The imaging properties and sensitivity of the facial pits of pitvipers as determined

by optical and heat-transfer analysis

Pitvipers possess facial organs that are sensitive to infrared radiation of approximately 5000–30,000 nm. These pit organs apparently function as pinhole cameras (similar to the nautilus eye). Thousands of receptors in a membrane at the back of the cavity can detect temperature differences of 0.003°C or less. Information from these receptors is processed in the medulla and, along with visual signals, mapped onto the optic tectum. Because of neural interconnections in the optic tectum and common pathways for visual and thermal signals, the visual system of pitvipers can justifiably be considered a single multispectral sense. Previous studies have examined behavioral correlates and neurophysiological design of the system (it has been shown to aid in the detection of prey and suitable sites for thermoregulation), but little is known of their optics; Bakken and Krochmal (2007) designed visual models based on physical optics and heat transfer properties with the aim of determining “what pitvipers can actually ‘see’ with the facial pits.”

As the text describes, infrared is a problematic subset of radiation to sense. The thermal environment can be noisy and heterogeneous, and tissues absorb heat before it can reach receptors, which require five times the photons that visual receptors do. For pitvipers, the brightness of targets is determined by angular resolution, which may be relatively poor. The authors analyzed the pit system as optical system; by converting irradiance of stimuli to membrane temperatures, they created thermograms that mimic the image created by the pit membrane in ecologically realistic environments. Because the radiance from a single source point is spread over several overlapping points on the membrane, the image on the membrane is fuzzy, with less contrast than the ideal image that would be expected from a pinhole camera. Larger pit apertures resulted in lower resolution but greater temperature contrast (as in eyes, a small pupil creates a sharp but dim image, while a large pupil makes it bright but blurred). Because of variable patterning in natural backgrounds, the strongest thermal signal may be the background rather than potential prey. The authors conclude that the sensitivity of pitvipers’ pit organs may be even greater than currently estimated, or less information is obtained by the sense than is commonly believed.