The brain stem is where the reticular activating system is found. It is a very old system and so is a very old part of the brain common to lower life forms as well as higher life forms.
See link below:
Reticular Activating System
Damage to the reticular activating system (RAS) in the brainstem can impair consciousness and disrupt the sleep-wake cycle. The RAS plays a crucial role in regulating arousal and alertness by filtering sensory information and sending signals to the cerebral cortex to maintain wakefulness.
The brainstem, particularly the reticular activating system, is responsible for habituation. It filters out repetitive or nonthreatening stimuli, allowing the brain to focus on more important information.
The prefrontal cortex, specifically the dorsolateral prefrontal cortex, plays a key role in helping you stay focused and ignore distractions. This area is responsible for executive functions such as attention, working memory, and cognitive control.
Midbrain, pons, and the medulla oblongata
midbrain.
Reticular Activating System
thalamus reticular activating system
The reticular activating system
The Reticular Activating System (RAS). It controls transitions between sleep and wakefulness, arousal, and attention.
reticular activating system
Damage to the reticular activating system (RAS) in the brainstem can impair consciousness and disrupt the sleep-wake cycle. The RAS plays a crucial role in regulating arousal and alertness by filtering sensory information and sending signals to the cerebral cortex to maintain wakefulness.
The reticular formation is a network of neurons located in the brainstem. It is involved in regulating arousal, attention, and sleep-wake cycles.
The brainstem, particularly the reticular activating system, is responsible for habituation. It filters out repetitive or nonthreatening stimuli, allowing the brain to focus on more important information.
Part of the mid-brain, it sends impulses to the cerebral cortex to keep it conscious and alert. It is also important for sleep and arousal.
Sleeping and waking depend on activities in the brain stem. Several areas of the brain seem to be involved in dreams and REM sleep, including the hippocamus and cortex. See attached links.
This is due in part to your reticular activating system - part of your brain - which alerts you to things (audio & visual) that may be dangerous. (It also does other things, too!)Your reticular activating system is listening out for dangerous sounds, and recognises an oncoming train as being potentially dangerous. Once the danger has passed, and nothing bad has happened, your reticular activating system is 'un-switched', and your brain doesn't need to hear it anymore, hence trains sound different moving away.The same is true of emergency vehicle sirens - if you hear them, you are alerted to them by your reticular activating system, and once they have passed and are no longer a danger, your brain doesn't need to register the siren anymore.I am doubtful but not dismissive of the information above regarding the reticular activating system. Whether or not there is any truth to it, the questioner may be making reference to the Doppler Effect, which has to do with the frequency of the sound waves hitting the ear when a train is approaching in comparison to the frequency when the train is moving away. While the sound waves themselves are moving at the same speed through the air no matter where the train is, the signal waves coming toward us as the train is advancing are compressed, and as a result we will perceive them as being at a higher pitch than when the train is receding, and the waves are less compressed.Its entirely the Doppler Effect and not the brain. The brain has no way to know if a sound is approaching or receding to then make it appear to sound different. Due to the Doppler Effect the actual frequency of approaching and receding sounds is different when they arrive at the ear as the waves are compressed and expanded, respectively.