The uncanny valley phenomenon, how our brain detects artificiality

The uncanny valley is a concept that describes the unsettling feeling that people experience when they encounter humanoid robots, that have subtle, barely perceptible non-human features.  

These creations may appear human-like, but small imperfections in their appearance or movements make them look disturbing and sometimes terrifying.
The uncanny valley concept can be extended to language. 

A story written by an artificial intelligence program may look and feel like any story written by a human author, but when reading it, something about it feels awry and  out of place. This makes the reader feel a sense of unease and discomfort. 

One of the characteristics of this "robotic" writing is its aseptic, over-polished nature that feel soulless and impersonal. It lacks human warmth, emotion.

Human writing often contains peculiar phrasings, unusual word combinations, and unexpected twists. 

These quirks give the author its unmistakable signature.

The uncanny valley becomes even more evident when an artificial intelligence program attempts to depict tight human bonds or emotional states. 

Grief, loneliness, love or melancholy feel dry, detached and dull. 

An artificial intelligence program can spawn language that accurately describes emotions and feelings but the reader will sense a void and a lack of substance behind the words. 
The closer an artificial intelligence program story comes to mimicking genuine human writing, the more noticeable the uncanny valley phenomenon becomes. 

A weird metaphor, an overly emotional description can illicit in us a slight feeling of revulsion due to the dishonest and deceptive nature of the account. 

We feel duped and misled, and we start to see the algorithm behind the plot manufacturing patterns and artificial phrases.

Human writing is not the result of sophisticated vocabulary and grammatical accuracy, it is an ordered, sophisticated chaos comprised of emotions, strife, and unexpected twists.