Sensation and Perception Ch. 4 (pp )

Let’s begin with some basic definitions… Sense – physical system that receives physical stimulation from surrounding environment and translates that stimulation into an electrochemical message Sense – physical system that receives physical stimulation from surrounding environment and translates that stimulation into an electrochemical message Sensation – detecting physical energy in environment and encoding it as neural signals. Neurons transmit the information from the sense organ to the brain. Sensation – detecting physical energy in environment and encoding it as neural signals. Neurons transmit the information from the sense organ to the brain. Perception – processing of information done by the brain – mental processes that organize and interpret sensory information that has been transmitted to the brain. Perception – processing of information done by the brain – mental processes that organize and interpret sensory information that has been transmitted to the brain.

Six stages in sensation & perception: Stimulus is received by sensory receptors Stimulus is received by sensory receptors Receptors translate stimulus properties into nerve impulses Receptors translate stimulus properties into nerve impulses Feature detectors analyze stimulus features Feature detectors analyze stimulus features Stimulus features are reconstructed into neural representation Stimulus features are reconstructed into neural representation Neural representation is compared with previously stored info in brain Neural representation is compared with previously stored info in brain Matching process results in recognition and interpretation of stimuli Matching process results in recognition and interpretation of stimuli

Transduction Communication between the brain & the rest of the body (& between different regions of the brain) occurs via neuron. We recently learned how communication between neurons occurs electrochemically (within neurons: electrical; between neurons: chemical). So the brain’s “language” is electrochemical! Communication between the brain & the rest of the body (& between different regions of the brain) occurs via neuron. We recently learned how communication between neurons occurs electrochemically (within neurons: electrical; between neurons: chemical). So the brain’s “language” is electrochemical! All senses involve something called receptor cells. Their job is to transduce (transform or even “translate”) physical stimulation/physical energy from the environment into electrochemical messages that can be understood by the brain. All senses involve something called receptor cells. Their job is to transduce (transform or even “translate”) physical stimulation/physical energy from the environment into electrochemical messages that can be understood by the brain.

So where do vision and hearing (& the other senses) happen? The Brain! The Brain!  The physical energy in the environment is detected by the eyes, ears, etc. but we can’t see, hear, etc. until the brain interprets them— i.e., makes sense of them. So in a way, we see, hear, smell, etc. in our brains!

The Visual System Stimulus = Light Stimulus = Light  Electromagnetic energy in the form of a lightwave  Amplitude-hue  Wavelength- brightness  Purity-saturation

Learning about the eye… Pupil – small adjustable opening – regulates the amount of light entering the eye Cornea – transparent membrane covering the front of the eye (protective) – the light enters the eye through the cornea (protective) – the light enters the eye through the cornea

Iris – muscle which surrounds the pupil and controls the size of the pupil Lens – focuses incoming rays on back surface of the eye (retina) - elastic, muscles stretch or thicken (thus changing the curvature of the lens) to bend the light at the right angle (so light is focused on retina, depending upon distance of object) this is called accommodation.

Common visual problems Myopia (nearsightedness) Myopia (nearsightedness)  Focus of light falls short of retina Hyperopia (farsightedness) Hyperopia (farsightedness)  Focus of light falls behind retina

The Retina  Network of neurons on rear surface inside the eye  Functions:  Absorb light  Process images  Send visual info. to brain

Sensory Receptor Cells Two types of sensory receptor cells in the retina Two types of sensory receptor cells in the retina  Rods enable us to see black and white, are more sensitive to light, and there are 100 million in each eye  Play key role in night vision and peripheral vision p. 95 fig. 4.5 gradually adapt, but improve vision the bestp. 95 fig. 4.5 gradually adapt, but improve vision the best

Sensory Receptor Cells  Cones enable us to see color, they work best in bright light, there are 6.5 million in each eye and they are located mostly at the center of the eye  Play key role in daylight vision and color vision  Rapidly adapt

More on Sensory Receptor Cells These cells transform light energy into neural impulses that are sent to various areas in the brain through the bundle of neurons called the optic nerve. These cells transform light energy into neural impulses that are sent to various areas in the brain through the bundle of neurons called the optic nerve. What is the place called in which the optic nerve leaves the eye? What is the place called in which the optic nerve leaves the eye?  The blind spot Why is it called “the blind spot”? Why is it called “the blind spot”?  Because there are no rods & cones on this area (there are neurons instead), images that fall on this area of the retina are not “seen”!

Fovea – center of retina with densely packed cones; visual acquity (picture sharpness) is the greatest here Optic Nerve – carries neural impulses to the brain Occipital Lobe – neural impulses are than sent to the primary visual cortex within the occipital lobe

Pathways from the Eyes to the Visual Cortex

The mystery of feature detectors… Feature detectors are cortical neurons that respond to specific features of a scene (e.g., particular edges, lines, angles and movements) Feature detectors are cortical neurons that respond to specific features of a scene (e.g., particular edges, lines, angles and movements) The brain then needs to assemble the whole image from these specific features. The brain then needs to assemble the whole image from these specific features. “…perceptions arise from the interaction of many neuron systems, each performing a simple task.” “…perceptions arise from the interaction of many neuron systems, each performing a simple task.”

