How We See the Outside World

                How we see the outside world.

                      The science behind it.

                      PKGhatak, MD

Looking straight ahead, we see a vast area- horizon to horizon. Using a visual field measurement chart and recording one eye at a time, we find each eye has a visual field of 155 degrees on the horizontal plane and 130 degrees on the vertical plane. By adding the visual fields of two eyes together, our visual field should be 310 degrees but in fact, the visual field is smaller because the nasal field of the two eyes overlaps.

That overlap gives us the dept of vision, meaning, we can judge the distance of an object in front. It is an essential quality for reading, writing, driving, painting, target shooting and other activities which require bringing objects in sharp focus.

The diagram above illustrates visual fields and where the images develop.

The visual field is divided into the Nasal field and the ear (Temporal) field. To make a clear distinction between the left and right visual fields, the right-hand side of the visual field is colored red and the blue color for the left.

Note that the nerve fibers carrying the image from the nasal half of the right eye, represented by blue lines, are crossing the midline and going to the left side to join the nerve fibers carrying the image from the temporal half of the left eye. The same is true for the left eye, marked with red color.

It is interesting to note that crossed and uncrossed nerve fibers join and carry images representing the right and left visual fields and not the left or right eyes.

The second point to note is that the nerve fibers make a connection at a nerve center called Lateral Geniculate Body (LGB). The right visual field image goes to the left side and the left visual field image to the right side of the LGB. The stratification of nerve fibers is maintained in the LGB, meaning the temporal fibers remain outer and nasal fibers on the inner nuclei of the LGB.

The third point is that the same stratification is also maintained in the Visual Cortex. The visual cortex is located in the back of the cerebral hemisphere, one on each side in the Occipital Lobe.

Not shown in the diagram, there is upper and lower field stratification on the visual cortex. The lower half of the visual field is represented in the upper part of the opposite cortex, and the upper half in the lower visual cortex.

By knowing the detailed anatomy, the doctors are capable of detecting any segmental visual field loss and treating properly without wasting much time.

How two images merge into one.

This fusion of two images in the visual cortex and perceived as one depends on unison eye movement and convergent reflexes that make image generation on the corresponding points on each retina. When initial images are close but not exactly one, then a further adjustment is made by eye muscles to focus properly.

Why does an object too close to the eyes appear blurred.

The eyes are situated 2 inches apart. The images of the two eyes are not precisely the same, the right eyes see a little more on the right-hand side and the left eyes see more on the left-hand side. The images fuse completely when the object is at a comfortable distance from the eyes. The disparity grows more and more when the object is brought closer and closer.

How images are formed on the retina.

Light rays travel in straight lines in all directions from an object. The pupil by changing its size brings light rays in focus on the retina. The photosensitive pigment of the retina converts the electromagnetic energy of light into electrical stimuli. Next, the rods and cones cells of the retina perceive the stimuli and then pass them to nerve cells which convert the electrical stimuli into nerve impulses and send out nerve impulses to the brain.

 How the color is represented in the brain.

In the daytime, sun rays are bright but colorless. But we know that daylight hides 7 colors from red to violet (color of the rainbow) in it. The color receptors of the eyes are cone shaped cells, called cones of the retina. Cones are plentiful in the central part of the retina and are only a few in the periphery. Cones are densely packed in the Fovea of the retina. The red, green and blue colors are detected by three different cone cells. The blue color generates maximum brightness of 2.56 to 2.76 eV and the red color is the least, generating 1.65 to 1.90 eV. These three cone cells make a unit known as the Trichrome unit. By various combinations of the impulse generation by the trichrome units, we are able to perceive the entire color spectrum. This is the basis of the printers' color cartridges we are so familiar with.  The pigment of the cone is called Opsin. It is a protein and it is most sensitive to light waves length of 550 nm, blue responds to light waves 450 to 485 nm and green to 500 to 565 nm wavelength.

White and Black color.

There is no black color receptor.  Instead, black, white and gray color perception depends on various shades of gray colors. The pigment of the gray scale is Rhodopsin, and the receptors are known as Rods. Rods are most plentiful at the periphery of the retina and are only sparsely distributed in the central part of the retina. Rhodopsin is most sensitive at light waves of 555 nm.

We see better in the corner of the eyes when light is very faint. There are no cones at the periphery of the retina, which is the reason we are unable to see color in the dark.

Why black and white images not as clear as a color pictures.

Each cone is connected with one neuron of the retina that sends out stimuli to the brain. 5 or 6 Rods are connected with one nerve cell, as a result, color picture has more pixels so to speak, and is sharper and clearer than black and white images.

Upside down and revered sidewise image.

