A mechanistic inter-species comparison of spatial contrast sensitivity

The validity of the Rovamo–Barten modulation transfer function model for describing spatial contrast sensitivity in vertebrates was examined using published data for the human, macaque, cat, goldfish, pigeon and rat. Under photopic conditions, the model adequately described overall contrast sensitivity for changes in both stimulus luminance and stimulus size for each member of this diverse range of species. From this examination, optical, retinal and post-retinal neural processes subserving contrast sensitivity were quantified. An important retinal process is lateral inhibition and values of its associated point spread function (PSF) were obtained for each species. Some auxiliary contrast sensitivity data obtained from the owl monkey were included for these calculations. Modeled values of the lateral inhibition PSF were found to correlate well with ganglion cell receptive field surround size measurements obtained directly from electrophysiology. The range of vertebrates studied was then further extended to include the squirrel monkey, tree shrew, rabbit, chicken and eagle. To a first approximation, modeled estimates of lateral inhibition PSF width were found to be inversely proportional to the square root of ganglion cell density. This finding is consistent with a receptive field surround diameter that changes in direct proportion to the distance between ganglion cells for central vision. For the main species examined, contrast sensitivity is considerably less than that for the human. Although this is due in part to a reduction in the performance of both optical and retinal mechanisms, the model indicates that poor cortical detection efficiency plays a significant role

Light, vision and the welfare of poultry

The visual system of domestic poultry evolved in natural light environments, which differ in many respects from the artificial light provided in poultry houses. Current lighting systems are designed mainly around human vision and poultry production, ignoring the requirements of poultry vision and the fimctional development of visual abilities during rearing. A poor correlation between the light provided and that required for effective vision may influence visually mediated behaviours such as feeding and social interaction, leading to distress and poor welfare. To understand fully the impact of the light environment on the behaviour and welfare of domestic poultry we need (i) to measure the physical properties of the light environment in a standard and relevant manner; (ii) to identify the limits of visual abilities in various light environments; (iii) to determine how light environments during rearing may disrupt the functional development of vision; and (iv) to resolve how visual abilities and lighting interact to affect visually mediated behaviour. Some conclusions can be drawn about the impact of current lighting regimes on bird welfare but there remains a pressing need to resolve various issues in this interaction. We propose, first, that dark periods should have a minimum duration of six hours; second, that bright light should be used in cases where pecking damage and cannibalism do not pose a problem; and third, that it is unlikely that the 100 Hz flicker associated with fluorescent light can be perceived by poultry. With less certainty, we can suggest that ultraviolet-supplemented lighting may have some welfare benefits, and that very dim lighting may adversely affect ocular development. We can only speculate on other issues, such as preferences and motivations for different coloured lighting or the ways in which lighting affects recognition of conspecifics. Several organisations and authorities have issued guidelines for poultry house lighting that strive to safeguard welfare and that are consistent with our current, but limited, understanding. One omission is a standard system for measuring light levels in poultry houses. Illumination with natural daylight would be an ideal solution to many lighting problems. Although some systems require artificial lighting for production purposes, we argue that it may be possible to rear birds humanely in artificial environments that contain some features of natural light. These features should be those for which poultry show some motivation, or whose exclusion would damage visual development