Chapter 4: Perception and Sensation

Introduction

In this chapter, you will also be able to identify the mechanisms that are responsible for your perceptual experiences.

The Perceptual Process

Stimulus

Stimulus in the environment has two aspects: environmental stimulus and attended stimulus. If you shift your attention, your attended stimulus also changes. The main principle of perception is based on electrical signals that are created in the receptors in our nervous system. Transduction is the transformation of one form of energy into another form of energy. This electrical energy is processed through networks of neurons, which is called neural processing. Recognition and action are the outcomes of the perceptual process. Sensation includes the information contained in the environmental stimuli that are captured by the relevant sensory system and transmitted to the brain. Perception, on the other hand, is the process of analyzing, recognizing, interpreting, and organizing this information. How physical stimuli are translated into psychological experiences, is measured by thresholds. While the difference threshold involves the ability to detect differences in stimulation levels, the absolute threshold refers to the smallest detectable level of stimulation.

Vision

We use vision to perceive the environment in various ways with our eyes as other primates. The light, which is a form of electromagnetic energy, affects the photoreceptors of the retina. This triggers processes that produce neural signals and then progresses through the neural networks. The lateral geniculate nuclei (LGN) in the thalamus and the signals from the cerebral cortex mediate different visual functions, such as movement, shape, color, and other distinctive features of the visual world. The pupil regulates the amount of light according to its brightness (by contracting and dilating). You can think of your eye as a camera. Light enters the eye through its pupil, focuses on the cornea and lenses, and then reaches the rods and cones, the receptor cells on the retina. The role of receptor cells is to convert the light energy into neural messages. The cones are plentiful in the fovea, but they become less and less prevalent as one moves away from the fovea to the retina’s periphery. The opposite is true for rods. The rods aren’t located in the fovea and the density increases toward the periphery. For this reason, the rods provide periphery detection while the cones are responsible for seeing the center. The rods and cones have different structural characteristics. The cones are less sensitive to light and responsible for seeing in daylight conditions, while rods are more sensitive to the light and responsible for the conditions with low light level.The stimuli are transmitted to the lateral geniculate nucleus of the thalamus first through the optic nerve and then to the visual cortex, after processing in the eye network system. At the point where the optic nerve leaves the eye, there are no photoreceptors, so the reflected light cannot be processed. For this reason, this point is called the blind spot.

Color Vision

The colors provide us information to identify and categorize things. We can distinguish objects from each other and from the background through our color perception. There are two main theories of color perception. These are the Trichromatic Theory and the Opponent-Process Theory. The trichromatic theory is based on the idea that the light of a particular wave length stimulates the three different receptor mechanisms, each with different spectral sensitivities (also called Young – Helmhotz theory of color vision). These three colors are red, green, and blue. According to opponent- process theory, color perception depends on the reception of pairs of antagonist colors.

Depth Perception

A two-dimensional image in the retina is perceived as three-dimensional with the deductions about depth or cues about distance. This is done by only one-eye (monocular) and two-eyes (binocular) depth cues. Oculomotor cues are based on our ability to sense the position of our eyes and the tension in our eye muscles. Monocular pictorial cues are the source of depth information:

• Occlusion: This is a result of nearer objects covering up part of farther objects.
• Relative height: Objects that are below the horizon and have their bases higher in the field of view are usually seen as being more distant.
• Relative size: When two objects are of equal size, the one that is farther away will take up less of your field view than the one that is closer.
• Familiar size: We use this cue when we judge distance based on our prior knowledge of the size of objects.
• Atmospheric perspective: It occurs when more distant objects appear less sharp accompanied with a slight blue tint.
• Linear perspective: Parallel lines appear to converge in depth.
• Shadowing or Shading: When objects appear in front of the shadow, the angle and sharpness of the shadow will influence the amount of depth perceived.
• Texture Gradient: Elements that are equally spaced in a scene appear to be more closely packed as distance increases.

Distance Perception

Distance perception is the act of knowing or recognizing a distance by previous experiences or recollective thought. Perception of depth is perceived by the distance cues: binocular cues and monocular cues. Binocular cues aid in depth perception and monocular cues provide the information necessary to feel the depth and the distance in a picture on a two-dimensional paper.

