Analytical methods for the geometric optics of thermal vision illustrated with four species of pitvipers

Abstract

SUMMARYThe pitviper facial pit is a pinhole camera-like sensory organ consisting of a flask-shaped cavity divided into two chambers by a suspended membrane. Neurophysiological studies and simplified optical models suggest that facial pits detect thermal radiation and form an image that is combined with visual input in the optic tectum to form a single multispectral image. External pit anatomy varies markedly among taxonomic groups. However, optical function depends on unknown internal anatomy. Therefore, we developed methods for relating anatomy to optical performance. To illustrate, we constructed detailed anatomical models of the internal anatomy of the facial pits of four individuals of four pitviper species using X-ray tomography sections of fresh material. We used these models to define the point spread function, i.e. the distribution of radiation from a point source over the pit membrane, for each species. We then used optical physics, heat transfer physics and computational image processing to define the thermal image formed on the pit membrane for each species. Our computed pit membrane images are consistent with behavioral observations if the sensitivity of membrane receptors equals the most sensitive (ca. 0.001°C) laboratory estimates. Vignetting (variation in optical aperture size with view angle) and differences between body and environmental temperatures can create temperature variation across the membrane that greatly exceeds image temperature contrasts, potentially impairing imaging. Spread functions plotted versus source point azimuth and elevation show distinct patterns that suggest new research directions into the relationships among the optical anatomy, ecology, behavior and sensory neurophysiology of pitvipers. Supplementary material available online at

Directional sensitivity in the thermal response of the facial pit in western diamondback rattlesnakes (Crotalus atrox)

“…Using this head holder -more precisely the position of the center of the rattlesnake's facial pit when in the apparatus -a modified polar coordinate system (consisting of vertical and horizontal angular coordinates) was mapped out on the surface of the vibration isolation table, and the inner walls of the surrounding Faraday cage. The coordinate system was designed in accordance with the topographic data on the facial pit of C. atrox (Bakken et al, 2012). The coordinate system extended horizontally from 0 to 80deg in front of the pit (using increments of 10deg) and vertically from -18deg below the pit to 30deg above the pit (using increments of 6deg); the center of the facial pit was used as a reference point for both planes (Fig.2).…”

“…Spatial maps of these spread functions indicate the directional sensitivity of the pit organ (Fig.1) [see also fig.3 in Bakken et al (Bakken et al, 2012)], and suggest that this directional sensitivity may vary among species. While rooted in sectional anatomy, the directional sensitivity profiles that we generated (Bakken et al, 2012) have not been tested experimentally to see whether they predict sensory response. The purpose of the present contribution was to use neuroelectrophysiology to quantitatively test the spatial map of directional sensitivity for the pit organ of Crotalus atrox (Fig.1).…”

“…In a recent study, described in the accompanying paper (Bakken et al, 2012), we examined the effect of interspecific variation in facial pit geometry on imaging. Using topographic information derived from X-ray computed tomography (CT) sections defining the facial pit's membrane, external opening and adjacent scalation, coupled with heat transfer physics, we were able to generate the optical spread functions (i.e.…”

“…Using topographic information derived from X-ray computed tomography (CT) sections defining the facial pit's membrane, external opening and adjacent scalation, coupled with heat transfer physics, we were able to generate the optical spread functions (i.e. plots of the area on the pit membrane illuminated by thermal radiation from a point source) needed to compute the distribution of temperatures over the facial pit membrane (Bakken et al, 2012). Spatial maps of these spread functions indicate the directional sensitivity of the pit organ (Fig.1) [see also fig.3 in Bakken et al (Bakken et al, 2012)], and suggest that this directional sensitivity may vary among species.…”

“…plots of the area on the pit membrane illuminated by thermal radiation from a point source) needed to compute the distribution of temperatures over the facial pit membrane (Bakken et al, 2012). Spatial maps of these spread functions indicate the directional sensitivity of the pit organ (Fig.1) [see also fig.3 in Bakken et al (Bakken et al, 2012)], and suggest that this directional sensitivity may vary among species. While rooted in sectional anatomy, the directional sensitivity profiles that we generated (Bakken et al, 2012) have not been tested experimentally to see whether they predict sensory response.…”