The Puzzle of Perception: Visual Agnosia Explored

Sejal R Singh, M. Optom

Research Optometrist, Sankara Nethralaya, Tamil Nadu, India

 

Visual agnosia stands as a neurological disorder that hinders an individual’s ability to recognise familiar objects solely through sight, even when their overall vision remains intact. It is a sub-type of agnosia, a more general neurological condition that impairs the use of one or more of the five senses to recognise familiar objects. “Agnosia” comes from the Greek word meaning “lack of knowledge.” People who suffer from visual agnosia have trouble using their eyes to identify things or certain features. This condition is caused by impairment to one of the two main visual pathways in the brain. The “where” pathway deals with spatial placement, whereas the “what” pathway deals with the recognition of visual inputs. Even though their eyesight and memory may be normal, people who have visual agnosia most likely have damage in particular areas of the cerebral cortex that process different aspects of vision. ⁽¹⁾

Types of Visual Agnosia Demystified

1. Apperceptive Agnosia ⁽²⁾

Perceiving the shapes or forms of observed items might be challenging for people with apperceptive agnosia (AA). The pathology that affects the ventral stream and the connections between the occipital and temporal lobes is the cause of AA. AA patients have trouble identifying an object as a whole, but they can name its characteristics. Upon seeing an illustration of a toothpick spelling the word “THIS,” an AA patient may report seeing the number “7415.”(Figure 1)

Figure 1: Gollin Form Test
[Image Courtesy: https://www.reviewofoptometry.com/article/an-ods-guide-to-visual-processing-disorders ]

2. Associative Agnosia ⁽²⁾

The condition known as associative visual agnosia is characterised by the inability to retrieve information associated with an object, including its name and its intended use. Nevertheless, when queried about the identity of common objects like a “lion,” “car,” or “chair,” they can verbally express the correct responses.

3. Prosopagnosia ⁽²⁾

The incapacity to recognise faces, known as prosopagnosia, is frequently associated with localised lesions near the temporal-occipital junction. The “famous faces test” is one of the diagnostic tools for PA. It is interesting to note that 2.5% of people are born with prosopagnosia. These individuals frequently have trouble remembering names of people and whether they have met someone previously.(Figure 2)

Figure 2: Famous Face Test
[Image Courtesy: https://www.cbsnews.com/news/do-you-have-trouble-recognizing-faces-take-a-test/]

4. Simultagnosia ⁽³⁾

Simultanagnosia (SA) involves the inability to recognise a group of objects, like a set of tools, or a scene, such as a landscape. Lesions in the dorsal stream, leading to difficulties in processing multiple details of an object simultaneously, characterise simultagnosia. The “cookie theft picture” serves as a classic test for SA, where patients may articulate details like dishes being cleaned but overlook water overflowing from the sink or a child stealing cookies.(Figure 3) SA patients also struggle with tests like the “vegetable man test,” where a person’s image is constructed from fruits and vegetables.(Figure 4)

Figure 3: Cookies theft test
[Image Courtesy: https://www.mdpi.com/brainsci/brainsci-12-00270/article_deploy/html/images/brainsci-12-00270-g001.png]

Figure 4: Vegetable Man test
[Image Courtesy: https://www.reviewofoptometry.com/article/an-ods-guide-to-visual-processing-disorders]

5. Topographic Agnosia ⁽⁴⁾

The inability to identify well-known or famous places, such as the Grand Canyon or a friend’s home room, is known as topographagnosia. People who have trouble navigating familiar environments or who get lost in their own homes could be suffering from topographic agnosia (TA). Approximately 30% of post-stroke patients report challenges in navigation, a factor strongly linked to their subjective quality of life.

Conclusion

In summary, visual agnosia poses significant hurdles in recognising everyday objects, faces, and locations due to disruptions in the brain’s visual processing. Understanding its various subtypes, we not only aid in diagnosis but also pave the way for targeted interventions to improve the quality of life for individuals affected by this neurological condition.

 

References

1. Atallah, O., Badary, A., Almealawy, Y. F., Farooq, M., Hammoud, Z., Alrubaye, S. N., … & Chaurasia, B. (2024). Insights into Anton Syndrome: When the brain denies blindness. Journal of Clinical Neuroscience, 120, 181-190.
2. Haque, S., Vaphiades, M. S., & Lueck, C. J. (2018). The visual agnosias and related disorders. Journal of Neuro-Ophthalmology, 38(3), 379-392.
3. Bellows, D., Chan, N., Chen, J., Cheng, H. C., Jindahra, P., Kaiyrzhanov, R., MacIntosh, P., Vaphiades, M., Weber, K. P., & Wong, S. (2019). Neuro-Ophthalmic Literature Review. Neuro-Ophthalmology, 43(1), 58–66.
4. Dalrymple, K. A., Barton, J. J., & Kingstone, A. (2013). A world unglued: simultanagnosia as a spatial restriction of attention. Frontiers in human neuroscience, 7, 145.