What happens to your brain when you watch videos online at faster speeds than normal

Many of us have got into the habit of listening to podcasts, audiobooks and other online content at increased playback speeds. For younger people, it might even be the norm. One survey of students in California, for instance, showed that 89% changed the playback speed of online lectures, while there have been numerous articles in the media about how common speedy viewing has become.

It is easy to think of some advantages to watching things more quickly. It can let you consume more content in the same amount of time, or go through the same piece of content a couple of times to get the most out of it.

This could be particularly useful in an educational context, where it might free up time for consolidating knowledge, doing practice tests and so forth. Watching quickly is also potentially a good way of making sure you sustain your attention and engagement for the entire duration to avoid the mind wandering.

But what about the disadvantages? It turns out that there are one or two of those as well.

When a person is exposed to spoken information, researchers distinguish three phases of memory: encoding the information, storing it and subsequently retrieving it. At the encoding phase, it takes the brain some time to process and comprehend the incoming speech-stream. Words must be extracted and their contextual meaning retrieved from the memory in real-time.

People generally speak at a rate of about 150 words per minute, though doubling the rate to 300 or even tripling it to 450 words per minute is still within the range of what we can find intelligible. The question is more about the quality and longevity of the memories that we form.

Incoming information is stored temporarily in a memory system called working memory. This allows chunks of information to be transformed, combined and manipulated into a form that is ready for transfer to the long-term memory. Because our working memory has a limited capacity, if too much information arrives too quickly it can be exceeded. This leads to cognitive overload and loss of information.

Speedy viewing and information recall

A recent meta analysis in this area examined 24 studies of learning from lecture videos. The studies varied in their design but generally involved playing a video lecture to one group at original speed (1x) and playing the same video lecture to another group at a faster speed (1.25x, 1.5x, 2x and 2.5x).

Just like in a randomised controlled trial used to test medical treatments, participants were randomly assigned to each of the two groups. Both groups then completed an identical test after watching the video to assess their knowledge of the material. The tests either required them to recall information, used multiple choice questions to assess their recall, or both.

The meta-analysis showed that increasing playback speed had increasingly negative effects on test performance. At speeds of up to 1.5x, the cost was very small. But at 2x and above, the negative effect was moderate to large.

To put this in context, if the average score for a cohort of students was 75% with a typical variation of 20 percentage points in either direction, then increasing the playback speed to 1.5x would bring down the average person’s result by 2 percentage points. And increasing the playback speed to 2.5x would lead to an average loss of 17 percentage points.

Older people

Interestingly, one of the studies included in the meta-analysis also investigated older adults (aged 61-94) and found that they were more affected by watching content at faster speeds than younger adults (aged 18-36). This may reflect a weakening of memory capacity in otherwise healthy people, suggesting that older adults should watch at normal speed or even slower playback speeds to compensate.

However, we don’t yet know whether you can reduce the negative effects of fast playback by doing it regularly. So it could be that younger adults simply have more experience of fast playback and are therefore better able to cope with the increased cognitive load. Similarly, it means we don’t know whether younger people can mitigate the negative effects on their ability to retain information by using faster playback more often.

Another unknown is whether there are any long-term effects on mental function and brain activity from watching videos at increased playback speeds. In theory, such effects could be positive, such as a better ability to handle increased cognitive load. Or they could be negative, such as greater mental fatigue resulting from increased cognitive load, but we currently lack the scientific evidence to answer this question.

A final observation is that even if playing back content at, say, 1.5 times the normal speed doesn’t affect memory performance, there is evidence to suggest the experience is less enjoyable. That may affect people’s motivation and experience at learning things, which might make them find more excuses not to do it. On the other hand, faster playback has become popular, so maybe once people get used to it, it’s fine – hopefully we’ll understand these processes better in the years to come.

Marcus Pearce does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.