This diagram illustrates why the upside down and left side on right side image projected on the retina. 

This is a pinhole camera taking a picture of a tree. The same Laws of Optics work for the eyes.

How Brain rights up images.

A newborn child grabs things with hand and tries to put them in his /her mouth. That is the beginning of the learning process of the brain and by the time the baby is 2yr old the proper interpretation of images by the brain is nearly complete but the learning process of the brain continues for lifelong.

Near and Distant vision.

Refractive errors of one or more components of the eye (cornea, anterior chamber, lens and vitreous) fail to bring light precisely on the retinal receptors and when the light rays fall short- then the condition is known as Near- Sightedness and when the light rays converge beyond the light receptors then it is called Far- Sightedness.

Double vision.

Double vision may be mono-ocular or binocular. Mono-ocular double vision comes from the dry-eyes, or contact lens injury, or foreign body injury of the cornea.

Binocular double vision is due to the failure of mages to merge into one. The defect may lie anywhere from the optic nerve to the visual cortex. Since eyes are extensions of the brain and when an eye is damaged it may not improve unless treatment starts immediately.

A quick way to determine mono-ocular or binocular double vision

The mono-ocular double vision disappears when the eye producing the double vision is closed and the person looks through the other eye. In binocular double vision persists by closing and opening one eye at a time.

In daylight, our world appears in full complements of spectrum colors - clear and beautiful and images up-righted and sides correctly positioned. In the dark the story is different. In fading light, the world appears in various shades of gray, fuzzy and best visible to us by the corner of the eye provided one is neither near-sighted nor far-sighted.

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Movement Disorder

 Movement Disorder

    PKGhatak, MD

Muscles move joints and joints move the body. Movement does not only refer to ambulatory movements but movements of part of the body when the person is stationary. In any movement, two groups of muscles work in a coordinated manner, one group contacts the other group must stay in a relaxed state for the movement to occur. If both groups contact simultaneously, the joint is held in a fixed position and is called spasm.

Well known Movement disorders:

Parkinson's disease, Parkinson syndrome, tremors, ataxia, chorea, tics, myoclonus and spasticity.

Parkinson's Disease (PD) and Parkinson Syndrome:

Parkinson's disease is due to the sickness of Dopamine (neurotransmitter) producing cells located in the Substantia nigra in the hindbrain. Slowly the cells die and many neurological abnormalities develop, including dementia and psychosis.

Movement abnormality in PD.

Slow movement and lack of associated movements, like swinging of arms while walking, are striking features. Increased muscle rigidity of neck, head, limbs and back muscles produces a stooping forward posture; and stiffness of legs and thighs produces a shuffling gait. Tremors of hands at rest produce pill-rolling movement of fingers; later writing, feeding, and grooming become difficult. Loss of balance causes frequent falls. The cause of PD is unknown.

Parkinson Syndrome. This term is reserved for Parkinson like features in patients with known causes, for example - poisoning from heavy metals and pesticides, repeated cerebral concussion, and adverse effects of drugs used in Psychosis, Schizophrenia, Depression, etc.

Parkinson's disease (PD) Subtypes.

Experts classify the movement abnormalities in two catigories.1. Tremor predominant. 2. Postural instability - gait difficulties.

1. Tremor-predominant.

Clinically tremor-predominant PD are of three kinds- Mild, Progressive malignant and intermediate varieties.

a. Mild.

Tremors are mostly seen while trying to work with hands.

B. Progressive malignant PD tremors.

Tremors are rapidly progressive type. The resting tremors are present during waking hours and in light sleep but during deep sleep, tremors disappear. Tremors prevent patients from doing most routine work - like cooking, cleaning, or answering telephone calls.

C. Intermediate

The symptoms of this subtype fall between the above two subtypes.

2. Postural instability-gait difficulty.

Due to increased rigidity, the patients assume a stooping posture. There is a lack of spontaneous corrective adjustments of posture. Decreased two-way communication with the cerebellum produces unsteady balance and a slight bump against a chair or wall the patient falls forward on the ground.

Subtypes of Parkinson's disease.

A. Progressive Supranuclear Palsy (PSP).

The most distinctive feature of PSP is recurrent sudden backward fall on attempted walking, however, during and after the fall the patient remains fully conscious.  Pseudobulbar palsy leads to difficulty in chewing and swallowing. Intranuclear ophthalmoplegia produces abnormal eye conjugate movements when looking downwards. The patient is unable to look upwards and on attempts to look upward produce horizontal nystagmus. The upper eyelids are kept half-closed. PSP patients have no rigidity and tremors unless the disease is far advanced.

