Biological Psychology: Vision

© Cengage Learning 2016 © Cengage Learning 2016

Chapter 5

Vision

© Cengage Learning 2016

5.1 Visual Coding

• How far one sees is dependent on how far light travels before it strikes one’s eyes

• Perception of vision is not in the eyes; it’s in the brain


General Principles of Perception

• Each of our senses has specialized receptors that are sensitive to a particular kind of energy

• Law of specific nerve energies states that activity by a particular nerve always conveys the same type of information to the brain– Example: impulses in one neuron indicate light;

impulses in another neuron indicate sound


The Eye and Its Connections to the Brain

• Light:– Enters the eye through an opening in the

center of the iris called the pupil

– Is focused by the lens and the cornea onto the rear surface of the eye known as the retina, which is lined with visual receptors

– From the left side of the world strikes the right side of the retina and vice versa

– From above strikes the bottom half of the retina and vice versa


Route Within the Retina – Bipolar Cells

• Cells, located closer to the center of the eye, that receive messages from visual receptors at the back of the eye

• These cells send messages to ganglion cells that are even closer to the center of the eye– The axons of ganglion cells join one another to

form the optic nerve that travels to the brain


Route Within the Retina – Amacrine Cells

• Additional cells that receive information from bipolar cells and send it to other bipolar, ganglion, or amacrine cells

• Control the ability of the ganglion cells to respond to shapes, movements, or other specific aspects of visual stimuli


Cross Section of a Vertebrate Eye


Visual Path Within the Eye


The Optic Nerve

• Consists of the axons of ganglion cells that band together and exit through the back of the eye and travel to the brain

• Leaves the back of the eye; the point at which it leaves is called the blind spot because it contains no receptors


Illustration of the Blind Spot


The Fovea, Part 1

• Is the central portion of the retina and allows for acute and detailed vision– Packed tight with receptors

– Nearly free of ganglion axons and blood vessels


The Fovea, Part 2

• Each receptor in the fovea attaches to a single bipolar cell and a single ganglion cell known as a midget ganglion cell

• Each cone in the fovea has a direct line to the brain which allows the registering of the exact location of input

• Our vision is dominated by what we see in the fovea


The Placement of Receptors on the Retina


The Periphery of the Retina

• In the periphery of the retina, a greater number of receptors converge into ganglion and bipolar cells– Detailed vision is less in peripheral vision

– Allows for the greater perception of much fainter light in peripheral vision


Convergence of Input onto Bipolar Cells


The Arrangement of Visual Receptors

• Highly adaptive– Example: predatory birds have a greater

density of receptors on the top of the eye; rats have a greater density on the bottom of the eye


The Difference Between Foveal and Peripheral Vision

CharacteristicFoveal vision Peripheral vision

Receptors Cones Proportion of rods increases toward periphery

Convergence of Input

Each ganglion cell excited by a single cone

Each ganglion cell excited by many receptors

Brightness sensitivity

Distinguishes among bright lights; responds poorly to dim light

Responds well to dim light; poor for distinguishing amongbright lights

Sensitivityto detail

Good detail vision because each cones own ganglion cell sends a message to the brain

Poor detail vision because many receptors converge their input onto a given ganglion cell

Color Vision Good (many cones) Poor (few cones)


Visual Receptors: Rods and Cones, Part 1

• The vertebrate retina consists of two kinds of receptors– Rods: most abundant in the periphery of the

eye and respond to faint light (120 million per retina)

– Cones: most abundant in and around the fovea (6 million per retina)

• Essential for color vision and more useful in bright light


Visual Receptors: Rods and Cones, Part 2

• Though cones are outnumbered, they provide about 90% of the brain’s input

• On average, 120 million rods and 6 million cones converge onto 1 million axons in the optic nerve

• The ratio of rods to cones is higher in species that are more active at dim light


A Comparison of Rods and Cones


Photopigments

• Chemicals contained by both rods and cones that release energy when struck by light– Consist of 11-cis-retinal bound to proteins

called opsins

– Light energy converts 11-cis-retinal quickly into all-trans-retinal

– Light is thus absorbed and energy is released that activates second messengers within the cell


