The second important result relating to the encoding stage is that some regions of the brain are specifically activated when information to be remembered is successfully encoded; however, this pattern of activations doesn’t take place when successful encoding relates to an item that the subjects were instructed to forget. The regions concerned are the right entorhinal cortex, which is part of the hippocampal complex, a key structure in associative memory, the anterior medial prefrontal cortex and the insula. “The involvement of the hippocampus during the encoding of words to be remembered, which was successfully achieved, shows that the subjects tried to create associations with each of these words and other words in the list or with personal events, thus favouring relatively in-depth encoding”, Christine Bastin points out. “Hence, in such or such a participant, the word “car” may well have been related to the need to change their car’s tyres soon or to pay for their car insurance.”

Richness or familiarity?

Let’s now turn to retrieval. The recognition of words to remember goes hand in hand with an activation of the left posterior hippocampus, the right precuneus, the left inferior parietal regions and the posterior cingulate cortex. “Once again, the hippocampus, and the parietal regions are typically activated when someone remembers every aspect of an episode, i.e., with the prevailing details at the moment of encoding”, Christine Bastin reminds us. “In other words, the episode was sufficiently in-depth to guarantee the retrieval of a memory rich in content.”

When items to be forgotten are nevertheless retrieved – this happens, as we have seen, in 50% of cases -, completely different cerebral regions are activated.  In this case, they are the left dorsomedial thalamus, the right posterior intraparietal sulcus and anterior cingulate cortex. The thalamic region involved belongs to a memory network specifically dedicated to a form of retrieval that neuroscientists call the feeling of familiarity. This is what we experience when we pass by someone and think that we have already seen them, but we are unable to remember their name or the circumstances in which we previously encountered them. In short, we are in the realm of rather vague memories. Furthermore, the anterior cingulate cortex is indissociable from the notion of effort. “Its activation suggests that the subjects who identified the words presented during the encoding phase, but to which the “to be forgotten” instruction was attached, had to make a greater effort to recognise them, probably because they were in greater doubt as to whether these items had been presented before or not", the CRC post-doctoral researcher points out.

According to Fabienne Collette, one of the conclusions of the experiment carried out at the University of Liège is that the phenomenon of directed forgetting is relatively complex, since it brings into play particular brain networks. Furthermore, it isn’t easy to intentionally forget a piece of information. If it was processed on an in-depth level during encoding, it has every chance of reappearing within our memories. Consequently, it would perhaps be ultimately erased from our memory as a result of erosion and interferences (unintentional forgetting).