Nikolic et al. (2007)
From Psy3241
Color Opponency in Synaesthetic Experiences by Danko Nikolic, Philipp Lichti, and Wolf Singer
What is Synaesthesia? Synaesthesia is a harmless perceptual condition in which there is a blending of the senses. Generally letters and numbers are seen as different colors or where music initiates the perception of color. A synaesthetic individual could be said to have the ability to see sound. Some synaesthetes are categorized as associators which is the perception of a color on an internal screen or in the mind, while others are considered projectors where color is seen on objects in space such as colored letters on a page.
The Stroop Test A Stroop test which was developed in 1935 by J. R. Stroop, an individual is instructed to name the ink color of a word that refers to a color. The reaction times to identify the ink color for the individual is measured and analyzed. Reaction times are generally longer if the color of the ink and the meaning of the word do not match (incongruent condition), for example if yellow is written in red ink. This test can be applied to study synaesthetic individuals by adjusting the meaning of the words to match the perceptual experience a person has to that word. In summary, a synaesthetic person would look at a word that evokes a red perception that would be colored in red ink and would be able to react faster than a non-synaesthetic individual.
Color Opponency in Synaesthetic Experiences In the current study researchers are taking advantage of the color opponent receptive fields of the brain. The color opponent receptive fields are cells that are excited by red and inhibited by green and cells that are excited by yellow and inhibited by blue. This fact allows researchers to study which receptive fields are active during real and synaesthetic perceptions.
Experiment 1 Subjects 6 synaesthetic individuals participated in the study, four were women and two were men and five of these people had other forms of Synaesthesia. 12 nonsynaesthetes participated in the study as the control group who matched the synaesthetes in gender and age.
Procedure The subjects were tested on color associations. There were three conditions: the congruent condition in which the color of each grapheme was the same as the synaesthetic color; the incongruent opponent condition in which the color of each grapheme was opposite to the synaesthetic color; the incongruent independent condition in which the color of each grapheme and the synaesthetic color were represented by different opponent-color channels; and the baseline condition in which the experimenters used a grapheme that did not have a synaesthetic color association. The experiment took place in a dimly lit room with a computer running the visual stimulation tool. The subjects were given 200 trials in which they were told to accurately name the real color of each grapheme as fast and as they could. The entire experiment took approximately 25 minutes.
What they found! The subjects response accuracy was very high (98%) but a Tukey HSD test for a post hoc comparison indicated that subjects named the correct color faster in the congruent condition than in the incongruent condition. The experimenters found significant results between the incongruent independent condition (which was named faster) and the incongruent opponent condition. They found that opponent incongruent colors produced more interference than the independent incongruent colors. They also found that the congruent synaesthetic colors helped the subjects to name the real colors of the graphemes.
Experiment 2 The experimenters examined semantic associations between shape and color using the Stroop task. The stimuli that they used were commonly known everyday associations between shape and color. The experimenters hypothesized that semantic associations do not involve the opponent-color system. They used four of the synaesthetes and 8 of the control subjects from experiment 1. They also used the same methods as for the synaesthetic Stroop test except that only three objects were used and each only appeared in three stimulation conditions. The subjects were presented each stimulus 25 times, given a total of 225 trials overall.
What they found! Once again, the response accuracy was very high, indicating that color opponency does in fact affect the semantic associations between shape and color differently than synaesthetic associations.
Overall, the experimenters concluded that opponent synaesthetic and real colors interfere the most with the naming and perception of a real color. Conversely, when synaesthetic and real colors are identical, the color-naming process is assisted and the response times are decreased. These findings show that the color experiences stimulated by this experiment involve color-opponent channels and thus neurons in the V1 to V4/V8 areas. The results of experiments 1 and 2 suggest that the semantic associations between graphemes and colors explain the interference between nonopponent colors.
