Avoidance of obstacles in the absence of visual awareness

The spatial character of our reaching movements is extremely sensitive to potential obstacles in the workspace. We recently found that this sensitivity was retained by most patients with left visual neglect when reaching between two objects, despite the fact that they tended to ignore the leftward object when asked to bisect the space between them. This raises the possibility that obstacle avoidance does not require a conscious awareness of the obstacle avoided. We have now tested this hypothesis in a patient with visual extinction following right temporoparietal damage. Extinction is an attentional disorder in which patients fail to report stimuli on the side of space opposite a brain lesion under conditions of bilateral stimulation. Our patient avoided obstacles during reaching, to exactly the same degree, regardless of whether he was able to report their presence. This implicit processing of object location, which may depend on spared superior parietal-lobe pathways, demonstrates that conscious awareness is not necessary for normal obstacle avoidance.

Microspectrophotometry of visual pigments and oil droplets in a marine bird, the wedge-tailed shearwater Puffinus pacificus: Topographic variations in photoreceptor spectral characteristics

Microspectrophotometric examination of the retina of a procellariiform marine bird, the wedge-tailed shearwater Puffinus pacificus, revealed the presence of five different types of vitamin A(1)-based visual pigment in seven different types of photoreceptor. A single class of rod contained a medium-wavelength sensitive visual pigment with a wavelength of maximum absorbance (lambda(max)) at 502 nm. Four different types of single cone contained visual pigments maximally sensitive in either the violet (VS, lambda(max) 406 nm), short (SWS, lambda(max) 450 nm), medium (MWS, lambda(max) 503 nm) or long (LWS, lambda(max) 566 nm) spectral ranges. In the peripheral retina, the SWS, MWS and LWS single cones contained pigmented oil droplets in their inner segments with cut-off wavelengths (lambda(cut)) at 445 (C-type), 506 (Y-type) and 562 nm (R-type), respectively. The VS visual pigment was paired with a transparent (T-type) oil droplet that displayed no significant absorption above at least 370 run. Both the principal and accessory members of the double cone pair contained the same 566 nm lambda(max) visual pigment as the LWS single cones but only the principal member contained an oil droplet, which had a lambda(cut) at 413 nm. The retina had a horizontal band or 'visual streak' of increased photoreceptor density running across the retina approximately 1.5 mm dorsal to the top of the pecten. Cones in the centre of the horizontal streak were smaller and had oil droplets that were either transparent/colourless or much less pigmented than at the periphery. It is proposed that the reduction in cone oil droplet pigmentation in retinal areas associated with high visual acuity is an adaptation to compensate for the reduced photon capture ability of the narrower photoreceptors found there. Measurements of the spectral transmittance of the ocular media reveal that wavelengths down to at least 300 nm would be transmitted to the retina.

Detection of fruit and the selection of primate visual pigments for color vision

Primates have X chromosome genes for cone photopigments with sensitivity maxima from 535 to 562 nm. Old World monkeys and apes (catarrhines) and the New World ( platyrrhine) genus Alouatta have separate genes for 535-nm ( medium wavelength; M) and 562-nm ( long wavelength; L) pigments. These pigments, together with a 425-nm ( short wavelength) pigment, permit trichromatic color vision. Other platyrrhines and prosimians have a single X chromosome gene but often with alleles for two or three M/L photopigments. Consequently, heterozygote females are trichromats, but males and homozygote females are dichromats. The criteria that affect the evolution of M/L alleles and maintain genetic polymorphism remain a puzzle, but selection for finding food may be important. We compare different types of color vision for detecting more than 100 plant species consumed by tamarins ( Saguinus spp.) in Peru. There is evidence that both frequency-dependent selection on homozygotes and heterozygote advantage favor M/L polymorphism and that trichromatic color vision is most advantageous in dim light. Also, whereas the 562-nm allele is present in all species, the occurrence of 535- to 556-nm alleles varies between species. This variation probably arises because trichromatic color vision favors widely separated pigments and equal frequencies of 535/543- and 562-nm alleles, whereas in dichromats, long-wavelength pigment alleles are fitter.