In detail

How visual perception occurs

How visual perception occurs

From the eye to the brain

Visual perception consists of ability to detect light and interpret it through the sense of sight.

Visual perception begins as soon as the eye focuses the light on the retina, where it is absorbed by a layer of photoreceptor cells. These cells convert light into electrochemical signals and are divided into two types, rods and cones, named for their shape. Normal vision depends on the integrity and proper functioning of all centers and visual pathways.

The main problem in visual perception is that what people see is not simply a translation of retinal stimuli (that is, the image in the retina), if it is not the result of a complex framework that decodes our brain.

Content

  • 1 The photoreceptors
  • 2 The visual field representation
  • 3 Primary visual route
  • 4 Primary visual cortex or V1
  • 5 Secondary visual area or V2

The photoreceptors

The Canes They are responsible for our night vision and respond well to dim light. They are found mainly in the peripheral regions of the retina, so most people will realize that they can see better at night if they focus their gaze right next to what they are observing.

The cones they are concentrated in a central region of the retina called fovea; They are responsible for tasks of great acuity, such as reading, and also for color vision. Cones can be subcategorized into three types, depending on how they respond to red, green and blue light. In combination, these three types of cone allow us to perceive color.

The visual field representation

Each eye Look at a part of the visual space, it's your visual field. Thus, each retina and its visual field are divided into four quadrants (vertical and horizontal lines that intersect in the fovea): two of nasal (upper and lower) and two of temporal (upper and lower).

Since the eye is spherical, the nasal retina of the left eye and the temporal retina of the right eye look towards the left half of the visual field (left hemisphere); and the nasal retina of the right eye and the temporal retina of the left eye look towards the right half of the visual field (right hemisphere). At the same time, the lower part of the retina looks towards the upper field of vision and the upper part of the retina, in the lower field of vision.

The nasal retina receives light from the temporal field of vision and the temporal retina receives light from the visual nasal field.

So that, when the light passes through the different optical elements of the eye, the images that are projected on the retina are inverted with respect to their original position in the visual field.

Projection of the image of the inverted visual field on the retina.

Primary visual route

The most important route of the visual system is the so-called retinogeniculooccipital or primary visual route. In this way, the resulting signals resulting from visual processing in the retina leave through the optic nerve and head towards the dorsal geniculate nucleus of the thalamus; and from there to the primary visual cortex that we found in the occipital lobe.

Fibers that originate in the nasal part of each retina cross in the opposite hemisphere, while those that originate in the temporal part project directly to the ipsilateral hemisphere.

This means that the fibers originating in the temporal retina of the left eye and those originating in the nasal retina of the right eye project towards the left hemisphere; and vice versa, the fibers originating in the nasal retina of the left eye and those originating in the temporal retina of the right eye project towards the right hemisphere.

Once the optic chiasm is crossed, the axons of the ganglion cells become part of the optic tract. These axons reach the NGLd of the thalamus, in an orderly manner and creating a map of the contralateral hemisphere. NGLd neurons maintain this topography in their projection towards the primary visual cortex.

Projection of the visual field on the retinas and crossing of the corresponding fibers in the optic chiasm.

Dorsal lateral geniculate nucleus

Each lateral geniculate nucleus (NGL) is made up of six cell layers. The arrangement of the nucleus in differentiated layers has to do with the synaptic relay of different types of retinal information.

Layers one and two contain neurons with a large cell body and are called magnocellular layers. These layers receive the information of the ganglion cells of the type M retina. The rest of the layers (from three to six) are formed by cells with a smaller soma and are therefore called parvocellular layers. These layers receive the information of the ganglion cells of the type P retina.

Thus, each NGL, the axons from the ipsilateral eye, synapse in layers two, three and five; and those of the contralateral eye in layers one, four and six.

Primary visual cortex or V1

The visual world is represented in what we call V1 and in thirty other different areas of the cerebral cortex (some of these exclusively visual and others, polymodal). The representation or cortical map of each of these cortical areas are connected to each other and give rise to a very complex network.

The primary visual cortex (also called striated cortex or V1) is located along the calcarine fissure of the occipital lobe and corresponds to area 17 of Brodmann.

Morphologically, V1 shows a band of myelinated fibers parallel to the cortical surface, which has given it the name of striated cortex. In V1 we find two organizational levels: layers and columns.

Although we know that there is a functional specialization of each area, visual perception originates from the global activity of the entire cerebral cortex. The information is segregated in different parallel processing channels specialized in the analysis of the different attributes of the visual stimulus.

