the optic pathway

In this chapter we advance in the processing of information and will focus on explaining the optical way.

The retina, the visual stimulus is encoded in electrical stimulation and leaves the eye, passes to the "Optic Pathway", to the Lateral Geniculate Nucleus and to the Brain, where the information is recoded and distributed in different regions of the cortex and extrastriate areas. Let's see what happens.

the optic pathway

So far we have seen what what happened in the retina, now the generated signal passes through the optic pathway and reaches the Lateral Geniculate Nucleus (LNG) in the thalamus, from where fibers will come out that will mostly reach the striated cortex of the occipital lobes.

We could say that 90% of the fibers that come out of the Optic nerve (NO) reach the lateral geniculate nucleus, while the other 10% go to the Superior Colliculi, responsible for regulating eye movements. The lateral geniculate nucleus not only receives fibers from the optic nerve, but also fibers from other structures, such as the cortex, brain stem or other regions of the thalamus.

An important fact that has been recently seen is that for each 10 stimulus that arrives from the retina, only four leave towards the cortex, which means that the lateral geniculate nucleus functions as a filter. It is also a differential fact that the lateral geniculate nucleus reaches fibers from both eyes, it is a bilateral organ, organized in 6 layers, each corresponding to one eye, alternately, so one eye sends fibers to the 1,4 and 6 layers and the other to the 2, 3 and 5 layers. 

In the layers of the lateral geniculate nucleus, the retinotopic map is maintained, so each point of the lateral geniculate nucleus corresponds to a specific point on the retina and, in turn, an adjacent point in the lateral geniculate nucleus will correspond to an adjacent point on the retina.

Types of cells that reach the lateral geniculate nucleus

There are three types of cells that reach the lateral geniculate nucleus from the retina:

• The P cells, of the parvocellular pathway, which respond to sustained stimuli, with synapses in layers 3,4,5, and 6.
• M cells, of the magnocellular pathway, which respond in the form of bursts and which synapse in layers 1 and 2.
• The K cells or koniocellulars, whose function is still not too clear.

Schiller in 1990 was the first to determine the functions of these fibers, establishing that the magno pathway was sensitive to movement while the parvo pathway was sensitive to color and detail detection, functioning as independent channels.

The optical path and the cortex

The signals from the lateral geniculate nucleus carry information to the cortex about the characteristics of the visual scene, ranging from simple changes in light intensity to complex patterns, such as faces or moving stimuli. All this information arrives through different channels, most of them independent, therefore the first function of the cortex is to integrate all the information until it has a “whole” that represents the visual scene.

Types of cortex neurons

In the cortex we find three types of neurons:

• Simple
• Complex
• Hypercomplexes

The Simple Cells are of the center-periphery type but their geometry is no longer circular but rectangular. Basically three different types are identified, some with an ON center flanked by an OFF periphery, others with the opposite situation and a third group that respond to a light-dark edge, when it falls on the border between the positive and negative region.

A positive response will be obtained when the stimulus falls on the excitatory zone and the more it fills its surface, without invading the adjacent inhibitory region, the more powerful the response will be, thus the ideal stimulus is a band of light located exactly on the zone + of excitation ( a).

In the same way, in figure (b), we have the maximum response with a dark band adjusted in size and direction to the central zone.

In figure (c), the best response corresponds to edges of dark light, with the precise orientation that corresponds to that cell, thus stimulation with diffuse light does not evoke any type of response in any of the simple cells described.

Simple cortex neurons

Simple cells work with afferents that would arrive from cells of the retina and lateral geniculate nucleus with a circular center and periphery, as shown in the figure, the set of several adjacent cells of this type would send information to a simple cell. The overlap of their camphe receptors could explain the behavior of a simple cell type (a) described above.

Complex cortex cells

Complex Cells constitute 3/4 of the striated cortical cells and their campReceptors also have a rectangular geometry and, like simple cells, are very sensitive to orientation, although they respond equally as long as the excitatory stimulus falls within their campor receptor, although the maximum response is obtained when the stimulus moves along the campor receiver.

In the same way as in simple cells, complex cells receive input from several simple cells, overlapping their campAdjoining receptors, with the same orientation, a response like those described for complex cells would be obtained.

These cells fire when selective stimulation occurs according to orientation, but it must also be selective to the direction of movement.

In the diagram there is a response when the direction is to the right, and it is null to the left, although the exciting bar corresponds to the orientation of the complex cell on which it moves.

A possible mechanism to explain this situation is the one that assumes the presence of intermediate cells between simple and complex cells. In the previous scheme this relationship is expressed, so the final result will be that there are no firings of the complex cells when the stimulus moves from left to right.

