All neurons in the brain belong to complex neural circuits, typically receiving and transmitting activity from and to multiple brain areas. A critical question in systems neuroscience is whether a brain region broadcasts the same information to multiple downstream areas, or if activity to distinct brain areas somehow conveys different information. Understanding this question is a key step forward in understanding the circuitry underlying cognition.
In a recent issue of Science, Ciocchi et al. demonstrate that the ventral hippocampus (vCA1) sends different types of information – anxiety, goal-directed activity, or task-related activity – to different areas of the brain, depending on the prevailing need. This result is critically important, because it demonstrates that the vCA1 is not sharing the same information with all downstream brain structures. Rather, the vCA1 is performing a specific neural computation and then passing that information along to specific brain regions.
Ciocchi and colleagues used a combination of optogenetics and in vivo recording of single neuron activity in animals engaged in an anxiety task (an elevated plus maze), a spatial exploration task (an open field), or a goal-directed navigation task in a single session. This was critical, as it allowed the researchers to compare neural activity across different tasks and determine exactly what each neuron was signaling. Further, the researchers used optogenetic stimulation of central hippocampal neuronal projections to the medial prefrontal cortex (mPFC), nucleus accumbens (Acb), and amygdala to identify where each neuron projected. By stimulating each projection site, projection neurons from vCA1 were induced to fire, and the neural activity was recorded back in the vCA1. In this way, neurons which projected to specific locations could be anatomically dissociated from other neurons in a living animal.
Neurons from vCA1 which projected to mPFC carried an elevated level of anxiety information, compared to neurons which projected to Acb or amygdala. This is contrasted with vCA1 neurons which projected to Acb. These neurons carried more goal-directed information, demonstrated by decreased activity when rats entered reward zones. However, vCA1 neurons which projected to both mPFC and Acb were significantly more active in reward zones. Overall, vCA1 neurons which projected to Acb carried more goal-motivated information than vCA1 neurons which projected to other brain regions. Finally, vCA1 neurons which projected to mPFC, Acb and amygdala were more active before any task, had higher activity during the task, and were more likely to be recruited during sharp wave ripples, a measure related to consolidation of memory in the hippocampus.
Taken together, these data represent a key answer to the question of how neural circuits perform computations. Rather than a single neural structure performing one computation and broadcasting this information to all downstream areas, it appears that neural structures route specific information to specific downstream parts of the circuit. This information is a big step forward in our understanding of how the brain works, from a systems perspective.
- Ciocchi S, Passecker J, Malagon-Vina H, Mikus N, Klausberger T (2015) Selective information routing by ventral hippocampal CA1 projection neurons. Science 348:560-563. doi: 10.1126/science.aaa3245