While doing some spring cleaning I came across this treasure that I haven’t looked at for quite some time which is my loss, really. Actually, I’ve looked at it a number of times but haven’t looked between its covers to rediscover why I loved Jonathan Trobe’s masterpiece on The Neurology of Vision.
First let me dispense with the only part of this Treasure Trobe that you’ll find objectionable if you’re involved in neuro-optometric rehabilitative care. And one may be able to excuse Jonathan’s limited view of Optometry based on the fact that this was published sixteen years ago. If you take a look at the Nook version of the book, you’ll note at the outset that Dr. Trobe divides the visual system in to three components:
- Optical: the eye’s focusing elements
- Retinocortical: the segment extending from the retina to the primary visual cortex. Neural signals must travel from one end to the other without interruption and only in response to external visual stimuli (classically “bottom-up”, though doesn’t fully factor in more recent research on stimulation of these pathways from internal stimuli as in visualization or visual imagery). The primary visual cortex must perform correct elementary encoding.
- Integrative: the segment extending from the primary visual cortex to the parietal and temporal vision -related cortex. Encoded elementary features must reach these association cortex regions and be converted into meaningful percepts (part of ongoing transformations and parallel as well as complementary processing; a feature of “top-down” processing as well and must be dovetail with embodied cognition).
And here is my only quibble with Dr. Trobe, as he continues: “This segmentation of the visual pathway is clinically useful because it lends itself to a proper division of labor among the specialists involved in vision care. If a problem is optical in nature, an optometrist or ophthalmologist will take charge. If a problem is retinocortical, a neurologist, ophthalmologist, and neurosurgeon may have to collaborate in localizing and managing the pathologic process. If the problem lies at the integrative level, detailed cognitive evaluation by a neurologist or neuropsychologist may be necessary.” That may have been the case in certain quarters in 2001, but in 2017 neuro-rehabiliative optometrists are quite skilled in addressing these three components of the visual system. With that out of the way, let’s get into a sampling of the pearls:
- What do you think of when I say “LGBT”? In this instance it’s the Lateral Geniculate Body of the Thalamus, and regarding its role, Dr. Trobe states: “The fact that 80% of the inputs to LGB come from nonretinal sources, including the mesencephalic reticular formation, other thalamic nuclei, posterior parietal cortex, and occipital cortex, suggests that the LGB may screen relevant from irrelevant visual information before it is sent on to visual cortex.”
2. Regarding Optic Radiations, here is a useful nugget: “The majority of fibers in the optic radiations do not actually come from the LGB but from other thalamic nuclei and the visual cortex. They may play a role in visually guided eye movements and visual attention.”
3. Regarding the principles of retinocortical transmission, Dr. Trobe differentiates between parallel and hierarchical processing as follows. Parallel processing: “Separate processing of form and/or color and spatial information begins at the retina and is carried out in distinct channels. Spatial channels may be selectively damaged in glaucoma, amblyopia, dyslexia, and Alzheimer’s disease.” Hierarchical processing: “Uncoding of visual information proceeds from a primitive to a more complex level. Retinal receptors encode an array of spots of different light intensities; retinal ganglion cells enhance edge contrasts and encode spectral information; the primary visual cortex encodes orientation of line segments.”
4. In subdividing the integrative component of vision, he identifies: “The occipitoparietal (“where”) pathway, which signals where an object is in space, how fast it is moving, and where it is going. It also mediates how attention is to be distributed acroositems in visual space.” The occipitotemporal (“what”) pathway, which identifies objects, symbols, and colors. Largely dependent on foveal parvocellular inputs, it is phylogenetically newer than the occipitoparietal pathway … Although two parallel integrative visual pathways clearly exist in humans, there is equally firm evidence that the two pathways converge and interact in the perceptual process.”
5. “Glasses-induced image distortion results from differential magnification by the glasses; it is always associated with a pulling sensation about the eyes and nausea or disequilibrium from visual-vestibular interactions.”
6. “A simple way to verify that the VOR is the cause of the symptom is to have a patient read the Snellen near card while moving his or her head rapidly from side to side. Normal VOR compensation permits clear vision to be maintained. If the VOR is defective, acuity will drop at least two lines.”
7. “Integrative visual deficits should never be blamed on elementary visual deficits unless the elementary visual deficits are very severe. Thus a visual acuity as poor as 20/100 in the better eye and visual fields limited to a maximum diameter of 5 degrees are still compatible with normal scores in integrative vision tests, although patients may take slightly longer than those with normal elementary vision to complete the tests. On the other hand, even the mildest integrative dysfunction may interfere with performance on visual acuity and visual field tests.”