Neuroscience Online (ii,15,7)

15. Visual Processing: Cortical Pathways
Part 7 of 7

Valentin Draogoi, Ph.D.
(Content adapted from material by Chiyeko Tsuchitani, Ph.D.)

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Clinical Examples

Figure 15.21

The cortical areas involved in color perception and face recognition are illustrated in the left hemisected brain with cerebellum removed (A). Distribution of the major branches of the anterior and posterior cerebral arteries viewed on the inferior and medial surfaces of the brain with the cerebellum removed (B). The location of the lesion is colored red.

Symptoms: A patient, who is stabilized after suffering a stroke two months earlier, is referred to a neuro-ophthalmologist for evaluation. The patient does not appear to be blind but has problems with processing visual information. For example, the patient cannot describe the color of an object presented to him or recognize faces. He has normal spatial orientation and motion detection. The patient is referred for perimetry testing.

Perimetry Test Results: The results indicate no consistent loss of vision. However, it is difficult to obtain consistent results because the patient tires easily and his attention appears to wander.

Side & Retinotopicity of damage: The patient

is not blind in either eye
does not have deficits in detecting the location or movement of objects
does not exhibit the symptom of "neglect" (i.e., visual inattention)
exhibits deficits in higher visual processing involving color and object recognition

You conclude that the neurological defect is

not related to damage in the visual pathway from the eye to the striate cortex
not related to damage in the middle or superior temporal gyrus
not related to damage in the parietal lobe
related to damage in the inferior temporal gyrus (Figure 15.21)
involving branches of the posterior cerebral artery that supply the inferior temporal gyri

Neural imaging results indicate damage to the caudal portion of the inferior temporal lobe, which normally receives blood from branches of the posterior cerebral artery.

Extrastriate or Association Cortex Damage: While destruction of the primary visual cortex produces blindness in the contralesional hemifield, damage to cortical areas surrounding the striate cortex does not Instead, they may produce profound deficits in the higher order-processing of visual information. For example, bilateral damage to a small area of the inferior temporal gyrus (Figure 15.21) produces a loss in the ability to recognize faces. Damage to more superior areas of the temporal lobe (area 39 in Figure 15.4) produces an inability to recognize or comprehend written words and/or passages. Damage to areas in the parietal cortex may result in the inability to see motion (i.e., a moving object will be seen in ``frames’’ in one place at one point in time and at another place in a following period of time). The object does not appear to move; rather it appears to have jumped from one place to the next. Damage to large areas involving the posterior parietal cortex and superior temporal cortex may result in the symptom of ``neglect", wherein objects in parts of the visual field are ignored or denied existence.

Summary

In this chapter, you have learned how the visual system is organized in the brain. You have learned that stimulus features extracted by the retinal neurons (color, brightness contrast, movement) are kept segregated in separate “information channels” and processed in parallel by different cells at all levels of the visual system. Information coded and carried by one million retinal ganglion cells are distributed to hundreds of millions of cortical neurons in the occipital, parietal and temporal lobes. The perception of a coherent visual image is recomposed out of these fragments of information by the simultaneous activation of large areas of cortex. You have also learned how the spatial representation of the visual image is maintained by the retinotopic organization of the visual system and learned how this information is useful in determining the location and extent of damage to the visual system by examining the visual fields. Finally, you have learned that neuronal responses in visual cortex exhibit plasticity at different time scales, short term (as adaptation and dynamics) and long term (as learning) – this plasticity allows visual cortex to construct an accurate picture of the world that can rapidly adapt to match the changes in the environment.

[1] An opthalmologist or well trained physician might observe abnormalities in the optic disc.

[2] An opthalmologist or well trained physician might observe abnormalities in the optic discs of one or both eyes.

Test Your Knowledge

Question 1
A
B
C
D
E

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance.

B. have color opponent receptive fields.

C. require a specific stimulus orientation.

D. have elongated receptive fields.

E. synapse with magnocellular lateral geniculate neurons.

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance. This answer is INCORRECT.

B. have color opponent receptive fields.

C. require a specific stimulus orientation.

D. have elongated receptive fields.

E. synapse with magnocellular lateral geniculate neurons.

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance.

B. have color opponent receptive fields. This answer is CORRECT!

C. require a specific stimulus orientation.

D. have elongated receptive fields.

E. synapse with magnocellular lateral geniculate neurons.

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance.

B. have color opponent receptive fields.

C. require a specific stimulus orientation. This answer is INCORRECT.

D. have elongated receptive fields.

E. synapse with magnocellular lateral geniculate neurons.

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance.

B. have color opponent receptive fields.

C. require a specific stimulus orientation.

D. have elongated receptive fields. This answer is INCORRECT.

E. synapse with magnocellular lateral geniculate neurons.

Which of the following are characteristic of the primary visual cortex "blob" neurons? They:

A. are binocular and exhibit ocular dominance.

B. have color opponent receptive fields.

C. require a specific stimulus orientation.

D. have elongated receptive fields.

E. synapse with magnocellular lateral geniculate neurons. This answer is INCORRECT.

Question 2
A
B
C
D
E