How do the nervous system and the brain integrate visual, auditory, and other stimuli, and which stimuli are prioritized over others? Hawkmoths are helping scientists gain insight into these questions.
“Sensory integration remains one of the more interesting tasks that even simple nervous systems accomplish,” said University of Washington biology professor Thomas L. Daniel. “From tasks like reaching in humans to nectar-feeding in insects, our challenge has been developing experimental ways to reveal the mechanisms and circuitry that underlie combined visual and mechanical sensing.”
A research team led by Daniel studied Manduca sexta, which uses a long proboscis to drink nectar meals while hovering above flowers. The moths have been observed adjusting their position to track with the flower’s position as it is swayed by wind.
Daniel’s group studied this behavior in the laboratory using artificial flowers constructed with small nectar pods. The moths respond to the artificial flowers much as they would to real ones and adjust their position to continue feeding as researchers move the flower. To see how input from different sensory systems contributed to this behavior, Daniel’s team modified the artificial flowers to deliver contradictory visual and tactile cues: the flower’s petals, which the moth follows using its eyes, move independently from the nectar pod, which the moth proboscis touches. By studying how moths respond to such discordant signals, the researchers hoped to gain insight into how the moth processes and combines inputs from both sensory systems. Their findings were published Oct. 24 in the Proceedings of the National Academy of Sciences.
“Typically, to study how a particular sense contributes to a behavior, scientists try to design experiments in which the animal only receives that one kind of sensory cue,” said University of Washington postdoctoral researcher Eatai Roth, who is lead author on the paper. “But this doesn’t reflect what’s happening when an ensemble of senses contribute concurrently.”
The team determined that when the nectar chamber moved but the rest of the flower was still, the moths were generally able to sway in response. But when researchers kept the nectar chamber still and moved the flower petals, the moths only swayed slightly. This indicated that, for feeding, tactile information transmitted by the proboscis may be a more important sensory input than vision.
“In nature, the visual and touch cues largely agree and either sense alone is enough for the job. Having both provides redundancy, a backup just in case,” said Roth. “But when we present the moth with conflicting stimuli, it must decide how to balance the mismatched information — which cue to follow. And it turns out, quite surprisingly, that touch beats out vision in this sensory tug-of-war.”
The researchers measured moth positions during the tests and used these data to describe the moths’ behavior in a mathematical model. Though the sense of touch appeared to play a greater role in tracking behavior, moths do not rely on this sense alone. Their mathematical model indicated that the moth brain uses a simple additive or linear summation model to integrate signals from the proboscis and the eyes. And though moths rely heavily on the touch cues from the proboscis, the model suggests that both the visual and touch senses are acute enough for the moth to follow the flower.
The team used this model to predict how the moths would behave in a new setting in which the nectar chamber and flower were both moving, but quite differently. The researchers tested these predictions on a different set of Manduca sexta, and they responded just as the model predicted. Daniel and his team believe that the mathematical underpinnings they describe here may represent a common mode of signal integration in animals.
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