Category: Vision

Sensory illusions before and after vision

smackenz/ / Haptic touch, Hearing, Illusions, Multisensory, Vision

Sometimes the brain gets it all wrong. It misinterprets the information from one or more of the senses. This phenomenon is commonly known as sensory illusions.

Revisiting S.B., who regained his eyesight after more than 50 years of being blind. Using vision, he now recognised simple shapes and ordinary objects as well as their size. But he closed his eyes in traffic. Perhaps more complex visual information overwhelmed him. Perhaps it did not match his memories from when he was still blind. Or perhaps both. (See our blog for the scientific approach, Vision, haptic touch, and hearing and Sensory mismatch.) A related issue is that of conflicting information within and between the senses. Did S.B. show an effect on sensory illusions based on or including visual information?

When S.B. was still blind, he would have been familiar with both tactile and auditory illusions.

But what about visual illusions?

Visual experience is not necessary to show an effect on all visual illusions1. Indeed, S.B. would have encountered some of them when he was still blind. Simply because certain illusions are both visual and tactile. And S.B. would, therefore, have shown an effect on these illusions immediately after he had started using vision. For example, on the Müller-Lyer Illusion2,3.

Visual illusion: 2 lines of equal length appear unequal when the ends have arrow shapes attached.
(Müller-Lyer Illusion, retrieved from elevers.us)

The Müller-Lyer Illusion consist of two horizontal lines that are identical in length: one with inwards-pointing and one with outward-pointing fins. People who show an effect on this illusion, perceive the line with the outwards-pointing fins as longer than the other line. The Müller-Lyer Illusion is found both in people who are born fully sighted and in people who are born blind. As well as in children (born with very low or no vision; 8–16 years old) after only 48 hours of seeing4. But this was not the case for S.B., who regained his eyesight at the age of 525.

S.B. showed a very weak effect on the visual Müller-Lyer Illusion.

For other visual illusions, visual experience is sometimes necessary and sometimes not. An example is the Ponzo Illusion. The Ponzo Illusion consists of two parallel lines that are converging. These two lines are crossed by several horizontal lines that are identical in length. Almost like a railway track that disappears into the distance. People who show the Ponzo Illusion perceive the crossing lines as becoming shorter and shorter the more the vertical lines converge.

Visual Illusion, perspective of the train tracks makes the 2 yellow lines appear different sizes
(Ponzo Illusion, retrieved from illusionsindex.org)

The Ponzo Illusion does not show and effect in people who rely on their sense of touch. And prior visual experience does not change that6. This illusion is not tactile. At the same time, the visual Ponzo Illusion is found in children (born with very low or no vision; 8–16 years old) after only 48 hours of seeing4. The illusion is visual, but prior visual experience is not necessary. In a parallel vein, the Ponzo Illusion has been translated into an auditory format. This auditory version of the illusion occurs in people who are fully sighed and wearing a blindfold. But not in people who have been blind since before they were 20 months old7. The Ponzo Illusion is not auditory without prior visual experience. S.B. who had been visually impaired from before he was two years old should, therefore, have shown an effect on the visual Ponzo Illusion immediately after he had regained vision. Or on a similar illusion.

Visual illusion using perspective to make figures appear larger.
(retrieved from richardgregory.org)

Instead of judging the length of two lines as in the Ponzo Illusion, S.B. was asked to describe the relative sizes of four men. People who have been fully sighted since birth typically perceive the men as increasing in height. S.B. described: “They don’t look far away, it’s just as though the men were standing underneath (? the buildings). The first man looks smaller, but the last three look the same.” 5 S.B. showed a very weak effect on the visual Perspective Size Changes Illusion. (Gregory & Wallace, 1969, p. 22)

After having regained vision, S.B. would also encounter multisensory illusions that include visual information. These illusions consist of conflicting information from vision, touch, hearing, smell, and/or taste. The brain now has to decide how to deal with this. It most often turns to previous learning. An alternative would be to ignore the visual information. Indeed, multisensory illusions that include visual information do not exist without vision. And also not if the visual information is not associated with the other sensory information in a certain way, for example, the lip movements and the sound of spoken words. Prior visual experience is necessary.

Immediately after having regained vision, S.B. would not show an effect on multisensory illusions that included visual information. But his susceptibility to them would probably increase as he learnt to associate and integrate visual information with other sensory information. (See our blog for Crossmodal correspondences between the senses and Multisensory processing.) That is, if he did not close his eyes.

Now, challenge your senses.

Tactile illusions:

 

Auditory illusions:

 

Visual illusions:

 

See our blog for Activities; especially 65-67.

