title: Blobs versus bars: psychophysical evidence supports two types of orientation response in human color vision.
abstract: The classic hypothesis of Livingstone and Hubel (1984, 1987) proposed two types of color pathways in primate visual cortex based on recordings from single cells: a segregated, modular pathway that signals color but provides little information about shape or form and a second pathway that signals color differences and so defines forms without the need to specify their colors. A major problem has been to reconcile this neurophysiological hypothesis with the behavioral data. A wealth of psychophysical studies has demonstrated that color vision has orientation-tuned responses and little impairment on form related tasks, but these have not revealed any direct evidence for nonoriented mechanisms. Here we use a psychophysical method of subthreshold summation across orthogonal orientations for isoluminant red-green gratings in monocular and dichoptic viewing conditions to differentiate between nonoriented and orientation-tuned responses to color contrast. We reveal nonoriented color responses at low spatial frequencies (0.25-0.375 c/deg) under monocular conditions changing to orientation-tuned responses at higher spatial frequencies (1.5 c/deg) and under binocular conditions. We suggest that two distinct pathways coexist in color vision at the behavioral level, revealed at different spatial scales: one is isotropic, monocular, and best equipped for the representation of surface color, and the other is orientation-tuned, binocular, and selective for shape and form. This advances our understanding of the organization of the neural pathways involved in human color vision and provides a strong link between neurophysiological and behavioral data.
authors.affiliation: McGill Vision Research, Department of Ophthalmology, McGill University, Montreal, Quebec, Canada.
authors.forename: Mina
authors.initials: M
authors.lastname: Gheiratmand
authors.forename: Tim S
authors.initials: TS
authors.lastname: Meese
authors.forename: Kathy T
authors.initials: KT
authors.lastname: Mullen
mesh_terms: D055253:Color Vision / Q000502:physiology*; D006801:Humans; D009949:Orientation / Q000502:physiology*; D010775:Photic Stimulation; D011601:Psychophysics / Q000379:methods*; D015348:Vision, Binocular / Q000502:physiology*; D015349:Vision, Monocular / Q000502:physiology*; D014793:Visual Cortex / Q000502:physiology*; D014795:Visual Pathways / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*