Chapter 4

Chapter 4: Sensation & Perception

Sensation & Perception

Psychophysics: The study of how physical stimuli are translated into psychological experiences. (Gustav Fechner (1860) - Elemente der Psychophysik)

Detection

  • Stimulus: Any detectable input from the environment

  • Absolute Threshold: For a specific type of sensory input, the minimum amount of stimulation that an organism can detect. Arbitrarily defined as the stimulus intensity detected 50% of the time

  • Sensation: Stimulation of the sense organs

    Perception: The selection, organization, and interpretation of sensory input


Just Noticeable difference (JND): Minimum amount you have to change the stimulus before a change is noticed, a.k.a. Difference Threshold

KNOW THE DIFFERENCE BETWEEN THE FOLLOWING LAWS QUESTION WILL BE ASKED ON THE SECOND MIDTERM

Weber’s Law: The size of a JND is a constant (Weber fraction) proportional to the size of the initial stimulus. These constants differ depending on the type of sensory input! delta(I)/I = k

Fechner’s Law: states that subjective sensation is proportional to the logarithm of the stimulus intensity. This means that with constant increases in stimulus intensity, smaller and smaller increases are perceived in the magnitude of that sensation! S = kln(I)

Signal Detection Theory: Detection of stimuli involves decision and sensory processes. Both methods are influenced by a variety of factors besides stimulus intensity

Perception & Awareness

Subliminal Perception: The registration of sensory input without conscious awareness. Started with James Vicary in 1957 — “Eat Popcorn”

Sensory Adaptation

A gradual decline in sensitivity due to prolonged stimulation. This is an adaptive process. Aware of changes in the environment rather than constants

The Auditory System

The auditory system is responsible for the sense of hearing.

Sound waves are vibrations of molecules travelling through a medium, such as air. These vibrations travel much slower than light.

Sound waves are characterized by their:

  • Amplitude (measured in decibels - dB) which determines loudness.

  • Frequency (measured in hertz - Hz) which determines pitch.

  • Wavelength (measured in distance units) which also determines pitch.

  • Purity (viewed as a sound envelope) which determines timbre.

The human ear has three major parts: the outer ear, middle ear and inner ear. Sound is conducted differently in each part.

The basilar membrane in the inner ear plays a crucial role in hearing.

Two main theories attempt to explain pitch perception:

Place theory: Different portions of the basilar membrane vibrate in response to different pitches.

Frequency theory: The entire basilar membrane vibrates at a rate corresponding to the sound's frequency.

Current understanding suggests that pitch perception relies on both place and frequency coding. Low-frequency tones are likely processed via frequency coding, while high-frequency tones rely on place coding. Complex tones involve a combination of both.

Auditory localization refers to locating the source of a sound in space. The separation of human ears aids in this process. Key cues for auditory localization are sound intensity (loudness) and the timing of sounds reaching each ear.

Deafness affects about 1 in 1000 people, and many others have hearing deficits. Causes can be genetic, disease, injury or exposure to loud noise.

The two main types of deafness are conductive deafness and nerve deafness. Noise-induced hearing loss is a significant problem.

Some degree of age-related hearing loss is normal.

Studies show that musicians' brains have larger motor, auditory, and visuospatial areas in the cerebellum compared to non-musicians. There are also links between music training and language abilities. Some research suggests music can enhance spatial abilities (the "Mozart Effect")

The Gustatory System

The gustatory system is responsible for the sense of taste.

Stimuli for taste are soluble chemical substances.

Taste receptors are clusters of taste cells located in taste buds found in the trenches around tiny bumps on the tongue.

There is debate over a fifth taste called umami.

Taste perception depends on complex patterns of neural activity initiated by taste receptors.

While basic taste preferences might be innate and regulated by physiological mechanisms, most are learned.

Taste sensitivity varies between individuals. People can be classified as non-tasters, supertasters, or medium tasters. This means that two people tasting the same food may not have the same sensory experience.

Taste sensitivity influences eating habits. Supertasters often have healthier habits than non-tasters. Women are more likely to be supertasters than men.

Flavour is a combination of taste, smell, and the tactile sensation of food in the mouth. Odours are particularly important for flavour perception.

These notes summarize the key information on the olfactory, auditory, and gustatory systems provided in the sources.

The Olfactory System

The olfactory system is the sensory system for smell.

Humans are relatively insensitive to smell.

Stimuli for the olfactory system are volatile chemical substances, which evaporate easily at room temperature.

Olfactory cilia, hair-like structures in the upper nasal passages, act as receptors.