How we see color… Based on the wavelength of light the object reflects rather than absorbs. Based on the wavelength of light the object reflects rather than absorbs. Thus if an object absorbs every color of light except red, then the object reflects red and therefore we see red. Thus if an object absorbs every color of light except red, then the object reflects red and therefore we see red. Color is also based on interpretation of context. Color is also based on interpretation of context. Is the red I see the same red you see? Is the red I see the same red you see?

Color Perception Theories  Trichromatic (three color) Theory  Young and Helmholtz  three different retinal color receptors respond in varying degrees to different colored stimuli  red  green  blue

Opponent-Process Theory Opposing retinal processes enable color vision  signals are processed by red-green, blue- yellow, black-white opponent process cells  stimulated by one wavelength and inhibited by its opponent

Each of the 3 cone types responds to two different wavelengths Each of the 3 cone types responds to two different wavelengths Activity in one set of cells in hypothalamus inhibit activity of “opposite” cells Activity in one set of cells in hypothalamus inhibit activity of “opposite” cells  (Green : Red; Blue : Yellow; Black : White) Active Green cells inhibit Red cells. Active Green cells inhibit Red cells. Thus, no such colors (to us) as “greenish-red” or “bluish-yellow” Thus, no such colors (to us) as “greenish-red” or “bluish-yellow”

Opponent Process- Afterimage Effect

It takes both theories to explain color perception…  Trichromatic  The eye has three cones that respond to color  Opponent Process  Cells in the retina, thalamus, and visual cortex respond in opposite ways to red-vs- green, blue-vs-yellow, etc…  Thus-both theories are needed in order to explain the perception of color!

Perception goes beyond vision. It involves what we see. Perception goes “right” more times than when it goes “wrong.” Perception goes “right” more times than when it goes “wrong.” The times it “goes wrong” however, can provide us with important info on how perceptions works! The times it “goes wrong” however, can provide us with important info on how perceptions works! Top-down processing Top-down processing  Sensory info is interpreted in light of existing knowledge, concepts, ideas, and expectations Bottom-up processing Bottom-up processing  System takes in individual elements of a stimulus and then combines them into a unified perception

What shapes our perceptual sets? What things do you think might affect our perceptual sets (i.e., our tendency to perceive things in certain ways)? Context effects Past experiences Effects of our culture

A perceptual set is… “a mental predisposition to perceive one thing and not another” “a mental predisposition to perceive one thing and not another” This is based on experiences, assumptions and expectations. This is based on experiences, assumptions and expectations.

Gestalt Principles Grouping  The perceptual tendency to organize stimuli into coherent groups  Grouping Principles  Phi phenomenon-illusion of movement created by presenting stimuli in rapid succession  proximity--group nearby figures together  similarity--group figures that are similar  continuity--perceive continuous patterns  closure--fill in gaps  connectedness--spots, lines, and areas are seen as unit when connected  simplicity--organize elements in simplest way

PerceptualOrganization: Grouping Principles

Perceptual Organization: Gestalt  Visual Capture  tendency for vision to dominate the other senses  Gestalt (Necker Cube) p. 107  an organized whole  tendency to integrate pieces of information into meaningful wholes

Perceptual Organization: Closure  Gestalt grouping principles are at work here.

PerceptualOrganization: Grouping Principles  Gestalt grouping principles are at work here

Perceptual Organization: Depth Perception Depth perception chalk drawings Depth perception chalk drawings

Perceptual Organization: Depth Perception  Depth Perception  ability to see objects in three dimensions  allows us to judge distance  Binocular Cues – Depth Cues  retinal disparity  images from the two eyes differ  closer the object, the larger the disparity  convergence  neuromuscular cue  two eyes move inward for near objects

Perceptual Organization: Depth Perception Visual Cliff

Perceptual Organization: Depth Perception  Monocular Cues – Distance Cues  relative size  smaller image is more distant  interposition  closer object blocks distant object  relative clarity  hazy object seen as more distant  texture coarse --> close fine --> distant

Schemas Another word used to describe perceptual set is schemas. Another word used to describe perceptual set is schemas. Schemas can influence our perception because we tend to see things that we expect to see. Schemas can influence our perception because we tend to see things that we expect to see. We interpret things based on similar things we have experienced in the past – this is called assimilation. We interpret things based on similar things we have experienced in the past – this is called assimilation.

Schemas….. Now what do you think would happen if we receive new information that doesn’t not fit our schemas? When we receive new information we can ignore it (!) or we can change our schemas to fit the new information. This is called accommodation. Schemas are almost like a box that we put information into. The way we organize compartments in the box is based on things like our past experiences, contexts, etc. and this influences our perceptions!