Perception is the process by which living beings capture information from the environment in order to process it and acquire knowledge about it, being able to adapt to the situations we live in.

However, in many cases, whether or not there is a mental disorder, perceptions are produced that do not correspond to reality, and these perceptual alterations can be grouped into distortions or deceptions, mainly.

While in perceptual distortions a real stimulus is perceived abnormally, in perceptual deceptions there is no stimulus that triggers the perceptive process. The clearest example of the latter type of perceptual alteration is hallucinations .

Hallucinations: defining the concept

The concept we have just mentioned, hallucination , has evolved throughout history and has been enriched by its description over the years. Hallucination can be considered as a perception that occurs in the absence of a stimulus that triggers it , with the sufferer having the sensation that it is real and that it occurs without the subject being able to control it (this characteristic being shared with obsessions, delusions and some illusions).

Although they are generally indicators of mental disorder (being a diagnostic criterion for schizophrenia and may appear in other disorders, such as during manic episodes or during depressions), hallucinations may also appear in many other cases, such as neurological disorders, substance abuse, epilepsy, tumors and even in non-pathological situations of high anxiety or stress (in the form of a nervous paroxysm for the object of our anxiety, for example).

An example of a hallucination

Here is an example to help us understand what a hallucination is

“A young man arrives at a psychologist’s office. There, he indicates to his psychologist that he has gone to him because he is very afraid. At first he is reluctant to talk to the professional, but throughout the interview he confesses that the reason for being in his office is that every time he looks in the mirror he hears a voice talking to him, insulting him, saying that he will never get anywhere in life and saying that he should disappear”.

This example is a fictitious case in which the supposed patient has perceived a stimulus that does not really exist from a specific situation (looking in the mirror). The young person has really had that perception, being for him a very real phenomenon that he can neither direct nor control . In this way, we can consider that it has all the characteristics mentioned above.

However, not all hallucinations are always the same. There is a wide variety of typologies and classifications, among which the one referring to the sensory modality in which they appear stands out. Furthermore, not all of them appear in the same conditions, and there are also multiple variations of the hallucinatory experience.

Types of hallucinations according to sensory mode

If we classify the hallucinatory experience according to the sensory mode in which they appear, we can find several categories.

1. Visual hallucinations

First of all you can find the visual hallucinations , perceived through the sense of sight. In this case the subject sees something that does not exist in reality. These stimuli can be very simple, such as flashes or lights. However, more complex elements can be seen, such as characters, animated beings or vivid scenes.

It is possible that these elements are visualized with different measures to those that would be perceived if these stimuli were real, being called lilliputian hallucinations in the case of smaller perceptions and gulliverian in the case of seeing them enlarged. Visual hallucinations also include autoscopy, in which a subject sees himself from outside his body, in a manner similar to that reported by patients with near-death experiences.

Visual hallucinations are particularly common in organic conditions, trauma and substance use, but also appear in certain mental disorders.

2.Auditory hallucinations

With respect to auditory hallucinations , in which the perceiver hears something unreal, they can be simple noises or elements with complete meaning such as human speech.

The clearest examples are second-person hallucinations, in which, as in the example above, a voice speaks to the subject, third-person hallucinations in which voices are heard that speak of the individual himself among them, or imperative hallucinations, in which the individual hears voices ordering him to do or not to do something. Hallucinations of this sensory modality are the most frequent in mental disorders , especially in paranoid schizophrenia.

3.Taste and smell hallucinations

As far as the senses of taste and smell are concerned, hallucinations in these senses are rare and are often related to the use of drugs or other substances, in addition to some neurological disorders such as temporal lobe epilepsy, or even in tumours. They also appear in schizophrenia, usually related to delusions of poisoning or persecution.

4.Haptic hallucinations

The haptic hallucinations are those that refer to the sense of touch. This typology includes a great number of sensations, such as temperature, pain or tingling (the latter being called paresthesias, and among them a subtype called dermatozoic delirium in which one has the sensation of having small animals in the body, being typical of the consumption of substances such as cocaine).

Apart from these, related to the senses, two other subtypes can be identified.

Firstly, cenesthetic or somatic hallucinations, which provoke perceived sensations with respect to the organs themselves, normally linked to strange delirious processes.

Secondly and finally, kinesthetic hallucinations refer to sensations of movement of the body itself that are not produced in reality, being typical of Parkinson’s patients and substance abuse.

As already mentioned, regardless of where they are perceived, it is also useful to know how they are perceived. In this sense we find different options.

Different modes of false perception

So-called functional hallucinations are triggered by the presence of a stimulus that triggers another, this time hallucinatory, stimulus in the same sensory mode. This hallucination is produced, begins and ends at the same time as the stimulus that originates it. An example would be the perception of someone who perceives the TV news tune every time he hears traffic noise.

The same phenomenon occurs in the reflex hallucination , only this time the unreal perception occurs in a different sensory mode. This is the case in the example above.

The extra-campine hallucination occurs in cases where the false perception is outside the individual’s perceptual field. That is, something is perceived beyond what could be perceived. An example is seeing someone behind a wall, without other data that could make one think of their existence.

Another form of hallucination is the absence of perception of something that exists, called negative hallucination . However, in this case the behaviour of the patients is not influenced as if they perceived that there is nothing, so that in many cases it has come to be doubted that there is a real lack of perception. An example is the negative autoscopy , in which the person does not perceive himself when looking at himself in a mirror.

Finally, it is worth noting the existence of pseudo- hallucinations . These are perceptions with the same characteristics as hallucinations with the exception that the subject is aware that they are unreal elements.

Why does a hallucination occur?

We have been able to see some of the main modalities and types of hallucinations but, why do they occur?

Although there is no single explanation for this, several authors have tried to shed light on this type of phenomena, some of the most accepted being those who consider that the subject who hallucinates wrongly attributes his internal experiences to external factors .

An example of this is Slade and Bentall’s theory of metacognitive discrimination, according to which the hallucinatory phenomenon is based on the inability to distinguish real from imaginary perception. These authors consider that this ability to distinguish, which is created and is possible to modify through learning, may be due to an excess of activation by stress, lack or excess of environmental stimulation, high suggestibility, the presence of expectations as to what will be perceived, among other options.

Another example, focusing on auditory hallucinations, is Hoffman’s theory of subvocalization , which indicates that these hallucinations are the subject’s perception of his or her own subvocal speech (i.e., our inner voice) as something alien to himself or herself (a theory that has generated therapies to treat auditory hallucinations with some effectiveness). However, Hoffman considered that this fact was not due to a lack of discrimination, but to the generation of involuntary internal discursive acts.

Thus, hallucinations are ways of “reading” reality in the wrong way, as if there were elements that are really there even though our senses seem to indicate otherwise. However, in the case of hallucinations our sensory organs function perfectly, what changes is the way our brain processes the information that arrives. Normally, this means that our memories are mixed with sensory data in an anomalous way, linking previously experienced visual stimuli to what is happening around us.

For example, this is what happens when we spend a lot of time in the dark or blindfolded so that our eyes don’t register anything; the brain starts to invent things because of the anomaly of not receiving data through that sensory pathway when we are awake.

The brain that creates an imaginary environment

The existence of hallucinations reminds us that we do not simply record data about what is happening around us, but that our nervous system has the mechanisms to “construct” scenes that tell us what is happening around us. Some illnesses can trigger hallucinations in an uncontrolled way, but these are part of our daily life, even if we don’t realize it.

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