Reflex arc of the pupil's response to light. Syndrome of impaired pupillary reflexes. Examination of the anterior chamber of the eye

The pupil (lat. pupilla, pupula) is a circle in the very center of the iris. It has a distinctive feature: thanks to the work of the muscles (sphincter and dilator), it becomes possible to regulate the flow of light directed at the retina. In bright sunlight or electric lighting, the sphincter becomes tense and the pupil narrows, blinding rays are cut off, the image becomes clear, without blur.

In twilight lighting, on the contrary, the pupil dilates (thanks to the dilator). All this is called the “diaphragmatic function”, which is provided by the pupillary reflex.

Pupillary reflex, symptoms of damage

Pupillary reflex: how it occurs

Any reflex has two directions:

Sensitive - it transmits information to the nerve centers;
Motor - transmits information from nerve centers directly to tissues. It is precisely this that is the response to an irritating impulse.

Pupil response(lat. pupilla, pupula) the irritating effects of light can be:

Direct - in which the light has a direct effect directly on the eye being examined;
Friendly - when the result of the influence of light is observed in the eye, which was not affected.

In addition to the reaction to lighting, pupula (Latin) reacts to the work of convergence (tension of the internal rectus muscles of the eyes) and accommodation - tension of the ciliary muscle, it occurs when a person moves his eyes from an object located in the distance to an object located nearby.

In addition, pupilla expansion can cause:

Pupula narrowing occurs:

With irritation of the trigeminal nerve;
With apathy, decreased excitability;
When taking medications that target the muscle receptors in the eyes directly.

The pupil is affected: symptoms

When the pupilla (Latin) is affected, its constant narrowing or expansion is monitored, regardless of exposure to light on the eyes.

Symptoms:

Change of form pupilla (Latin);
Hippus - the shape of the pupil changes in attacks that last several seconds;
Fixed (amaurotic) - a direct reaction occurs in the blind eye, which is exposed to light, and a friendly reaction in the sighted eye;
Nystagmus is involuntary rapid repetitive eye movements;
"Jumping pupils" - periodic dilation of the pupilla (Latin) in both eyes, while the reaction to light is normal;
Anisocoria - pupils of different sizes in the right and left eyes.

Diagnosis of the lesion

Visual inspection, determination of pupil equidistance;
Studying the reaction to exposure to a light source;
Studying the reaction of the pupula when studying the work of other muscles of the organs of vision;
Pupillometry (in case of pathology) - study the size of the pupil and the dynamics of its change.

Diseases that affect the pupillary reflex

Diseases that may cause a change in the reaction of the pupula (Latin) to a light source, as well as

Reflexes are the most important function of the body. Scientists who studied reflex function mostly agreed that all conscious and unconscious acts of life are essentially reflexes.

What is a reflex

Reflex is the response of the central nervous system to irritation of the recipes, which ensures the body's response to changes in the internal or external environment. The implementation of reflexes occurs due to irritation of nerve fibers, which are collected in reflex arcs. Manifestations of the reflex are the occurrence or cessation of activity on the part of the body: contraction and relaxation of muscles, secretion of glands or its stop, constriction and dilation of blood vessels, changes in the pupil, etc.

Reflex activity allows a person to quickly react and properly adapt to changes around him and within. It should not be underestimated: vertebrate animals are so dependent on the reflex function that even its partial disruption leads to disability.

Types of reflexes

All reflex acts are usually divided into unconditional and conditional. Unconditional ones are transmitted hereditarily; they are characteristic of every biological species. Reflex arcs for unconditioned reflexes are formed before the birth of the organism and remain in this form until the end of its life (if there is no influence of negative factors and diseases).

Conditioned reflexes arise in the process of development and accumulation of certain skills. New temporary connections are developed depending on conditions. They are formed from unconditioned ones, with the participation of higher brain regions.

All reflexes are classified according to different criteria. According to their biological significance, they are divided into nutritional, sexual, defensive, orientation, locomotor (movement), postural-tonic (position). Thanks to these reflexes, a living organism is able to provide the main conditions for life.

In each reflex act, all parts of the central nervous system are involved to one degree or another, so any classification will be conditional.

Depending on the location of irritation receptors, reflexes are:

exteroceptive (external surface of the body);
viscero- or interoreceptive (internal organs and vessels);
proprioceptive (skeletal muscles, joints, tendons).

