First, we need to understand the difference between sensation and perception. Let’s take as an example a pen. When we look at the pen, through our eyes we receive separate streams of information about the color, shape, and size of the observed object. This process is called visual sensation. Then, merging separate streams of information together into one stream allows us to recognize this object as a pen. In other words, by combining different types of information together, we understand that the object of this color, and this shape, and this size is called a pen. That’s called perception.
Essentially, sensation is a first step of perception. It involves the detection of physical energy (like light, sound, or touch) by our sensory organs and conversion of this energy into electrical signals. These signals are then sent to the brain, where they are consciously experienced as seeing a color or hearing a sound. There are at least 5 senses - vision, hearing, taste, smell, and touch.
For illustration purposes let’s take a closer look at visual sensation.
The physiology of visual sensation involves several key structures and processes in the eye and brain:
First, light refracted from the object enters the eye. When light hits the photoreceptors in the retina, it triggers a biochemical process that converts the light into electrical signals.
The electrical signals are then sent to the primary visual cortex (V1) in the occipital lobe of the brain. Here, basic features of the visual stimuli, such as edges, colors, and motion, are processed. Further processing occurs in adjacent areas of the cortex, leading to higher-level interpretations and recognition of objects.
The next step in the analysis of sensory information is called perception. Perception is the process by which we interpret and make sense of sensory information. It involves organizing, identifying, and interpreting sensory data received from our environment to form a coherent understanding of what we see. It largely depends on prior knowledge, experiences, and expectations (e.g., recognizing a flower by its colors and general shape).
The pathology of sensations and perceptions.
Pathology of sensations includes symptoms such as hyperesthesia, hypoesthesia, paresthesia and cenesthopathy.
Hyperesthesia is a condition characterized by an increased sensitivity to sensory stimuli. This heightened sensitivity can affect various senses, including touch, pain, temperature, and sound. Individuals with hyperesthesia may experience sensations more intensely than usual, which can be uncomfortable or even painful. For example, if you have inflammation in some parts of the eye, due to infection, your eye can become hypersensitive to light.
Hypoesthesia is contrary to hyperesthesia. It is a condition characterized by decreased sensitivity to sensory stimuli. Individuals with hypoesthesia may not feel sensations as strongly or may have difficulty perceiving them altogether. For example, one of the symptoms of COVID infection is reduced smell and taste.
Paresthesia refers to abnormal sensations in the skin, such as tingling, prickling, burning, or numbness. These sensations can occur anywhere in the body, but they are most commonly felt in the hands, feet, arms, or legs. Such symptoms may be present when there is damage to the peripheral or central nervous system.
Cenesthopathy is a term used to describe a type of sensory disturbance or abnormal sensation that is often difficult to classify. It can involve feelings of discomfort or sensations in the body that do not correspond to any identifiable physical cause. These sensations may be described as tingling, burning, crawling, or other unusual feelings, and they are mainly experienced inside the body. One of the main features of cenesthopathy is that such sensations are physiologically impossible. For example, itching inside of the brain, or freezing of the liver are physiologically impossible, because the brain itself does not have pain receptors, so it cannot directly sense sensations like itching. Similarly, the liver does not have temperature receptors, therefore sensations like freezing of the liver are physiologically impossible. Cenesthopathies are relatively rare and can be observed in patients with schizophrenia.
Now let’s move on to the pathology of perceptions. Pathology of perceptions can be divided into three groups of symptoms: psychosensory distortions, illusions, and hallucinations.
In psychosensory distortions the object of perception is recognized correctly, but some features of the object may appear in a distorted way. For example, when looking at a pen, it is correctly recognized as a pen, but the pen may be seen as disproportionately large. Psychosensory distortions include a wide range of symptoms that can be divided into two large groups: 1) symptoms of derealization and 2) symptoms of depersonalization. In derealization it is the objects of the external world that are perceived in a distorted way. These symptoms include micropsia (when visual objects appear smaller), macropsia (when they appear larger). Objects can also be perceived as closer or farther than they really are. Distorted perception of time, such as slowing of the time or its acceleration, and such phenomena as déjà vu which is a feeling of having already experienced the present situation are also considered as symptoms of derealization. In case of depersonalization, features of one’s own body or the sense of Self can appear in a distorted way. For example, your own legs may be perceived as very short, or there might be a feeling that there are two personalities in you. Psychosensory distortions may be caused by intoxication (including intoxication with hallucinogenic drugs), different neurological conditions such as stroke or Alice in Wonderland Syndrome, and some of them may be observed even in healthy people (for example, déjà vu).
Looking ahead, it should be noted that many psychiatric symptoms can be found in healthy people. There is no clear-cut border between normal and pathological in psychiatry, it more depends on the degree and significance of the disturbance. Just like sneezing can be both a normal physiological reaction or a symptom of allergy or flu. Similarly, many psychiatric phenomena can be normal reactions or symptoms of a mental disorder.
The second group of disorders of perceptions is illusions. In illusions the real object is recognized incorrectly. For example, when looking at the pen, instead of the pen one sees, let’s say, a knife. Illusions can be normal and pathological. Normal illusions include physical illusions like seeing water in a desert also called mirage. They occur due to physical processes.
