Sensations

Basic Principles of Sensation

Sensation is how our body takes in information from the environment using our sensory organs. Perception is how the brain organizes and understands that sensory information so it makes sense.

Transduction

Transduction is the process of changing physical energy (like light waves, sound waves, or smells) into brain signals (neural impulses) that the brain can read.

Sensory Thresholds

• Absolute Threshold: The smallest amount of stimulation needed to notice a stimulus $50\%$ of the time.

• Difference Threshold (Just Noticeable Difference or JND): The smallest change between two stimuli needed to notice a difference $50\%$ of the time.

• Weber's Law: The rule that to notice a difference, two stimuli must change by a constant percentage rather than a set amount. Shown as: $\frac{\Delta I}{I} = k$ where $\Delta I$ is the change in intensity, $I$ is the starting intensity, and $k$ is a constant.

• Signal Detection Theory: Explains how and when we notice weak signals among background noise. It states there is no fixed absolute threshold; instead, detection depends on things like fatigue, experience, motivation, and expectations.

  • Hit: Signal is present and detected.

  • Miss: Signal is present but not detected.

  • False Alarm: Signal is absent but reported present.

  • Correct Rejection: Signal is absent and correctly ignored.

Sensory Adaptation and Habituation

• Sensory Adaptation: Becoming less sensitive to a constant, unchanging stimulus over time (like getting used to a cold pool or a room's smell).

• Habituation: A decrease in response to a repeated stimulus that happens in the central nervous system rather than in the sensory organs.

Vision: The Visual System

Vision turns light waves into brain signals. Brightness comes from light wave height (amplitude), and color comes from wavelength.

Structure of the Eye

• Cornea: The clear outer layer that protects the eye and bends light to focus it.

• Pupil: The adjustable opening in the center of the eye where light enters.

• Iris: The colored muscle ring around the pupil that controls its size.

• Lens: The clear part behind the pupil that changes shape to focus images on the retina.

• Retina: The back surface of the eye containing light sensor cells (rods and cones).

Photoreceptors: Rods and Cones

• Rods: Sensor cells that detect black, white, and gray; needed for side (peripheral) vision and seeing in dim light.

• Cones: Sensor cells in the center of the retina (fovea) that work best in daylight to see fine details and color.

Signal Transmission to the Brain

1. Light triggers chemical activity in rods and cones.

2. Signals pass to bipolar cells, which activate ganglion cells.

3. Ganglion cell axons join to form the optic nerve, which carries signals to the brain.

4. Blind Spot: The area where the optic nerve leaves the eye, where no light sensors exist.

5. Signals travel along the optic nerve, cross at the optic chiasm, pass through the thalamus, and land in the visual cortex in the occipital lobe.

6. Feature Detectors: Brain cells in the visual cortex that respond to basic features like lines, shapes, angles, or movement.

Theories of Color Vision

• Trichromatic (Young-Helmholtz) Theory: The retina has three types of color sensors (red, green, and blue) that combine to create all perceived colors.

• Opponent-Process Theory: Color vision works through opposite pairs (red-green, yellow-blue, black-white). Some cells are stimulated by one color and stopped by its opposite, which explains afterimages.

Audition: The Auditory System

Audition is the sense of hearing. Sound waves are air vibrations. Wave height determines volume (decibels), and wave frequency determines pitch (Hertz).

Structure of the Ear

• Outer Ear: Collects sound waves.

  • Pinna: The visible outside part of the ear.

  • Auditory Canal: The tube guiding sound to the eardrum.

  • Tympanic Membrane (Eardrum): A thin membrane that vibrates when hit by sound waves.

• Middle Ear: Amplifies and passes along vibrations.

  • Ossicles: Three tiny bones—the hammer (malleus), anvil (incus), and stirrup (stapes)—that send vibrations to the inner ear.

• Inner Ear: Turns physical vibrations into neural signals.

  • Cochlea: A fluid-filled, coiled tube in the inner ear.

  • Basilar Membrane: A layer inside the cochlea containing hair cells (sound sensors). Hair cell movement triggers nerve impulses sent to the auditory cortex in the temporal lobe.

Pitch Perception

• Place Theory: We hear pitch based on the exact spot on the basilar membrane that gets stimulated (explains high pitches best).

• Frequency Theory: Pitch depends on how fast nerve impulses travel up the auditory nerve (explains low pitches best).

• Volley Principle: Neurons take turns firing in quick succession to match medium pitches.

Hearing Impairment

• Conduction Hearing Loss: Damage to the physical parts that bring sound waves to the inner ear (like a damaged eardrum or bone issue).

• Sensorineural Hearing Loss (Nerve Deafness): Damage to the inner ear's hair cells or the auditory nerve.

Other Senses

Chemical Senses: Taste and Smell

• Gustation (Taste): Sense of taste using receptors on the tongue. Includes sweet, salty, sour, bitter, and umami (savory).

• Olfaction (Smell): Sense of smell where molecules bind to nose receptors. Smell signals skip the thalamus and go straight to emotional and memory areas of the brain (limbic system).

Touch and Body Position Senses

• Skin Senses: Skin receptors that feel pressure, warmth, cold, and pain.

• Gate-Control Theory of Pain: The spinal cord acts like a neurological "gate" that blocks pain signals or lets them go to the brain.

• Kinesthesis (Proprioception): The system for knowing where individual body parts are and how they move using sensors in muscles and joints.

• Vestibular Sense: The sense of balance and full-body movement, controlled by fluid in the inner ear.