Humans spend a considerable portion of their lives engaged in ‘stimulus-independent thoughts' (SIT), or mental activity that occurs independently of input from the immediate external environment. Although such SITs are, by definition, different from thoughts that are driven by stimuli in one's external environment (i.e. stimulus-dependent thoughts; SDTs), at times, the phenomenology of these two types of thought appears to be deceptively similar. But how similar are they? We address this question by comparing the content of two types of SIT (dreaming and waking SITs) with the content of SDTs. In this 7 day, smartphone-based experience-sampling procedure, participants were intermittently probed during the day and night to indicate whether their current thoughts were stimulus dependent or stimulus independent. They then responded to content-based items indexing the qualitative aspects of their experience (e.g. My thoughts were jumping from topic to topic). Results indicate substantial distinctiveness between these three types of thought: significant differences between at least two of the three mental states were found across every measured variable. Implications are discussed. This article is part of the theme issue ‘Offline perception: voluntary and spontaneous perceptual experiences without matching external stimulation’.

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Humans spend a considerable portion of their lives engaged in “stimulus-independent thoughts”(SIT), or mental activity that occurs independently of input from the immediate externalenvironment. Although such SITs are, by definition, different from thoughts that are driven bystimuli in one’s external environment (i.e., stimulus-dependent thoughts; SDTs), at times, thephenomenology of these two types of thought appears to be deceptively similar. But how similarare they? We address this question by comparing the content of two types of SIT (dreaming andwaking stimulus-independent thoughts) with the content of SDTs. In this seven-day, smartphonebasedexperience-sampling procedure, participants were intermittently probed during the day andnight to indicate whether their current thoughts were stimulus dependent or stimulus independent.They then responded to content-based items indexing the qualitative aspects of their experience(e.g., My thoughts were jumping from topic to topic). Results indicate substantial distinctivenessbetween these three types of thought: significant differences between at least two of the three traitswere found across every measured variable.

No abstract