color has fascinated neuroscientists for a long time, as our brain influences our perception of it to such a degree that colors could be called an illusion. One mystery was: What happens in the brain when we look at black-and-white photographs? Do our brains fill in the colors?

Neuroscientists Michael Bannert and Andreas Bartels of the Bernstein Center and the Werner Reichardt Centre for Integrative Neuroscience in Tübingen addressed these questions. In their work, published in the leading scientific journal Current Biology, they showed study participants black-and-white photos of bananas, broccoli, strawberries, and of other objects associated with a typical color (yellow, red and green in the examples above). While doing so, they recorded their subjects’ brain activity using functional imaging. The true purpose of the study was unknown to the subjects, and to distract their attention they were shown slowly rotating objects and told to report the direction in which they were moving.

After recording brain responses to the black and white objects, the scientists presented real colors to their subjects, in the shape of yellow, green, red and blue rings. This allowed them to record the activity of the brain as it responded to different, real colors.

It turned out that the mere sight of black-and-white photos automatically elicited brain activity patterns that specifically encoded colors. These activity patterns corresponded to those that were elicited when the observers viewed real color stimuli. These patterns encoded the typical color of the respective object seen, even though it was presented in black and white. The typical colors of the presented objects could therefore be determined from the brain’s activity, even though they were shown without color.

“It was particularly interesting that the colors of the objects were only encoded in the primary visual cortex,” says Michael Bannert. The primary visual cortex is one of the first places a visual signal arrives in the brain. Scientists had assumed it simply passed on information about the physical properties of things seen, but was not able to recognize objects or to store color knowledge associated with objects. “This result shows that higher-level prior knowledge – in this case of object-colors – is projected onto the earliest stages of visual processing,” according to Andreas Bartels.

This study represents a significant contribution to answering the question of how prior knowledge contributes to perception on a neuronal basis. The projection of prior knowledge onto the earliest processing stages of the visual brain may facilitate the recognition of objects in difficult and noisy environments, such as in fog, and be relevant for colors in changing light conditions over the course of the day, when the weather is overcast, when we are indoors and so on. On the other hand, if prior knowledge or expectations have too much influence on early visual processing stages, this may account for hallucinations and the pathological perception of illusions.

A hypnotic suggestion can generate true and automatic hallucinations

A multidisciplinary group of researchers from Finland (University of Turku and University of Helsinki) and Sweden (University of Skövde) has now found evidence that hypnotic suggestion can modify processing of a targeted stimulus before it reaches consciousness. The experiments show that it is possible to hypnotically modulate even highly automatic features of perception, such as color experience.  The results are presented in two articles published in PLoS ONE and International Journal of Clinical and Experimental Hypnosis. The Finnish part of the research is funded by the Academy of Finland.

The nature of hypnotically suggested changes in perception has been one of the main topics of controversy during the history of hypnosis. The major current theories of hypnosis hold that we always actively use our own imagination to bring about the effects of a suggestion. For example the occurrence of visual hallucinations always requires active use of goal directed imagery and can be experienced both with and without hypnosis.

The study published in PLoS ONE was done with two very highly hypnotizable participants who can be hypnotized and dehypnotized by just using a one-word cue.
The researchers measured brains oscillatory activity from the EEG in response to briefly displayed series of red or blue shapes (squares, triangles or circles). The participants were hypnotized and given a suggestion that certain shapes always have a certain color (e.g. all squares are always red). Participant TS-H reported constantly experiencing a change in color immediately when a suggested shape appeared on the screen (e.g. seeing a red square when the real color was blue). The researchers found that this experience was accompanied with enhanced high-frequency brain activity already 1/10 second after the stimulus appeared and it was only seen in response to the shapes mentioned in the suggestion. The second participant did not experience the color change or the enhanced activity. However, she reported a peculiar feeling when a suggestion-relevant shape was presented: “sometimes I saw a shape that was red but my brain told me it had a different color”.

This enhanced oscillatory brain activity is proposed to reflect automatic comparison of input to memory representations. In this case the hypnotic suggestion “all squares are red” led to a memory trace that was automatically activated when a square was presented. Furthermore, for the participant TS-H the effect was strong enough to override the real color of the square. The matching must have occurred preconsciously because of the early timing of the effect and the immediacy of the color change. Also, both participants reported having performed under posthypnotic amnesia without conscious memory of the suggestions.

In the article published in International Journal of Clinical and Experimental Hypnosis TS-H was tested in a similar type of setting, however, only behavioral data, including accuracy and response times in color recognition, were collected. These results further support that a hypnotic suggestion affects her color perception of targeted objects before she becomes conscious of them. Furthermore, TS-H was not capable of changing her experience of visually presented stable images without the use of hypnotic suggestions i.e. by using mere mental imagery.

Importantly, both of these experiments were done by using a posthypnotic suggestion. The effect was suggested during hypnosis but the experience was suggested to occur after hypnosis. Thus all the experiments were carried out while participants were in their normal state of consciousness.

This result indicates that all hypnotic responding can no longer be regarded merely as goal directed mental imagery.  It shows that in hypnosis it is possible to create a memory trace that influences early and preconscious stages of visual processing already about 1/10 second after the appearance of a visual target. This result has important implications in psychology and cognitive neuroscience especially when studying visual perception, memory and consciousness.

The stalked eyes of mantis shrimp species that live in shallow water can have up to 16 kinds of photoreceptor cells, 12 of which are specialized for different colors. People make do with four kinds, three of which pick up colors.

Hanne Thoen of the University of Queensland in Brisbane, Australia, tested the color vision of mantis shrimp by training them to scoot out of their burrows toward a pair of optical fibers and punch at the one glowing a particular color. As she narrowed the color gap between the two fibers, she could tell when the animals no longer discerned a difference. 

So far, Thoen has tested her mantis shrimp on six target colors ranging from a 425-nanometer purple to a 628-nanometer red. If the animals perform just as poorly at distinguishing colors in other wavelengths, then mantis shrimp may be using some unknown system of color perception.

People and other animals studied so far distinguish colors through brainpower by interpreting competing activity in different kinds of light-receptor cells. Instead of doing such fancy brainwork, mantis shrimp may just rely on what a particular specialized cell responds to strongly. Wavelengths that tickle the purple-sensitive cells may be just plain purple regardless of whether they’re more toward the blue or the ultraviolet.