Summary: Researchers describe the neural events that occur immediately before a decision is made.
Source: University of Oxford
Monitoring the real-time dynamics of molecules and electrical signals inside living organisms is central to understanding disease and behavior. Achieving this in a minimally invasive, sensitive and high-resolution way remains challenging, and has driven the development of new tools for tracking biological signals as they change.
In earlier experiments that required fruit flies to tell apart increasingly similar concentrations of an odor, the team led by Professor Gero Miesenböck identified a very small group of roughly 200 nerve cells that are critical for making those sensory decisions.
Building on that work, the researchers now show that these decision-relevant neurons accumulate sensory evidence as tiny voltage changes across their membranes. Those small voltage shifts sum up over time until they reach a threshold — a hair-trigger point — at which the neuron fires a large electrical spike. That spike serves as the signal that a decision has been reached.
The research appears in the journal Cell. Funding was provided by the Wellcome Trust and the Gatsby Charitable Foundation.
“We have identified a straightforward physical mechanism that underlies a cognitive process,” says Dr. Lukas Groschner, the study’s lead author. “Our findings indicate that analogue electrical signals are an important component of cognition. Although the brain is often likened to a digital system of spikes and pauses, a great deal of computation happens in the graded, analogue activity that fills what might appear to be silence.”
The neurons implicated in the flies’ decisions are marked by the expression of a genetic regulator known as FoxP.
FoxP controls how incoming sensory inputs are integrated and how long the accumulated evidence is retained. Flies that carry a defective FoxP gene produce an excess of a potassium channel subunit that acts like an electrical shock absorber, damping the neurons’ membrane voltage so it is less responsive to new inputs. As a result, evidence accumulates more slowly and decisions are delayed — the flies become less decisive.
Fruit flies have a single FoxP gene, while humans possess four related FoxP genes. Two of the human forms, FoxP1 and FoxP2, have previously been linked to aspects of cognitive development and language, suggesting there may be shared principles across species in how FoxP-family proteins shape neuronal computation.
“Drosophila has repeatedly made difficult biological questions tractable,” says Professor Miesenböck. “Studies of these insects have revealed fundamental mechanisms of development and circadian rhythms, among other discoveries recognised by Nobel Prizes. Now, work on fruit flies is beginning to illuminate long-standing puzzles in cognitive science and psychology, and its influence may prove equally significant.”
Source: University of Oxford
Publisher: Organized by NeuroscienceNews.com
Image source: Image adapted from the University of Oxford news release.
Original research: Open access research article titled “Dendritic Integration of Sensory Evidence in Perceptual Decision-Making” by Lukas N. Groschner, Laura Chan Wah Hak, Rafal Bogacz, Shamik DasGupta, and Gero Miesenböck, published in Cell on March 14, 2018.
DOI: 10.1016/j.cell.2018.03.075
University of Oxford. “How Decisions Form in the Brain.” NeuroscienceNews, May 1, 2018.
Abstract
Dendritic Integration of Sensory Evidence in Perceptual Decision-Making
Perceptual decisions depend on accumulating sensory information until a criterion for action is reached. Most prior explanations emphasize evolving patterns of neuronal spiking. Here, the authors report that subthreshold membrane voltage changes — those that do not yet generate spikes — can represent the gradual accumulation of evidence prior to a choice. In the mushroom body of fruit flies, αβ core Kenyon cells (αβc KCs) integrate odor-evoked synaptic inputs at timescales that match the speed required for olfactory discrimination. The forkhead box P transcription factor (FoxP) sets both the neurons’ integration properties and the flies’ decision times by controlling the abundance of the Shal (KV4) voltage-gated potassium channel in αβc KC dendrites. Through this specific combination of biophysical properties, αβc KCs tailor the generic process of synaptic integration to the temporal demands of sequential sensory sampling.