Cortical hypercomplex cells

The third type of cortical cells are Hypercomplexes, with campreceptive older and more sensitive to the movement of the stimulus and its size.

The type of cell known as "final inhibition" stands out, where the moving stimulus must be adjusted in its place or length to the size of the campor receptor of the cell and if it exceeds it, a weaker or null inhibitory response will be produced:

This assumes that next to theampor excited receiver there is a campor lateral inhibitory, as it appears in the previous figure, where three ordinary complex cells can be seen that send information, afferents, to a hypercomplex cell of final inhibition.

With this composition, situations such as the following can occur: in (a) the response is maximum since the stimulus is adjusted to the excitatory zone of the campor receiver. In (b) the answer is poor, since the exciter bar invades the campor inhibitory lateral receptor and in (c) a maximum response occurs again since two stimuli appear, one optimal, similar to the one that appeared in (a) and another in the zone of inhibition, but without corresponding to the orientation of this hypercomplex cell Thus, it produces neither stimulation nor inhibition.

This situation is useful to explain the perception of corners or curved figures, where the extension of the curved area over the campThe lateral inhibitors do not weaken the response generated.

Processing of visual information in the cortex

We see how the cells of the cortex respond to stimuli in the form of bars, with specific orientation and movement, but the recognition process requires more things, including the determination of the details of the objects that are in the visual scene.

To do this, the cortex performs a decomposition of the scene in contrast frequencies, each element of the scene can be analyzed by means of a scan in which its contrast is determined point by point. When we observe an object with many details, the frequency bands are very narrow, each one corresponds to a point, and we speak of high frequencies, while when we observe an empty space or a very regular object, the points that constitute it are very similar and the contrast bands it generates are amplias, thick, then we talk about low frequencies. What has been seen is that there are cells in the cortex adapted to respond to different frequencies, high, medium or low, establishing specific channels of perception, related but independent.

Vision and spatial frequencies

The spatial frequencies correspond to the size of the objects or the details of thesethe higher, the greater the detail of the information it transmits. This allows to determine the degree of vision by analyzing the detection of the size of the frequency bands and for this we use the concept of angle of vision, which is the angle of an object with respect to the eye of the observer, as shown in the figure, where as the person moves away from the observer's eye (b), the visual angle decreases.

Visual angle

The visual angle depends on the size of the object and the distance from this to the observer. The visual angle indicates the size of the object in the retina. As a general rule, the size of the spatial frequencies are expressed in cycles per degree and correspond to the angle that determines the bands in the retina, the degrees that correspond to each contrast band, thus an angle of one cycle is equivalent to a band size of a grade and a lattice with this band size, we will say that it has a value of one cycle per degree.

To work with frequency lattices we can use different mathematical systems that represent them. The most used system is Fourier analysis, which assigns a value according to the contrast intensity of each band, being able to decompose each object according to the band frequency that we want, as we see in the figure.

We see how in the image that corresponds to high frequencies small details are appreciated, while if we only use low frequencies, the detail disappears and the image is coarser.

Space frequency analyzers

The cortex has cells specialized in the analysis of specific spatial frequencies. They are known as space frequency analyzers and constitute independent information channels. Through psychophysical studies it has been determined that there are three main channels, those of low frequencies (0.1 c/g) that correspond to a viewing angle (visual acuity), approximately 0.1, medium frequency channel (0.3-0.5 c/g), corresponding to visual acuity angles of 0.3 and the high frequency channel (0.7-1 c/g) corresponding to visions from 0.8 to 1 Analyzing the level of perception, the contrast sensitivity in each of these frequency bands, low, medium and high, we can generate a curve that we call the Contrast Sensitivity Function, like the one in the figure.

Central processing of visual information in the optical path

Recent studies show how orientation detection is not as accurate as Hubel and Wiesel assumed. Lamme 2000 showed how neurons in V1 are context sensitive.

Neurons with campor receptor for a vertical line, they responded better when the vertical stimulus was surrounded by other similar stimuli, vertical lines, while if the surrounding stimuli were lines with random orientation, the response of the neuron had a lower intensity, it is what is called , Contextual Modulation and which is based on the concept of stimulus salience.

This situation could be explained only by the hypothesis that the information in V1 must link to a process that is not limited to the cells of this region, that the signal must travel out of V1 and, perhaps, return to V1 to finish the process, in any case, the output of the signal out of V1 was to regions we call the extrastriatal brain.

