Sensation and Sensory Thresholds

Sensation

• Involves the acquisition of information from the external environment
• Sensory receptors, located in sense organs like eyes, ears, nose, skin, and taste buds, detect stimuli
• Sensory receptors are neurons that respond to various stimuli, including light, vibrations, pressure, temperature, or chemical molecules
• Transduction is the process where physical stimuli are converted into neural signals that are sent to the brain

Sensory Thresholds

• Defined as the limits of what can be detected through our senses
• Weber (1795-1878) identified the 'just noticeable difference' as the smallest detectable difference between two stimuli, detectable 50% of the time, and is constant
• Fechner (1801-1887) defined 'absolute threshold' as the lowest level of stimulation needed for detection 50% of the time

Subliminal Stimuli

• Subliminal stimuli are present but do not reach conscious awareness
• These stimuli activate sensory receptors without conscious perception
• Subliminal stimuli can affect automatic reactions, such as increasing facial tension

Habituation

• Habituation involves a reduced response to repeated stimuli
• The brain learns to ignore unimportant stimuli
• Sensory signals from receptors are not sent to the cortex for processing during habituation
• An example of habituation is not hearing planes land and take off after being exposed to them repeatedly

Sensory Adaptation

• Sensory adaptation occurs when receptors become less responsive to constant stimuli
• The receptors stop sending signals to the brain
• Sensory adaptation is exemplified by no longer feeling ice-cold water after some time

Vision

• Vision uses light, which consists of photons with wavelengths that the eye registers
• Photoreceptors detect light stimuli
• Humans can see wavelengths from 380 to 760 nm
• Brightness is determined by the amplitude of the wavelength, with higher amplitudes resulting in brighter light
• Colour determined by wavelength, with shorter wavelengths (nm) corresponding to blue and longer ones to red
• Saturation depends on colour purity, comparing a pure red to a red mixed with blue

Anatomy of the Eye

• The cornea allows light to enter the eye
• The pupil, controlled by the iris, regulates the amount of light entering the eye, enlarging in dim conditions
• The lens, located behind the iris, contracts and relaxes and this is called accommodation
• Light travels to the retina, where photoreceptors (rods and cones) are located
• The Lateral Geniculate Nucleus (LGN) receives information from the optic nerve and sends it to the occipital lobe for processing

Photoreceptors

• Photoreceptors include cones and rods
• Cones: 6 million cones and are responsible for colour vision and function best in daytime
• Cones provide sharpness (acuity) in vision and are located in the fovea which is the central part of the retina
• Rods: 120 million rods, sensitive to low light levels and are used for night vision, without colour (no hue)
• Rods have poor acuity and are located in the periphery of the retina

Photoreceptors: Optic Disk

• The optic disk is located at the back of the eye, where visual information is sent to the optic nerve
• The blind spot is where there are no receptors and light cannot be seen if it falls there

Perception of Colour: Trichromatic Theory (Hermann von Helmholtz, 1852)

• States that there are three types of cones (blue, red, and green), which are the primary colours of light and enable colour vision
• Each cone registers different wavelengths
• Perceiving different colors results from the combination of activation of different cones

Perception of Colour: Opponent-Process Theory

• There are different ganglion cell pairs: red-green, yellow-blue, black and white
• Colour red activates the red cone, excites red cells, and inhibits green cells in the red-green pair so it is seen as red
• The two pairs of cones cannot be activated simultaneously, meaning colours such as a reddish-green, are not seen

Colour Blindness

• Colour blindness is a defect in the pigments within the cones
• Results in an inability to see some hues

Taste

• Taste buds on the tongue contain about 20 receptors
• Molecules of food are broken down by saliva, detected by receptors, and sent to the brain (insula and frontal operculum)
• The chemical sense is responsible for taste
• Taste buds are replaced every 10 to 14 days
• There are six types of taste: bitter, sour, sweet, salt, umami, and fat

