Even buzzing, hyperactive students can learn. Insects, including Drosophila, show associative learning. However, learning deficits are seen in Drosophila with mutations in the cAMP-dependent protein kinase pathway genes dunce, which encodes cAMP phosphodiesterase, and rutabaga, which encodes an adenylate cyclase. Now van Swinderen reports attention deficits in Drosophila with mutations associated with learning deficiency in a recent article in Science.
The author has previously shown local field potential (LFP) responses in the Drosophila brain in the 20-30 Hz range during visual choice behaviors. This LFP activity is attenuated by sleep and anesthesia, increased by classical conditioning and regulated by dopamine, suggesting that it is similar to mechanisms of visual attention in vertebrates.
In the current study, the author recorded brain LFP responses in Drosophila tethered in the center of an arena with rotating walls. When symbols rotated directly in front of Drosophila, he measured peak LFP responses in the 20–30 Hz range. However, he found reduced and delayed LFP response in dunce mutant relative to wild-type Drosophila.
Drosophila attend to novelty. The author habituated Drosophila to seeing two boxes for over one minute. When he changed one of the boxes to a cross, wild–type Drosophila showed increased LFP responses in the 20–30 Hz range. In contrast, dunce and rutabaga mutant Drosophila showed absent and impaired 20-30 Hz LFP responses, respectively. Instead, both mutant Drosophila lines showed LFP responses in the 10–20 Hz range. Therefore, although dunce decreases and rutabaga increases cAMP, mutating either gene disrupts mechanisms associated with attention.
Mutations associated with learning deficits do not affect vision. Because several learning-associated mutations in Drosophila affect flight, the author designed an 'optomotor' task that tested vision independent of flight. He made an enclosed, transparent maze with different choice points allowing Drosophila to walk to the left or right of center. Underneath the maze, he projected a movie of green and black stripes moving from left to right. Wild-type Drosophila deviated to the right, in the direction of the moving lines. Surprisingly, dunce and rutabaga mutant Drosophila deviated even further to the right than did wild-type Drosophila. Seven Drosophila mutant lines with impairments in learning and memory showed strong optomotor responses, with dunce mutant Drosophila showing the strongest response.
Dunce and rutabaga mutant Drosophila cannot be distracted. A solid bar painted on the maze apparatus disturbed wild–type Drosophila when it was placed on the side opposite to the apparent direction of motion. So, wild-type Drosophila stopped moving to the right when a new visual object appeared on the left. However, the new visual object had no effect on the optomotor performance of dunce and rutabaga mutant Drosophila. Expression of the wild–type dunce gene product throughout development rescued distraction in dunce mutant Drosophila, but expression induced in adulthood did not, suggesting that dunce is important during development.
Although the inability to be distracted might seem like a good thing, it indicates that dunce and rutabaga mutant Drosophila may be deficient in the plasticity required to weigh the importance of competing visual cues, according to the author.