Module 7: Physical Stimuli for Perception
Understanding the physical stimuli required for different sensory modalities is fundamental in biopsychology.
Vision
Physical Stimulus
Light Waves: Vision begins with the detection of light waves, which are forms of electromagnetic radiation. The human eye is sensitive to wavelengths approximately between 380 nm (violet) and 750 nm (red).
Intensity and Wavelength: The intensity (brightness) and wavelength (color) of light determine the visual experience. Light enters the eye through the cornea, passes through the lens, and is focused on the retina.
Processing
Photoreceptors: The retina contains photoreceptors (rods and cones) that convert light into electrical signals. Rods are sensitive to low light levels and are crucial for night vision, while cones are responsible for color vision and function best in bright light.
Optic Nerve: Electrical signals from the photoreceptors are transmitted via the optic nerve to the brain's visual cortex, where they are processed and interpreted as images.
Audition
Physical Stimulus
Sound Waves: Audition begins with the detection of sound waves, which are vibrations of air molecules. These waves vary in frequency (pitch) and amplitude (loudness).
Frequency and Amplitude: Human hearing typically ranges from 20 Hz to 20,000 Hz. The frequency of sound waves determines the pitch, while the amplitude affects the loudness.
Processing
Outer Ear: Sound waves are collected by the outer ear (pinna) and funneled through the ear canal to the eardrum (tympanic membrane), causing it to vibrate.
Middle Ear: Vibrations are transmitted through three small bones (ossicles: malleus, incus, and stapes) to the oval window of the cochlea in the inner ear.
Inner Ear: The cochlea, filled with fluid, converts these vibrations into neural signals via hair cells. These signals are then transmitted via the auditory nerve to the brain's auditory cortex for processing.
Somatosensation
Physical Stimulus
Mechanical Stimuli: Include pressure, vibration, and stretch, which are detected by mechanoreceptors in the skin and other tissues.
Thermal Stimuli: Include changes in temperature, which are detected by thermoreceptors sensitive to heat and cold.
Chemical Stimuli: Include substances that can cause a chemical reaction on the skin, detected by chemoreceptors.
Processing
Receptors: Different types of receptors in the skin detect various stimuli:
Mechanoreceptors: Detect touch, pressure, and vibration (e.g., Meissner's corpuscles, Pacinian corpuscles).
Thermoreceptors: Detect temperature changes (e.g., free nerve endings).
Nociceptors: Detect pain, responding to potentially damaging stimuli.
Nerve Fibers: Sensory information from these receptors is transmitted via peripheral nerves to the spinal cord and then to the brain.
Somatosensory Cortex: In the brain, the somatosensory cortex processes and interprets these signals, providing a perception of touch, temperature, and pain.
Summary
Vision: Requires light waves, detected by photoreceptors in the retina, processed by the visual cortex.
Audition: Requires sound waves, detected by hair cells in the cochlea, processed by the auditory cortex.
Somatosensation: Requires mechanical, thermal, and chemical stimuli, detected by various receptors in the skin, processed by the somatosensory cortex.