Medical Physics and Biomedical Engineering


‘Once Santa Claus shows himself, the magic seems to die.’

13 December 2018

Scrooge or Believer?

Researchers in the Department of Medical Physics and Biomedical Engineering have been measuring colleagues’ brain activity, including that of our very own Dean of Engineering Professor Nigel Titchener-Hooker, as part of a tongue-in-cheek experiment to detect the absence, or presence of Christmas spirit.

Dr Kirill Aristovich, Dr Anna Witkowska-Wrobel (Electrical Impedance Tomography Group) and Lindsay Wright, UCL Department of Medical Physics & Biomedical Engineering


Many studies have found a relationship between electrical activity of the brain recorded via electrodes placed on the scalp (Electroencephalogram, or EEG) and processes such as alertness, arousal, attention, memory, executive functions and cognitive processing. 

One of the commonly used techniques for understanding brain function during EEG is event-related potentials (ERPs) – electrical activity occurring as a response to specific events or stimuli.

ERPs are thought to reflect the summed activity of all the electrical responses in the brain when thousands of millions of neurons in the cerebral cortex all fire at once while processing information. There are usually two stages of response in this process: an early one (around 100ms) and a later, cognitive response (around 300ms) which only appears if a subject is actively engaged in the task.

At the same time, EEG in general is described in terms of the occurring wavelengths, their voltage, synchrony and morphology, which all reflect the underlying cortical activity. Alpha activity, which has 8 to 14 Hz frequency, is typically recorded towards the back of the head, when a person is closing their eyes and relaxing. Alpha rhythm is sensitive to event-related stimulation and its spread is expected to decrease during such stimulation.

Brain activity can be also detected and imaged via Electrical Impedance Tomography (EIT), an emerging, non-invasive type of medical imaging which uses electrodes to deliver a series of safe, low doses of current. It is beginning to be used for imaging healthy brain activity and pathology in patients affected by epilepsy, stroke and brain injury, and can be used alongside conventional EEG monitoring methods.

We wanted to use these techniques to measure the electrical responses in the brain when people are shown a series of obvious, neutral and hinted Christmas-related visual stimuli. An obvious stimulus might be a picture of Santa Claus, a hinted one is an image suggesting the presence of Santa Claus but without him actually being in the picture (for example, a roaring fire with a plate of mince pies next to it), and a neutral stimulus would be an image completely unrelated to Christmas.

We expected that images with the ‘hinted’ Christmas stimulation would require more intensive cognitive processing, as the stimulation is complex rather than straightforward, and would produce higher ERP, lower alpha rhythm of EEG, and functional changes in EIT images.

Materials and Methods


Twelve healthy volunteers, all staff and students from the department, and including Dean of Engineering Professor Nigel Titchener-Hooker (pictured here) participated in the study. The study was conducted under the local ethical committee approvals.

Recording procedures and visual stimulation

Subjects were seated in an armchair, 18 Ag/AgCl cup scalp electrodes were placed according to the standard 10-20 International EEG montage with ground and reference electrodes on the forehead. Prior the electrode placement, skin was prepared with alcohol and scrub gel. Electrodes were secured with EEG paste and tape to the head.

There were 50 stimuli for each group, all randomly generated, separated with black screen, and presented for 5 seconds. 

Andy O'Reilly (left), Departmental Manager for the Department of Medical Physics and Biomedical Engineering, and Professor Nigel Titchener-Hooker process an 'obvious' Christmas image


There was a significant reaction from all subjects to the presented pictures (fig. 2). When comparing event-related potentials (ERPs), the averaged response was approximately 30±10% larger for stimulation with pictures than for the black screen. Christmas-related pictures elicited roughly 20±10% larger response than the neutral stimuli. Finally, when the Christmas stimulation required more cognitive processing, i.e. the ‘hinted’ group, the response was around 10±5% larger than for Santa Claus images, the ‘obvious’ group.

A similar trend was also observed when the alpha spectrum was analysed (fig. 3). A maximum of alpha activity spread was seen over the occipital channels when the black screen was shown, which then decreased by 20±10% with cognitive activation during visual stimulation. During qualitative assessment, the largest reduction of the alpha spectrum was found to be within the ‘hinted’ group, suggesting that this group of pictures required the most cognitive effort.


With all scientific research, there are caveats. Dr Aristovich comments: ‘The study was quick and limited. We had a small sample, so although within the subject the results were consistent, they were not statistically significant across the subjects, and more research is needed. Regarding the EIT imaging, however, we have acquired useful data which will help us improve the technique and transfer it to clinical use. Although there were no first-glance results, we are now analysing the images and anticipate more detailed conclusions in two to three months’ time.

'Overall, subjects showed a significant reaction in ERPs and alpha EEG spectrum to the Christmas-related visual stimulation. Further analysis showed that hinted Christmas stimuli required more cognitive effort and therefore produced a stronger response. We were expecting ‘hidden Santa’ to be more exciting than ‘obvious Santa’, because higher-order neural processing between perception, storage in memory and emotional components of hinted information induce a significantly larger cortical response observed with EEG. We were not able to find any difference between the ‘Scrooges’ and the “Believers’, but that is likely to be explained by the small sample tested.

Dr Aristovich concludes: ‘It was interesting that the ‘hidden’ images, where the presence of Santa Claus was suggested but where he wasn’t actually shown in person, evoked a stronger response than those where featured in the picture. These were all adult subjects too, so it almost suggests that, even when you have long since given up believing in Santa Claus, the magic of Christmas never really disappears. It was the mystical promise of Santa Claus arriving that seemed to appeal to our subjects, rather the reality. Once Santa Claus shows himself, the magic seems to die.’