VENKATESWARA NETHRALAYA ** Eye Hospital & Lasik Centre***
Pediatric Eye Care
Vision has around 10 Components that need to be tested especially in children who may suffer from anyone of these problems..
1. Central Distance Vision (Snellen/ETDRS Chart)
2. Central Near Vision(Near Vision Chart)
3. Peripheral Vision (perimetry)
4. Brightness perception (Mesopic Pupil, Pupillary reflexes and RAPD)
5. Contrast Sensitivity(PelliRobson Chart)
6. Higher Order Abberations(Abberometry)
7. Colour Vision(Ishiharas Plates)
8. Ocular SensoriMotor Evaluation (Saccades, Smooth Pursuit)- by Saccades by normal 30-2 perimeter disabling the fixation monitors and pursuit by goldman perimetry disabling the fixation monitors.
9. Squint (phoria/tropia/fusion amplitude evaluation using synoptophore)
10. Binocularity (SMP, Fusion, Stereopsis) using a Fly test, TNO, etc
While we test distance vision, near vision, pupillary reflexes regularly as a part of routing ophthalmic evaluation, Perimetry, Colour vision, Synoptophore evaulation is done only if signs are present in further evaluation.
Contrast Sensitivity, HOAs, is done mostly in Pre LVC patients as a part of work up to evaluate if Wavefront correction is required and if patient will be happy with 20/20 post LVC vision.
Other tests like Sensorimotor evaluation, Binocularity is almost never done unless it is done as a part of treament of Amblyopia.
Saccades/Pursuit is not done because most people do not understand the Magnocellular and parvocellular pathways.
The vision is a very complicated photochemical process that has a beginning in the retinal photoreceptors, where the light energy is converted to chemicals that stimulate neural tissue, the ganglion cells which convert these chemical into electrical neural impulses and they carry forward the neural impulses, to the ultimate destination, visual cortex and beyond. V1 (Calcarine cortex) is the first destination of most, if not all retino/geniculo cortical impulses . The ganglion cell output is divided in to nasal and temporal halves, the vertical line splitting the fovea. Nasal fibres cross over to the other side in a complex pathway and end in the Lateral geniculate body (LGB), part of thalamus. Ganglion cells are of mainly two types central P cells and peripheral M cells.P and M cells both these end in different layers of LGB. LGB is a layered structure with 6 layers.1,2,3,4,5,6, and parvo cells discharge in to 4 layers and Magno cells in to 2 layers. This is consistent with central retinal area getting greater representation as opposed to the peripheral retina. Central retina has different function compared to peripheral retina, and so the retinal receptors also have different morphology. Geniculocalcarine fibres end in V 1 mostly.
The right and left fibres ( Nasal crossed and temporal uncrossed) Central P cells and peripheral M fibres course separately and do so till they reach V1. At V1 the Peripheral M cells and Central P cells end in different layers. P cells end in the layer 4 C beta, and peripheral M cells in layer 4 C alfa .The former are involved i “ what of vision “ and the latter “where” of vision. Nearly 80% of the cortical cells are binocular. Each side of cortex receives one half of field of vision. Right occipital cortex receives, left hemi field of vision and left cortex receives the right half of field of vision. The brain compares the two halves of impulses via the connection in the posterior part of carpus callosum,in layer 2 and 3 of V 1. This is the site of stereopsis.
From V 1, 4 C beta the central parvo cellular out put ( called feed forward impulses), goes to V 2 ,V3, V4 and inferior temporal lobe ,is involved in what of vision and fine stereopsis
The peripheral magno cellular out put from level 4 C Alfa in V1, goes to supero parietal area and is involved in motion perception and gross stereopsis . These two “ what of vision” and “where of vision” path ways are intricately connected at different levels in cortex. A considerable amount of information is exchanged between the two pathways via the vertical occipital flocculus (VOF)
The Central P cell discharges at V 1 , 4 C beta are called image processing feed forward pathway and are influenced considerably by the top down impulses, coming from parietal lobe attention area . The parietal lobe is activated a few milliseconds before V 1!
The peripheral magnocellular out put reaches the supero parietal area, and then reaches the attention area in the IPS 1 and IPS 2 in the inferior parietal area. The peripheral Magnocellular output reaches the attention area in parietal lobe much before the P cell output reaches temporal lobe. From the temporal lobe also there is an input in to the attention area in parietal lobe, which is close to the saccade area, in parietal lobe. The saccadic area and attention area influence each other and saccadic area is connected to frontal eye field area. The top down impulses from Parietal area to V 1 are more than the bottom up impulses from V 1 to parietal attention area
The Stimulation of peripheral retina by the peripheral light, stimulates supero parietal area and attention area in the same parietal lobe. The attention area sends top down impulses (called image modulating impulses) to V 1 area, to facilitate and accelerate the image processing that happens in V 1 at level 4 C beta and the image process is greatly hastened.
