The brain can learn to suppress instinctive fear reactions through neural plasticity. This process occurs due to the long-term suppression of inhibitory connections in the ventrolateral geniculate nucleus, mediated by endocannabinoids. The findings show how the sensory cortex can modulate automatic responses, offering new insights into the regulation of fear and anxiety.
Rapid, instinctive reflexes to environmental stimuli are essential for the survival of many animals, helping them to react quickly to threats. However, in certain situations, these responses can be exaggerated or unnecessary.
To cope with this, animals have the ability to learn to control and even suppress certain automatic reactions. The study analyzed investigates how the brain carries out this learning process and which neural circuits are involved in this behavioral adaptation.
Researchers at University College London, UK, have discovered that a specific region of the brain, called the posterolateral superior visual areas (pHVAs), plays a key role in learning how to suppress instinctive flight responses to visual threats.
This region connects to another brain structure, known as the ventrolateral geniculate nucleus (vLGN), via a descending neural pathway.
During learning, the posterolateral superior visual areas help to modify the response of the ventrolateral geniculate nucleus, enabling it to regulate the fear response.
The “What” pathway turns out to be a huge anatomical stream from the primary visual cortex projecting to V4 and the temporal lobe dealing with the analysis of shape, color, contour, discrimination, and recognition of objects. As such, it processes “What” information, relying on the memory circuitry within the temporal lobe, sending its sensory neurons to the prefrontal cortex. This stream is also known as the ventral stream and the vision-to-perception pathway. The “What” system operates along the junction of the occipital and temporal lobes, primarily involved in object recognition. The “Where” system operates along the junction of the occipital and parietal lobes, primarily involved in localizing information. Interconnected in parallel with the “What” or ventral pathway running from V1 to the temporal lobe is the dorsal stream, also known as the “vision-to-action pathway,” blending “Where” and “How” on its journey from V1 to the parietal lobe. Image: BY LEONARD J. PRESS, O.D., FAAO, FCOVD
However, once learning is consolidated, the participation of the posterolateral superior visual areas is no longer necessary, and the control of learned behavior depends exclusively on the changes that have occurred within the ventrolateral geniculate nucleus.
These changes in the ventrolateral geniculate nucleus are the result of neural plasticity, a process in which the brain's circuits adjust based on experience.
The researchers found that, throughout learning, neurons in the ventrolateral geniculate nucleus that receive information from the posterolateral superior visual areas begin to respond more intensely to threatening stimuli.
This happens because these neurons undergo a specific type of modification: their inhibitory connections become weaker over time due to the action of endocannabinoids, chemicals that act in the communication between neurons.
With this long-term suppression of inhibitory signals, the response of the ventrolateral geniculate nucleus to visual threats is adjusted, allowing for more refined control of instinctual reactions.
This study reinforces the idea that the sensory cortex plays an active role in regulating the body's automatic responses. Rather than just processing visual information, it can also modify instinctive behaviors by influencing subcortical brain circuits.
The discovery of this mechanism of subcortical plasticity reveals new insights into how the brain learns to regulate emotions and defensive behaviors, which may have important implications for the study of fear- and anxiety-related disorders.
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Overwriting an instinct: Visual cortex instructs learning to suppress fear responses
SARA MEDEROS, PATTY BLAKELY, NICOLE VISSERS, CLAUDIA CLOPATH, AND SONJA B. HOFER
SCIENCE, 6 Feb 2025, Vol 387, Issue 6734, pp. 682-68 (2025)
DOI:10.1126/science.adr2247
Abstract:
Fast instinctive responses to environmental stimuli can be crucial for survival but are not always optimal. Animals can adapt their behavior and suppress instinctive reactions, but the neural pathways mediating such ethologically relevant forms of learning remain unclear. We found that posterolateral higher visual areas (plHVAs) are crucial for learning to suppress escapes from innate visual threats through a top-down pathway to the ventrolateral geniculate nucleus (vLGN). plHVAs are no longer necessary after learning; instead, the learned behavior relies on plasticity within vLGN populations that exert inhibitory control over escape responses. vLGN neurons receiving input from plHVAs enhance their responses to visual threat stimuli during learning through endocannabinoid-mediated long-term suppression of their inhibitory inputs. We thus reveal the detailed circuit, cellular, and synaptic mechanisms underlying experience-dependent suppression of fear responses.
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