USMLE - First Aid - Eye question

[Phloston]

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Hi, I'm just looking to confirm details here:

In First Aid 2012, it states on p.463 that Meyer's loop (temporal lobe, per the inferior horn of lateral ventricle) supplies the INFERIOR retina and that the dorsal optic radiation (parietal lobe, via internal capsule) supplies the SUPERIOR retina.

That being said, I had always thought the opposite: that damage to Meyer's loop causes a SUPERIOR contralateral quadrantanopsia and that damage to the dorsal optic radiation causes an INFERIOR contralateral quadrantanopsia.

Could someone please help me out here?

I already understand that temporal --> superior deficit and that parietal --> inferior deficit. Now I'm just looking to have it elaborated in terms of Meyer's loop, etc.

The only other explanation I could possibly think of is that this is because the image gets inverted once it passes through the visual tracts?

I'd really appreciate it. Thanks!

~Phloston

 

[loveoforganic]

Hi, I'm just looking to confirm details here:

In First Aid 2012, it states on p.463 that Meyer's loop (temporal lobe, per the inferior horn of lateral ventricle) supplies the INFERIOR retina and that the dorsal optic radiation (parietal lobe, via internal capsule) supplies the SUPERIOR retina.

That being said, I had always thought the opposite: that damage to Meyer's loop causes a SUPERIOR contralateral quadrantanopsia and that damage to the dorsal optic radiation causes an INFERIOR contralateral quadrantanopsia.

Could someone please help me out here?

I already understand that temporal --> superior deficit and that parietal --> inferior deficit. Now I'm just looking to have it elaborated in terms of Meyer's loop, etc.

The only other explanation I could possibly think of is that this is because the image gets inverted once it passes through the visual tracts?

I'd really appreciate it. Thanks!

~Phloston

You're both right. The image on the retina is inverted and reversed, so the lower retina has the superior visual field represented on it

 

[AndyRSC]

It seems you've already figured out the answer: the image gets flipped as it enters the eye. Light entering from above hits the lower part of the retina, and light from below hits the upper part. Thus, when Meyer loop is knocked out, the input received from both retinas responsible for capturing the superior contralateral field goes blind.

There is an excellent diagram illustrating the concept here:
http://en.wikipedia.org/wiki/File:ERP_-_optic_cabling.jpg

 

[Phloston]

Thank you for the clarification.