Most leech photoreceptor neurons, like that shown here at the right are very broadly tuned to respond best to green light. However, we discovered that leeches have color vision and can also discriminate near ultraviolet light (UV)! About 5-10% of the cephalic photoreceptors respond best to UV, while the ventral sensilla are very narrowly (almost 100%) tuned to respond to UV light.
We can record the electrical responses from whole eyes or sensilla, or from individual photoreceptors, or from individual neurons within the CNS receiving light input. In this way we are listening in on the conversations these neurons have with each other when exposed to light and we are coming to understand the way leech eyes encode images and respond to the visual world.
Although short wavelengths (blue, violet, near UV) scatter in water more than red, yellow green, they also penetrate most readily (are less absorbed). So, while scattering reduces the ability to use these short wavelengths for long distance viewing, it may actually increase their availability by diffusing them. Thus, short wavelengths in an aquatic environment may be readily available for spatial discrimination of close objects, and UV, violet and blue may be more readily available at a given depth where red, yellow and even green are heavily attenuated.
In many aquatic environments there are a range of wavelengths available, including near UV. UV may be associated with damaging amounts of light, or low oxygen or other factors. Leeches respond accordingly. We also suggest that to avoid visually guided predators, it is important for the counter-shaded leech to be dorsal upward (for camouflage) when light is available. Consistent with this, we discovered that for leeches the "concept" of UP is a color! Leeches appear to use color vision to determine which way is up. We know the leech does not "feel" these relatively dim lights since we record no visual responses from those neurons that encode "feel" in the leech.
Click here for a short video clip of a leech reacting to ventral UV!
We continue to examine the sorts of visual cues that leech eyes can detect. For example we also learned that the eyes can respond preferentially to temporal and spatial frequencies, in other words, moving bars of light and dark as well as threatening shadows.
We are interested in mapping these visual responses into the CNS to understand how simple brains can extract visual features and act on them.