||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Varsha Sathappan, Baylor College of Medicine Class of 2022
||So today we're going to be talking about visual association cortex. So in your brain, as you know, the information comes into the eye and is transmitted, processed in thalamus, passes through the geniculate body, through the radiations, to the ultimate destination which is the primary visual cortex. The primary visual cortex is in the occipital lobe, and the occipital lobe receives this information. But then then processing, the secondary processing, and the primary processing of that information that has been transmitted to the occipital lobe occurs in the parietal and temporal lobes. And the dorsal stream and the ventral stream of information processing is how we refer to this. It's dorsal because this used to be the dorsal part of the brain. Now that we have stood up, the top part is now dorsal. And the temporal lobe is now the ventral pathway. The dorsal pathway is sometimes referred to as the where pathway. And the ventral stream is sometimes referred to as the what pathway. Because the dorsal where pathway is about localization in space and visual space processing of the information. Versus the what pathway, tells us more about space, color, recognition of features like in faces. And so, when we are dealing with processing problems, like including neurodegenerative disorders or strokes in the dorsal or ventral streams, that might produce very specific eye complaints that have nothing to do with the visual acuity per say. So, someone could literally be 20/20 in both eyes, have a normal pupil reaction because the pupil pathway is intact, and have a normal visual field, and yet be complaining about processing abnormalities that are arising from the deficit in the visual association cortex. And so, some examples of those in what pathways: someone might not be able to recognize what color something is, or what person is this based on their face, which is prosopagnosia. Versus in the dorsal pathway, the where localization could be wrong. So, a patient with a dorsal problem might not be able to localize an item in space, so we call that optic ataxia. So, the pen is held here, and you might ask them to grab it but they might mis-localize because their visual-spatial where pathway is abnormal. Or the patient might have other visual-spatial processing abnormalities. Where they have difficulty with localization of items and those have different names, including simultanagnosia, where they can't see the forest of the trees, they can only see individual pieces. But when you try to pile all the pieces on top of one another they get lost. Or with Ishihara color plates, they can see the little colored dots, but they cannot see that it is making up the number 12 or 20. So, in patients who have visual association cortex problems, its processing that is the problem, its agnosia's. Agno- I don't know, I don't know how to read, I don't know how to recognize this person's face: that's prosopagnosia, I don't know cannot look at things simultaneously: that's simultanagnosia, I don't know what color that is: that's a color agnosia. So the different types of agnosia occur from processing difficulties in the both the dorsal stream and ventral stream, which represent the where and what pathway. Even though the transmission of the information is intact, the primary visual cortex, and occipital lobe.