Color Vision

• Visible light is a portion of the electromagnetic spectrum

• The perception of color is dependent upon the wavelength of the light

• “Visible” wavelengths are dependent upon the species’ receptors

• Humans perceive wavelengths between 400 and 700 nanometers (nm)


Visible Light on the Electromagnetic Spectrum


Specificity of Color Vision

• Depends on specific receptors within the eye

• Two major interpretations of color vision – Trichromatic theory/Young-Helmholtz theory

– Opponent-process theory


Trichromatic Theory, Part 1

• Color perception occurs through the relative rates of response by three kinds of cones– Short wavelength

– Medium-wavelength

– Long-wavelength


Wavelength-Sensitivity Functions


Trichromatic Theory, Part 2

• Each cone responds to a broad range of wavelengths, but some more than others

• The ratio of activity across the three types of cones determines the color

• More intense light increases the brightness of the color but does not change the ratio

• Three kinds of cones are unevenly distributed


Distribution of Cones in Two Human Retinas


The Opponent-Process Theory

• Suggests that we perceive color in terms of paired opposites

• The brain has a mechanism that perceives color on a continuum from red to green and another from yellow to blue– A possible mechanism for the theory is that

bipolar cells are excited by one set of wavelengths and inhibited by another


An Afterimage as an Effect of Context


Limitations of Color Vision Theories

• Both the opponent-process and trichromatic theory have limitations– Color constancy, the ability to recognize color

despite changes in lighting, is not easily explained by these theories

• Retinex theory suggests the cortex compares information from various parts of the retina to determine the brightness and color for each area


The Context of Color Perception


Brightness Constancy


Color Vision Deficiency

• An impairment in perceiving color differences– Gene responsible is contained on the X

chromosome

– Caused by either the lack of a type of cone or a cone that has abnormal properties

– Most common form is difficulty distinguishing between red and green

• Results from the long- and medium-wavelength cones having the same photopigment


5.2 How the Brain Processes Visual Information

• Senses, such as vision, provide psychological experiences

• Neuroscientists have developed a relatively detailed understanding of vision

• Understanding the mechanisms of vision provides a model of what it means to explain something in biological terms


An Overview of the Mammalian Visual System, Part 1

• Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells

• Horizontal cells are cells in the eye that make inhibitory contact onto bipolar cells

• Bipolar cells make synapses onto amacrine cells and ganglion cells

• Different cells are specialized for different visual functions


An Overview of the Mammalian Visual System, Part 2

• Ganglion cell axons form the optic nerve

• The optic chiasm is the place where the two optic nerves leaving the eye meet

• In humans, half of the axons from each eye cross to the other side of the brain

• Most ganglion cell axons go to the lateral geniculate nucleus, a smaller amount to the superior colliculus, and fewer to other areas


The Vertebrate Retina


The Path of Visual Input


Processing in the Retina

• The lateral geniculate nucleus– Part of the thalamus

– Specialized for visual perception

– Destination for most ganglion cell axons

– Sends axons to other parts of the thalamus and to the visual areas of the occipital cortex

• The cortex and thalamus constantly feed information back and forth to each other


Lateral Inhibition in the Retina

• Sharpens contrasts to emphasize the borders of objects

• The reduction of activity in one neuron by activity in neighboring neurons

• The response of cells in the visual system depends upon the net result of excitatory and inhibitory messages it receives


An Illustration of Lateral Inhibition


Further Processing

• The receptive field refers to the part of the visual field that either excites or inhibits a cell in the visual system of the brain

• For a receptor, the receptive field is the point in space from which light strikes it

• For other visual cells, receptive fields are derived from the visual field of cells that either excite or inhibit– Example: ganglion cells converge to form the

receptive field of the next level of cells


An Example of a Receptive Field


Primate Receptive Fields

• Ganglion cells of primates generally fall into three categories– Parvocellular neurons

