The researchers then sought to determine how the tadpoles process different stimuli. To do that they held the tadpoles in place while presenting a variety of simple animations via a fiber optic cable held next to an eye. The animations included a flashed circle, an apparently approaching circle (it became larger and larger), and a couple of "in between" animations, such as a circle that was faded in, rather than simply flashed into being.
While the tadpoles watched the animations, the researchers tracked their tail movements with a high-speed camera (to determine if the tadpoles were executing a fleeing maneuver) and recorded electrical signals along the visual processing circuitry: at the optic nerve leading from the retina to the brain's optic tectum region, at "excitatory" and "inhibitory" synaptic inputs of neurons in the optic tectum, and at the outputs of the tectal neurons.
What the scientists found was that the tectum, rather than the retina, appears to be where the tadpoles determine that something is approaching rather than merely present. How did they know? The strongest difference between responses to the apparently approaching circle, versus responses to other stimuli, such as flashed or faded circles, was detected at the stage of output from tectal neurons.
Moreover, the difference in activity related to approaching vs. flashed circles increased as the signal propagated from the optic nerve, through tectum input, and to tectum output.
"The tectum is the first place that responded to approaching stimuli not just differently, but stronger," Khakhalin said.
Inhibition moderates the conversation
|Contact: David Orenstein|