Steps along the Way from Spectrum to Object
Distinguishing the notions of coding and perception is essential in analyzing the neural mechanisms mediating perception. Coding concerns the relation between the stimulus and neural activity while perception concerns our capacity to interpret brain events in terms of objects and events outside of us. Nevertheless, these notions are still often confused in discourse concerning color vision. In color, coding concerns the neural activity evoked by spectra while perception concerns our attribution of redness, greenness, etc. to world events that we interpret as lights and surfaces. A central persisting question is how coded neural activity is integrated to produce perception of objects. Three experiments on color will be presented, aimed at different stages of this process, to elucidate some of the complexities involved. First, hue-cancellation using cone-isolating stimuli reveals that there is no fixed hue percept that can be associated with the activity coded in individual photoreceptor classes. Second, I will show that observers can integrate chromatic information over very narrow regions of color space during the decision process in a chromatic discrimination task. These results are consistent with the participation of cortical level mechanisms. Third, I will present cerebral imaging data that implies that the integration of local chromatic differences to extract a color transparent region involves the participation of a region previously associated with responding to object properties of color, a region different from the one typically evoked as being essential for color.