Depending on the location of neurons, reflexes are:

spinal (spinal cord);
bulbar (medulla oblongata);
mesencephalic (midbrain);
diencephalic (diencephalon);
cortical (cerebral cortex).

The reflex acts carried out by neurons of the higher parts of the central nervous system also involve fibers of the lower parts (intermediate, middle, medulla oblongata and spinal cord). In this case, the reflexes that are produced by the lower parts of the central nervous system necessarily reach the higher ones. For this reason, the presented classification should be considered conditional.

Depending on the response and the organs involved, reflexes are:

motor, motor (muscles);
secretory (glands);
vasomotor (blood vessels).

However, this classification applies only to simple reflexes that combine certain functions within the body. When complex reflexes occur that irritate the neurons of the higher parts of the central nervous system, different organs are involved in the process. This changes the behavior of the organism and its relationship with the external environment.

The simplest spinal reflexes include flexion, which allows you to eliminate the stimulus. This also includes the scratching or rubbing reflex, knee and plantar reflexes. The simplest bulbar reflexes: sucking and corneal (closing of the eyelids when the cornea is irritated). Mesencephalic simple ones include the pupillary reflex (constriction of the pupil in bright light).

Features of the structure of reflex arcs

A reflex arc is the path that nerve impulses travel, carrying out unconditioned and conditioned reflexes. Accordingly, the autonomic reflex arc is the path from irritation of nerve fibers to the transmission of information to the brain, where it is converted into a guide to the action of a specific organ. The unique structure of the reflex arc includes a chain of receptor, intercalary and effector neurons. Thanks to this composition, all reflex processes in the body are carried out.

Reflex arcs as parts of the peripheral nervous system (the part of the nervous system outside the brain and spinal cord):

arcs of the somatic nervous system, which provide nerve cells to skeletal muscles;
arcs of the autonomic system that regulate the functionality of organs, glands and blood vessels.

Structure of the autonomic reflex arc:

Receptors. They serve to receive irritating factors and respond with excitation. Some receptors are presented in the form of processes, others are microscopic, but they always include nerve endings and epithelial cells. Receptors are part of not only the skin, but also all other organs (eyes, ears, heart, etc.).
Sensory nerve fiber. This part of the arc ensures the transmission of excitation to the nerve center. Since the nerve fiber bodies are located directly near the spinal cord and brain, they are not included in the central nervous system.
Nerve center. Here, switching between sensory and motor neurons is ensured (due to instantaneous excitation).
Motor nerve fibers. This part of the arc transmits a signal from the central nervous system to the organs. The processes of nerve fibers are located near internal and external organs.
Effector. In this part of the arc, signals are processed and a response to receptor stimulation is formed. The effectors are mostly muscles that contract when the center receives stimulation.

The signals of receptor and effector neurons are identical, since they interact following the same arc. The simplest reflex arc in the human body is formed by two neurons (sensory, motor). Others include three or more neurons (sensory, intercalary, motor).

Simple reflex arcs help a person involuntarily adapt to changes in the environment. Thanks to them, we withdraw our hands if we feel pain, and our pupils react to changes in lighting. Reflexes help regulate internal processes and help maintain a constant internal environment. Without reflexes, homeostasis would be impossible.

How the reflex works

A nervous process can provoke or increase the activity of an organ. When nervous tissue receives irritation, it goes into a special state. Excitation depends on differentiated concentrations of anions and cations (negatively and positively charged particles). They are located on both sides of the membrane of the nerve cell process. When excited, the electrical potential on the cell membrane changes.

When a reflex arc has two motor neurons in the spinal ganglion (nerve ganglion), the cell's dendrite will be longer (a branched process that receives information through synapses). It is directed towards the periphery, but remains part of the nervous tissue and processes.

The excitation speed of each fiber is 0.5-100 m/s. The activity of individual fibers is carried out in isolation, that is, the speed does not transfer from one to another.

Inhibition of excitation stops the functioning of the site of stimulation, slowing down and limiting movements and responses. Moreover, excitation and inhibition occur in parallel: while some centers fade away, others are excited. Thus, individual reflexes are delayed.

Inhibition and excitation are interconnected. Thanks to this mechanism, the coordinated operation of systems and organs is ensured. For example, the movements of the eyeball are carried out by alternating the work of muscles, because when looking in different directions, different muscle groups contract. When the center responsible for muscle tension on one side is excited, the center on the other slows down and relaxes.

