Leaves, fruit, shadows, and lighting in Kibale Forest

@InProceedings\{TPL2003,
  author       = "Tom Troscianko and C. Alejandro Parraga and U. Leonards and R. Baddeley and D.J. Tolhurst",
  title        = "Leaves, fruit, shadows, and lighting in Kibale Forest",
  booktitle    = "Perception, European Conference on Visual Perception (ECVP\´03)",
  volume       = "32",
  pages        = "51b",
  year         = "2003",
  address      = "Uganda",
  abstract     = "There has been much debate about the links between primate colour vision and the properties of fruit and leaves eaten by trichromatic primates living in the wild. However, attempts to explore the spatio-chromatic properties of such stimuli are more recent. P\'arraga et al (2002 Current Biology 12 483 - 487) showed a correspondence between the luminance and red - green Fourier spectra of images containing green leaves and red fruit, and the contrast sensitivity for luminance and red - green gratings, particularly at normal grasping distance. However, the fruit sampled in that study was growing in England, and not a tropical rainforest. Furthermore, the sample did not contain any red, unripe leaves of the type eaten by monkeys and also argued to be of importance for monkey vision (Dominy and Lucas, 2001 Nature 410 363 - 365). We therefore obtained 223 images in Kibale Forest, Uganda, in the dry season of 2002. A further 80 images of two scenes, at intervals of 10 - 20 min, were also obtained to investigate changes in illumination. The images were obtained with a digital camera calibrated to output L, M, S cone responses for each pixel. The results are in keeping with those obtained earlier. Interestingly, there was no difference between images with red fruit and those with red leaves in this regard. The results are due to the strong invariance of the red - green opponent system to shadows. This extends, in the time-of-day images, to an invariance against natural changes in illumination allowing the relative colour of objects to be encoded in spite of substantial changes in the intensity and spectral content of the illuminant. The conclusion is that the red - green system has several important properties that make it fit for a variety of foraging operations.",
  ifactor      = "1.259",
  quartile     = "3",
  area         = "PSYCHOLOGY",
  url          = "http://cic.uab.cat/Public/Publications/2003/TPL2003"
}