B. Restless Leg Syndrome (RLS).

Patients develop an uncontrollable urge to move. Symptoms may be intermittent, and the degree may vary. Restlessness may also be felt in the arms, neck and head. Sitting on a chair for a long period, driving an automobile and sleeping at night become difficult. Patients also describe a strange sensation of insects crawling under the skin, pins and needles, throbbing and aching pain deep in the legs and lower body.

The use of Dopaminergic drugs -  Pramipexole, Ropinirole and Rotigotin  produce some relief in the early phase of the disease. Later L-dopa may be helpful.

C. Lewy body dementia.

Abnormal muscle movements are generally periodic, bizarre, uncontrollable and purposeless. Apathy and profound loss of memory lead to total dependence on others for activities of daily living.

Other Neurological Conditions.

Neurological conditions produce muscle disorders, for example, Multiple System Atrophy (MSP), Cortico-Bulbar syndrome, Low pressure hydrocephalous, Huntington disease, Wilson disease, Tourette syndrome, Motor stereotypies and Multi System Atrophy.

Multi System Atrophy. 

The impairment of the autonomic nervous system produces a loss of spontaneous adjustment of BP with postural changes and with physical activities. Postural hypotension is common. Of the GI symptoms constipation and difficulty in swallowing are common. Men may develop erectile dysfunction and difficulty in voiding urine. Olivopontocerebellar atrophy produces stiffness and incoordination of movements. loss of balance and slowness of movements.

Corticobulbar syndrome.

Degeneration of neurons in the motor and sensory cortex produces symptoms of poor muscle coordination, slowness of thought processing, language difficulty and speech abnormality. Increased and prolonged increased muscle tome produces stiffness of posture.

Huntington disease.

Huntington disease is an autosomal dominant inherited condition. Progressive severe dementia, obsessive compulsive disorder, mania, and bipolar disorder are major components. Writhing slow dance like movements, jerky movements of limbs, (called Chorea) language and speech abnormalities are present. Slow eye movements, muscle rigidity, impaired gait, balance and posture resemble PD. 

Wilson disease. It is a recessive mode of inherited disease involving excess copper accumulation in the brain and other vital organs. The muscle disorder includes tremors, difficulty in swallowing and speech and trouble in ambulation. Cirrhosis of the liver at an advanced stage is common. Copper accumulation at the periphery of the cornea of the eye produces a distinct diagnostic feature called the Kaiser Flacherie ring.

Tourette syndrome. It is an inherited disease, but the precise genetic abnormality is not known. Involuntary movements commonly involve the face and uncontrollable vocal sounds followed by the arms, legs, or trunk. Vocalizations of embarrassing words called Coprolalia often accompanied by obscene jesters, Copropraxia, make patients difficult to socialize with. The symptoms usually begin in childhood and spontaneously may disappear when older.

Motor stereotypes.

These are fixed repeated movements with no useful purpose, appear at the same predictable location and time. Movements last a minute or longer and occur several times a day. Typical motor stereotypes are thumb sucking, lip biting, hair twirling, teeth grinding and head banging. Symptoms start in childhood and rarely persist in adult life. Sometimes more complex motor dysfunctions involving hand and arm movements like waving arms and wiggling fingers in front of the face and closing and opening hands.

Essential tremors. In this disorder, rhythmic involuntary fine shaking movements of hands generally appear in midlife. Tremors mostly occur when the patient is in the company of others and performing normal tasks like drinking a cup of coffee or buttoning a shirt. Tremors may also happen in the head producing nodding of the head as yes-yes or no- no jesters.

Spasticity. Prolonged muscle contractions lead to increased tone of muscles that interfere with normal walking or performing activities of daily living. Spasticity is a feature of cerebral stroke, Multiple sclerosis, spinal cord injury, spina bifida and severe brain injury.

Dystonia. Dystonia is an involuntary muscle spasm producing twisting motion of the limbs or trunk producing abnormal posture and positions. An example is Writer's cramp. The onset of dystonia is gradual and always follows a specific muscle action. Muscles fail to relax in between contractions preventing smooth functioning.

Ataxia. Ataxia means uncoordinated muscle actions, mostly noticeable in arms and legs. Cerebellar ataxia is more common. In the younger age group, viral infections, tumors and cysts and vascular disruption in elderly people are common causes.

Tardive Dyskinesia (TD). Tardive means late onset. Dyskinesia is defined as difficulty in performing voluntary functions. Muscles of the face, tongue and upper extremity show repeated involuntary movements like grimacing, lip smacking, pursing of lips, eye blinking, and rapid involuntary arm, torso and finger movements. Long term use of anti-Schizophrenia, Anti choline and anti-depression drugs use are associated with TD. Discontinuation of drugs improves symptoms if done early.

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