Smell

• Smell odours are called olfaction using the chemical sense
• Olfactory receptor cells in the nasal cavity convert smell molecules into a signal the brain registers
• Cilia, little hairs are receptor sites and send a signal to the olfactory bulb, which is under the frontal lobe
• Smells gets sent to the limbic cortex, amygdala, and orbitofrontal cortex

Somatosenses (Body Senses): Touch

• Skin receptors register touch sensations and send them to the somatosensory cortex for processing
  • Sensations include light touch, deep pressure, hot and cold temperatures, and pain
  • There are different receptors for each sensation
  • Thermal receptors exist for temperature
  • Free nerve endings are receptors under the skin that respond to temperature, pressure, and pain

Visceral Pain

• Visceral pain receptors register pain (pressure) from internal organs

Somatic Pain

• Somatic pain arises in the skin, muscles, tendons, and joints
• Large nerve fibres transmit sharp and fast pain
• Small nerve fibres transmit slower, ache-type pain

Somatosenses (Body Senses): Pain

• Gate-Control Theory (Melzack & Wall, 1965; 1996)
  • Messages need to pass a gate at the spinal cord to travel to the brain
  • The gate (neural activity) can be closed via brain signals
  • Endorphins released inhibit transmission of pain messages

Body Movement and Position

• Kinesthesia refers to body movement, and proprioception relates to body position
• Both are based on sense information
• Receptors in muscles, tendons, and joints detect movement, muscle stretching, and body location in space
• Vestibular System relating to balance in the inner ear
  • Otolith organ: head moves crystal and leads fluid to vibrate and receptors (hair cells) are activated, revealing head movement with comparison of information from the brain and eyes
  • Semicircular organ: detects motion where information from this organ and the eyes cause mismatch resulting in motion sickness, also called sensory conflict theory

Perception

• Size constancy allows objects to be perceived as the same size regardless of distance
• Shape constancy allows objects to be perceived in its actual shape even when the shape of an object on our retina may change
• Brightness constancy allows objects of the same brightness to be perceived even with changes in light

Gestalt Principles

• These principles help us to see patterns rather than random arrangements
• Figure-ground relationships
  • The figure has a distinct shape with clearly defined edges, while the ground forms the background
  • Features can be grouped to figure out which features are part of the same object
  • Common region: same region - we group as one

Depth Perception

• Involves perception in three dimensions
• Monocular cues
  • Linear perspective: Two parallel lines merge in the distance
  • Relative size: Smaller objects percieved as in the distance
  • Interposition: One object blocks another where the blocked object is behind and further away
• Aerial perspective: objects get hazier further away
• Texture gradient: objects in the distance have less texture
• Motion parallax: objects that are close pass faster than those in the distance
• Accommodation relates to muscular cues where changing of eye lens occurs for focusing

Binocular Cues

• Convergence happens when both eyes turn to focus on an object
• Less convergence for distant objects
• Binocular disparity: information is different from both eyes depending on if they are close or far
• Illusions are distortions of what we are seeing and differs from reality

Illusions

• Illusions: differ from reality and are distortions of what we are seeing from the brain processing
• Some neurons (feature detectors) in the primary visual cortex respond to: light, orientation and movement, corners/curves/edges, and lateral inhibition
• Lateral inhibition involves excited neurons in the brain inhibiting activity of other close by neurons
• At intersections, more light is processed leading to more inhibition, which can lead to darker spots

Müller-Lyer Illusion

• Environmental adaptation involves corners
• Building on the outside lines appear closer and get perceived as sorter, while the inside ones get perceived further away and longer

Ebbinghaus Illusion

• Context: the comparison of other objects relative to the target object

Moon Illusion

• Moon in the sky: there are no cues to help with depth perception
• Horizon: Horizon in the distance is behind houses, making it seem far away
• Objects in distance that look big: It looks bigger because if it wasn't it would appear small
• It gets perceived as slightly bigger than it really is