Source: gamera_obscura CC BY-NC 2.0

You take in lots of information about the world through your sensory systems. Your eyes detect light bouncing off surfaces. Your ears are sensitive to sound waves in the local environment. Sensory organs on your skin detect heat, touch, and pain. Sensors in your muscles detect the degree of muscle strain required to hold and lift objects.

Even though you have these separate sources of information, you perceive the world in a unified way. You quickly integrate information across various sources to get a sense of what is going on in the world. Indeed, mismatches among sources of information can be jarring, as when the soundtrack of a movie is slightly desynchronized from the audio and the words spoken by characters do not match the movement of their mouths. This situation is annoying because when you perceive speech, you actually combine information from what you hear with visual information of people speaking.

An interesting place where this integration of information happens is when you judge the weight of an object you are lifting. As you might expect, when you lift an object with your arms, sensors that detect the tension in your muscles provide information about the weight of the object. However, you also make judgments about the weight of an object from aspects of what it looks like, such as its size or the material you think it is made of. You use that information to gauge how much effort to put in when starting to lift an object.

You also combine that information when judging the overall weight of an object. An interesting observation is the size-weight illusion, in which people lift a small object and a large object that have the same weight. They judge the small object to be heavier than the large object. Presumably, this reflects that people generally feel that small objects should require less effort to lift than large objects. So, a larger-than-expected amount of muscle tension needed to lift the small object leads to a judgment that it is heavier than the large object.

An interesting question explored in a paper in the June 2019 issue of Psychological Science by Myrthe Plaisier, Irene Kuling, Eli Brenner, and Joren Smeets is when this visual information needs to be available in order to get this illusion.

In these studies, participants had to lift an object. The object was a tall or a short box. The boxes were constructed with weights inside so that one of the tall and short boxes was heavy (but the two heavy boxes had the same weight) and one of the tall and short boxes was light (but the two light boxes had the same weight). The objects were constructed so that participants couldn’t figure out whether the box was tall or short just by lifting it.

There were several experiments, but in the most clever version, participants wore special goggles that prevented people from seeing most of the time but would open for one-fifth of a second (200 milliseconds) at some point during the process of lifting the object.

The experimenter placed the object in front of the participant and guided the participant’s hand to the handle of the object. Participants were given a signal, after which they had to lift the object. On each trial, they got a glimpse of the object that came before the signal to lift, as they were starting to lift the object, or well into the lift. After lifting the object and putting it down, they estimated how heavy the object was.

The researchers were particularly interested in when they would get the size-weight illusion, because that would be an indication that participants had integrated what the object felt like when lifting it with information about what it looked like.

The pattern of results is interesting. If participants got a glimpse of the object before they started lifting, then there was a size-weight illusion. That is, the smaller object was judged to be heavier than the larger object with the same weight.

If participants got a glimpse of the object well after they started lifting, then the size-weight illusion went away. That is, eventually, the visual information did not influence the judgment of weight. However, if the glimpse came within the first few hundred milliseconds after they started lifting, then the size-weight illusion was still there.

These results suggest that people are combining visual information with what it feels like to lift something for about a quarter of a second to make a judgment about how heavy it is. As long as people get some visual information before the lift or just afterward, then the visual information affects judgments of weight.

It may seem strange that visual information affects judgments of weight after you already have information about how much something weighs. But the purpose of our senses is not to give us a readout of the properties of the world. That is why we developed devices like scales to give us accurate measurements of the world.

Instead, our sensory system provides us with information we need to act on the world. We care about how heavy things are because it influences how much force we need to generate with our muscles to move or lift the object. It also influences the kind of work we might have to do to stay balanced while holding the object. The better the job we can do of predicting what we will have to do to lift and hold something, the more likely we are to be successful when we pick things up.

And it is particularly interesting that our sense of the weight of an object develops over time.