As the name suggests, pathological illusions are mostly observed in various pathological conditions. Pathological illusions can be divided into verbal, affective, and pareidolical. In verbal illusions the real words in a conversation between two people are misperceived. For example, imagine two people are actually talking about weather, but a third person who has verbal illusions hears them talking about him or her.
Affective illusions may appear when one experiences extremely strong emotions, most commonly, fear. In such emotional state, for example, a tree may be misperceived as a perpetrator.
In pareidolic illusions, there is a misperception of real patterns or random visual stimuli. For example, one might see a snake instead of the line on the floor, or see a face instead of some pattern on a wallpaper. When static objects on paintings or photographs start moving are also examples of pareidolic illusions. Importantly, when we perceive something as similar to something else, it is not a pathological illusion. For example, one may say that the cloud looks like a face. That’s normal. But if instead of the real cloud one sees a face and does not see the cloud it is a pathological pareidolic illusion.
Unlike psychosensory distortions and illusions where there is a real stimulation of the sense organ, but objects are perceived incorrectly, a hallucination is a perception that occurs without an external stimulus, meaning a person experiences something that isn't present in the environment. There are many types of hallucinations. Depending on their modality there are:
Auditory Hallucinations: Hearing sounds or voices that are not there.
Visual Hallucinations: Seeing things that do not exist, such as objects, people, or lights.
Tactile Hallucinations: Feeling sensations on the skin, such as bugs crawling or pressure, without any physical cause.
Olfactory Hallucinations: Smelling odors that are not present, often unpleasant.
Gustatory hallucinations: experiencing usual or strange tastes.
Depending on their complexity hallucinations can be elementary, simple, and complex.
Elementary hallucinations – for example, seeing flashes of light, geometric shapes, or hearing sounds like gunshots, yelling.
Simple hallucinations are more complicated than elementary hallucinations but limited to only one sensory modality. For example, only seeing a dead person, or only hearing them talking.
Complex hallucinations are characterized by involvement of two or more senses. For example, not only seeing a dead person but also hearing their voice.
There are also hypnogogic hallucinations that appear in a transition from wakefulness to sleep, and hypnopompic hallucinations that happen in a transition from sleep to wakefulness.
As you can see, there are many types of hallucinations, but most important distinction should be made between true and false hallucinations or pseudohallucinations.
The main difference between true and false hallucinations is that true hallucinations are located in the external environment, while false hallucinations are experienced inside of the mind. Because of external localization, a person experiencing a true hallucination cannot separate it from reality and often behaves in accordance with the hallucination. For example, when seeing a glass of water that does not exist in reality, they might try to grab and drink it. There is a sense of objectivity, meaning that a person believes that other people also experience the same thing.
Exact mechanisms of true hallucinations are unknown, but they partially can be explained by erroneous activation of cortical sensory centers due to some pathological processes. Remember when we discussed the process of visual sensation, we talked about how light entering the eye transforms into electrical signals in the retina which sends these signals via optical nerves to the brain where visual perception is generated. So, in true visual hallucinations primary visual cortex gets activated without previous steps, i.e. without external signals from the retina. This can occur due to various reasons. For example, there might be a tumor growing close to the primary visual cortex, or there might be vascular pathology, or infection, or something else that triggers activation of the visual cortical center.
Now let’s examine false hallucinations. In contrast to true hallucinations, false hallucinations are experienced inside the mind, not in the external environment. They are similar to inner imagery, fantasies or inner dialog, but unlike these normal phenomena they lack the sense of agency. In fact, they appear because of the inability of a patient to recognize self-generated experiences. For example, false verbal hallucinations or voices are in fact patients’ own thoughts that lost their connection with the self. There is a sense of coercion, as if someone forcefully inserts these voices or images into their head.
Mechanisms of false hallucinations are also poorly understood, but they are associated with working memory and attention disturbances. Working memory is the brain’s ability to hold information temporarily that allows us to work with information without losing track of what we are doing. It is akin to Rapid Access Memory in smartphones or computers.
Imagine that you have been watching YouTube videos for several hours and then remember that you have an important task to do. In this situation you might say to yourself: “Stop wasting your time!”. We often talk to ourselves like that, which is normal. But we know that it is our self-generated inner voice. How do we know it? The working memory allows us to do that. The sequence of events preceding the inner voice is stored in our working memory. The fact that you have been watching YouTube for several hours, then remembering about the important task to do, and the rest of the train of thoughts preceding and explaining you saying to yourself “Stop wasting your time” is all stored in working memory. In some pathological conditions, such as schizophrenia, working memory is impaired. Due to glitches in working memory they forget that it was them who produced the voice and this is how self-generated inner speech becomes someone else’s voice in the head. Working memory function is performed by the prefrontal cortex. Damage to the prefrontal cortex may disrupt working memory. This may happen due to schizophrenia or other reasons, for example, substance abuse, head injury, brain infection, and others.
In summary, sensation and perception are complex processes, that consist of many steps and components. They are also strongly interconnected with other brain functions such as emotions. Every sensation causes an emotional reaction as every sensation can be pleasant, unpleasant or neutral. Emotions can also influence how we perceive things. For example, strong fear might cause incorrect perception of reality. And as we saw it in the case of false hallucinations, perceptions are also connected with cognitive functions such as working memory.