Sensation and Perception: Chapter 9

• Chapter 9 explores how we sense, perceive, and visually experience the world.
• The study of sensation and perception seeks to understand how we interpret the world around us.

Nature of Sensation and Perception

• Sensory information is received by the brain in the form of neural impulses.
• Nerves convert forms of energy, like light waves, into nerve impulses.
• Neural impulses travel along specific pathways to reach the brain.
• How we perceive these nerve impulses as a representation of the world remains unknown.

Sensory Receptors

• Sensory receptors are involved in transduction, which converts sensory input into neural activity.
• Sensory systems use specialized receptors to respond to specific types of energy.
• Vision relies on light energy converting to chemical energy.
• Auditory sense relies on air pressure converting to mechanical energy.
• Somatosensory relies on mechanical energy.
• Taste and olfaction rely on chemical molecules.

Receptive Fields of Sensory Receptors

• A receptive field includes the region of sensory space where a stimulus can alter a receptor's activity.

Receptor Density and Sensitivity

• The number of sensory receptors varies across the body.
• Receptor density determines the sensitivity of a sensory system.

Neural Relays

• All receptors connect to the cortex through a sequence of intervening neurons.
• Vision passes from the retina, to the thalamus, to V1 to V5.
• Auditory senses pass from receptors in the ears, to the hindbrain, midbrain, thalamus, and then to the cortex.
• Neural relays enable sensory systems to interact with each other.

McGurk Effect

• Integration of auditory and visual information that impacts speech perception.

Sensory Coding and Representation

• Sensory information is encoded by action potentials that travel to the central nervous system (CNS) via peripheral nerves.
• Stimulus presence is encoded by both an increase or decrease in a neuron's discharge rate.
• Stimulus intensity is also coded by increase or decrease of rate.
• How action potentials encode different kinds of sensations like vision and touch requires further exploration.

Sensory Information Processing

• Different brain regions encode different aspects of sensory information.
• McGurk effect shows how experience shapes sensory distinction.
• Sensory distinctions are not always clear, shown by synesthesia.

Homunculus and Topographic Maps

• The neocortex represents the sensory world by organizing sensory input like vision, hearing, touch, smell, and taste.

Sensation vs Perception

• Sensation registers physical stimuli via sensory organs from the environment.
• Perception is based on the brain's subjective interpretation of these sensations.
• Our experience is a subjective reality constructed by the brain.

Perception and Plasticity

• Perception is influenced by experience and can change over time.

Perceptual Illusions

• Perceptual illusions displays how our brain interprets sensory information and constructs our perception of reality.
• Ambiguous images, such as Edgar Rubin's vase/faces, can be perceived in multiple ways.
• Context affects perception.
• Gestalt psychology informs how humans perceive components as part of the bigger picture.
• The Cornsweet illusion shows how perception of lightness/darkness depends on edge gradients.

Visual Systems

• Vision is our primary sensory experience
• A large part of the human brain is dedicated to vision
• Understanding the visual system is key to understanding the brain

Visible Light and the Structure of the Eye

• Light is electromagnetic energy that is visible to humans.
• The range of visible light is defined by the wavelengths humans can perceive.

Central Focus & Retina

• The fovea shows a depression with a high concentration of photoreceptors, making it the location where vision is clearest.

Acuity Across the Visual Field

• Vision is better in the center of the visual field.
• The periphery is less sharp than the center.

Photoreceptors Types

• 419 nm (blue, or short electromagnetic wavelengths)
• 531 nm (green, or middle electromagnetic wavelengths)
• 559 nm (red, or long electromagnetic wavelengths)
• There are approximately equal numbers of red and green cones, but fewer blue cones.

Retinal Receptors

• Cones are more responsive to bright light, responsible for color vision, and our ability to see fine detail.
• Rods are more numerous than cones and are responsible for night vision.

Retina Structure

• The optic disc is where axons form the optic nerve and exit the eye, and blood vessels enter and leave.
• The optic disc has no photoreceptors
• The visual system compensates for the blind spot
• The optic disc is in a different location in each eye.
• The optic disc is lateral to the fovea in each eye, lying to the left of the fovea in the left eye and to the right of the fovea in the right eye.

