Perception is the psychic process that operates the synthesis of sensory data into forms with meaning. Assumptions to the study of perception vary depending on theories and historical moments. The main disciplines that have dealt with perception are psychology, medicine and philosophy.

While our sensory receptors are constantly collecting information from the environment, it is ultimately how we interpret that information that affects how we interact with the world. Perception refers to the way sensory information is organized, interpreted, and consciously experienced. Perception involves both bottom-up and top-down processing. Bottom-up processing refers to the fact that perceptions are built from sensory input. On the other hand, how we interpret those sensations is influenced by our available knowledge, our experiences, and our thoughts. This is called top-down processing.

It is possible to make an initial distinction between sensation linked to the immediate and elementary effects of the contact of sensory receptors with signals coming from outside and able to elicit a response and the perception that corresponds to the organization of sensory data in a complex experience or the final product of a process of processing of sensory information by the entire organism.

Associationist psychology considered perception as the sum of several simple stimuli directly linked to the physiological substrate of the sensory apparatus. With the development and consolidation of Gestalt psychology, the focus of the investigation of perceptual processes shifted from the previous elementary conception to perception as the result of an interaction and global organization of various components. Before expounding on the various theories that have dealt with perception, it is appropriate to understand the processes that underlie it. These processes are of two types: categorization and identification.

The concept of perception goes back a long way and was first conceptualized in philosophy; it refers to becoming aware of something, that is, being conscious of the fact that there are other things than ourselves. In fact, the word perceive means precisely the collection of information that can confirm the existence of an external world. Perception, therefore, has the task of mediating between reality and its representation; it is a process that, in summary, leads to the formation of new forms of knowledge derived from sensory data or real.

In psychology, however, perception is understood as a mental process aimed at converting sensory data into concepts with meaning. Often it happens to confuse the concept of perception with the concept of sensation, using indistinctly the two terms, which however imply very different processes.

From perception to sensation

Sensation is a basic or elementary process, which cannot be further broken down. Sensation, therefore, derives from what the sense organs, present on our body, detect and then translate into physiological stimuli, sent to the brain as electrical signals. This process is called “sensory transduction”, that is the transformation of sensory information into electrical stimulus.

Perception, on the contrary, is something more complex, since it is a process that aims to attribute meaning to the perceived sensory data. Perception, therefore, is a process aimed at identifying, ordering and classifying sensory stimuli from the external world.

Clearly, the distinction between sensation and perception is not immediate, so much so that some consider it as a single psychic process, defined precisely as sense-perception. For this reason, we could consider it as a function that is arranged along a continuum ranging from simple sensory perception to the assumption of specific meanings to the same.

Distal and proximal perception and percepts

Every day we perceive reality exactly as it is shown to our eyes. In this way we obtain a representation of the physical world exactly as perceived through the senses. The perception of the world as it appears is called distal stimulus, or perceived physical object, rich in information from the external environment, such as light, shape, colors and other stimuli available to the eye. When the visual stimulus reaches the retina, it will be called the proximal stimulus. Thus, an external object, (e.g. a house) represents the distal stimulus, while the image projected onto the retina constitutes the proximal stimulus.

So, the sensory information after being encoded and processed is called the percept. In this way, a psycho-physical chain is generated that leads to connect the external environment (distal stimuli) to the retinal projection (proximal stimuli), to which meanings will be attributed (percepts).

Theories of perception

Perception represents a direct connection channel from our mind to the surrounding reality. For this reason many theories have been elaborated in order to obtain a more precise and detailed scientific interpretation of this phenomenon.

First, Hermann von Helmholtz created the empirical theory, according to which the perception of the world, and therefore of objects, happens through experience and learning, coming from the contact with the external world. The elementary sensations, or simple sensory, transmitted to the brain from the external world, after being integrated, constitute the set of knowledge acquired. It is obtained, so a process that takes the name of inference, or the deduction of meanings of elements learned from the external world.

According to Gestalt, instead, the meaning of perceptions derives from the innate laws originated by the organization of the perceptual field, on which neither the subjective experience nor the future expectations of individuals weigh. For gestaltists, stimuli are fragments (a series of parts), which lead to the organization of everything in an automatic way, until they form a perceptual field on the basis of internal dynamics (principle of automatic self-distribution). These phenomena allow the perception of objects in their totality.