Perceptual Organization

Perceptual organization involves the grouping of elements in an image to create larger objects. Gestalt psychologists proposed the idea that the whole differs from the sum of its parts. The simplest and the most consistent arrangements are made to provide a perceptual organization. One of the important perceptual organization laws is law of similarity, which states that similar things appear to be grouped together. Another important perceptual organization law is law of proximity. There are four principles of law of proximity that the Gestalt psychologists proposed to explain how perceptual grouping occurs: Grouping by (a) proximity; (b) common region; (c) connectedness and (d) synchrony.

• Proximity: When things are near each other, they appear to be grouped together.
• Elements that are within the same region of space appear to be grouped together, and this phenomenon is called principle of common region.
• The principle of uniform connectedness states that a connected region of visual properties, such as lightness, color, texture, or motion, is perceived as a single unit.
• The principle of synchrony states that visual events occurring at the same time are perceived as belonging together. So far, we have considered how we know where an object is and what it is doing. It seems like the most important aspect of perception is the sequence of steps through which we perceive and recognize what an object is.

Hearing

There are two definitions of sound: physical and perceptual. The physical definition of sound refers to the pressure change in the air, whereas the perceptual definition refers to the experience that we have during hearing. Human ear consists of three parts inner, outer and middle ear. Sound travels at a constant speed in the air. A sound wave is defined by three components: frequency, amplitude and phase. Sound waves first reach the outer ear, which is the visible part of the hearing system. The outer ear is the visible part of the hearing system in the shape of a megaphone. Unlike the megaphone, it transmits the sound waves inward, not outward. The main function of the outer ear is to catch the voice from the outside world and transmit it to the other parts of the ear. The sound wave reaches the eardrum through an ear canal. When the sound waves reach the eardrum (tympanic membrane), it causes vibration. As a result of this, the vibrations coming to the center are transmitted to the middle ear. The middle ear contains three bones: anvil, stirrup and hammer. The sound wave arriving at the inner ear reaches the structure cochlea. the cochlea that contains the hearing neurons called the hair cell.

Auditory Perception

The pitch is important because it plays an important role in distinguishing people’s voices and their emotions during speech. There are several theories that are related to pitch perception: place theory and temporal theory. How do we know where the sound comes from? Most of the mammals have two ears that provide information about the locaiton of sound. Like the monocular and binocular cues that provide information about depth, the auditory system uses both monaural (one-eared) and binaural (two-eared) cues to localize sound.

Other Sensations

Smell

Smell is often the primary source of perceiving the environment. The animals, which have a highly developed sense of smell are macrosmatic. The sense of smell is not that crucial to our survival, so humans are microsmatic. Basicly in humans, the sense of smell plays a role in behaviors such as finding nutrition and orienting themselves in the environment. It has also a role on social relations. Some forgotten events or memories can be remembered with the help of smells that are associated with memories. Losing the ability to smell is called anosmia. The reason for that is the close connection between smell and flavor. Also, olfaction is more important for our survival than we think. It serves as an olfactory warning system in dangerous situations, such as gas leakage or rotten food.

Taste Perception

In general, the taste sensation has the following categories: sweetness, sourness, saltiness and bitterness. Recently, a category of taste called umami has been mentioned. Umami is perceived when we taste some salts. These salts are known as monosodium glutamate and are mostly used in ready-made soups and packaged foods. Taste has a complex structure beyond the recognition of flavors. When the taste of food is imagined, appetite swells. The surface of the tongue contains many ridges and valleys caused by the presence of structures called papillae. All of the papillae contain taste buds and the whole tongue contains about 10,000 taste buds. There are also individual differences in tasting some foods. Most of the people describe their taste preferences in terms of sweet, sour, salty and bitter. However, there are also genetic differences that affect people’s ability to sense the taste of certain substances.

Touch

Humans are sensitive to mostly four tactile sensations: pressure (touch), pain, warmth and cold. These sensations are conveyed thorough receptors on the skin and in our internal organs. The skin receptor cells have an unequal distribution on the skin. The skin prevents dangerous stimuli from entering the body. It also has other functions such as regulating the body temperature. Although researchers cannot completely explain the pain, they can explain the process of injury and brain’s role. In the 1960s, Ronald Melzack and Patrick Wall proposed a theory about pain that is called the gate-control theory of pain. According to this theory, pain signals travel from body to the brain through a gate in the spinal cord. Gate refers to the pattern of neural activity which functions as a gate. This activity either stops pain signals or allows them to pass. Openness or closeness of the gate is determined by the signals of the brain.