Retinal Neurons Types

• Bipolar cells receive input from photoreceptors.
• Horizontal cells link photoreceptors and bipolar cells.
• Amacrine cells link bipolar cells and ganglion cells.
• Retinal ganglion cells (RGC) give rise to the optic nerve.

Visual Pathways - Optic Chiasm

• The optic nerves from each eye cross at the optic chiasm.

Types of Ganglion Cells

• Magnocellular cells (M-cells) are large, receive input from rods, and are sensitive to light and motion.
• Parvocellular cells (P-cells) are small, receive input primarily from cones, and are sensitive to color.

Main Visual Pathways

• The optic nerve transmits via two routes
• The geniculostriate pathway carries visual information to the primary visual cortex.
• The tectopulvinar pathway projects to the temporal and parietal lobes.
• Only M Cells are carried in the tectopulvinar pathway, P Cells are not.

Three Routes to the Visual Brain

• Geniculostriate System
• Tectopulvinar System is considered primitive
• Retinohypothalamic tract that synapses in the suprachiasmatic nucleus and regulates circadian rhythms and pupillary reflex.

Striate Cortex

• The primary visual cortex.
• Two paths emerge
• One route goes to vision-related regions of the parietal lobe, with the other route going to vision-related regions of the temporal lobe.

Visual Streams

• Dorsal visual streams go from the occipital cortex to the parietal cortex and determines "how" to guide actions.
• Ventral visual streams go from the occipital cortex to the temporal cortex and determines "what" an object is.

Tectopulvinar Pathway Information

• Magnocellular cells from the retina project to the superior colliculus.
• Information is sent to the pulvinar region of the thalamus.
• Information regarding location (where) is supplied.
• Medial pulvinar sends signals to the parietal lobe.
• Lateral pulvinar sends signals to the temporal lobe.

Visual Streams Process

• Information for color, form, and motion remains separate as it moves from the striate V1 cortex and back out.

Visual Field

• The field is the region of the visual world that is seen by the eyes.
• The field is divided into left and right fields.
• Information from the left visual field goes to the right hemisphere.
• Information from the right visual field goes into the left hemisphere.

Retina Location coding

• Retinal ganglion cells (RGC) respond to stimulus on a circular retina patch-the receptive field.

Visual Corpus Callosum

• Corpus callosum connects the two hemispheres but only certain brain structures.
• Frontal lobes are connected to one another.
• The occipital lobes have almost no callosal connections.
• Exception: cells along the midline of the visual field are connected via the callosum with receptive fields that overlap.

Processing Shape in the Temporal Cortex

• Cells are maximally excited by complex visual stimuli (e.g., faces or hands).
• There may be selectivity to particular faces, posture, and even certain facial expressions.
• Stimulus equivalence enables recognition of the object regardless of viewing.

Shape Processing in the Temporal Cortex

• Tanaka (1993) found that complex features are necessary to activate several neurons in area TE.
• The features include combinations of characteristics like orientation, size, color, and texture.
• Similar neurons cluster in columns.

Trichromatic Theory of Color

• Color is explained by coding based on three primary colors: red, green, and blue
• Colors are seen based on the responses of cone types
• Different types of colorblindness can be explained
• Limitations include not explaining the four basic colors: red, green, yellow, and blue.
• Also cannot explain afterimages of red-green and blue-yellow.

Opponent-Process Theory of Color

• Color vision is emphasized, including opponent colors (red vs. green, blue vs. yellow).
• Opponent processing occurs within the retinal ganglion cells.
• On-off and center-surround receptive fields are components.
• About 60% of retinal ganglion cells use such components.

Injuries To the "What" Pathway

• Agnosia is not knowing.
• Visual-form agnosia the inability to recognize objects.
• Color agnosia (achromatopsia) is the inability to recognize colors.
• Face agnosia (prosopagnosia) is the inability to recognize faces.

Visual Guidance

• We can consciously reach for an object such as a pen or a mug.
• The hand forms the appropriate posture automatically, without conscious awareness.

Injury to the How Pathway

• Optic ataxia is a deficit in visually controlled reaching/movement
• Damage to the parietal cortex
• Ability to recognize objects is still typical