Perception can be defined as conscious sensory experience. Everything we see, hear, taste, smell, and feel comes through our sensory receptors (= specialized cells that respond to particular environmental energies, such as light, sound, chemical stimuli, and pressure).
It is often said that sensation is the process of detecting the elementary properties of a stimulus (= anything that causes a sensory response) and perception is the process of interpretation. But this distinction is not always clear or useful, and so, the term perception can be used to encompass both.
Perceptual process
The perceptual process consists of a sequence of seven steps. These are –
- Stimulus in the environment
- Stimulus in the receptors
- Receptor processing
- Neural processing
- Perception
- Recognition
- Action
Before describing the steps, it’s important to note that the perceptual process doesn’t always follow these chronologically. There can be three exceptions –
- Perception (“I see something.”) and recognition (“That’s a pigeon.”) may not always happen one after another. They can happen together or even in reverse order.
- Action may affect recognition (and perception). For example, walking closer to the pigeon (an action) may reveal that it’s actually a dove.
- Action itself can create new stimuli in the environment. For example, walking close to the bird may cause it to fly away, which creates another stimulus.
To account for these exceptions, the diagram (which features a tree instead of a bird) showing the perceptual process (for vision) below uses a two-sided arrow to show the relationships between perception – recognition and recognition – action. Additionally, an arrow shows the connection between action and stimulus, turning the perceptual process into a cycle.
Stimulus in the environment and receptors
Distal stimulus
The stimulus in the external environment is called distal stimulus. Though there are stimuli in our body producing pain or helping us sense our limbs, we generally consider only the distal ones in our discussion of the perceptual process. In the diagram above, the tree is the distal stimulus.
Proximal stimulus
Light reflected from the tree enters our eyes and reaches the visual receptors. The stimulus in the receptors is called proximal stimulus. In this example, the image of the tree on the retina (sensory receptors) is the proximal stimulus.
Principle of transformation and Principle of representation
The principle of transformation states that information from distal stimulus keeps changing as it moves through the perceptual process toward perception.
The principle of representation says when information from distal stimulus finally reaches perception, it is no longer the actual distal stimulus but an internal representation of it.
According to the transformation principle, distal stimulus gets transformed or changed into proximal stimulus. For example, a tree (distal stimulus) will appear different in our retina (proximal stimulus) based on the properties of the light that is being reflected from the tree (e.g., midday vs. dusk), the properties of the atmosphere through which light is transmitted (e.g., clean air vs. dusty), etc. The eyes’ optical system also affects how the tree appears in the retina. According to the representation principle, proximal stimulus is a representation of the distal stimulus, but they are never identical.
Receptor and neural processing
Receptor processing
When the proximal stimulus reaches sensory receptors, they (A) transform it into electrical energy and (B) respond differently depending on its properties, which ultimately shapes perception. This combined activity is called receptor processing.
Transduction
Transformation of environmental energy (like light or sound) into electrical energy/signals is called transduction. For example, through transduction, the proximal stimulus of a tree is converted into electric signals by the sensory receptors.
Lobes of the brain
After transduction, electrical signals are transmitted to the cerebral cortex (= the 2mm thick outer layer of the brain). The cerebral cortex is divided into four main primary receiving areas (image below) –
(A) Occipital lobe (electrical signals of vision are sent here).
(B) Temporal lobe (electrical signals of hearing are sent here).
(C) Parietal lobe (electrical signals of skin senses are sent here).
(D) Frontal lobe (coordinates the senses).
Neural processing
When electrical signals from sensory receptors are transmitted (to and within the brain) through neurons, the signals get processed or changed (= some signals are amplified, some are reduced, and some are prevented from getting through). This is called neural processing.
Perception, recognition, and action
Perception vs. recognition
Recognition is the ability to place an object into a meaningful category. For example, while perception is the conscious awareness of an object (like a tree), recognition is the ability to categorize the object (as a tree). This difference is further illustrated by neurological conditions such as visual form agnosia. Someone with this condition can perceive an object and describe its properties, but can’t recognize what it is as a whole. A famous example is a patient who, upon seeing a glove, could describe it as “a continuous surface unfolded on itself… appears to have five out-pouchings… a container of some sort…” yet could not recognize it as a glove.
Action
Action is an important part of the perceptual process. It is defined as the motor activities in response to a stimulus. While perception may not always lead to action (since one can perceive and yet not respond), action almost always leads to new perception. For example, the visual representation of a tree changes each time the person moves. Researchers suggest that visual processing in animals evolved not simply to let them form a picture of the environment, but to guide actions such as catching prey or avoiding predators.
Knowledge
Knowledge is defined as any information that the perceiver already possesses, whether acquired long ago or recently. Although not listed as a step in the perceptual process, knowledge plays a significant role. For example, our ability to recognize objects depends heavily on the knowledge we already have.
In fact, recognition almost always requires knowledge-based processing (= top-down processing) because it can’t happen without using the existing knowledge of the perceiver. Receptor processing, however, is considered data-based processing (= bottom-up processing) because it relies solely on data from proximal stimulus. Except for very simple cases that can be handled by bottom-up processing alone, both top-down and bottom-up processings typically work together in the perceptual process.