The mechanisms of odour recognition were clarified by Buck & Axel (1991), who discovered a set of 1000 genes affecting olfactory receptor cells. This work won them the Nobel Prize in 2004.

While humans can distinguish around 10,000 different odours, these cannot be classified as neatly as tastes. We have approximately 350 types of olfactory receptors.

Like other sensory systems, the sense of smell exhibits sensory adaptation. The perceived strength of an odour usually fades to less than half its original intensity within four minutes.

Females tend to be more accurate than males in odour recognition tasks.

Pheromones are chemical messages, often imperceptible, that can be sent and received between members of the same species. Humans both emit and are susceptible to pheromones.

The Tactile System

The Tactile System

Physical stimuli for touch are mechanical, thermal, and chemical energy that affect the skin

  • Can produce perceptions of tactile stimulation (the pressure of touch against the skin), warmth, cold, and pain • Human skin has at least six (6) types of sensory receptors


Perceiving Pressure

  • Tactile localization is very accurate

  • Nerve cells carry information about tactile stimulation through the spinal cord to the brainstem, then project through the thalamus onto the somatosensory cortex

  • Sensory adaptation also occurs with our perception of

Perceiving Pain

Perception of pain can be influenced greatly by expectations, personality, mood, and other factors involving higher mental processes:

  • e.g., placebo effect (Benedetti, 2008)

  • e.g., culture (Jordan, 1983)

  • e.g., contextual factors (Master et al., 2009; Roy et al., 2009; Wegner, 2008)

Attention and pain perception interact! A reaction from people around you can make pain more intense or less intense based on if its positive or negative.

Blocking Pain

Gate Control Theory (Melzack & Wall, 1965): Holds that incoming pain sensations must pass through a “gate” in the spinal cord that can be closed, thus blocking ascending pain signals. Gate is NOT an anatomical structure, but a pattern of neural activity

Blocking Pain

Endorphins and serotonin seem to modulate pain perception’ Endorphin: endogenous, or internally produced morphines

The Kinesthetic & Vestibular Systems

Kinesthetic System

  • Monitors the positions of the various parts of the body.

  • Most kinesthetic stimulation is transmitted to the brain along the same pathway as tactile stimulation.

Vestibular System

Responds to gravity and keeps you informed of your body’s location in space.

  • Shares space in the inner ear with the auditory system.

  • Semicircular canals make up the largest part of the vestibular system.

  • Critical for keeping you oriented and balanced in space!

The Visual System

Visual System●

  • The visual system is the sensory system for sight.●

  • The stimulus for the visual system is light, which is a form of electromagnetic radiation that travels in waves.

Key Properties of Light●

  • Amplitude: The amplitude of a light wave affects the perception of brightness.●

  • Wavelength: The wavelength of a light wave affects the perception of colour.●

  • Purity: The purity of light influences the perception of the saturation or richness of colours.

How the Eye Works1.

  • Light enters the eye: Light first enters the eye through the cornea, a transparent outer layer.2.

  • Passing through the lens: The light then passes through the lens, which can adjust its shape to focus the light on the retina. This process is called accommodation.○

  • When focusing on a close object, the lens becomes fatter (rounder) to provide a clear image.○

  • When focusing on distant objects, the lens flattens out.3.

  • Regulating light: The eye regulates the amount of light reaching the retina using:○

  • Iris: This coloured ring of muscle surrounds the pupil and controls its size.○

  • Pupil: The black centre of the eye, located in the middle of the iris, regulates how much light enters the rear chamber of the eye.

  • The Retina and Visual Receptors●

  • Retina: This neural tissue lines the back of the eye and performs several crucial functions:○

  • Absorbs light○

  • Processes images○

  • Sends visual information to the brain●

  • Cones: These specialised visual receptors are essential for:○

  • Daylight vision○

  • Colour vision○

  • Cones are most concentrated in the centre of the retina, with density decreasing towards the periphery.○

  • The fovea, a tiny spot in the centre of the retina, contains only cones, making it the area of greatest visual acuity.●

  • Rods: These visual receptors are crucial for:○

  • Night vision○

Peripheral vision○

Rods are most densely packed just outside the fovea, with density gradually decreasing towards the periphery.

Adaptation to Light Conditions●

Dark adaptation: This process allows the eyes to become more sensitive to light in low-light conditions.○

Complete dark adaptation takes around 30 minutes.○

Most of the adaptation occurs within the first 10 minutes.

Colour Vision●

The perception of colour is influenced by all three properties of light:1.

Wavelength: Determines the hue of the colour.2.

Amplitude: Influences the brightness (value) of the colour.3.

Purity: Affects the saturation of the colour