Imagine you have a small TV on your table, and you are watching an episode of “The Honeymooners” (look it up). Nice. Now imagine a second, identical TV on the table, tuned to the same episode of the show, but on a different channel, so the two shows are out of sync with each other. Imagine what it would be like to try and follow both programs at the same time. Now set the second TV to a different program altogether, and imagine trying to watch both. Imagine trying to watch two different tennis matches on two different Jumbotrons, simultaneously. I could go on…

All of these imagined scenarios should feel unpleasant. And it should not be hard to make the case that under any of these conditions, neither one of the two inputs would get the attention it deserved. Notice the almost irresistible tendency to tune out one source in favor of the other, just to be able to properly attend to something.

This tendency to tune out one source, and this feeling of unpleasantness, is what your audience experiences when they are asked to read one thing (a bunch of text on a slide) and listen to something else (you, speaking) at the same time. If what is being spoken is the same as what is on the slide (i.e., you are reading the slide content verbatim out loud), then it is annoying and unpleasant, but the audience will simply tune out one source (usually the slide) and pay attention to your voice, because your voice requires less effort.

But if you are saying something that is entirely different from what is on the slide—almost inevitable if a slide is left on display while you go on with your talk, your audience will still (for the same reason) give up on the slide and attend to your voice only. But now your audience is ignoring content that you went to a lot of trouble to put in front of them. This is not a preferred outcome, but it is a scenario that is repeated with depressing regularity in presentations everywhere. It bears writing about because it keeps showing up.

The cognitive fact is that for an audience, listening to your voice and reading a slide are both audio inputs. When you read, the words are heard inside your head. Take a look and see if this is not happening even now, as you read this. Putting a slide in front of an audience that they have to read while you continue to speak is like sitting them down at that table with the two television shows Inevitably, one of the shows is sacrificed so that the other can be attended to. Most audiences will ignore the slide so they can follow the talk.

The solution is simple: be aware of where you want to direct your audience’s attention, and don’t unwittingly divide it. If you want a slide to be read, stop talking and let the audience read it. Otherwise, keep any text on a slide short and telegraphic, and set up the content so the audience only has to take in one piece of information at a time. Avoiding this one surprisingly common misstep opens the door to all kinds of improvements in the flow of a presentation and levels of audience comprehension and retention. Which is kind of the point.