Sensation and Perception Basics

Sensation and Perception Overview

• Sensation is the detection of physical energy by our sense organs like incoming light or sound waves.
• Perception is the brain's interpretation of raw sensory inputs, such as recognizing an object or voice.

Basic Principles of Sensation and Perception

• Sensory stimuli influence receptors, which code the stimulus in terms of electrical signals.
• Sensory transduction is the process of converting sensory stimuli into electrical signals.
• Sensory systems are designed to detect change
• Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation; this keeps us attuned to changes rather than constants.
• Psychophysics is the study of how we perceive sensory stimuli based on their physical characteristics
• Gustav Fechner made contributions to psychophysics in 1860 and described the requirements for sensation, like a stimulus.
• Absolute threshold is the lowest level of a stimulus detectable on 50% of trials when no other stimuli are present.
• The stimulus must reach the absolute threshold to be detected.
• Just Noticeable Difference (JND) is the smallest detectable change in stimulus intensity, also known as the difference threshold.
• Ernst Weber developed Weber's law, and as stimulus magnitude increases, so does the JND.
• Weber's Law states that the JND is a constant fraction.
• Signal detection describes how we detect stimuli under uncertain conditions, accounting for cognitive factors and response biases.
• In standard signal detection experiments, one half of trials presents a near-threshold stimulus and the other half presents no stimulus; trials are randomized to prevent prediction.
• In signal detection theory a "hit" is to detect a stimulus that was present.
• A "miss" is failing to detect a stimulus that was present.
• A "false alarm" is indicating a stimulus was present, when it was not.
• A "correct rejection" indicates there was no stimulus, when there was no stimulus.
• Sensory systems process their own information; Johannes Müller proposed the doctrine of specific nerve energies in 1826.
• Sensations are determined by sensory receptor properties, such as phosphenes.

Cross-Modal Processing and the McGurk Effect

• Many examples exist of cross-modal processing, where signals from different senses interact.
• The McGurk Effect is a classic example of signals from two different senses interacting with one another.
• Processing speech requires brain calculations of the most probable sound by combining information from vision and audition.
• The McGurk Effect occurs when a mismatch between seen and heard information leads to inaccurate perception.
• Synesthesia is a condition where people experience cross-modal sensations and perceptions; over 60 types have been identified.
• Grapheme-color synesthesia is the most common type, where the experience of numbers and letters associates with color.

The Role of Attention in Perception

• Attention interacts with perception, focusing on certain stimuli in the environment to the exclusion of others.
• Selective attention allows us to select one sensory channel and ignore or minimize others.
• Reticular Activating System (RAS) and the cerebral cortex are involved in selective attention.
• Dichotic listening is selective attention originally studied
• Shadowing requires participants to repeat the attended message aloud.
• Initial experiments suggested shadowing only one ear at a time was possible.
• Participants were largely unaware of messages in unattended ears, as shown in Cherry's 1953 research.

Broadbent's Filter Model and Parallel Processing

• Broadbent's (1958) filter model explains selective attention, noting unattended messages are completely blocked.
• Broadbent's filter model does not explain the cocktail party effect - ability to detect important words in conversations that do not involve the listener.
• Parallel processing helps to attend to many sensory modalities simultaneously.
• Bottom-up processing involves constructing a whole stimulus from its parts (stimulus-driven).
• Top-down processing influences perception based on beliefs, expectations, and past experiences (conceptually driven).
• Perceptual hypotheses involve making educated guesses about what our sensory system is telling us (top-down).
• Educated guesses are correct most of the time.
• Gestalt principles explain rules governing how we perceive objects as a whole within context.
• Proximity, Similarity, Continuity, Closure, Symmetry, and Figure-Ground are the six main Gestalt principles.

The Binding Problem

• Complex stimulus aspects are processed in different brain areas, but we perceive the stimulus as a single unit.
• Selective attention may be considered "glue" that binds features together.

Vision - The Stimulus of LIght

• Vision begins when light in the form of electromagnetic radiation that travels as a wave.
• Visible light has a wavelength of 380 - 760 nm.
• Brightness related to the amount of light reflected off objects into our eyes.
• Wavelength affects mainly the perception of color (hue).

Parts of the Eye

• Reflected light enters the pupil
• The cornea and lens then focus the light on the retina to form clear images.
• Once light hits the retina photoreceptors are activated.
• The fovea is the area of the retina with highest visual acuity.
• The optic nerve relays these signals from the eye to the brain.
• Sharp vision occurs when the image focuses properly in the retina.
• Myopia (nearsightedness) - when image is focused in front of the retina, and is caused by the eyeball being too long or the cornea curved too much.
• Hyperopia (farsightedness) - when the image is focused behind the retina, and is caused by the eyeball being too short or the cornea not curved enough.