– Magnocellular neurons

– Koniocellular neurons


Parvocellular Neurons

• Mostly located in or near the fovea

• Have smaller cell bodies and small receptive fields

• Highly sensitive to detect color and visual detail


Magnocellular Neurons

• Distributed evenly throughout the retina

• Have larger cell bodies and visual fields

• Highly sensitive to large overall pattern and moving stimuli


Koniocellular Neurons

• Have small cell bodies

• Found throughout the retina

• Have several functions, and their axons terminate in many different places


Characteristics of Receptive Fields

• Cells of the lateral geniculate have a receptive field similar to those of ganglion cells:– An excitatory or inhibitory central portion and a

surrounding ring of the opposite effect


The Primary Visual Cortex, Part 1

• The primary visual cortex (area V1) receives information from the lateral geniculate nucleus and is the area responsible for the first stage of visual processing

• Some people with damage to V1 show blindsight: an ability to respond to visual stimuli that they report not seeing


The Primary Visual Cortex, Part 2

• Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex– Simple cells

– Complex cells

– End-stopped/hypercomplex cells


Simple Cells

• Fixed excitatory and inhibitory zones

• The more light that shines in the excitatory zone, the more the cell responds

• The more in the inhibitory zone, the less the cell responds

• Bar-shaped or edge-shaped receptive fields with vertical and horizontal orientations outnumbering diagonal ones


Complex Cells

• Located in either V1or V2

• Have large receptive field that can not be mapped into fixed excitatory or inhibitory zones

• Responds to a pattern of light in a particular orientation and most strongly to a moving stimulus


Simple and Complex Receptive Fields


The Receptive Field of a Complex Cell


• Similar to complex cells but with a strong inhibitory area at one end of its bar shaped receptive field

• Respond to a bar-shaped pattern of light anywhere in its large receptive field, provided the bar does not extend beyond a certain point

End-Stopped or Hypercomplex Cells


The Receptive Field of an End-Stopped Cell


Properties of Simple, Complex, and End-Stopped Cells

Characteristic Simple Cells Complex Cells End-Stopped Cells

Location V1 V1 andV2 V1 and V2Binocular input Yes Yes YesSize of receptive field

Smallest Medium Largest

Receptive field Bar- or edge-shaped, with fixed excitatory and inhibitory zones

Bat- or edge-shaped, without fixed excitatory or inhibitory zones

Same as complex cell but with strong inhibitory zone at one end


Columnar Organization of the Visual Cortex, Part 1

• In the visual cortex, cells are grouped together in columns perpendicular to the surface

• Cells within a given column process similar information– Respond either mostly to the right or left eye,

or respond to both eyes equally

– Do not consistently fire at the same time


Columnar Organization of the Visual Cortex, Part 2


Visual Cortex Cells as Feature Detectors, Part 1

• Cells in the visual cortex may be feature detectors, neurons whose response indicate the presence of a particular feature/stimuli

• Prolonged exposure to a given visual feature decreases sensitivity to that feature


Visual Cortex Cells as Feature Detectors, Part 2


Development of the Visual Cortex

• Animal studies have greatly contributed to the understanding of the development of vision

• Early lack of stimulation of one eye: leads to synapses in the visual cortex becoming gradually unresponsive to input from that eye

• Early lack of stimulation of both eyes: cortical responses become sluggish but do not cause blindness


Critical Periods in Development

• Sensitive/critical periods are periods of time during the lifespan when experiences have a particularly strong/enduring effect– Ends with the onset of chemicals that inhibit

axonal sprouting

– Changes that occur during critical period require both excitation and inhibition of some neurons

• Cortical plasticity is greatest in early life, but never ends


Stereoscopic Depth Perception

• A method of perceiving distance in which the brain compares slightly different inputs from the two eyes– Relies on retinal disparity or the discrepancy

between what the left and the right eye sees

– The ability of cortical neurons to adjust their connections to detect retinal disparity is shaped through experience


Strabismus

• A condition in which the eyes do not point in the same direction– Usually develops in childhood

