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The attention and its physiological bases

The attention and its physiological bases



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They have been able to relate over time and intense neurophysiological studies, various brain structures that help maintain the general attention state. Thalamic lesions that affect the intralaminar nuclei or reticular nucleus, or lesions of some polymodal association areas of the cortex (such as the prefrontal cortex, the ventral temporal cortex or the posterior parietal cortex), produce alterations of general attention.

These different activator systems are formed by groups of neurons with the soma located in the Brain stem that send their axons to multiple diencephalic and telencephalic locations. In the case of cholinergic projections, although it is true that some of these originate in the brain stem, there is a very important group of axons that provide innervation in the cortex and the hippocampus They have their soma located at the basal anterior brain.

Equally, histamine, another neurotransmitter substance, also modulates the activity of other areas of the brain, but in this case, the histaminergic axon somas are located in the hypothalamus.

Traditionally, however, brain stem reticular formation has been discussed as the most important activating system of the brain. We currently divide the reticular formation into different neuronal groups that differ according to the neurotransmitter they use (acetylcholine, dopamine, serotonin or norepinephrine). These activating systems send projections, some of which through the thalamus, in multiple diencephalic and telencephalic structures, such as the cortex, modulating their activity.

Main noradrenergic pathways of the brain originating in the locus coeruleus (locus cerulean).

We will focus on the activating role of norepinephrine, as it is one of the best studied systems. Most noradrenergic pathways originate in the cerulean locus, located in the bulge of the brain stem.

Neurons can show two types of differentiable activity. First, all neurons respond when they are stimulated by some neurotransmitter substance. This response is called a signal, since it is a consequence of some internal or external event that has affected the organism. Second, many neurons show a certain frequency of basal discharge, in the absence of stimulation, which we call noise. Norepinephrine increases the information processing capacity by increasing the signal / noise ratio, since it increases the neural activity that follows a stimulation (signal) without affecting the noise. In this way, it is achieved that the neural activity resulting from a stimulation stands out above the basal activity, thereby facilitating the detection of stimuli.

Norepinephrine increases the information processing capacity in various areas of the brain, increasing the response of neurons when they are stimulated.

Low activity of locus coeruleus

Thus, in conditions of low activity of the cerulean locus, neurons in other areas of the brain will react poorly to stimulation, decreasing the ability to detect stimuli; that is, decreasing the general attention state. This is characteristic of the sleep phase, for example.

High activity of locus coeruleus

Under conditions of high activity of the cerulean locus, however, neurons will show an increased response to stimulation. This situation causes a hypervigilant general attentional state, in which a large number of stimuli are easily detected, but the difficulty of selective attention and the distraction increases. This is characteristic of Dangerous, innovative and general situations of anxiety states.

Moderate locus coeruleus activity

Finally, if the activity of the cerulean locus is moderate, relevant stimuli can be easily detected while preventing interference from irrelevant stimuli. In this condition, moreover, attention can be easily focused, since the appearance of an overhanging stimulus causes a timely hyperactivation of the cerulean locus, which increases the probability of detection of this stimulus.

The role of parietal lobe in attention

The electrophysiological studies in primates they have detected neurons in the parietal lobe that increase their frequency of discharge when the animal focuses its attention on a significant stimulus. This increase in activity is maintained throughout the period during which the animal focuses its attention on the stimulus, and disappears when it stops paying attention.

The parietal lobe It contains neurons that respond specifically during periods of attention to relevant stimuli.

The syndrome of contralateral negligence or hemispheric negligence

After a brain injury, some patients show an attentional disorder known as contralateral negligence syndrome.

Contralateral negligence syndrome is usually caused by lesions in the parietal cortex of the right hemisphere.

Patients presenting with this syndrome do not attend to the objects presented by any sensory modality on the side contralateral to the lesion. In some cases, they do not pay attention to the parts of their body contralateral to the lesion. In no case do not present any sensory or motor deficit that may explain the disorder.

Drawings made by patients with contralateral negligence syndrome. You can see how they almost completely omit the left side of the drawing.

The attention deficit towards the side contralateral to the injury does not only affect the perception of the stimuli, but also their mental representation. Thus, in one experiment, subjects with neglect syndrome were asked to describe by heart a space known to all of them (the Duomo square in Milan). It was observed that the patients only described the details located next to the ipsilateral lesion. When they were asked to mentally stand on the opposite side of the square, they only described those details that they had previously omitted.

Given the the lesion causing this syndrome usually affects the right hemisphere, these patients do not react to stimuli presented on the left side, and sometimes completely neglect the left side of their body, forgetting to wash, dress, shave, etc. The most extreme cases they come to deny its existence. Some patients, for example, have come to assert that in their bed there was a leg or arm that was not theirs (referring to their left leg or arm).

As we have said, most of the time contralateral negligence syndrome is a consequence of parietal lesions in the right hemisphere. This is because while the left hemisphere controls the attention that is directed to the right side, the right hemisphere controls the attention that is directed to both sides. In this way, the attention deficit that would be observed after left parietal lesions is compensated by the bilateral control of the attention exercised by the right parietal lobe.

Physiological basis of selective attention

The physiological mechanisms of selective attention have been very well studied alongside the visual system. For example, When we simultaneously present two stimuli to the receptor field of a neuron in a visual area, they will compete with each other so that one's perception is favored.

The mechanisms of selection of the attention stimulus are quite complex, and include regulatory afferents from the prefrontal cortex and other association areas. Thus, for example, we can select a stimulus on which to focus attention depending on whether previously its appearance has been followed by the presentation of a reinforcer. In this case, it is the areas of the prefrontal cortex where this stimulus-reinforcing association is stored, which are responsible for the neuron receptor field of the visual association areas being reduced around this conditioned stimulus.

Selective attention is very useful in many situations of everyday life. Imagine, for example, that we are in the cinema and we are looking for a friend who is blonde. Automatically, the neurons of the visual association areas shrink their receptor field around the stimuli that coincide with the characteristic "blonde", so it will be much easier for us to find our friend.

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