Biological Psychology - Chapter 5: Vision

Biological Psychology

Introduction

• Our perception of colors, sounds, tastes, and smells is the brain's interpretation of messages from receptors.
• Each sense has specialized receptors sensitive to a particular type of energy.
• Law of specific nerve energies: Activity by a particular nerve always conveys the same type of information to the brain.
  • Example: Impulses in one neuron indicate light, while impulses in another indicate sound.

Principles of Vision

• Perception is in the brain, not in the external object.
• Seeing occurs when light from an object alters brain activity.
• Feeling occurs when touch information reaches the brain; the sensation is felt in the brain, not the fingers.

The Eye and Its Connections to the Brain

• Light:
  • Enters the eye through the pupil, an opening in the center of the iris.
  • Is focused by the lens and cornea onto the retina, the rear surface of the eye lined with visual receptors.
• The left side of the visual world strikes the right side of the retina and vice versa.
• What comes from above strikes the bottom half of the retina and vice versa.

Route Within the Retina

• Bipolar cells: Receive messages from visual receptors at the back of the eye.
• Ganglion cells: Receive messages from bipolar cells; their axons form the optic nerve that travels to the brain.
• The point where the optic nerve exits the eye (also where blood vessels enter and leave) creates a blind spot.

Route Within the Retina (cont.)

• Amacrine cells: Receive information from bipolar cells and send it to other bipolar, ganglion, or amacrine cells.
• Amacrine cells control the ability of ganglion cells to respond to shapes, movements, or specific aspects of visual stimuli.

The Periphery of the Retina

• In the periphery, a greater number of receptors (primarily rods) converge onto ganglion and bipolar cells.
• Peripheral vision has less detailed vision but allows for greater perception of faint light.

The Arrangement of Visual Receptors

• Highly adaptive arrangement.
• Examples:
  • 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

Visual Receptors: Rods and Cones

• Rods: Most abundant in the periphery of the eye; 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; provide 90% of the brain’s input.
• The ratio of rods to cones is higher in species that are more active in dim light.

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, activating second messengers within the cell.

The Trichromatic (Young-Helmholtz) Theory

• Color perception occurs through the relative rates of response by three kinds of cones:
  • Short-wavelength
  • Medium-wavelength
  • Long-wavelength
• 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; long- and medium-wavelength cones are far more abundant than short-wavelength (blue) cones.

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.
• Support for the opponent-process theory includes negative color afterimages.

The Retinex Theory

• 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.

Color Vision Deficiency

• An impairment in perceiving color differences.
• The 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.
• The most common form is difficulty distinguishing between red and green, resulting from the long- and medium-wavelength cones having the same photopigment.

An Overview of the Mammalian Visual System

• Rods and cones of the retina make synaptic contact with horizontal cells and bipolar cells.
• Horizontal cells make inhibitory contact onto bipolar cells.
• Bipolar cells make synapses onto amacrine cells and ganglion cells.
• Different cells are specialized for different visual functions.
• 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.

Processing in the Retina

• Lateral Inhibition: 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.

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.

Primate Receptive Fields

• Ganglion cells of primates generally fall into three categories:
  • 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

The Primary Visual Cortex

• 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.
• One proposed explanation for blindsight is that small islands of healthy tissue may survive in an otherwise damaged area.

The Primary Visual Cortex (2 of 2)

• Hubel and Weisel (1959, 1998) distinguished various types of cells in the visual cortex.
  • Simple cells:
    • Have fixed excitatory and inhibitory zones.
    • Response increases with light in the excitatory zone and decreases with light in the inhibitory.
    • Prefer bar or edge shapes, mostly vertical or horizontal.
  • Complex cells:
    • Located in V1 or V2, these cells have large receptive fields.
    • Respond to specific orientations and react best to moving stimuli.
  • End-stopped/hypercomplex cells:
    • Like complex cells, but with a strong inhibitory end.
    • Respond to bar-shaped light within a set range of their large receptive field.

Are Visual Cortex Cells Feature Detectors?

• Cells in the visual cortex may be feature detectors, neurons whose response indicates the presence of a particular feature/stimuli.
• Prolonged exposure to a given visual feature decreases sensitivity to that feature.

Uncorrelated Stimulation in the Two Eyes

• 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.

Uncorrelated Stimulation in the Two Eyes (2 of 2)

• 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, the cortical cell strengthens connections with only one eye.
  • Development of stereoscopic depth perception is impaired.

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.
• Seventy percent of infants have astigmatism.

The Ventral and Dorsal Streams

• The secondary visual cortex (area V2) receives information from area V1, processes information further, and sends it to other areas.
• The ventral stream refers to the path that goes through the 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.
• 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.

Shape Perception

• 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.

Visual Agnosia

• The inability to recognize objects despite satisfactory vision.
• Caused by damage to the pattern pathway, usually in the temporal cortex.

Recognizing Faces

• Face recognition occurs relatively soon after birth.
• Newborns show strong preference for a right-side-up face and support the idea of a built-in face recognition system.
• Facial recognition continues to develop gradually into adolescence.

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.
• At the opposite extreme, “super-recognizers” have richer-than-average connections between the fusiform gyrus and occipital cortex and easily recognize people they saw only once or twice long ago.

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 and are color-insensitive.

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.

Activity 5-2: The Eyes Have It

• During voluntary eye movements (saccades), area MT and parts of the parietal cortex decrease their activity.
• Neural activity and blood flow in MT and part of the parietal cortex begin to decrease 75 milliseconds (ms) before the eye movement and remain suppressed during the movement; you become motion blind for a split second.