Jamie Moffatt, 10th January, 2020

PhD Student, University of Sussex, ECHR 2019 Bursary Awardee

In psychology research, the most common way to study behaviour is by recreating that behaviour in a controlled setting. Some memory researchers will ask their study participants to recall a list of words, vision researchers will ask people to look at various images and emotion researchers might think of ways to recreate feelings of sadness, disgust or joy in their participants. 

To study hallucinations and other unusual experiences, one technique might be to recreate those experiences. Researchers can use cutting-edge technology, sophisticated experimental manipulations, or even just some clever sleight of hand to create perceptions of things that do not match reality. This comes with the caveat that these recreated experiences are usually mild and temporary, which is normally not the case with hallucinations that occur in the context of psychosis or other psychiatric disorders. 

However, these techniques do allow researchers to investigate potential causes of unusual experiences. Furthermore, methods for recreating unusual experiences may also have therapeutic value. For example, using virtual reality to create the experience of standing at a great height has proven to be effective in therapies for treating fear of heights (Freeman et al., 2018). One day it may be possible to use similar methods as part of treatments for more complex unusual and unwanted experiences, such as those experienced in psychosis.

In my research, we induce sensations of touch using a task called the Somatic Signal Detection Task (SSDT for short), which was developed just over a decade ago by researchers in the UK (Lloyd, Mason, Brown & Poliakoff, 2008). The setup for this task can be seen in Figure 1. In this task, participants place their finger on a small tactile motor, which will sometimes vibrate to cause a sensation of touch. Next to the motor is a small LED light. Past research has repeatedly demonstrated that when the light flashes, people are more likely to report feeling a sensation of touch, even when there is no actual touch delivered at all (Lloyd et al., 2008). These can be thought of as light-induced touch sensations.

Figure 1. Setup for the Somatic Signal Detection Task. Participants place their finger on a small, coin-shaped tactile motor, next to a small LED light

What causes these phantom touch experiences? Our perceptions of the world comes from two sources. There is sensory information that we detect in the world, such as a flash of light or the sound of a ringing telephone. But what we perceive is also influenced by our prior beliefs and expectations that we hold about the way of the world. The influence of prior beliefs can be demonstrated with an illusion, such as the one in Figure 2. In this drawing, you might initially see a duck. However, if you think of the duck’s beak as rabbit’s ears, you might start to see a rabbit instead. Some people might experience it the other way – if you initially see a rabbit, try to think of the rabbit’s ears as a duck’s beak. You might then experience something like a feeling of “switching” between the picture being a rabbit or a duck. Both perceptions are clearly true, but it demonstrates that what we perceive is not simply a result of our sensory input, but is also influenced by what we expect to see in the world.

Figure 2. Famous Duck-Rabbit Illusion, which was first published in German humour magazine Fliegende Blatter in 1892. Image taken from: Donaldson, J. (July 2016), “The Duck-Rabbit Ambiguous Figure” in F. Macpherson (ed.), The Illusions Index. Retrieved from https://www.illusionsindex.org/i/duck-rabbit.

The influence of prior beliefs on perception can explain the light-induced tactile sensations on the SSDT. In day-to-day life, sensory events that occur close together are usually caused by the same event – for example, the flashing lights and blaring sirens of a speeding ambulance both originate from the ambulance. This leads us to develop prior beliefs that sensory events that occur close together are causally linked. Similarly, in our task, when a participant sees the flash of light happening very close to their finger, they might also feel and report a sensation of touch even if there was no touch sensation at all. Indeed, if you move the light away from the finger, the light-induced sensations of touch disappear (Mirams, Poliakoff & Lloyd, 2012). An influential line of research suggests that hallucinations may occur when people place more weight on their prior beliefs and expectations, compared to sensory information, when perceiving the world. 

My research will be exploring whether people who report frequent hallucinations may be more susceptible to light-induced tactile sensations. By doing so, I hope to shed a little more light on the mechanisms that might be underlying the occurrence of hallucinations.


Freeman, D., Haselton, P., Freeman, J., Spanlang, B., Kishore, S., Albery, E., … & Nickless, A. (2018). Automated psychological therapy using immersive virtual reality for treatment of fear of heights: a single-blind, parallel-group, randomised controlled trial. The Lancet Psychiatry5(8), 625-632. https://doi.org/10.1016/S2215-0366(18)30226-8

Lloyd, D. M., Mason, L., Brown, R. J., & Poliakoff, E. (2008). Development of a paradigm for measuring somatic disturbance in clinical populations with medically unexplained symptoms. Journal of Psychosomatic Research64(1), 21–24. https://doi.org/10.1016/j.jpsychores.2007.06.004

Mirams, L., Poliakoff, E., & Lloyd, D. M. (2017). Spatial limits of visuotactile interactions in the presence and absence of tactile stimulation. Experimental Brain Research235(9), 2591–2600. https://doi.org/10.1007/s00221-017-4998-0