Biological Psychology - Vision

Biological Psychology: Vision

Introduction

• Perception of colors, sounds, tastes, and smells are the brain’s interpretation of receptor messages.
• Each sense has specialized receptors sensitive to specific energy types.
• Law of specific nerve energies: Activity by a particular nerve conveys the same type of information to the brain.
  • Example: Impulses in one neuron indicate light, while others indicate sound.

Principles of Vision

• Perception occurs in the brain, not in the external object.
• Visual experience requires light altering brain activity.
• Tactile experiences are 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 iris).
• The lens and cornea focus light onto the retina (rear surface of the eye lined with visual receptors).
• The left visual field strikes the right side of the retina, and vice versa.
• The upper visual field strikes the bottom half of the retina, and vice versa.

Route within the Retina

• Bipolar cells receive messages from visual receptors.
• Bipolar cells send messages to ganglion cells.
• Axons of ganglion cells form the optic nerve that travels to the brain.
• The optic nerve exits through the back of the eye, creating 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 specific visual stimuli (shapes, movements, etc.).

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 greater sensitivity to faint light.

The Arrangement of Visual Receptors

• Highly adaptive arrangement
• Examples:
  • Predatory birds have a greater receptor density on the top of the eye.
  • Rats have a greater receptor density on the bottom of the eye.

The Difference Between Foveal and Peripheral Vision

Visual Receptors: Rods and Cones

• Vertebrate retina consists of two kinds of receptors:
  • Rods: abundant in the periphery, respond to faint light (120 million per retina).
  • Cones: 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.
• Ratio of rods to cones is higher in species that are more active in dim light.

Photopigments

• Chemicals in 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 into all-trans-retinal.
• Light is 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 brightness but does not change the ratio.
• Long- and medium-wavelength cones are more abundant than short-wavelength cones.

The Opponent-Process Theory

• Suggests that we perceive color in terms of paired opposites.
• The brain perceives color on a continuum from red to green and another from yellow to blue.
• A possible mechanism is that bipolar cells are excited by one set of wavelengths and inhibited by another.
• Support includes negative color afterimages.

The Retinex Theory

• Both the opponent-process and trichromatic theories 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.
• Gene responsible is contained on the X chromosome.
• Caused by the lack of a type of cone or a cone with abnormal properties.
• Most common form is difficulty distinguishing between red and green.
• Results from the long- and medium-wavelength cones having the same photopigment.

An Overview of the Mammalian Visual System

• Rods and cones 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 where the two optic nerves 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.
• 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 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 indicate 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
• 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.

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 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 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 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; 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.