The first article that revealed the presence of an extrastriate pathway was published by Leslie Ungerleider and Mortimer Mixhkin in 1982. In it, the authors showed that there were two processing streams, one for "what" and one for "where." , ventral and dorsal pathways, respectively, the former going to the temporal lobe and the dorsal to the parietal lobe.

The ways of what and where

These ways of what and where, are related to the neurons that come from the retina. In this way:

• The way of what, selective for the details of the object, starts with the parvocellular pathway from the retina and lateral geniculate nucleus.
• The path of where, related to the location of the object, with movement, would start with the retinal magnum cells and the lateral geniculate nucleus.

These two routes, although with different functions, would not be totally independent, there is much evidence that shows a close collaboration between both routes.

How we take an object

The most recent studies have focused on the path of where, and point out that rather than the location of objects, this route should be called a "how" route, since it would be destined to how we perform certain movements or actions, such as catching, grasping , one thing. It is evident that in the task of action, where the object is involved is involved, but now there is a reason for the location, the process of acting with an end (physical interaction with the object).

Brain injuries that affect the optic pathway

Studies with brain-injured patients have shown that perception and action work differently, that they follow different pathways. The ventral way would be the what, while the dorsal track it would be the how, or path of action, since it determines the way in which a person performs an activity.

Where are the recognition of how and what we see in the optical path

The way of "how" is closely related to the detection of movement, a function found in the medial temporal region (TM).

The detection of forms, typical of the path of what, is represented in the inferotemporal region (ITEM).

Inferotemporal region in vision

In the inferotemporal region, neurons were found that responded to stimuli in specific ways and cells that responded to more elaborate stimuli, such as an apple or a house, and even specific cells for faces were found.

In this case, when a human figure was presented, these cells responded specifically with respect to each face, while if the head was covered, when the rest of the figure was presented, they no longer responded. fMRI studies locate these specific cells of the face in a region of the IT called the Fusiform Facial Area (FFA). 

How the brain interprets the images that arrive through the optical path

Visual perception is not simply a process in which points and edges are detected, we have said that it is an active process, generally intended for action, and that implies a series of connotations.

An important aspect is what we call "Sensory code”, which corresponds to the way the brain has of processing images of objects, of how it constitutes mental representations.

Sensory code

The sensory code is the information contained in the firing pattern of the neurons that represent what we perceive. 

Among the neurons that make up the sensory code, we find size-invariant neurons, location-invariant neurons, and sight-invariant neurons, that is, they are neurons that continue to fire even if some aspects of the object we perceive change. This is essential for your identification.

If we are seeing a car that is approaching us, as it approaches, its size on the retina is larger, so if there were not these size-invariant cells, that change in size on the retina, the brain could interpret it as that it is of another object, whereas now we are informed that it is the same car approaching us. In fact, there is a collaboration between different neurons, some detect what kind of objects we are seeing and, through connections with the invariants, they inform us that this object remains the same, even if it changes its appearance, size or location.

Specific neurons and the optical path

A question that the researchers asked themselves was the specific neurons, such as those detected by the faces, we had them from birth, that is, with hereditary character as a result of evolution or, they appeared as the result of each one's experience. The answer is not completely conclusive and, although there is evidence that we are born with a certain potential to recognize faces, it seems that this ability can be improved throughout life.

Visual attention: visual and neuronal selectivity

The assumption that every time a stimulus arrives on the retina, that stimulus is perceived centrally, we currently know to be false.

Not every time a stimulus arrives on the retina it is processed until it becomes conscious, in fact, in many cases this final stage does not occur. A lot of information reaches the eyes, but we are only aware of what we see in certain situations. This phenomenon is closely linked to attention.

Magicians and illusionists know this fact very well.

Blindness due to lack of attention

When we pay attention to something, although a well-defined visual stimulus is presented to us in front of us, in most cases, despite the fact that our visual system has captured it, we are not aware of having seen it, it is what is called blindness due to lack of attention.

Attention flicker

In this line we have what is known as attentional flicker which is revealed when a series of stimuli, such as cardboard with letters, is presented, changing every 100 msec. If between the letters we introduce two different cardboards, with numbers, the first number will be remembered well but the second will not, as long as it is presented within a space of 1 second after presenting the first number.

This phenomenon is due to the fact that the attentional process has a minimum time to change, or disengage, as it is technically called, during which we cannot remember, it is like when we blink, at that moment we cannot see, hence its name, attentional blinking .

With all the information that we have been seeing in these four chapters, we are now in a position to better understand how we see the objects of the visual scene, the mechanisms that our perceptual system uses to differentiate an object from the background where it is located, or from the shadow it causes. We will go into matters of a more practical order.