Sudden appearance of stimulus in the peripheral field of vision, directs the attention of fovea to this and the eye makes a saccadic movement to locate and analyze the peripheral stimulus and during the saccade, the two areas stimulated are the peripheral retinal area and the fovea and the intervening retina is suppressed to prevent image smearing due to fast movement. The fovea is stimulated a few milli seconds before the onset of saccade and persists after completion of movement. This is possible via the top down impulses from attention area in parietal lobe to the V1. Repeated stimulation of the magnocellular fibres, leads to repeated stimulation of V1 via top down impulses as mentioned above. The top down impulses accelerate the feed forward impulses at V 1 beyond the suppression at layer 4 C beta level.
So, the pathway for Central P cells/fibres is CP4CT (Central,Parvocellular,4Layered in thalamus,Colour vision,Temporal cortex) while for Peripheral M cells/fibres ,it is PM2MP (Peripheral,magnocellular, 2layered in thalamus, monochrome or black and white, parietal cortex). After the geniculate body, the fibres go to V1 in the calcarine cortex to layers 4C alpha(Magnocellular) and 4C beta (parvocellular). From V1 they go to temporal and parietal lobe.
For Saccadic stimulation of the peripheral Magnocellular pathway in the mid retinal periphery, 30-2 or 24-2 perimeter is better than a 1m chart. This is because the farther the chart, the more central is the stimulus initiating a pursuit rather than Saccade. In a pursuit movement, foveation of the stimuli is less accurate than a Saccade. The onset and amplitude of a saccade is less when a far retinal peripheral point esp when the attention is strong and such stimuli are likely to be ignored.
Stimuli size and brightness in the peripheral Magnocellular pathway is important in eliciting a saccade. The greater the saccade, greater is the occipital stimulation leading to foveal stimulation upto a point and after that the occipital stimulation tapers down as the saccade is not initiated at all as the eye tries to avoid extremely large or extremely bright stimuli in the periphery..
Again Inferior Stimulus points do better foveal stimulation as compared to Superior Stimulus points.
All these can be achieved by modifying certain parameters in the Perimeter. A perimeter is basically designed to block a saccade by its fixation monitoring systems. So disable the fixation
Amblyopia is a condition where foveal stimulation of the occipital cortex is weak either due to strabismus, anisometropia, stimulus deprivation. It can also be described as Attention deficit disorder and the top down impulses ( alfa and Beta ) probably appear randomly due to non programmin in childhood with normal vision and after the appearance of the gamma oscillations (due to image processing activity) and hence the image processing acceleration by the Central parvocellular pathway to V2,3,4 temporal cortex identifying the “what” of vision does not happen.
Traditionally treatment of Amblyopia depended on patching the better eye and forcing the defective lazy eye to intiate foveal stimulation by central stimuli so that image processing acceleration by the Central parvocellular pathway to V2,3,4 and temporal cortex can happen. This was prolonged, and not 100% effectove as the top down impulses from the parietal cortex, frontal eye fields were random as they were not programmed properly to respond in a coherent manner to central stimuli due to out of focus images from sensory deprivation, strabismus, or anisometropic eye having led to amblyopia in the first place. Further forced stimulation of the same pathways with a defective programming of the image processing neural network from the top down signals arising from the parital cortex to VI will only lead to partial results as was happening with patching the better eye. ..Instruments like the Perimeter or Santans instrument depend on Peripheral Stimuli to initiate a saccade which initiate the top down signals from latera interparietal cortex to V1which inturn alerts the macula via the occipital cortex as well as Calcarine-temporal cortex through the FEF which stimulates the occipital lobe about the impending arrival of the object on the fovea. This is possible because the saccades as well as pursuit sensorimotor pathways are already programmed early in childhood as strabismus,anisometropia do not affect saccades/pursuit. Total Sensory deprivation amblyopia may again affect recovery in this manner. Hence in Total sensory deprivation amblyopia combined treatment with patching and peripheral stimulation may work better than just patching alone.
In fact in strabismus and anisometropia, peripheral stimulation may be attempted first followed by patching while in sensory deprivation both may be attempted simultaneously.
Repeated stimulation will lead to better image processing signallation programming to the point where the central parvocellular pathway too starts using the same neural networks to deliver better central vision or the what of the vision.
This explains why the instrument works so rapidly and consistently, making the top down impulses appear 100 milli secs before the bottom up impulses.
Top–down flow of visual spatial attention signals from parietal ( IPS 1 and IPS 2) to occipital cortex stimulates early visual areas in occipital cortex V1 ( layer 4 c Beta)
Our results show that sustained visual spatial attention in the absence of visual stimulation is accompanied by widespread increases in coherency magnitude for many pairs of parietal and occipital cortical areas and that attention-related activity in IPS1 and IPS2 leads that in the visual cortex by a few hundred milliseconds. These findings imply a top–down flow of information from IPS1 and IPS2 to earlier areas in the visual cortical processing hierarchy during sustained attention and support the hypothesis that IPS1 and IPS2 transmit attention signals from higher brain areas to visual cortex.
This is consistent with a model in which attention accelerates feedforward processing relative to fixation
effects of attention are first observed in extra striate cortex and only later in V1, consistent with a top–down flow of visual spatial attention signals to cortical area V1