In recent years, reading and studying on electronic devices has become more common. Although electronic devices have benefited mankind tremendously, they cause eyestrain and headaches^1,2 and lead to poor reading comprehension^3,4. The link between visual environment and cognitive performance has been reported in basic research^5,6. The decline in comprehension when reading from an electric device might be due to poor concentration levels or different sensory processing circuits, which might be associated with physiological states, including brain and physiological activity levels. Even if the content of the text is the same, reading comprehension may be different depending on the visual context. Because vision has a dominant influence on other senses when it comes to sensory integration or cross-modality^7,8,9, visual input might also affect brain state and physiological condition. As such, brain activity and physiological changes likely exist as mediating variables in the relationship between the visual environment and cognitive performance.

We focused on respiration and brain activity as potential mediators. Previous research indicates that overactivity in the brain is associated with poor narrative content comprehension^10,11,12. Many studies have reported associations between respiration and various cognitive functions. In these studies, a reciprocal relationship between respiration and cognitive function was found. Respiration is affected by cognitive load^13,14 or emotion (stress and anxiety)¹⁵, which alters the depth and rhythm of breathing. Conversely, respiratory patterns affect frontal cortex and hippocampus functions and, consequently, the formation of memories^16,17,18. Attention to breathing has also been shown to enhance memory performance¹⁹. Furthermore, brain activity is entrained to the respiratory cycle²⁰. However, no studies have examined the relationship between the visual environment and respiration. According to inter-sensory integration mechanisms, it is known that visual information can alter information from other senses (touch, taste, smell, and auditory). However, information from other senses rarely affects visual sense^7,8,9. These findings indicate that vision is dominant over other senses, and it is quite possible that visual environments influence various functions including that of the respiratory system.

Given the findings that the use of digital devices lowers cognitive performance^3,4, it is probable that the visual environment affects cognitive performance. Regarding cascade processes between the visual environment and cognitive performance, the relationships between the visual environment and brain function^7,8,9; brain function and cognitive performance^10,11,12; respiration and cognitive performance^16,17,18,19; and respiration and brain function²⁰ have been clearly established. However, it is unclear how the visual environment affects respiration and brain function. Therefore, this study investigated the involved mechanism in an exploratory manner. Specifically, we aimed to explore and demonstrate the relationship between the visual environment and respiration and that between the visual environment and brain function to elucidate the phenomenon postulating that the use of digital devices lowers cognitive performance.

If a particular visual environment negatively affects respiratory function and/or brain function, coupled with the interactive relationship between brain and respiratory activity, it is possible that the said process negatively influences cognitive performance (Supplementary Fig. 1). This exploratory study examined the effects of electronic device use on reading comprehension by measuring brain and respiratory activity simultaneously in 34 healthy individuals.

To investigate the causes for reading comprehension decline when using electronic devices, we prepared a repeated-measured (cross-over) design consisting of four conditions involving combinations of two media (smartphone and paper) and two sentences extracted from different texts (novels A and B; see Supplementary Text). One trial consisted of four sessions: resting state before reading, reading, resting state after reading, and reading test (Fig. 1a). Frontal brain activity (two channels) was measured by functional near-infrared spectroscopy (NIRS), while respiratory activity (six indexes) and metabolism pattern (2 indexes) were measured by a respiratory Aeromonitor (see “Methods”).

Reading score is decreased by using smartphone compared to paper medium. (a) Each trial consisted of four sessions: resting state before reading, reading, resting state after reading, and the reading test. Reading time was unlimited, and two min were spent in the resting state before/after reading. (b) For reading test scores, the main effect of medium was significant, while the main effects in novel and the interaction were not. Scores with the smartphone medium were lower than those with the paper medium in sentences from both novel A and B (*P < 0.05). (c,