Clinical Examples

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. 

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.

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