The Retina In Detail

• The retina is neural tissue lining inside back surface of the eye, and it absorbs light, processes images, and sends visual information to the brain via the optic nerve
• The fovea is contained in the retina, and is responsible for acuity
• The retina contains both rod and cone receptors
• Rods are for peripheral vision, and is in lower light, low acuity, high sensitivity, and monochromatic
• Cone are for central vision (fovea), and is in brighter light (day), high acuity and colour

Visual Pathway

• Damage to V1 in primary visual cortex can result in blindness, even though the eye can still work
• Patients with damaged V1 can have blindsight, which is the ability to process limited visual information outside of awareness
• Damage to visual areas in the temporal lobe can result in problems with object or face perception
  • ie. Visual agnosia and prosopagnosia is impaired abilities in recognizing objects or faces

Perceiving Shape and Contour

• Hubel & Weisel (1960s) found cells in primary visual cortex (V1) that fired for specific orientations
  • This allowed for the discovery of simple vs. complex feature detector cells
• Trichromatic theory - colour vision depends on activity of three different colour types, blue (S), green (M) and red (L)
• Opponent-process theory - colour vision is related to opposing responses by blue and yellow, and red and green
• Humans are normally trichromats, meaning they have 3 kinds of cones
• They have cones for short, medium, and long wavelengths.
• Some people are dichromats meaning they only have 2 kinds of cones, affecting ~1% of males and .01% of females. Its typically caused by red-green colourblindess
• Very rarely, people are monochromats meaning that only have 1 kind of cone, therefore no coulour vision
• Trichromatic cannot explain Afterimages (when you look at a colour for a long time, and then look away, you may see a differently coloured replica of the same image)
• Due to the aforementioned problems with both theories, Dual-Process Theory incorporates both trichromatic and opponent-process theory to explain colour vision.

Auditory Processing

• Sound is produced by vibrations of air molecules that travel in waves through a medium, such as air
  • They are characterized by amplitude, their wavelength, and complexity
• The range of human hearing is from 20-20,000Hz

Structure and Function of the Ear

• Outer ear - pinna, auditory canal, eardrum
• Middle ear - ossicles
• Inner ear – cochlea (hearing) and semicircular canals (vestibular)
• Sound is conducted differently in each section
• Cochlea is filled with fluid
• Hair cells- the sensory receptors in the ear
• Basilar membrane- the membrane which contains the hair cells
• Vibrations in the cochlear fluid (from sound waves) cause the cilia of hair cells to bend resulting in action potentials

Place Theory

• Place theory suggests that perception of pitch corresponds to the vibration of different portions, or places, along the basilar membrane
  • This works for frequencies between 5000-20,000Hz
• Frequency theory holds that perception of pitch corresponds to the rate, or frequency, at which the entire basilar membrane vibrates
  • This works for tones from 20-100 Hz

Auditory Cortex

• Signals from cochlea (inner ear) are transmitted to the thalamus, then to primary auditory cortex (A1)
• Higher order auditory processing occurs in auditory association cortex

Deafness

• Conductive deafness is due to a malfunctioning of the ear, especially a failure of the eardrum or the ossicles of the middle ear.
• Nerve deafness (sensorineural) is due to damage to the auditory nerve
• Common causes of hearing loss are noise-induced hearing loss and age-related hearing loss (presbycusis)

Taste and Smell

• Olfaction (smell) is intimately linked with gustation (taste)
• Heavily linked to memory and recognition
• Allows us to develop associations between objects in the world and expected smells

Gustatory System

• The gustatory system is a sensory system for taste.
• Physical stimuli for the sense of taste are chemical substances that are present in our saliva
• Gustatory receptors are clusters of taste cells that are found in the taste buds that line the trenches around tiny bumps on the tongue Qualities of Taste
• The 5 main qualities of taste:

1. Bitterness
2. Sourness
3. Sweetness
4. Saltiness
5. Umami- (savory, MSG)

• The perception of taste is related to a complex pattern of firing across taste receptors that are coded in gustatory cortex

Smell and Physical Stimuli

• Physical stimuli are chemical odorants that travel through the air
• Receptors for smell are olfactory cilia, hair-like structures located in the upper portion of the nasal passages
• There are ~350 different types.
• Signals are sent to the olfactory bulbs, and then to pyriform (smell) cortex
• Odours are coded as a complex pattern of firing
• Damage to olfactory bulbs can result in the inability to identify smells (anosmia)
• Flavoured is really the combination of taste and smell, for example eating with a cold makes everything tasteless
• Odorant molecules cannot get through the mucus in your nose
• Taste and smell interact in the orbitofrontal cortex to enable the cognitive processing of smells and tastes

Body Senses - The Somatosensory System

• The Somatosensory system is a system that is responsible for sensation of touch, temperature and pain
• The four main components of the somatosensory system are:.
  • Nocioception - the perception of pain and temperature
  • Hapsis - perception of objects using touch and pressure
  • Proprioception - knowledge of the position of your limbs in space
  • Balance - controlled by the vestibular system in the inner ear
• The skin is one of the largest organs in our body
• The skin has multiple layers
• Epidermis: the skin's outermost layer, and is comprised of dead skin cells
• Dermis: below the epidermis
• Subcutaneous: below the dermis
• Somatosensory system responds to stimuli applied to the skin
• Physical stimuli is mechanical, thermal, and chemical energy that impinge on the skin and it is detected by Mechanoreceptors.
• Touch, vibration, skin indentation
• Free nerve endings are responsible for pain and temperature
• They are Distributed unevenly across the body surface

Pain Peception

• Activation of somatic nerves synapse with spinal cord
• Touch signals travel faster than pain
• Both often activate spinal reflexes before travelling to the brain
• Signals are sent to thalamus, and then to somatosensory cortex (S1)

Proprioception

• Proprioceptive system monitors the positions of the various parts of the body, allowing us to move efficiently
• Proprioceptors sense muscle stretch and force
• Most proprioceptive information is transmitted to somatosensory cortex along the same pathway as tactile stimulation

Vestibular System

• Part of the inner ear
• Responds to gravity and keeps you informed of your body's location in space
• Allows us to maintain balance
• Tilt and orientation of the head
• Enables a stable image on the retina
• Semicircular canals make up the largest part of the vestibular system and are filled with fluid
• Signals processed in the brainstem and cerebellum