Sometimes it is difficult to tell what you see. For example, you may wonder whether the person on the other side of the street is your friend Robert. You will continue looking at him to gather more information and at some point will decide that this person indeed is Robert – or not. In this case the perceptual decision “Robert or not” is done by summing up, or accumulating, pieces of information across time.

Models of perceptual decision-making assume that this is how perceptual decision-making works in general. In essence, the assumption is that humans accumulate information even when it is obvious what they are seeing. For example, when Robert stands right in front of them they would still accumulate, but the process would just go a lot faster.

In one study we found that parietal cortex - commonly believed to carry out this process of accumulation - carried less information about perceptual choices when choices were easier, while there was a concurrent increase of information in visual cortex. This finding is in line with a more flexible perceptual decision-making mechanism where accumulation of evidence happens only when it is really necessary. Importantly, the information in parietal cortex was shown to be independent of the hand with which the choice was carried out, i.e. the signal was based on a perceptual decision, rather than on the decision how to respond.