– Also known as “lazy eye”

• If two eyes carry unrelated messages, cortical cell strengthens connections with only one eye

• Development of stereoscopic depth perception is impaired

Two Examples of Lazy Eye


Early Exposure to a Limited Array of Patterns

• Leads to nearly all of the visual cortex cells becoming responsive to only that pattern

• Astigmatism refers to a blurring of vision for lines in one direction caused by an asymmetric curvature of the eyes– 70% of infants have astigmatism


Restricting Early Visual Experience


An Informal Test for Astigmatism


Long-Term Consequences of Impaired Infant Vision

• Study of people born with cataracts but had them removed at age 7 or 12 indicate that vision can be restored gradually, but problems persist– Difficulty in recognizing objects

– Unable to tell that components are part of a whole

– Best prognosis is for children whose vision problems are corrected early in life


5.3 Parallel Processing in the Visual Cortex

• Neuroscientists have identified at least 80 brain areas that contribute to vision in different ways

• One part of your brain sees its shape, another sees color, another detects location, and another perceives movement


The Ventral and Dorsal Paths

• The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas

• Information is transferred between area V1 and V2 in a reciprocal nature

Visual Pathways in the Monkey Cerebral Cortex


The Ventral and Dorsal Streams, Part 1

• The ventral stream refers to the path that goes through temporal cortex– The “what” path

– Specialized for identifying and recognizing objects

• The dorsal stream refers to the visual path in the parietal cortex– The “how” path

– Important for visually guided movements.


The Ventral and Dorsal Streams, Part 2

• Normal behavior makes use of both pathways in collaboration

• Damaging either stream will produce different deficits– Ventral stream damage: can see where objects

are but cannot identify them

– Dorsal stream damage: can identify objects but not know where they are


Detailed Analysis of Shape

• Receptive fields become larger and more specialized as visual information goes from simple cells to the complex cells and then to other brain areas

• The inferior temporal cortex contains cells that respond selectively to complex shapes but are insensitive to distinctions that are critical to other cells

• Cells in this cortex respond to identifiable objects


Transformations of a Drawing


Visual Agnosia

• The inability to recognize objects despite satisfactory vision– Caused by damage to the pattern pathway

usually in the temporal cortex


Face Recognition – The Fusiform Gyrus


Recognizing Faces

• Face recognition occurs relatively soon after birth– People with cataracts removed at 2-6 months

develop nearly normal vision but have slight difficulties in distinguishing faces

– Newborns show strong preference for a right-side-up face and support idea of a built-in face recognition system

• Facial recognition continues to develop gradually into adolescence


Amount of Time Infants Spend Looking at Patterns


How Infants Divided Their Attention Between Faces


Prosopagnosia

• The impaired ability to recognize faces– Occurs after damage to the fusiform gyrus of

the inferior temporal cortex

– The fusiform gyrus responds much more strongly to faces than anything else


Color Perception

• Dependent on both the light reflected on an object and how it compares with objects around it– Area V4 may be responsible for color

constancy and visual attention

– Color constancy: the ability to recognize something as being the same color despite changes in lighting


Motion Perception

• Involves a variety of brain areas in all four lobes of the cerebral cortex– The middle-temporal cortex (MT/V5) responds

to a stimulus moving in a particular direction

– Cells in the dorsal part of the medial superior temporal cortex (MST) respond to expansion, contraction, or rotation of a visual stimulus

– Both receive input from the magnocellular path; color-insensitive


Stimuli That Excite the Dorsal Part of Area MST


Motion Blindness

• The inability to determine the direction, speed and whether objects are moving– Likely caused by damage in area MT

• Some people are blind except for the ability to detect which direction something is moving– Area MT probably gets some visual input

despite significant damage to area V1


Saccades

• Several mechanisms prevent confusion or blurring of images during eye movements– Saccades are a decrease in the activity of the

visual cortex during quick eye movements

– Neural activity and blood flow decrease 75 milliseconds before and during eye movements

Science

Biological Psychology: Vision