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Illusory Perceptions



The 2024 Global Year will examine what is known about sex and gender differences in pain perception and modulation and address sex-and gender-related disparities in both the research and treatment of pain.

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A person, let’s say his name is Pat, views a staircase in front of him; he is delivering a parcel (Postman Pat) and must climb three flights of stairs to deliver it to flat 15. Pat has knee pain and stairs aggravate his pain. Just as Pat is surveying the stairs, a (pain-free) resident sidles passed him and makes his way up; Pat follows behind him. The decision made by both individuals is the same; this cannot be said for the processes that led them there.

Postman Pat

We are constantly making decisions; ranging from the mundane to the critical. No matter how whimsical they may seem the decisions that we make are never isolated events. Indeed they are reliant on the information that we gather. We make the assumption that this information is accurate, an unbiased representation of how things are. Yet, since the late 1800’s and Hermann Helmholtz’s idea of perception as unconscious inference [2], there has been doubt rightly cast over the information we have access to; what we see is less how the world is and more how the world is expected to be.

The inferences that we make, whilst efficient, can be seen as a form of illusion, the foundations of which lie in primitive mechanisms; Richard Gregory brilliantly demonstrated this throughout his career [1]. When it comes to making decisions based on these inferences, our priorities can be closely related to survival, one such primitive mechanism- we endeavour to minimise the costs and maximise the benefits when interacting with our environment; in short, we economise our actions [3], perceiving our world based on our utility within it.

Back to Pat.

His decision resulted in the same action as the anonymous resident, however if we consider it in line with the idea of perception as inference and the economy of action theory, the process of getting there is more complex. Pat associates the stairs with pain, a cost that he has incurred before and anticipates incurring again. He is, however, required to climb the stairs as part of his work; the benefit of delivering the parcel being that he will keep his job. It is postulated that these top-down processes could impact the way that bottom-up sensory information is conveyed. As Helmholtz postulated, inferences made about a visual scene are at the mercy of our unconscious processing. Although Pat is aware of the costs and benefits associated with the action, what is not clear is the impact that these analyses have beyond conscious awareness.

This opens up the possibility that what Pat ‘sees’, for example the length, number and gradient of the stairs, could be a subtly different from reality, an illusion created prior to his conscious awareness, which impacts on the decision that is made.

Based on these thoughts we carried out an experiment that looked at whether the perception of distance estimation would be altered in the presence of pain. Using experimental pain and a pain-relieving switch we found that when people are in pain and must judge the distance to a pain-relieving switch, they significantly underestimate that distance they are required to reach compared to when they are pain-free [4]. In other words, when participants viewed a scene whilst experiencing a costly stimulus, the perception of distance changed, the beneficial object was seen as closer.

Bringing this back to Pat, and indeed people who experience pain, could it be that the world is altered when you are in pain? That is, what you ‘see’ is manipulated before it reaches your conscious awareness, yet reflects the utility that you have within that environment.

This would be an important consideration for people in pain, as it potentially offers a different perspective on why particular choices are made and actions executed. For now we have plenty of fodder for further investigations, not least in people who experience pain persistently.

Abby Tabor

Abby Tabor BiM

Abby has a very posh English accent, and clearly doesn’t like granola bars.  What brought Abby out to Oz? Abby first got an inkling for what pain really was after listening to Mick Thacker during her undergraduate degree at King’s College London. It was his inspiration, along with a certain Moseley style Explain Pain experience that led her to the other side of the world to delve deeper into the world of pain.

She finally agreed to return to the UK, on one condition, that they would let her come back to the land down under to do part of her PhD. Still in the early days of research she is nurturing a nice little coffee habit and asking a lot about how philosophy can fit into our understanding of pain.  Here is Abby talking more about what she does.


1. Gregory RL. Knowledge in perception and illusion. Philos Trans R Soc Lond B Biol Sci 1997;352(1358):1121-1127.

2. Helmholtz H. Concerning the perceptions in general. East Norwalk, Connecticut: Appleton-Century-Crofts, 1867.

3. Proffitt DR. Embodied Perception and the Economy of Action. Perspect Psychol Sci 2006;1(2):110-122

4. Tabor A, Catley MJ, Gandevia S, Thacker MA, & Lorimer Moseley G (2013). Perceptual bias in pain: a switch looks closer when it will relieve pain than when it won’t. Pain, 154 (10), 1961-5 PMID: 23726934

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