Blog post author: Dr Torø Graven

 

1. Bean, C H (1938) The blind have “optical illusions.” Journal of Experimental Psychology, 22(3), 283–289. https://doi.org/10.1037/h0061244

2. Heller, M A, … [et al.] (2002). The haptic Müller-Lyer illusion in sighted and blind people. Perception, 31(10), 1263-1274. https://doi.org/10.1068/p3340

3. Millar, S, & Al-Attar, Z (2002) The Müller-Lyer illusion in touch and vision: Implications for multisensory processes. Perception & Psychophysics 64(April), 353–365. https://doi.org/10.3758/BF03194709

4. Gandhi, T, Kali, A, Ganesh, S, & Sinha, P (2016) Immediate susceptibility to visual illusions after sight onset. Current Biology, 25(9), R358-R359. https://doi.org/10.1016/j.cub.2015.03.005

5. Gregory, R L, & Wallace, J G (1969) Recovery from Early Blindness A Case Study. Experimental Psychology Society Monograph, No. 2. https://www.richardgregory.org/papers/recovery_blind/recovery-from-early-blindness.pdf

6. Heller, M A, & Ballesteros, S (2012) Visually-impaired touch. Scholarpedia, 7(11), 8240. http://www.scholarpedia.org/article/Visually-impaired_touch

7. Renier, L, …  [et al.] (2005) The Ponzo Illusion with Auditory Substitution of Vision in Sighted and Early-Blind Subjects. Perception, 34(7), 857-867. https://doi.org/10.1068/p5219

Touch and Nature Learning

tgraven/ / Crossmodal correspondences, Haptic touch, Multisensory, Posts, Vision

Some information is sensory specific, for example, colour to vision and temperature to touch. But most is perceived by several senses. An example is shape. Shape is perceived by vision, touch, and hearing. It transfers between the senses. And when used together, the senses perceive supplementary and overlapping information. (See our blog for Crossmodal correspondences between the senses, Vision, haptic touch, and hearing, and Multisensory processing).

In this post, I have invited the research team behind TOUCH to shed some light on how sighted people use their sense of touch when exploring real objects. The blog post is written by Dr Lisa Bowers, Open University, Professor Andrew Manches, University of Edinburgh, and Professor Laura Colucci-Gray, University of Edinburgh. Here, they present their previous research, leading up to TOUCH, on the role of touch when sighted children learn about nature.

Given the worsening state of our planet, an important challenge for education is how to support and promote a renovated relationship between humans and the natural world. Whilst schools offer an ideal context for this challenge, current teaching approaches tend to emphasise children’s visual experiences, exaggerated through increased engagement with screen devices: the world is presented as if ‘at a distance’, disconnected from everyday experiences and the matters we deeply care about. Although an education through the senses is predicated to support various hands-on initiatives, including outdoor and play-based experiences, its role is typically conceived as a means to actively explore visual information, rather than holding any intrinsic value in its own right.

Our research team has examined the role of touch in how children interacted with and described a selection of nature objects (e.g., a leaf, a shell, and a feather). We have found three key dimensions:

  • Propensity to Touch. Children differ in how much prompting they need to physically pick up objects – ranging from those who immediately touch objects to those who are reluctant even with prompting.
  • Touch interaction. Children differ in the
    © University of Edinburgh

    richness of their touch interaction – from simple tapping to rich exploration using both hands.

  • Touch communication. Children differ in the extent to which they use tactile language in their description of objects, as well as how they simulate touch through gesture when describing what they see.

 

As well as revealing intriguing similarities and differences between children, our research has highlighted the way that touch wasn’t simply a means for children to visually explore objects (although this was important, e.g., manipulating objects to inspect them from all angles) but offered much value as a unique mode of interaction. Indeed, many children would look away from objects whilst exploring them through touch. For example, a child might rub the inside and outside of the shell whilst reflecting on why one was smooth and the other rough and this process would recall prior experiences in other places or in the company of significant others – often positive emotional recollections.

Our research also revealed challenges children often had in describing tactile properties of objects – often drawing on analogies with other touch experiences when they lacked vocabulary. The identification of gestures simulating touch was particularly powerful in revealing how children had internalised touch experiences in their concepts of objects as they described them. This internalisation of sensory experience in conceptual thinking is a key claim of embodied theories of cognition. However, the benefits of tactile learning extend beyond basic sensory exploration. Somatosensory perception is central to early human development and the explicit incorporation of touch as a way to feel, interact, and communicate through the body can enhance learning, with increased engagement and memorability.

Somatosensory Cortex Stimulation by Touch: Somatosensory Cortex - Nerve impulse - Touch
Created by Lisa Bowers, 2025

The foundational work of research studies such as the one described above has highlighted various possible avenues for design – including ways to encourage children’s propensity to touch, ways to encourage children’s exploration of tactile properties in greater depth, or ways to develop children’s tactile language (and possibly gestures) as a means to communicate tactile properties. We have explored some of these opportunities through classroom and outdoor activities, as well as the development of haptic prototype designs where children receive haptic feedback from a special stylus moved over a screen device. For these haptic designs, tactile feedback not only provides a means for children to tactilely explore ‘less accessible’ objects like a bee or unfamiliar plant, but can prime children to be more curious about the feel of real-world natural objects. We are also exploring the potential of this work to quantify the role of touch in order to examine how different factors influence children’s touch interaction and communication (e.g., their age, confidence or nature experiences), and to evaluate how, and if, our interventions influence the role of children’s touch in children’ science learning.

In our future research, touch is conceived of as a medium that builds on human capacities to feel and perceive what might be distant and inaccessible, but also to pay attention to aspects of the world that may go unnoticed or taken for granted. We are now asking ourselves and all we are engaging with a key question – what is the role of touch in what it means to be curious about, explore, think, feel, communicate, and be connected to our natural world?

See our blog for Activities; especially 62-64.