According to a new study, our gut feelings can enhance the retrieval of explicitly encoded memories – those memories which we encode actively – and therefore lead to improved accuracy in simple decisions. The study, which is published online in Nature Neuroscience, also provides evidence that the retrieval of explicit and implicit memories involves distinct neural substrates and mechanisms.
The distinction between explicit and implicit memory has been recognized for centuries. We know, from studies of amnesic patients carried out since the 1950s, that implicit memories can influence behaviour, because such patients can learn to perform new motor skills despite having severe deficits in other forms of memory. Thus, the term implicit memory refers to the phenomenon whereby previous experience, of which one is not consciously aware, can aid performance on specific tasks.
Ken Paller of Northwestern University and Joe Voss, who is now at the University of Urbana-Champaign in Illinois, set out to investigate further the influence of implicit recognition on decision-making, and used electroencephalography (EEG) to try to identify the brain activity associated with it.
12 healthy participants were presented with kaleidoscopic images under two different conditions. In one set of trials, they paid full attention to the images, and then perform what is referred to as a forced-choice recognition test, in which they were shown another set of images and asked to decide whether or not they had seen each of them before. In the other condition, they were made to perform a working memory task whilst the initial first set of images were presented to them – they heard a spoken number and were asked to keep it in mind, so that during the next trial they could indicate whether it was even or odd. Thus, in these trials, their attention was diverted away from the stimuli.
Thus, under the first condition, the participants are consciously aware of having seen some of the images before, and had formed explicit memories of the presented images. During the forced-choice test, they could base their decisions on these memories, and report afterwards that they remembered or knew whether or not they had seen each image before.
By contrast, during the diverted memory trials, they did not form explicit memories of the stimuli, and under these conditions reported either that they had guessed at the answer. This therefore signifies that the participants were unaware of any memories of the images presented to them.
Nevertheless, the decisions which were reportedly made by guessing were found to be significantly more accurate than those based on explicit memories of the visual stimuli,. This suggests that visual information can be encoded accurately even when one is not paying attention to it -something which has been demonstrated before – and also leads to the counterintuitive conclusion that retrieval of a memory is actually enhanced one’s attention is diverted during encoding of that memory.
The researchers measured event-related potentials as the participants performed these tasks – that is, they used an elastic cap, containing 59 electrodes evenly distributed over the scalp, to measure the electrical activity of the brain. This showed that each type of decision – those based on explicit and implicit memory – was associated with a distinct spatio-temporal pattern of brain waves (above). During trials in which decisions were based on explicit memories, bursts of activity were recorded from one cluster electrodes, beginning approximately 400 milliseconds after the onset of the stimuli.
By contrast, trials involving implicit memory (during which the participants’ attention was divided) were associated instead with a decrease in activity recorded from another electrode cluster, which began at around 200 ms after the stimulus onset.
That distinct electrophysiological signatures were associated with explicit and implicit memories suggests that the retrieval of each involves distinct neural mechanisms – retrieval of the former was associated with reduced activity in the occipital lobes and left prefrontal cortex. One would therefore predict that implicit memory retrieval can be disrupted by perturbing activity in those parts of the brain, and this could be tested to confirm these findings, perhaps by using transcranial magnetic stimulation. Future work may also identify the specific occipital and prefrontal areas involved.
This study, then, suggests that when we try to remember something, we actually know more than we think we know, because of implicit memory recall of which we are unaware, and that what we call intuition may in fact play a large role in decision-making.
Voss, J. L. & Paller, K. A. (3009). An electrophysiological signature of unconscious recognition memory. Nat. Neurosci. DOI: 10.1038/nn.2260.