Overview

Agnosia, which is simply a modality-specific disorder of recognition, is understood best in the context of visual form agnosia—where identification of an object is unable to occur. The gross anatomy of the visual system is intact, but there is a deficit in the modalities that piece together recognition of objects.

Studies on visual agnosia once claimed that agnosia may impact general object recognition*. Yet new research has shown increasingly more evidence that indicates that visual form agnosia is specific to an element*. Visual agnosia’s can vary from the inability to recognize faces, colors, or landmarks in the environment.*

Anatomy of a functioning visual system:

Cells that are depolarized by light waves will either synapse at magnocellular or parvocellular layers, which will then send axons to the primary visual cortex (V1) and synapse at different layers. In the secondary visual cortex (V2), the form will go to the thick stripe to which it will go through dorsal pathways and terminate in dorsolateral parietotemporal cortex. The form and color will come into V1 by way of parvocellular layers and synapse in layer IV beta, it will synapse to either the thin strip (color) or interstripe (form) of the secondary visual cortex and terminate in the inferior occipitotemproal cortex.
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Figure 1

Signs of Dysfunction:

In visual form agnosia patients express an inability in understanding what an object is. They know what an object is and what it looks like and how it works, but they cannot connect that what they are seeing to said object.

Imagine a can of soup, a patient with visual form agnosia would know what a can of soup is. They could even tell you that it usually comes in a medium sized cylindrical container with metal ridges and a papery covering along the side. They could pick up the can and feel that exact description and express to you exactly what they perceive. Yet if you asked them what they were holding even with all the evidence they have in front of them, they would have no idea this was a soup can.

As stated earlier visual form agnosia can associate itself with almost any object. Prosopagnosia, or “face blindness” is a form of agnosia where patients cannot recognize faces, even of people they know intimately. The key to agnosia is that there is a deficit in recognition but no deficit in.

prosopagnosia.jpg

Figure 2: What face blindness may be like: above are upside down faces--at first we wouldn't be able to recognize them, yet as they are turned right side up we realize these are highly recognizable faces of celebrities
Anatomy of Visual System:
Within our spinal cord we have two clusters of afferents that deal specifically with visual processing—these areas are designated the dorsal and ventral stream. The ventral stream links the primary visual cortex to inferior regions of the occipitoparietal cortex and has been found to be associated with the “what” of our visual processing. Visual form agnosia is a disorder of the “what” pathway. “What” would comprehend the form and color of an object. Their pathways derive from Parvocellular ganglion cells of the retina that would innervate the thalamus and primary visual cortex before entering the interstripe or thin stripe of the secondary visual cortex. The interstripe of the secondary visual cortex would hold the comprehension of form and therefore could be the missing piece needed when agnosia patients cannot recognize an object or face. Those that suffer from prosopagnosia would most likely have damage in the area surrounding the interstripe, whereas patients suffering from color agnosia would have damage in the thin stripe of V2.
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Figure 3


Conclusion:

Visual form agnosia can easily be classified as one of the most bizarre conditions one can acquire. How is it even possible that one can see something and know in their minds what this object is used for, yet they cannot recognize it in front of them? It sounds completely impossible, there’s no way someone wouldn’t recognize a face of a loved one or an every day object. Yet, when brain damage gets down to specific pathways and cellular areas made just for recognition, we learn that indeed there is a scary reality of inability to recognize. Humbling since we so often take everyday recognition for granted.

Glossary:

Interstripe: Layer 2 & 3 of interblob regions of the secondary visual cortex
Prosopagnosia: Cognitive disorder of face preception.
Thin Stripe: Layer 2 & 3 of blob regions of the secondary visual cortex
Parvocellular: Small ganglion cells of the lateral geniculate nucleus that process contrast and shape discrimination.
Ventral Stream: inferior temporal visual association cortex. Processes parvocellular channel information.
Dorsal Stream: Neurons in parietal association cortex that have biocular receptive fields and processes parvocellular information.

Suggested Readings

"Face Blindness: Where everyone is a stranger". http://www.cbsnews.com/news/60-minutes-face-blindness-when-everyone-is-a-stranger/
--This is a CBS documentary piece on prosopagnosia that has given the disorder a larger understanding.

"Face Blind" by Oliver Sacks http://www.newyorker.com/magazine/2010/08/30/face-blind
--World renowned neuroscientist Oliver Sacks sheds light on the unique disorder of prosopagnosia


Quiz

True/False
1. Visual form agnosia is when patients cannot recognize faces.
2. Visual agnosia is a deficit in vision.
3. Recognition of motion would be interpreted by parvocellular cells of the lateral geniculate nucleus

Multiple Choice
1. Which pathway would object recognition go through?
a. Ventral Stream
b. Dorsal Stream
c. Lateral stream
d. anterior stream

2. Color form agnosia would be caused by deficits in what area?
a. interstripe
b. Thin stripe
c. lateral geniculate nucleus
d. interblob regions

3. The dorsal stream of the visual system would help in what aspect?
a. recognition of an object
b. reach and grasp of an object
c. contrast of color of the object
d. all of the above

4. The ventral steam of the visual system would help in what aspect?
a. recognition of an object
b. reach and grasp of an object
c. contrast of color of the object
d. all of the above

5. Visual form agnosia is?
a. acqurired
b. congenital
c. a & b
d. none of the above

Short Answer:
1. What is visual form agnosia? What are symptoms of the disorder?
2. What anatomical pathways are impacted by visual form agnosia?


Answers:
T/F
1. False
2. False
3. False

MC
1. A
2. B
3. B
4. A
5. C




References

1. Coslett HB. (2011) "Sensory Agnosias". In: Gottfried JA, editor. Neurobiology of Sensation and Reward. Boca Raton (FL): CRC Press/Taylor & Francis. Chapter 10. Available from: https://www.ncbi.nlm.nih.gov/books/NBK92800/

2. Karnath, Hans-Otto; Rüter, Johannes; Mandler, Andre; & Himmelbach, Marc. (May 6 2009). The Anatomy of Object Recognition—Visual Form Agnosia Caused by Medial Occipitotemporal Stroke.

3. Mishkin M, Ungerleider LG (1982). "Contribution of striate inputs to the visuospatial functions of parieto-preoccipital cortex in monkeys.". Behav Brain Res. 6 (1): 57–77.

4. Milner, A.D. (2012) "Is Visual Processing in the dorsal stream accessible to consciousness?"


5. Milner, AD.; Goodale, MA. (February 2008). "Two visual systems re-viewed.". Neuropsychologia. 46 (3): 774–85

Image References:

1. "Neuroanatomy through clinical cases" by Hal Blumenfeld
2. www.cbs.com
3. "Neuroanatomy through clinical cases" by Hal Blumenfeld