HDBR and ISS: What They Are and Why They Matter in Neuroscience Research

What is HDBR?

HDBR stands for Head-Down Tilt Bed Rest.

It is a ground-based experimental model widely used in space and neuroscience research to simulate some physiological effects of microgravity on the human body.


HDBR and ISS - What They Are and Why They Matter in Neuroscience Research

How HDBR works:

• Participants remain in bed for days or months with the body tilted about −6°, with the head lower than the feet.

• This position induces a cephalad fluid shift (blood and cerebrospinal fluid moving toward the head), similar to what happens in space.

What HDBR models well:

• Fluid redistribution

• Cardiovascular adaptations

• Muscle and bone deconditioning

• Changes in sleep, autonomic balance, and internal bodily state (interoception)

What HDBR does not fully model:

• Real microgravity

• Continuous sensorimotor adaptation

• Dynamic proprioceptive and motor control without body support

In BrainLatam terms, HDBR strongly affects Tekoha (extended interoception and internal regulation), but only weakly challenges APUS (extended proprioception and spatial movement).

What is the ISS?

ISS stands for International Space Station.

It is an orbital laboratory operating in real microgravity, where astronauts live and work for long durations, typically around six months, in low Earth orbit (~400 km).

What characterizes the ISS environment:

• Continuous microgravity (not zero gravity, but permanent free fall)

• The body is in a free-floating condition, without stable postural support

• Strong decoupling of gravitational input from vestibular and proprioceptive systems

• Constant need for motor planning, stabilization, and spatial orientation

Neurophysiologically, the ISS environment:

• Forces continuous sensorimotor recalibration

• Alters vestibular, somatosensory, and visual integration

• Demands active adaptation of movement, posture, and coordination

In BrainLatam terms, the ISS directly challenges APUS (extended proprioception), while also impacting Tekoha (internal regulation) in a dynamic and task-dependent manner.

Why the distinction matters for neuroscience

HDBR and the ISS are often treated as interchangeable models of microgravity, but they are not equivalent.

• HDBR primarily simulates a passive internal physiological state, characterized by immobility and reduced sensorimotor demands.

• ISS microgravity represents an active adaptive state, where the brain must continuously reorganize motor control and proprioceptive mapping in real space.

This distinction helps explain why EEG studies often find:

• Increased delta/theta activity in HDBR, consistent with reduced sensorimotor engagement and altered internal regulation.

• Increased beta activity in ISS studies, especially in somatosensory and motor regions, consistent with ongoing proprioceptive and motor adaptation.

BrainLatam synthesis

HDBR models internal shifts.
The ISS reveals embodied adaptation.

Both are valuable, but they answer different neuroscientific questions. Understanding their limits is essential when interpreting brain data related to microgravity, movement, and regulation.

The body does not adapt only by resting.
It adapts by moving, sensing, and reorganizing itself in space.

Re.:

‌Sevilla-García, M., Quivira-Lopesino, A., Cuesta, P., Pusil, S., Bruña, R., Fiedler, P., Maestu, F., Cebolla, A. M., Cheron, G., Brauns, K., Stahn, A. C., & Funke, M. E. (2025). Brain power comparison between microgravity and head-down tilt bed rest: an electroencephalography approach. Scientific Reports, 15(1). https://doi.org/10.1038/s41598-025-26291-8