In most cases, sensory neurons transmit information directly to the brain using a reflex arc and several interneurons. The brain not only processes sensory information, but also stores it for future use. In parallel, the brain sends impulses along the descending pathway, initiating a response from effectors (the target organ that performs the tasks of the central nervous system).

Visual path

The anatomical structure of the visual pathway is represented by a number of neural links. In the retina, these are rods and cones, then bipolar and ganglion cells, and then axons (neurites that serve as a path for impulses emanating from the cell body to the organs).

This circuit represents the peripheral portion of the visual pathway, which includes the optic nerve, chiasm, and optic tract. The latter ends in the primary visual center, where the central neuron of the visual pathway begins, which reaches the occipital lobe of the brain. The cortical center of the visual analyzer is also located here.

Components of the visual pathway:

The optic nerve begins at the retina and ends at the chiasm. Its length is 35-55 mm, and its thickness is 4-4.5 mm. The nerve has three sheaths and is clearly divided into halves. The nerve fibers of the optic nerve are divided into three bundles: axons of nerve cells (from the center of the retina), two fibers of ganglion cells (from the nasal half of the retina, as well as from the temporal half of the retina).
The chiasm begins above the area of the sella turcica. It is covered with a soft shell, length is 4-10 mm, width is 9-11 mm, thickness is 5 mm. This is where fibers from both eyes connect to form the optic tracts.
The visual tracts originate from the posterior surface of the chiasm, go around the cerebral peduncles and enter the external geniculate body (the unconditional visual center), the visual thalamus and the quadrigeminals. The length of the optic tracts is 30-40 mm. The fibers of the central neuron begin from the geniculate body and end in the sulcus of the bird's spur - in the sensory visual analyzer.

Pupillary reflex

Let's consider the reflex arc using the example of the pupillary reflex. The path of the pupillary reflex passes along a complex reflex arc. It starts from the fibers of the rods and cones, which are part of the optic nerve. The fibers cross in the chiasm, passing into the optic tracts, stop in front of the geniculate bodies, partially twist and reach the pretectal region. From here, new neurons go to the oculomotor nerve. This is the third pair of cranial nerves, which is responsible for the movement of the eyeball, the light reaction of the pupils, and the raising of the eyelid.

The return path begins from the oculomotor nerve to the orbit and the ciliary ganglion. The second neuron of the link emerges from the ciliary ganglion, through the sclera into the perichoroidal space. A nerve plexus is formed here, the branches of which penetrate into the iris. The sphincter of the pupil has 70-80 radial neuron bundles entering it sectorally.

The signal for the muscle that dilates the pupil comes from the ciliospinal center of Budge, which is located in the spinal cord between the seventh cervical and second thoracic vertebrae. The first neuron goes through the sympathetic nerve and sympathetic cervical ganglia, the second starts from the superior ganglion, which enters the plexus of the internal carotid artery. The fiber that supplies the pupillary dilator nerves leaves the plexus in the cranial cavity and enters the optic nerve through the trigeminal ganglion. Through it, the fibers penetrate the eyeball.

The closedness of the circular work of the nerve centers makes it perfect. Thanks to the reflex function, the correction and regulation of human activity can occur voluntarily and involuntarily, protecting the body from changes and danger.

Pupil called the hole in the center of the iris through which all light rays entering the eye pass. Pupil It ensures clarity of the image of objects on the retina, transmitting only central rays and eliminating the so-called spherical aberration.

Spherical aberration is that rays striking the peripheral parts of the lens are refracted more strongly than the central rays ( rice. 209). Therefore, if peripheral rays are not eliminated, circles of light scattering should be obtained on the mesh.

The muscles of the iris are able to change the size of the pupil and thereby regulate the flow of light into the eye. If you cover your eye from the light and then open it, the pupil, which dilated during darkening, quickly narrows. This narrowing occurs reflexively.

In the iris there are two types of muscle fibers surrounding the pupil: some are circular (m. sphincter iridis), innervated by parasympathetic fibers of the oculomotor nerve, others are radial (m. d ilatator iridis), innervated by sympathetic nerves ( rice. 210). Contraction of the former causes constriction of the pupil, contraction of the latter causes dilation of the pupil.

