Perceptual and Motor Systems - Basic Senses

• Module 2 focuses on perceptual and motor systems.

Basic Senses

• Vision (seeing)
• Audition (hearing)
• Gustation (tasting)
• Somatosensation (touching/pain)
• Olfaction (smelling)
• Vestibular (balancing)

Commonalities

• There is a core set of commonalities among sensory systems despite their differences.
• An illustrative example of commonalities in sensory systems is vision.

Why Vision?

• A large number of cortical areas are devoted to vision, occupying slightly less than 1/2 of the human cortex.
• Vision is the dominant sensory modality in humans.
• More research is conducted on vision than any other human modality.
• Stimuli can be easily manipulated, allowing for experimentation.

Key Elements

• Stimulus represents forms of energy in the world including:
  • Photic
  • Mechanical
  • Thermal
  • Electrical
  • Chemical

Electromagnetic Spectrum

• For humans, visible light is a narrow band of the spectrum of electromagnetic radiation.
• Receptors are a specialized interface with stimuli.
• Receptors convert energy into a change in electrical potential across their membrane.

Receptor-Photoreceptors in Retina

• Photoreceptors are the receptors in the eye.
• Eye components include:
  • Eyelid
  • Lacrimal caruncle
  • Tear duct
  • Lateral rectus muscle
  • Sclera
  • Choroid
  • Retina
  • Macula lutea
  • Fovea centralis (central depression)
  • Optic disc (blind spot)
  • Optic nerve and retinal blood vessels
  • Iris
  • Pupil
  • Cornea
  • Anterior chamber (filled with aqueous humor)
  • Lens
  • Vitreous body (filled with vitreous humor)
  • Posterior chamber
  • Suspensory ligaments
  • Ciliary body and muscle
  • Medial rectus muscle

The Eye

• The extraocular muscles move the eye.
• The human retina contains layers of ganglion, bipolar, and photoreceptor cells.

Transduction

• Transduction is the process by which energy from the environment is converted to a change in membrane potential in a neuron.
• Transduction converts external stimuli to internal stimuli
• This is the first step in visual perception.
• In complete darkness, the resting potential of photoreceptors is -40 mv.
• This is less negative intracellularly than is typical.
• Depolarization is caused by the constant influx of Na+ ions through special channels in the photoreceptors which is known as the dark current.
• There is a messenger produced intracellularly that opens the Na+ gates.
• Photoreceptors hyperpolarize in response to light.
• This is initiated by the absorption of light by a photopigment in the membrane of the photoreceptors called rhodopsin (in rods) and cone-opsin (in cones).
• The effect of light on -opsin serves to activate an enzyme that breaks down the messenger responsible for opening the Na+ channels.
• Channels close and the membrane hyperpolarizes.
• Light striking a photoreceptor produces hyperpolarization, so the photoreceptor releases less neurotransmitter.
• The neurotransmitter normally hyperpolarizes the membrane of the bipolar cell, so the reduction causes a depolarization.
• The depolarization causes the bipolar cell to release more neurotransmitters, which excites the ganglion cell.

Differences Between Rod and Cone

• There are 120 million rods/eye
• There are 6 million cones/eye.
• Rods work under low-intensity light, offering high sensitivity but low acuity.
• Cones are responsible for acuity and require more light to function.

Fovea

• The fovea is a thinning (“pit”) of the retina at the center.
• There is lateral displacement of ganglion cells from the fovea.
• This allows light to strike the photoreceptors without passing through other retinal layers.
• There are no rods, only cones.
• Ganglion cells in the fovea receive input from a smaller number of photoreceptors than those in the periphery and provide more acute visual information.

Receptive Field

• Receptive fields are an important concept.
• Neurons exhibit selectivity in the stimuli to which they respond
• For example, a neuron might be excited by particular properties of light, sound, etc.
• The location of the receptive field of a particular neuron depends on the location of photoreceptors that provide it with visual information.

Perceiving Color

• The trichromatic theory involves mixing the three receptor types (Blue, Green, Red) via their relative activity.
• Color vision genetic defects result from anomalies in one or more of the three types of cones.
  • Protanopia is lacking a red cone.
  • Deuteranopia is lacking a green cone.
  • Tritanopia is lacking a blue cone.
  • Monochromatic vision exists.
• The trichromatic theory accounts for much data, but is incomplete.
• The opponent process theory includes the idea that when ganglion cells are excited or inhibited for a prolonged period of time, they later show a rebound effect, firing slower or faster than normal.