Caroline Williams reported in the Oct Australian New Scientist Magazine (2015). According to Ms Williams, Russel Hurlburt of the University of Nevada, Las Vegas, has sampled individual trains of thought since the 1970's. To date he has discovered very few "big" thoughts as a result of his sampling studies, which consisted of beeping people six times a day to write down their thoughts. The outcome of his research suggests there are five basic modes of thinking.
Hearing words in your own voice.
Seeing a visual image in your mind's eye
Any strong emotion
A sensory signal that overwhelms all else. Can be imagined e.g. thinking of something soft
A concept unattached to any mental words or images.