New research from Fujita Health University reveals that speech can subtly delay the eyes’ ability to detect and stabilize visual information. In experiments comparing speaking, listening, and control conditions, only speech produced slower reaction, movement, and fixation times during rapid eye movement tasks. Because driving is highly dependent on rapid shifts of gaze, these delays can affect hazard detection and delay physical responses. The findings highlight the hidden dangers of engaging in conversations while driving.
Talking while driving is widely recognized as a major source of distraction, but the specific ways in which conversation interferes with the early stages of visual processing have remained largely unclear. While previous research has shown that cognitive distraction can slow braking or reduce situational awareness, the question of whether speech disrupts fundamental gaze processes that precede bodily responses has remained unanswered.
Now, researchers from Fujita Health University have shown that speaking imposes a cognitive load strong enough to delay basic eye movement responses, potentially affecting the rapid visual assessments needed for safe driving. A study led by Associate Professor Shintaro Uehara and team, including Mr Takuya Suzuki and Professor Takaji Suzuki, published online on October 6, 2025, at PLOS ONEexamined how speech changes the temporal dynamics of gaze behavior.
Gaze behavior is particularly important because approximately 90% of the information used for driving is acquired visually. Any delay in the initiation or completion of eye movements can result in slower recognition of hazards, reduced accuracy of visual scanning, and delayed motor responses. “We investigated whether the effect of speech-related cognitive load on gaze behavior varies with eye movement direction.” explains Dr. Uehara.
To investigate this, researchers asked 30 healthy adults to perform center-out rapid eye movement tasks under three different conditions: speaking, listening, and a no-task control. Participants were instructed to look as quickly and accurately as possible toward a peripheral visual target presented in one of eight directions. In the speaking condition, participants answered general knowledge and episodic questions adapted from the Wechsler Adult Intelligence Scale and additional adapted prompts. In listening conditions, participants heard excerpts from the Japanese novel I’m a cat. The order of conditions was randomized on three separate days. Across all participants, speech produced clear and consistent delays in three key temporal components of gaze behavior: the time required to initiate an eye movement after target appearance (reaction time), the time required to reach the target (movement time), and the time required to fixate gaze on the target (fixation time). None of these effects were observed during the listening or control conditions, suggesting that the act of speaking and the cognitive effort required to search for and produce verbal responses create substantial interference with gaze control mechanisms.
These delays seem small in isolation, but during driving, they can accumulate into slower hazard detection and delayed initiation of physical reactions. Even hands-free conversations can introduce a cognitive load strong enough to interfere with the neural processes that initiate and guide eye movements. Because drivers often need to look down at pedestrians, debris, or objects in the road, these delays highlight the major risks of conversing during visually demanding driving scenarios.
The authors note that their findings do not imply that speech is the sole or dominant cause of slowing down of bodily reactions behind the wheel. Driving performance is affected by many cognitive and perceptual factors, including inattentional blindness, divided attention, and the broader interference that occurs when the brain is forced to manage two demanding tasks simultaneously. Even so, the study shows that speech introduces delays in the early stage of visual processing before recognition, decision-making or physical action, meaning it can quietly undermine driving performance in ways that are not immediately obvious to the drivers themselves. “These results show that the cognitive demands associated with speech interfere with the neural mechanisms responsible for the initiation and control of eye movements, which represent the critical first stage of visuomotor processing during driving.” concludes Dr. Uehara.
This information has significant implications for public safety. By understanding that the cognitive effort involved in conversation can degrade gaze accuracy and timing, drivers may become more mindful of when and how they choose to speak while driving. Over time, this knowledge could support safer driving behaviors, inform driver education frameworks, inspire improvements in vehicle interface design, and guide policymakers in formulating future recommendations about cognitive distraction.