Rice. 210. Scheme of innervation of the iris and ciliary muscle. 1 - ciliary ganglion; 2 - short ciliary nerves; 3 - superior cervical sympathetic node; 4 - sympathetic nerves; 5-ciliary muscle; 6 - fibers of the lens capsule; 7 - iris; 8 - cornea; 9 - lens; 10 - ring muscles of the iris; 11 - radial muscles of the iris.

Accordingly, adrenaline causes pupil dilation, and acetylcholine and eserine cause pupil constriction. With emotions accompanied by excitation of the symiatric system (fear, rage, pain), the pupils dilate.

The pupils also dilate during asphyxia. Therefore, dilation of the pupils during deep anesthesia indicates impending asphyxia and is an ominous sign indicating the need to reduce anesthesia.

The pupils of both eyes in healthy people are dilated or constricted equally. When one eye is illuminated, the pupil of the other also narrows; such a reaction is called friendly.

Constriction of the pupil also occurs when viewing close objects, when accommodation and convergence of the visual axes of both eyes occurs (convergence).

In some cases, the pupil sizes of both eyes are different (anisocoria). This may occur due to damage to the sympathetic nerve on one side, which entails a narrowing of the pupil (miosis) and at the same time a narrowing of the palpebral fissure (Horner's sign). Dilation of the pupil (mydriasis) of one eye can occur due to paralysis of the n. ociiloinatorius or due to irritation n. sympathicus.

The physiology of sensory systems is based on reflex activity. The pupillary reflex is a friendly reaction of both pupils to light. Its adequacy is determined by the coordinated activity of all components of the neural arch, consisting of 4 neurons and the brain center. The eyes do not immediately react to flash or darkness. It takes a fraction of a second for the impulse to reach the brain areas. Too sluggish a reaction indicates pathology at some stage of the reflex chain.

Normally, the direct reaction of constriction and dilation of the pupillary fissure in response to fluctuations in lighting around the human head depends on the adequate activity of afferent and efferent nerve fibers. It is also influenced by the functioning of the center in the optical cortex of the occipital hemispheres of the brain.

Anatomy of the eyeball and nerves

The arc of the pupillary reflex begins on the retina and passes through several nerve regions. In order to better navigate the sequence of movement of impulses to the target point, below is a diagram of the anatomical structure of the eye analyzer:

Cornea. It is the first obstacle in the path of the light beam. This transparent structure consists of a dense row of cells, the structure of which is dominated by cytoplasm.
Front camera. It does not contain liquid. This cavity limits the pupillary opening in front.
Pupil. It is a hole surrounded on all sides by the iris. It is the pigmentation of the latter that gives the eyes their color.
Lens. It is considered the second refractive structure after the cornea. According to its anatomy, the lens is a biconvex lens, capable of changing curvature due to the contraction and relaxation of the accommodative muscles and the ciliary body.
Rear camera. It is filled with vitreous humor, which is a gel-like mass that conducts light rays.
Retina. This is a collection of nerve cells - rods and cones. The former capture light, the latter determine the color of objects around.
Optic nerve. It conducts light impulses accumulated by rods and cones to the optic tract.
Bactrian bodies. They are structures of the central nervous system.
Axons heading to the Jakubovich or Edinger-Westphal nuclei. These fibers represent the afferent site of the unconditioned reflex.
Axons of parasympathetic oculomotor nerves to the ciliary ganglion.
Short fibers of neurons of the ciliary ganglion to the muscles that constrict the pupil. They close the reflex arc.

What is he?

Depending on the lighting, a person’s pupil changes: in weak light it expands, in strong light it narrows.

The normal reaction of the pupil to light or photoreaction is a narrowing of the pupillary slit when there is an abundant supply of light photons and its widening in low light. The pupillary reflex pathways begin on the light-refracting structures of the eyeball. Captured by the light-sensitive cells of the retina - rods and cones - photons of light are recorded by specific pigments and arrive in the form of nerve impulses to the optic nerve. From there, through neurotransmitters along myelinated fibers, the impulse passes into the afferent part of the nerve pathway. Afferentation ends at the level of the midbrain nuclei of Yakubovich or Edinger-Westphal. They are also called accessory nuclear structures of the oculomotor nerve. From the tegmentum of the brain stem, impulses through the conductive area enter the muscle fibers, causing the pupillary fissure to expand and contract.

