What causes words repeated in your head to start fading away from consciousness?

Question

Background: I tried to do a self-control exercise that involved doing something boring like folding clothes while the exercise part involved repeatedly saying "focus" in my head, to stop daydreaming. However, after 15 minutes, my internal voice suddenly became weaker and weaker until it stopped to echo in my head. I tried to scream "focus" inside my head and it felt like moving a hand while in sleep paralysis, nothing echoed, while my front head (not forehead) felt weird. I am confused as knowing nothing about what actually happened and why.

Off-Question Notes: I have ADD, that sometimes something feels so boring I can't focus on it (The purpose of the exercise), but it doesn't seem to be the case, because I couldn't say anything else in my head nor think.

Question

What is the cause of the phenomena whereby repeating words inside your head begin to fade from consciousness?

Answer 1

As @Steven Jeuris has said, the phenomena is best known as semantic satiation. It's not as popular a topic of study as it used to be (most references I can find for it come from the 1960s), but, to the best of my memory, the actual cause of the phenomena is down to how meaning is represented in the brain.

I'll explain this by loosely paraphrasing an account of the phenomena in Michael Spivey's book, The Continuity of Mind, but there are some (interesting) theoretical claims in that book which I won't bother with for the scope of this answer.

The general consensus on neural representations of meaning is that semantic information (i.e. the meaning of a word, although this can include much more than that) is encoded as a pattern of activation across many neurons. Exposure any given word (let's use giraffe as our example, following Spivey) causes that specific pattern to become partially activated, and so we experience the "meaning" of the word - the more accurately the pattern is recreated, the more intense our conscious experience of the meaning.

However, individual neurons are subject to adaptation, or fatigue (Wikipedia): if a particular neuron has been firing for a prolonged period of time, it begins to weaken, and it's firing rate slows, until it has time to recover (think of a drummer's arm muscles, performing the same action repeatedly until he tires out). When you repeat a specific word (giraffe), the neurons used in the corresponding pattern become fatigued in this way, and so the corresponding meaning ("long-necked African mammal") fades, or is replaced by other, possibly related meanings.

_{Spivey, M. (2007). The continuity of mind. Oxford University Press.}

Answer 2

If you use a slightly different (and to my own experience more common) phrasing of this phenomenon to look for information you find immediate relevant results on Google. Rather than referring to words fading from consciousness I've usually heard it being stated as repeating a word often makes it lose its meaning.

The first hit is a Wikipedia article on semantic satiation.

Semantic satiation (also semantic saturation) is a psychological phenomenon in which repetition causes a word or phrase to temporarily lose meaning for the listener, who then perceives the speech as repeated meaningless sounds.