Our brain, though looks simple but yet complicated; is center of our conscious and subconscious being. Each mechanism of our body whether voluntary or involuntary; whether motor or sensory is controlled by our brain. Though small, this organ of ours control each and every part of the body, but it’s still surprising that around 70 % of total brain area is involved in visual processing. There are about 14 levels of visual processing and 305 interconnections of which 25 connections serve to relay pure visual information.
Our primary visual system starts from photoreceptor excitation which in turn stimulus three types of ganglion cells magno ,parvo and konio .Ganglion cell axons combine to form optic nerve which would end in either pretectum , superior colliculus or lateral geniculate nucleus .For orientation of visual attention we have got pulvinar pathway in which nerve fibers enter pulvinar nucleus then superior colliculus and cerebral paracentral cortex.
Another pathway called retinohypothalamic tract starts with melanopsin producing photosensitive ganglion cells called ipRGCs (intrinsically photosensitive ganglion cells) which ends up into suprachiasmatic nucleus in hypothalamus. ipRGCsdonot take information from photoreceptor cells and get automatically excited when visible light ray enters the eye. This is the pathway which concerns with day and night sleep/wake cycle.
Secondary visual system consists of various neocortical modules like primary visual cortex, primary olfactory cortex , primary somatosensory cortex etc. Primary visual cortex connect with other visual association areas via unimodal fibers forming unimodal association cortex .These visual unimodal cortex connect with other sensory unimodal association areas of touch, smell, hearing, etc. forming multimodal association cortex.
Anterior multimodal association area is located in prefrontal cortex and visual percept from here are sent to higher visual cortical center.
Posterior multimodal association area allows temporal and spatial integration of vision which other sensory areas esp. language.
Limbic system is associated with emotion, memory, thought, creativity and acts as interface between visual world and internal environment.
Neural wiring within brain
Neural wiring within brain consist of association fibers, striatal fibers and subcortically directed fibers.
Long association fibers like superior longitudinal fasciculus connects angular gyrus and occipital –parietal area with dorsolateral pre-frontal cortex. This mediates visual awareness ,maintenance of attention in spatial field of view .
Fronto-occipital fasciculus connects occipital and parietal lobes with dorsal prefrontal cortex and is involved in peripheral vision processing.
Inferior longitudinal fasciculus conveys information is bidirectional manner between occipital and temporal lobe. Processing of object, color,face recognition and object memory takes place here.
Arcuate fasciculus connects temporal lobe with prefrontal area meditating language with input from visual system. Cingulum bundle connect caudal cingulate with Parahippocampal area. This area is involved in motivation, emotion and spatial working memory. Commissural fibres communicate between two hemisphere (largest: corpus callosum) for exchange of visual information between two hemisphere.
Apart from other Visual-motor integration, simple visual function can be categorized into different steps;
- Receiving stimulus for clear vision.
- Controlling of gaze toward particular stimulus
- Accommodating for more clear vision and binocularity
- Maintaining visual attention
Receiving Stimulus means ability to send clear visual signal through clear optical system transferring it from retina to primary visual cortex and see clear fused image sent from two eyes.
Controlling Gaze: Supranuclear gaze center mediates vergence. It is mediated in midbrain and integrate with cranial nerve III, IV and VI and horizontal /vertical eye centers in Reticular formation. The center also forms vestibular and proprioceptive multisensory integration. Infranuclear gaze center co-ordinate movement of 12 extraocular muscles. Lesions in this center causes diplopia.
Accommodation is controlled by premotor area for focusing and cortical visual areas for convergence and divergence during the process. From primary visual cortex circuitry continues to Ediner Westphal nucleus where it is changed into motor command by oculomotor nerve.
Attention network: consists of filtering unnecessary information, focusing mainly in an object of regard. The process involves disengaging focus from current object/location and fixating to other by generating saccades by Frontal eye field.
Higher Visual Processing: starts at primary visual cortex(v1) in occipital lobe. Having 6 neo-cortical layers, each with specific visual function; it’s also called striate cortex. These layers contain vertical column of cells each with different functions.i.e. orientation cell, blob cell for color processing, dominance cell for binocular integration ( thus stereopsis begins at primary visual cortex). Also parvo, magno and konio cellular projection from LGN passes through here for extra-striatal processing.
There are seven identifiable visual association areas namely:
- V1: primary visual cortex
- V2: 1st region within visual association area stores object recognition memory and long term memory
- V3: motion , depth perception, contrast
- V4 : color and size estimation
- V5: motion detection
- V8: higher analysis of color
Ventral ( what ) stream: Inferior longitudinal fasciculus connects primary visual cortex with V4 and specific areas of temporal lobe mediating recognition of object and face.
Dorsal (where) stream: connects (V1-V3) and V5 areas with parietal areas and involved in visuo-spatial processing. This system connects with prefrontal/premotor with superior longitudinal fasciculus and frontal occipital fasiculus for visual spatial awareness.
Problems with processing in any one of the above circuitry may cause visual Defects. Problems can be with regard to reception ( blurry, distorted or no perception), attention (attention deficits, inaccurate shift of attention) or higher visual cortical processing. Problems in ventral system processing includes loss of cognitive abilities, agnosia, alexia, anomia, amnesia , hyperemotionality etc. Problem I’m dorsal system processing affect where and how to system resulting in: abnormal motion perception ( akineptosia), visuo- spatial neglect, optic ataxia ( lag in reach), ocular motor apraxia.problems with primary and secondary visual system causes positive visual phenomena like illusion and hallucination.
Our goal as an optometrist thriving to learn neuroptometry portion is to have detailed knowledge in each of these vision processing areas and deficits arising from their abnormal processing. Knowledge on this domain would help in analysis and formulation of proper rehabilitation approach in managing the visual dificits.
Rupesh Poudel, M-Optom Tilganga Eye Institute