What happens to our brains when we watch videos at faster speeds than normal?

Many of us have gotten into the habit of listening to podcasts, audiobooks, and other online content at faster playback speeds. For younger people, it may even be the norm. For example, a survey of California students found that 89% changed the playback speed of online lectures, while numerous media articles have appeared about how fast-paced viewing has become.

It's easy to think about the advantages of viewing things faster. It allows us to consume more content in the same amount of time or review the same content multiple times to get the most out of it.

This could be especially useful in an educational context, where it could free up time to consolidate knowledge, take practice tests, etc. Watching videos quickly is also potentially a good way to ensure you maintain attention and interest throughout the duration, thus preventing your mind from wandering.

But what about the downsides? It turns out there are more than one.

When a person is exposed to oral information, researchers distinguish three phases of memory : encoding the information, storing it, and then retrieving it. During the encoding phase, the brain needs some time to process and understand the stream of words it receives. The words must be extracted, and their contextual meaning must be retrieved from memory in real time.

People typically speak at a speed of about 150 words per minute, although doubling that speed to 300 or even tripling it to 450 words per minute is still within the range of what we might consider intelligible. The issue is more about the quality and longevity of the memories we form.

Incoming information is temporarily stored in a memory system called working memory . This allows fragments of information to be transformed, combined, and manipulated until they reach a form ready to be transferred to long-term memory. Since our working memory has a limited capacity, if too much information arrives too quickly, it can overflow. This leads to cognitive overload and information loss.

Quick display and retrieval of information

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

Similar to a randomized controlled trial used to test medical treatments, participants were randomly assigned to each of two groups. Both groups then took an identical test after watching the video to assess their knowledge of the material. The tests consisted of recalling information, answering multiple-choice questions to assess their recall ability, or both.

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

To put this into context, if the average score for a group of students was 75%, with a typical variance of 20 percentage points in either direction, increasing the playback speed to 1.5x would reduce each person's average score by 2 percentage points. And increasing the speed to 2.5x would result in an average loss of 17 percentage points.

Age matters

Interestingly, one of the studies included in the meta-analysis also studied 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 individuals, suggesting that older adults should view content at normal speed or even at slower playback speeds to compensate.

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

Another unknown is whether watching videos at higher playback speeds has long-term effects on mental function and brain activity. In theory, these effects could be positive, such as an increased 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 scientific evidence to answer this question.

One final observation is that even if playing content at, say, 1.5 times normal speed doesn't affect memory performance, there is evidence to suggest that the experience is less enjoyable. This may affect people's motivation and experience when learning things, which could lead them to find more excuses not to do so. On the other hand, faster playback has become popular, so perhaps, once people get used to it, there won't be a problem. Hopefully, in the coming years we'll have a better understanding of these processes.