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Vision in the peafowl (Aves: Pavo cristatus)

Nathan S. Hart

Vision, Touch and Hearing Research Centre, School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072, Australia

View larger version (154K): [in a new window]   Fig. 1. Photomicrograph of Nissl-stained cells in the retinal ganglion cell layer of the peafowl Pavo cristatus. Ganglion cells are characterised by their large polygonal cell bodies, darkly staining abundant cytoplasm and pale staining nucleus. Putative displaced amacrine cells in the ganglion cell layer (such as those indicated by arrowheads) are usually round, oval or teardrop shaped and much smaller than the ganglion cells. Scale bar, 20 µm.

View larger version (17K): [in a new window]   Fig. 6. (A,B). Topography of cells in the ganglion cell layer of the peafowl retina (right eye). (A) All cells excluding glia; (B) Presumptive ganglion cells only (see Fig. 1 and text for details of criteria used). In both cases a prominent area centralis, with the possibility of a visual streak of high density extending from the area centralis towards the nasal retina, is indicated. Lines represent isodensity contours; numbers represent x103 cells mm-2. The black dot at the centre of the retina represents the location of the highest density count obtained (35.6x103 ganglion cells mm-2 in B). D, V, T and N refer to the dorsal, ventral, temporal and nasal aspects, respectively. The approximate location of the pecten (p) is also shown.

View larger version (41K): [in a new window]   Fig. 2. (A—G) Normalized pre-bleach (filled squares) and post-bleach (open circles) absorbance spectra of visual pigments in the peafowl Pavo cristatus. Pre-bleach spectra are overlayed with best-fitted rhodopsin templates (bold line; Govardovskii et al., 2000). Post-bleach spectra are fitted with a variable point running average (thin line). (H) The spectral distribution of the wavelengths of maximum absorbance (max) of the individual cone visual pigment absorbance spectra used to create the mean spectra. The group labeled as LWS in the histogram includes LWS single cones and both the principal and accessory members of the LWS double cone pair. VS, SWS, MWS and LWS refer to violet, short wavelength, medium wavelength and long wavelength sensitive single cones, respectively.

View larger version (34K): [in a new window]   Fig. 3. (A—G) Normalized mean difference spectra (symbols) and best-fitted rhodopsin visual pigment templates (lines) for the visual pigments in the peafowl Pavo cristatus. Difference spectra represent the change in absorbance of the outer segment on bleaching with white light (see text for details). (H) The spectral distribution of the wavelengths of maximum absorbance (max) of the individual cone visual pigment difference spectra use to create the mean spectra. For further details see legend for Fig. 2.

View larger version (20K): [in a new window]   Fig. 4. Mean absorptance spectra (A) of oil droplets found in the single and double cone photoreceptors of the peafowl Pavo cristatus. T, C, Y, R, P and A refer to the T-type (`transparent'), C-type (`colourless'), Y-type (`yellow'), R-type (`red'), P-type (`principal/pale') and A-type (`accessory') oil droplets found in the VS, SWS, MWS and LWS single cones and the principal and accessory members of the double cone pair, respectively. Subscripts D and V refer to whether the P-type oil droplets were measured in the dorsal or ventral retina, respectively, as noticeable differences in the spectra obtained from these two retinal regions were observed. (B) The spectral distribution of the cut-off wavelengths (cut) of the C-, Y-, R- and P-type oil droplets used to create the mean spectra. Note that P-type oil droplets measured in the ventral retina have a cut at longer wavelengths than those measured in the dorsal retina.

View larger version (14K): [in a new window]   Fig. 5. Transmittance of the individual components and combined ocular media of the peafowl Pavo cristatus. C, L, A and V refer to the cornea, lens and aqueous and vitreous humours, respectively. Spectral transmittance measurements of small portions of aqueous and vitreous humours were adjusted for in vivo pathlengths along the optic axis using a frozen sectioned eye (see text for details). Adjusted transmittances for the aqueous and vitreous were summed with measurements of the intact cornea and lens to give the transmittance along the optic axis of the combined pre-retinal ocular media (Eye). The wavelength of 0.5 transmittance T0.5 was 365 nm.

View larger version (15K): [in a new window]   Fig. 7. Calculated relative photon catches for each of the single cone types in the retina of the peafowl Pavo cristatus. Visual pigment spectral absorptance was modeled using mathematical templates of the appropriate max (Govardovskii et al., 2000). The outer segments of all single cones were assumed to be 16 µm long (as for the chicken, Morris and Shorey, 1967) and contain a visual pigment with an end-on specific absorbance of 0.015 µm-1 (Bowmaker, 1977). The calculated spectral absorptance of each visual pigment was multiplied by the spectral transmittance of the combined ocular media (Eye in Fig. 4), the spectral transmittance (1-absorptance), and cross-sectional area (diameters from Table 1) of the relevant oil droplet, and normalized to the SWS cone.

View larger version (10K): [in a new window]   Fig. 8. Predicted relative threshold spectral sensitivity for the potentially tetrachromatic colour vision system of the peafowl Pavo cristatus. The model used is that given by Vorobyev and Osorio (1998) and assumes that thresholds are determined by photoreceptor noise, that noise in a given colour channel is proportional to the reciprocal of the square root of the relative proportion of that cone type in the retina, and that the level of noise in a cone is independent of spectral sensitivity. VS, SWS, MWS and LWS single cones were assumed to be present in the ratio 1:1.9:2.2:2.1, respectively (Hart, 2001a). Spectral sensitivities for the different cone types were those presented in Fig. 7.