Topic 1.6 Notes – Sensation

What Sensation Is

Sensation means detecting stimulation from the environment and transducing it into neural signals. Transduction happens in every sense. Light, sound waves, pressure, and chemicals do not go into the brain unchanged. Receptors convert them into the nervous system’s language.

Perception is the next step. It organizes and interprets those signals. If light hits your retina, that is sensation. Realizing it is your friend’s face is perception.

Psychophysics studies the link between physical stimuli and what you experience.

Thresholds and Sensory Change

• Absolute threshold is the minimum stimulation you can detect 50% of the time.
• Subliminal stimuli are below that threshold.
• Difference threshold or just-noticeable difference (JND) is the smallest change you can detect, also 50% of the time.

Weber’s law says the JND is a proportion of the original stimulus, not a fixed amount.

$$\frac{\Delta I}{I}=k$$

If the starting stimulus is bigger, the change needed to notice it is also bigger.

• Sensory adaptation means reduced sensitivity to constant stimulation.
  • You stop noticing your hoodie on your skin.
  • You notice a new smell at first, then awareness fades.
• This helps you pay attention to change, which is usually more useful.

Sensory Interaction

Your senses work together.

• Flavor depends heavily on taste + smell.
• Seeing lip movements can change what sound you think you heard.
• Synesthesia is when one kind of stimulation automatically triggers another sensory experience, like hearing music and seeing colors.

How Vision Works

Visible light is electromagnetic energy. Wavelength relates to hue or color. Amplitude/intensity relates to brightness.

A quick cross-section of the eye helps you place the main structures before you trace the path of light.

Light follows this path through the eye:

1. Cornea
2. Pupil
3. Iris adjusts pupil size
4. Lens
5. Retina

The lens changes shape by accommodation so images focus on the retina.

• Myopia means distant images focus in front of the retina.
• Hyperopia means near images would focus behind the retina.

Retina and Visual Cells

The retina is the light-sensitive surface where visual transduction begins.

• Rods matter for dark adaptation.
• Axons of ganglion cells form the optic nerve.
• The blind spot is where the optic nerve leaves the eye. There are no receptors there.
• You do not see a hole because the brain fills in missing information.

Color Vision

These two theories fit together. The cone types are shown across the top, and the opponent color channels are shown below.

Two theories explain color vision at different stages:

• Trichromatic theory says you have three cone types sensitive to
  • short wavelengths blue
  • medium wavelengths green
  • long wavelengths red
• Opponent-process theory says color is processed in opposing pairs
  • red/green
  • blue/yellow
  • black/white

This explains afterimages. Stare at one color, then look at white, and the opponent color appears.

• Dichromatism means one cone system is missing or impaired.
• Monochromatism means little or no color discrimination.

Visual Brain Problems

Vision also depends on the brain, especially the occipital lobe.

• Prosopagnosia means face blindness.
• Blindsight means responding to visual stimuli without conscious awareness of seeing them.

How Hearing Works

Sound is pressure waves in air. Frequency relates to pitch. Amplitude relates to loudness.

To follow the process, it helps to picture the sound pathway through the outer, middle, and inner ear.

Path of sound:

1. Pinna
2. Ear canal
3. Eardrum
4. Hammer, anvil, stirrup
5. Oval window
6. Cochlea
7. Basilar membrane bends hair cells
8. Auditory nerve → thalamus → temporal lobe

Pitch and Location

• Place theory says pitch depends on where the basilar membrane vibrates most. Best for high pitches.
• Frequency theory says firing rate matches sound frequency. Best for low pitches.
• Volley theory says groups of neurons take turns firing. Best for middle ranges.

Sound localization happens because the brain compares tiny differences in timing and loudness between the two ears.

Hearing Loss

• Conduction deafness comes from problems transmitting sound in the outer or middle ear.
• Sensorineural deafness comes from damage to hair cells or the auditory nerve.
• Aging and loud noise often cause sensorineural loss.
• Cochlear implants can help some people with sensorineural loss.

The Other Senses

Smell and Taste

• Olfaction is smell. Airborne molecules activate receptors in nasal tissue.
• Smell is the only sense that does not first route through the thalamus.
• Pheromones are chemical signals, clearly established in many nonhuman species.
• Gustation is taste from chemicals dissolved in saliva.
• The six tastes are sweet, sour, salty, bitter, umami, and oleogustus.
• There is no true tongue map.
• Supertasters, medium tasters, and nontasters differ in receptor sensitivity.
• Flavor depends strongly on smell + taste together.

Touch and Pain

• Skin receptors detect pressure, warmth, and cold.
• Touch is processed in the somatosensory cortex.
• Feeling hot can come from warm and cold receptors firing together.
• Nociceptors detect possible tissue damage.

Gate control theory says the spinal cord has a “gate” that can increase or reduce pain signals. That is why rubbing an injured spot can lessen pain.

Pain is shaped by attention, emotion, and expectation, not just injury.

• Phantom limb sensation is feeling an amputated limb. Sometimes it is painful.

Balance and Body Position

• The vestibular sense helps with balance, head movement, and orientation.
• Semicircular canals detect rotational movement.
• Kinesthesis is awareness of body-part position and movement.
• Smooth movement depends on vestibular, kinesthetic, visual, and touch input working together.