Writing Memories In the Brains of Flies

Source for the following post: BBC NEWS | Science & Environment | Bad memories written with lasers

The brains of flies are far simpler than the brains of humans. Previously, researchers had discovered that only 12 neurons were involved in the formation of associative memories in flies. This most recent study builds on this knowledge. If these 12 neurons are involved in forming memories, could researchers trigger these neurons and create memories?

According to a recently published paper in Cell, the answer is “Yes.” Using genetically-modified flies with adenosine-5′-triphosphate (ATP) activated neurons (the ATP is triggered by lasers), the researchers were able to affect the flies such that “the flies associated the smell with a bad experience, so the laser flash gave the fly a memory of a bad experience that it never actually had.”

Here’s a link to the journal article (requires a subscription).

Here’s the abstract:

“Dopaminergic neurons are thought to drive learning by signaling changes in the expectations of salient events, such as rewards or punishments. Olfactory conditioning in Drosophila requires direct dopamine action on intrinsic mushroom body neurons, the likely storage sites of olfactory memories. Neither the cellular sources of the conditioning dopamine nor its precise postsynaptic targets are known. By optically controlling genetically circumscribed subsets of dopaminergic neurons in the behaving fly, we have mapped the origin of aversive reinforcement signals to the PPL1 cluster of 12 dopaminergic cells. PPL1 projections target restricted domains in the vertical lobes and heel of the mushroom body. Artificially evoked activity in a small number of identifiable cells thus suffices for programming behaviorally meaningful memories. The delineation of core reinforcement circuitry is an essential first step in dissecting the neural mechanisms that compute and represent valuations, store associations, and guide actions.”

You can also listen to an interview with one of the researchers on this episode of NPR’s Science Friday.

As we learn more about how memories are created we might be able to understand and fix problems when memories fail.

The Modal Model of Memory and the Serial Position Effect

I’m continuing my recent trend of basic cognitive psychology posts. The following post is about the Modal Model of memory, which has been highly influential for a number of decades but it is slowly being modified over time. I won’t get into the more modern modifications of the modal model, rather, in my post I present the very traditional view of memory, even if it is somewhat controversial today. For example, a number of psychologists do not believe that short term memory really exists (working memory fills in the gap). In any case, my post serves as a brief introduction to a classic view of memory and of the primacy and recency effects.

The modal model of memory has three main components. They are: sensory register, short-term memory (STM), and long-term memory (LTM). This Atkinson and Shiffrin model of memory assumes that the processes of moving information from the sensory store to short-term and then long-term memory takes place in discrete stages. At any of these stages information can be lost through interference or decay. Another assumption of this model is that information processing has to start in the sensory register and be attended to, then move to STM, and then to LTM with rehearsal.

The serial position effect (split into the primacy and recency effects) is that the first few and last few items in a word list, for example, are the easiest to remember. A graph of this effect would be roughly parabolic (i.e., U-shaped). The primacy effect occurs because people have time to rehearse the first few items until the STM capacity is reached. The recency effect occurs because the last items are still in STM and have not decayed yet so they are easy to remember. The items in the middle of lists are easy to forget because STM capacity is too full for much rehearsal by then and as more items are presented, older items in STM are “pushed out.”

Serial Position EffectThere are ways to hinder the primacy or recency effects though. If items are presented rapidly then there is not time to rehearse the items and the primacy effect fades away. If there is a distracting task given at the end of the main task (similar to Peterson and Peterson’s 1959 study testing the decay rate of STM), then the recency effect disappears due to STM capacity being taken up by the distracters, which leads to decay of the information in STM. These findings indicate that the systems governing primacy and recency effects are separate. The findings also gave support to the modal model because researchers identified the primacy effect with the transfer of STM into LTM. The recency effect is just an example of information being in STM.