ELECTROPHYSIOLOGICAL FINDINGS OF COLOR SE LECTION PROCESSES

In perception, certain criteria have to be used to select and process stimuli prior to conscious recognition. One possible selection criteri on is the color of the stimuli. In previous work it has been shown tha t color and color selection influence visual event-related potentials (ERP). A frontally localized positivity occuring about 200 ms post sti mulus and a more widely distributed negativity from 150 to 350 ms post stimulus have been described. We further explored the mechanisms of c olor selection with the ERP-technique. The specificity of the effects of color selection and the topographical distribution of the effects b y means of isovoltage maps were of particular interest. In two experim ents the same 8 healthy subjects viewed a randomized order of stimuli appearing in one of two possible colors. The subjects had the task to attend to stimuli of one color while ignoring those of the other color and to press a button whenever a defined target stimulus appeared in the attended color. Between both experiments the physical aspects of t he stimuli varied with respect to form and definition of the target st imuli. This allowed a separation of color-specific and color-nonspecif ic selection processes. Color specific selection processes have to be independent of physical stimulus aspects, while the effects of nonspec ific selection processes vary with the experiments. The earliest effec ts of color selection on the ERP were found from 200 ms after stimulus presentation. While the latency of the frontal P2-component varied si gnificantly between the two experiments, a superimposed frontal positi vity in the range from 200 to 250 ms, which did not vary between the t wo experiments, could be separated. Also, a more widely distributed ne gativity between 250 and 500 ms post stimulus was found. While the lat ency and amplitude of the frontal positivity did not vary between the two experiments, the amplitude of the negativity was significantly lar ger in one experiment than in the other. These findings are interprete d in terms of a color specific frontal positivity, which is also indep endent of the P2-component and a color nonspecific negativity. The iso voltage maps showed a frontal maximum for the positivity. These result s are discussed with respect to the results of other ERP- and Positron Emission Tomography (PET) -studies of color selection.