How does the verification take place?

Pupillary response to light is studied in ophthalmology clinics or physical therapy offices. Its demonstration is made possible with the help of a special lamp that supplies pulsating light with different frequencies and strengths. Under the influence of light rays, the nerves that conduct impulses are excited and the doctor registers reflex movements. Using the same technique, convergence and divergence are studied. Their adequacy indicates full binocular vision. Before starting the study, it is necessary to take into account the concomitant medical history. If the diagnosis is carried out in a person who abuses psychoactive substances, is intoxicated, or has a complicated neurological history, adjustments should be made in advance for these characteristics. Physiology studies the mechanics of testing and the boundaries of normal and pathological results.

Normal limits

The change in diameter with normal vision occurs synchronously; in another case, pathology is diagnosed.

The reaction of the pupils to an increase or decrease in the intensity of the glow should be bilateral and synchronous. A slight difference in diameter is allowed if a person has previously been diagnosed with unilateral myopia or hypermetropia. These medical terms refer to nearsightedness or farsightedness in one eye. In such patients, the affected eyeball must capture slightly less or more light, thereby regulating the number of photons reaching the retina. In healthy people, the pupillary diameter varies between 1.2-7.8 mm. In a brown-eyed person, this value will always be higher, since the dark pigment melanin additionally protects the retina from excessive insolation.

VISUAL PATH

The anatomical structure of the visual pathway is quite complex and includes a number of neural links. Within the retina of each eye there is a layer of rods and cones (photoreceptors - the first neuron), then a layer of bipolar (second neuron) and ganglion cells with their long axons (third neuron). Together they form the peripheral part of the visual analyzer. The pathways are represented by the optic nerves, chiasm and optic tracts.

The latter end in the cells of the external geniculate body, which plays the role of the primary visual center. From them originate the fibers of the central neuron of the visual pathway, which reach the region of the occipital lobe of the brain. The primary cortical center of the visual analyzer is localized here.

The optic nerve is formed by the axons of retinal ganglion cells and ends at the chiasm. A significant part of the nerve is the orbital segment, which in the horizontal plane has an 8-shaped bend, due to which it does not experience tension when the eyeball moves.

Over a considerable distance (from the exit from the eyeball to the entrance to the optic canal), the nerve, like the brain, has three membranes: hard, arachnoid, soft. Together with them, its thickness is 4–4.5 mm, without them – 3–3.5 mm. At the eyeball, the hard shell fuses with the sclera and the Telon capsule, and at the optic canal, with the periosteum. The intracranial segment of the nerve and the chiasm, located in the subarachnoid chiasmatic cistern, are dressed only in a soft shell. The intrathecal spaces of the orbital part of the nerve (subdural and subarachnoid) are connected to similar spaces of the brain, but are isolated from each other. They are filled with fluid of complex composition (intraocular, tissue, cerebrospinal).

Since intraocular pressure is normally two times higher than intracranial pressure (10–12 mm Hg), the direction of its current coincides with the pressure gradient. The exception is cases when intracranial pressure increases significantly (for example, with the development of a brain tumor, hemorrhages in the cranial cavity) or, conversely, the tone of the eye significantly decreases.

All primary fibers that make up the optic nerve are grouped into three main bundles. The axons of ganglion cells extending from the central (macular) region of the retina constitute the papillomacular fascicle, which enters the temporal half of the optic nerve head. Fibers from the ganglion cells of the nasal half of the retina run along radial lines into the nasal half of the disc. Similar fibers, but from the temporal half of the retina, on the way to the optic nerve head “flow around” the papillomacular bundle from above and below.

In the orbital segment of the optic nerve near the eyeball, the relationships between nerve fibers remain the same as in its disk. Next, the papillomacular bundle moves to the axial position, and the fibers from the temporal squares of the retina move to the entire corresponding half of the optic nerve. Thus, the optic nerve is clearly divided into right and left halves. Its division into upper and lower halves is less pronounced. An important clinical feature is that the nerve is devoid of sensory nerve endings.

In the area of the skull, the optic nerves connect above the area of the sella turcica, forming a chiasm, which is covered with the pia mater and has the following dimensions: length 4-10 mm, width 9-11 mm, thickness 5 mm. The chiasma borders below with the diaphragm of the sella turcica (the preserved portion of the dura mater), above (in the posterior section) with the bottom of the third ventricle of the brain, on the sides with the internal carotid arteries, and behind with the pituitary infundibulum.