Although the neuroscience Currently, it still cannot explain many aspects of visual perception, many advances have been made in recent years to answer the question of how they represent neurons the different aspects of our visual world.

Scheme of the cytoarchitecture of the striatum cortex or V1.

In both humans and macaques, approximately half of the V1 cortex is dedicated to collecting information from the fovea and its surrounding region. This enables the great acuity of spatial discrimination in the central part of the visual field.

Apart from the primary visual pathway, axons from the retina give rise to other different pathways:

  • Projections in the hypothalamus: Participates in the synchronization of biological rhythms.
  • Projections to the mesencephalic pretectum: coordination of some eye movements, the iris and the muscles that control the lens.
  • Projections of the superior collicles or optical tectum: orientation of the eyes in response to new stimuli in the visual periphery.

Lesions in the primary visual pathway

Defects in the visual field due to the injury of different points of the primary visual pathway. On the right we have schematized the defects in the perception of the visual field associated with each lesion (marked in the horizontal brain drawing on the left). In black we have indicated the area of ​​the visual field in which there is loss of vision.

Different retinal or central lesions can lead to defects in the perception of the visual field. In the following illustration you have examples of how affectations of different points of the primary visual pathway can affect our perception of some part of the visual field.

The perception of movement, saccadic movements

The optic nerve directs information through the thalamus to the cerebral cortex, where visual perception occurs, the nerve also carries the necessary information for vision mechanics to two sites in the brainstem. The first of these sites is a group of cells that controls pupil size in response to the intensity of light. The information concerning the moving targets and the information that governs the scanning of the eyes travels to a second site in the brain stem, a nucleus called the superior colliculus. The upper colliculus is responsible for moving the eyes in short jumps, called saccadic movements. These movements allow the brain to perceive a series of relatively fixed images solving the problem of blurring that occurs when staring. Thus, humans do not look at a scene statically. Instead, the eyes move, looking for interesting parts of a scene and building a mind map referring to it. Moving the eye so that small parts of the scene can be noticed with greater resolution.

Depth perception

In-depth vision involves the conversion of two-dimensional images into three-dimensional images. Since the retina is two-dimensional, the perception of a three-dimensional world depends on obtaining information about distance.

Although different ways of perceiving depth depend on monocular signals (perspective, relative retinal size, etc.), a tendency shown by striated cortex neurons is binocularity. Binocular vision gives us a better perception of depth through the process of stereoscopic vision or stereopsis.

When an object appears in the visual field, eye movements are responsible for directing attention to it. When the stimulus is projected on both foveas, there is a perception of a unique image (binocular fusion).

Binocular disparity is the basis of depth vision.

Due to the horizontal separation between the two eyes, each retina receives a slightly different image of the world around us. These differences are called retinal disparity.

Association Bark

Apart from V1 there are many other cortical areas that participate in visual perception, they are called visual areas of association or extra striated cortex. In these hierarchically arranged areas the information of the individual modules of V1 is combined (although there are some of these association areas that receive direct inputs from the NGL); therefore, its function is that we have a complete perception of visual objects and scenes.

Many of these areas of association are still very unknown, but what does become clear is that there are two major currents of visual cortical processing:

  • Ventral current: from V1 to the lower temporal lobe. Important role in object recognition.
  • Dorsal current: from V1 to the posterior parietal lobe. Important role in spatial vision.

Secondary visual area or V2

In any case, the divergence of information in all major currents would start from area V2 (area eighteen of Brodmann). V2 is the area adjacent to V1, and most of its neurons have properties very similar to those of V1 neurons. It seems that many of your cells respond to illusory contours.

Perception of color and form

The area called V4 was discovered by Zeki (1973) who identifiedneurons with a very marked chromatic selectivity. Most of his references come from V2 and V3.

It is currently accepted that V4 participates in the analysis of color and form. V4 cells help maintain color constancy.

We understand how color constancy the stable perception of object colors, in spite of the variations of the spectral composition of the lighting that it affects. The organization of the receptor fields of V4 neurons (large and with an antagonistic chromatic organization) helps to reduce the changes in the illumination of the environment that affects a given object.

Sample of stimuli that cause high activation of V4 cells.

Bilateral lesions of V4 cause alterations in the ability of color discrimination and in the ability to discriminate patterns and orientations.

The V4 area projects to the inferotemporal cortex; Therefore, we speak of ventral current.

It is precisely in the temporal cortex lower than the recognition of visual patterns and the identification of particular objects.

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