Spanish researchers have mapped several direct pathways between the thalamus and the amygdala, brain regions believed to be involved in rapid responses to threat. The work was published in the journal NeuroImage. The thalamus is a central hub that receives signals from the senses, while the amygdala is responsible for emotional learning and defensive reactions.
In animals, such short routes have already been described: they allow an alarm signal to reach the amygdala without passing through the cerebral cortex, where more detailed analysis takes place. These pathways are sometimes called "low roads." In humans, it had not yet been possible to assemble them into a single map.
A group led by Emanuela Costeletu-Cassotaki of the University of Barcelona examined 113 healthy right-handed people (mean age 24.5 years, 65 women). The scientists used high-resolution diffusion-weighted MRI, a method that estimates the direction of nerve fibers from the movement of water in brain tissue. Then, using tractography, that is, reconstructing probable fiber pathways from these data, they built connections between several thalamic nuclei and the basolateral part of the amygdala, which is key for fear learning.
In 24 participants, a repeat scan was performed about 15 days later to check whether the pathways were reproduced in a new examination.
The most reliable connections ran from the posterior part of the thalamus to the basolateral amygdala. One pathway began in the inferior colliculus (the auditory center of the midbrain), passed through the medial geniculate nucleus, an important auditory node of the thalamus, and reached the amygdala. This resembles the auditory fear pathway well studied in rodents; in humans it was found in almost all participants.
Connections between the amygdala and the medial, inferior and lateral parts of the thalamic pulvinar (a structure involved in visual attention and the integration of signals from different senses) were found in all 113 people and confirmed on repeat scanning, especially for the inferior part. Connections with the anterior part of the pulvinar, however, were rare and unstable. A direct pathway from the inferior colliculus to the amygdala bypassing the medial geniculate nucleus could not be reliably reconstructed.
A notable connection was also identified between the mediodorsal nucleus (in the middle part of the thalamus) and the basolateral amygdala. However, the authors caution that signals along this pathway may mainly travel from the amygdala rather than to it.
Connections from the intralaminar thalamic nuclei, especially the parafascicular and central medial nuclei, were also found. But they varied more between people and between the two scans. This suggests that some pathways are stable, while others may be weaker, harder to detect or more variable in anatomy.
Different areas of the thalamus connected to different parts of the basolateral amygdala, and some pathways were more common in the left hemisphere than in the right. The authors warn, however, that this asymmetry may partly be explained by the method's characteristics.
Overall, the results support the idea that humans have several direct pathways between the thalamus and the amygdala, rather than one simple route. They may quickly transmit sensory and emotional signals, helping to respond flexibly to danger. The authors describe their work as "a unifying anatomical basis for multiple direct thalamo-amygdalar pathways in the human brain."
The method has limitations. Tractography may falsely show pathways that do not exist or fail to detect existing ones, especially where fibers cross, so the connections need to be confirmed by studies of brain tissue. The method also does not show the direction of signals and provides information about structure rather than real-time brain activity, so the link to fear is largely inferred from animal experiments.