Sensation & Perception

The Visual System

• visual area of the thalamus - visual cortex - retina - optic nerve

The Retina: Sensory Processing

3 major layers - cone, rod, and neural impulse

outside energy —> action potential = transduction

Photoreceptors (rods)

• Concentrated in the periphery of retina

• Function in dim light

• Essential for peripheral & twilight vision

Photoreceptors (Cones)

• Highly concentrated in center of retina (fovea)

• Function in bright or day light

• Essential for color vision & visual clarity

Visual Pathways in the Brain (optic chiasm)

The two optic nerves meet at the optic chiasm

Thalamus and primary visual cortex

Sensory information is then routed through the thalamus —> thalamus sends info to the primary visual cortex (area v1)

Contralateral Processing

• Left visual cortex processes information from right visual field

• Right visual cortex processes information from left visual field

Feature Detection in Primary Visual Cortex

• Neurons located in area B1 called feature detector cells are found in the primary visual cortex.

• These neurons fire selectively in response to stimuli that have very specific characteristics (e.g., shape, angle, or movement).

High-order visual processing (dorsal visual stream)

– from occipital up to parietal lobes

• The “where” pathway

• Location and motion of objects in visual field

High-order visual processing (ventral visual stream)

– from occipital to temporal lobes

• The “what” pathway

• Features and identity of objects in visual field

face perception and prosopagnosia

• occipital lobe

• fusiform gyrus —> recognizes faces

disease where you cannot recognize faces

Top-down Processing

• Perception can often be guided by our previous experiences,

  stored knowledge, expectations, and assumptions.

Experts in Visual Search

• Radiologists were asked to read a series of CT scans to screen for

signs of lung cancer

• A small subset of these x-rays contained images of a cartoon gorilla

• Only 17% of the radiologists successfully detected the gorilla; the

vast majority missed it

Inattentional blindness

a psychological phenomenon where you fail to notice a fully visible, unexpected object because your attention is intensely focused on another task

tested this with radiologists and added cartoon gorillas in the images and only 17% detected it

Sensory Processing in the Ear

• Outer ear— sound waves are collected (ear lobe, pinna, auditory canal, eardrum)

• Middle ear— sound waves are amplified (ossicles)- 3 tiny bones

• Inner ear— sound waves are converted into neural impulses - semicircular, auditory nerve to brain, cochlea

Cochlea

• Coiled, fluid-filled tube in the inner ear lined with sensory neurons called hair cells that transform sound waves into auditory signals

The Taste (Gustatory System)

• The human mouth contains thousands of taste buds evenly distributed over the tongue

• Each taste bud contains an assortment of sensory neurons called taste receptor cells

• These taste cells transmit taste sensations to the gustatory cortex in the brain

Olfaction – our sense of smell

• Odorants enter the nasal cavity to stimulate millions of olfactory

receptor neurons

• Humans have about 400 different subtypes of olfactory receptors

• Sensory information is then relayed to the olfactory bulb which connects with the limbic system and olfactory cortex

Synesthesia

• The perceptual experience some people have in which their sensory experiences overlap. One sensation will evoke another simultaneous sensation.

• Seeing certain letters, numbers, or words in particular color

Nociception – Pain Perception

Pain sensations are transmitted by two different types

of sensory neurons called nociceptors (A-delta fibers, C fibers)

A-delta fibers

• Fast-acting, myelinated axons

• transmit sharp, shooting,

localized pain sensations

C fibers

• slow-acting, unmyelinated

• transmit longer lasting, duller,

aching pain sensations

• injury —> pain impulse —> cell body of nociceptor —> nerve cell send signals to the spinal cord —> brain

Nociception and Empathy

Seeing other people in pain triggers the same neural circuits that are involved in the processing of first-hand experience of pain.

The socially rejected brain

Research also shows that there is a neural overlap between social rejection and physical pain

Congenital Insensitivity to Pain

• a rare genetic condition marked by the inability to perceive

  physical pain

  • The most famous case is a young girl named Ashlyn Blocker