In the area of the chiasm, the fibers of the optic nerves partially intersect due to portions associated with the nasal halves of the retinas.

Moving to the opposite side, they connect with fibers coming from the temporal halves of the retinas of the other eye and form the visual tracts. The papillomacular bundles also partially intersect here.

The optic tracts begin at the posterior surface of the chiasm and, going around the outer side of the cerebral peduncles, end in the external geniculate body, the posterior part of the visual thalamus and the anterior quadrigeminosus of the corresponding side. However, only the external geniculate bodies are an unconditional subcortical visual center. The remaining two entities perform other functions.

In the optic tracts, the length of which in an adult reaches 30–40 mm, the papillomacular bundle also occupies a central position, and crossed and uncrossed fibers still run in separate bundles. In this case, the first of them are located vectromedially, and the second - do-rheolaterally. The optic radiation (central neuron fibers) originates from the ganglion cells of the fifth and sixth layers of the lateral geniculate body.

First, the axons of these cells form the so-called Wernicke's field, and then, passing through the posterior thigh of the internal capsule, they fan out in the white matter of the occipital lobe of the brain. The central neuron ends in the avian spur groove. This area represents the sensory visual center - the seventeenth cortical area according to Brodmann.

The path of the pupillary reflex - light and for placing the eyes at a close distance - is quite complex. The afferent part of the reflex arc of the first of them begins from the cones and rods of the retina in the form of autonomous fibers running as part of the optic nerve. In the chiasm they intersect in the same way as the optic fibers and pass into the optic tracts. Pupillomotor fibers leave them in front of the external geniculate bodies and, after partial crossover, end at the cells of the so-called pretectal region. Next, new interstitial neurons, after partial decussation, are sent to the corresponding nuclei (Yakutovich - Edinger - Westphal) of the oculomotor nerve. Afferent fibers from the macula of the retina of each eye are represented in both oculomotor nuclei.

The efferent pathway of innervation of the iris sphincter begins from the already mentioned nuclei and runs as a separate bundle as part of the oculomotor nerve. In the orbit, the sphincter fibers enter its inferior branch. And then through the oculomotor root - into the ciliary ganglion. Here the first neuron of the path in question ends and the second begins. Upon exiting the ciliary ganglion, the sphincter fibers as part of the short ciliary nerves, passing through the sclera, enter the perichoroidal space, where they form a nerve plexus. Its terminal branches penetrate the iris and enter the muscle in separate radial bundles, i.e., innervate it sectorally. In total, there are 70–80 such segments in the pupillary sphincter.

The efferent pathway of the dilator (expander) of the pupil, which receives sympathetic innervation, begins from the ciliospinal center of Budge. The latter is located in the anterior horns of the spinal cord. From here connective branches arise, which reach the superior ganglion through the borderline trunk of the sympathetic nerve, and then the lower and middle sympathetic cervical ganglia. Here the first neuron of the path ends and the second one begins, which is part of the plexus of the internal carotid artery. In the cranial cavity, the fibers innervating the pupillary dilator leave the mentioned plexus, enter the trigeminal (Gasserian) ganglion, and then leave it as part of the optic nerve. Already at the top of the border they pass into the nasociliary nerve and then, together with the long ciliary nerves, penetrate the eyeball. In addition, the central sympathetic pathway departs from the Budge center, ending in the occipital cortex of the brain. From here begins the corticonuclear pathway of inhibition of the sphincter of the pupil.

Regulation of the function of the pupillary dilator occurs with the help of the supranuclear hypothalamic center, located at the level of the third ventricle of the brain in front of the pituitary infundibulum. Through the reticular formation it is connected with the ciliospinal center of Budge.

The reaction of the pupils to convergence and accommodation has its own characteristics, and the reflex arcs in this case differ from those described above.

During convergence, the stimulus for pupil constriction is proprioceptive impulses coming from the contracting internal rectus muscles of the eye. Accommodation is stimulated by the blurriness (defocusing) of images of external objects on the retina. The effective part of the arc of the pupillary reflex is the same in both cases.

The center for setting the eye to a close distance is believed to be in the eighteenth cortical area according to Brodmann.