According to the New Look movement, founded by the Americans Bruner, Postman and Mc Ginnies, perception arises from the encounter between external stimuli and expectations, or the values and interests of the subject. Each person, therefore, becomes a dynamic builder of their own perceptual experiences.

According to the theories of direct or ecological perception, originated from Gibson’s theory, information is derived from the perceived stimulation and can be inferred from this without any particular additional processing. Therefore, the subject does not have to rework the perception, nor integrate it with information already present, but must only grasp the perceptual information existing in the environment. Gibson defines this process with the term affordances or availability.

Another theory is the Neisser’s theory of the perceptual cycle, according to which the schemes present in the mind orient attention and allow the exploration of the environment. The subject prepares himself, receives information and selects the most important parts of the objects that serve to achieve individual goals.

Perceptual organization

Based on the theories previously listed, it seems quite obvious that the perceived must be organized in order to allow the human mind to have a starting point with which to interact and organize the stimuli coming from the external world. This perceptual organization is oriented by the specific characteristics presented by the stimulus and the context in which it is immersed. The characteristics of the object activate a psychic function that allows to organize the stimulus coming from outside. At this point, a new process takes over: attention, which selects the stimuli of interest while excluding others. The exclusion, most often, is obtained on the basis of personal needs, motivations, emotions experienced and knowledge already acquired by the perceiver.

For example, in a supermarket we are able to perceive what we consider most interesting, extrapolating information that will remain in our memory at the expense of other non-informative information (cocktail party effect). Shifting perceptual attention to what we are interested in is a process determined by a limited amount of channels, imputed to information processing, which allow, consequently, a selective processing of stimuli in a salient way. According to the filter theory, when several messages are received at the same time, attention allows the selection of the most significant message and allows only this one to pass to the next stages of information processing.

Another effect related to perception is the Stroop effect. It consists in a delay in response times, when the subject is asked to say the name of the color with which a word indicating a different color is written. For example, when the subject is faced with the word “yellow” written in red, he or she must say red and not yellow. Attentional selection occurs when we have to select the answer to give. It happens to activate automatisms that would lead us to say exactly what is not asked in Stroop’s task, i.e. the written word. In this case, there is a sensory processing that can carefully select the information of interest.

Another perceptual effect is figure-background articulation, which involves relating each perceived stimulus, the figure, to a background. This process makes it possible to automatically bring out the figure on which to focus attention, which will be characterized by a precise shape, unlike the background. There are some figures, called reversible, from which both the figure and the background could emerge, depending on how you shift your attention. Therefore, an attentional effort must be made to ensure that the figure always emerges from the background.

Finally, another psychological phenomenon that facilitates the perceptual organization of our mind is the perceptual constancy, according to which a stimulus appears to us identical even if the conditions of stimulation of sensory receptors vary. Therefore, a book with a green cover will always be perceived as green even if in particular light conditions it might seem to tend to yellow.

Depth perception and movement

The perceived world is characterized by three dimensions, but our eye receives information in a two-dimensional way. The brain, however, with the help of additional sensory information can bridge this discrepancy. Depth, in fact, is perceived through different ocular processes, namely the accommodation or monocular process, the pictorial clues and the binocular process. The former consists of the focusing of an object by the crystalline lens. The pictorial clues, however, can be of different types, for example: the overlap between two stimuli that overlap only partially, the height on the horizon plane where the most distant stimuli appear higher, the chiaroscuro to indicate the depth of the stimulus, the linear perspective (such as the train tracks that tend to meet near the horizon) and the tissue gradient, according to which the closer an object is to the observer the less the latter will perceive clearly all the details. Examples of binocular processes are retinal disparity, which allows us to process objects that are located far away from the observer, while convergence allows us to interpret the information coming from the retinal muscles in order to recognize very close objects.

The external world does not only consist of static objects, but they are often in motion. Moving stimuli are perceived thanks to distance, defined as absolute and relative.

Sometimes, however, our information processing system can be misled, as in the case of the train illusion: if we are in a train and the neighboring one is about to leave, we actually perceive a movement by our vehicle; this phenomenon is due to the few perceptive clues received that make it difficult to compare relative movements.

In any case, it is necessary to refer not only to the movements perceived on the retina, but we can also make use of other indications, for example the relationship of the stimulus with the background (based on illumination and speed of the perceived movement), or the motion parallax, i.e. the movement of an object compared with a static object.

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