Scuttling around in the dead of night, American cockroaches tend to rely most on touch and their sense of smell for guidance. However, the insects always keep an eye open for a dark bolt hole to scamper into. Given that the animals are fine-tuned to head for dark corners, Matti Weckström and his colleagues from the University of Oulu, Finland, were curious to find out how sensitive the cockroaches' vision is in extremely dim light. The team decided to use the insects' tendency to turn in the same direction as their surroundings – when the image on their eye of the surroundings moves – to find how sensitive their eyes are and to learn more about how they process visual information at very low light intensities (p. 4262).

The team placed individual cockroaches on a roller ball – which the insects could only touch with their feet to indicate in which direction they were moving. Then they displayed images of moving gratings illuminated by light at intensities ranging from a brightly lit room (500 lx) to a dark moonless night (0.005 lx) and measured the insects' reactions. They were impressed to see that the cockroaches' vision was fantastically sensitive, allowing the animals to see gratings moving in light as low as 0.005 lx when each photoreceptor was only picking up one photon every 10 s. And when the team analysed the cockroaches' responsiveness, they realised that the insects were pooling and processing the signals from thousands of light-sensitive cells to detect motion at these low light levels.

Weckström and his colleagues say, ‘The cockroach visual system for motion detection has to rely on unknown neural processing in the deeper ganglia in order to cope with the inescapably deteriorating spatial resolution’, adding that they hope to apply the lessons that they learn from the sensitive insects to design better automatic nocturnal vision systems.

Cockroach optomotor responses below single photon level
J. Exp. Biol.