The “uncanny valley” refers to the discomfort brought on by seeing human image replicated artificially or incorrectly. While there are many theories as to why this is, the consensus seems to be that it's easy to tell if something seems fake and there can be a negative emotional response. The problem of the uncanny valley is usually found in media content with computer-generated characters or environments.

Video games and movies have historically had trouble trying to get an audience emotionally invested in a computer generated character whose movements call attention to the fact they are real because they aren’t subtle (like blinking in the wrong ways). Perhaps one of the most famous examples of this problem is “The Polar Express” (2004) with CGI characters that move strangely and whose emotions in their voice don’t always match their physical expressions or actions.

Technology has since gotten better. “Avatar” (2009) is considered by many critics to be a watermark for how motion capture and world building can be done. But, the success of CGI humanoid characters in movies depends on the quality of their creation and the ability of the audience to suspend disbelief of their existence (due to them being mostly in fantasy roles), which ranges across the board in terms of who puts more effort into the process. Films like “Avatar” (2009) are successful because they are constructed based on the technology. However, seven years later “Suicide Squad” brings The Enchantress and her brother, both of whom are constructed by CGI so bad it cannot be taken seriously. Maybe the point was to make The Enchantress look inhuman, but it comes off as unnatural not in a sinister way, but in a ridiculous one. The recent “Dr. Strange” which has technically impressive effects in some areas, but has a main villain who is pretty much just a giant CGI face that is not too great to look at and takes away any menace. The point is that the audience is taken out of their emersion to realize that there is something unnatural in the character's technical construction and not the fact that there is a somewhat human-looking alien.

Internet surrealist art uses low-tech CGI to take advantage of the unnatural look it creates. Some have used robots like in the “I Feel Fantastic” video where Tara the Android sings in a broken Microsoft Sam voice to produce the effect of unreality that is disturbing to the audience for reasons that they can only “feel.” The Brothers Quay, early pioneers of the method before the creation of the Internet, use dolls with pieces missing and jerky movements in stop motion for a similar effect.

So probably the reasons for negative perceptions in the uncanny valley is that it makes the human image into an Other. Movies that incorporate CGI are successful when they build the world around those characters (many times in movies with bad CGI there is just an actor wandering around a green screen with their mouth open in wonderment at all the “aliens” they are being directed to see). It also takes a high degree of skill and technology in a director to be able to create these kinds of characters and to have their actions make sense in relation to the real actors. It is ironic that science fiction writer Philip K. Dick who wrote about the potential problems of human interaction with androids has been turned into a into one by a robotics company. While there is some nostalgic emotional reaction for his readers, in general, the robot’s actions don’t inspire confidence that it can pass for a real person. The androids in “Blade Runner” (1982) are played by human actors with no CGI enhancement, perhaps because director Ridley Scott realized the audience’s emotional connection to them necessary for the plot depended on the discovery that there is something human in the androids, negating the uncanny valley's effects.