Scientists still argue if the emergence of new neurons, neurogenesis, takes place in adult human brain but they all agree that the opposite process, neurodegeneration, happens for different reasons, including aging. More and more scientists emerge, however, which demonstrate with their experiments and research that both processes are constantly reshaping human brain across life span in a process of brain’s biological adaptation to the environment.

Healthy human brain uses two different strategies in navigating physical places and mental models (maps) on competitive but complementary basis. Response strategy comprises sequences of short loops of single stimuli and instant response. Spatial strategy comprises longer loops of a number of linked stimuli leading to a delayed response. Functional magnetic resonance imaging confirms that people who use spatial memory strategies showed increased activity in the hippocampus, whereas response strategies are associated with activity in the caudate nucleus.

Furthermore, goal-directed and habit-based behaviors are driven by different decision making systems involving the hippocampus and caudate nucleus. On repetitive tasks, behavior transitions from goal-directed to habit-based with experience. Hippocampus has been implicated in initial learning and caudate nucleus in automating behavior.

Human brain selects different learning strategies in conformity with Bayesian probabilistic reasoning. According to it, brain initiates explorative behavior whenever it finds available information insufficient or not credible. In order to perform relevant computations brain encodes separately three levels of uncertainty: expected uncertainty (risk), estimation uncertainty (credibility), unexpected uncertainty (prediction error, surprise). Surprise and credibility both modulate exploration.

Brain uses spatial learning and episodic memory systems to generate, encode, use and update its model of the world. Spatial learning once used to navigate places using landmarks is now used to store and retrieve knowledge using values and notions as landmarks.

Episodic memory sets up scenes, dynamically encodes narratives, and networks of items, places and events, which occur in our life and are emotionally accented to be remembered as individual experiences.

Surprising events are emotionally accented because brain rewards prediction error with dopamine. Brain is learning by surprise. This learning is vital for survival. Thus it should be well rewarded. The emergence of individuality also requires environments enriched with surprise.

As you can see, many vitally important cognitive functions are supported by brain networks moderated by hippocampus. As muscles, anatomical areas of the brain which support these functions depend on workout. Physical volume of gray matter in hippocampus grows when we use it, and shrinks when we don’t. It has been confirmed many times by fMRI imaging of human brains.

Automation of human behaviour in a predictable environment is a biologically adaptive mechanism that permits our brain to free up cognitive resources, in particular, hippocampus for more computationally expensive cognitive functions. This drive toward efficiency may be an important underlying factor behind the shift in strategies. With the repetition of successful behavior, a response strategy emerges, leading to the automatization of behavior or habit formation.

This shift toward response strategies comes at a cost. Less engaged hippocampus leads to decreased hippocampal gray matter. As atop of many other functions hippocampus performs also a decision-making role in the event of conflicts, when it shrinks below a certain threshold brain loses the ability to make value based decisions or to switch to goal-oriented behaviour.

In 2012 a group of researchers from the Centre for Cognitive and Neural Systems of the University of Edinburgh and the German Center for Neurodegenerative Diseases discovered a strategy switching deficit exhibited by old adults compared to young adults. Old adults clearly demonstrated one particular difficulty in switching from response (egocentric, model free) learning strategy to spatial (allocentric, model based) learning strategy while they had no difficulty in switching in reverse order. They couldn’t specify the exact cause of this deficit but were surprised by the fact that only one way switching from response strategy to spatial strategy was affected. Participants of the experiment were able to easily switch on response strategy but failed to switch it to spatial strategy when the environment required it from them for successful achievement of their goals. It was not age but long successful experience in a predictable environment that lead to such a one-sided deficit.

After a long and successful life in a predictable environment, response strategy and habits hijack the entire brain. A human machine emerges. Deprivation of surprise doesn’t only make older people comfortably numb but also hinders the emergence of individuality in children and teens. If nothing will be done their dehumanisation will advance faster. Is it how the end of human species will happen?