Kapitel 6: Det Visuelle System

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Chapter 6: The Visual System

• Light Enters the Eye and Reaches the Retina (LO 6.1):

  • The pupil, controlled by the iris, regulates the amount of light entering the eye.
  • The lens focuses light onto the retina.
  • Accommodation is the process of adjusting the lens shape for focusing on objects at different distances.
  • Light can be thought of as photons (discrete particles of energy) or as waves of electromagnetic energy (380-760 nanometers for humans).
  • Wavelength is important for color perception, intensity for brightness.

• Eye Position and Binocular Disparity (LO 6.2):

  • Some vertebrates have eyes on the sides of their heads for a wider field of view.
  • Other vertebrates have eyes on the front of their heads creating binocular vision, which allows for depth perception.
  • Binocular disparity is the difference in the images projected onto each retina. The brain uses this difference to determine depth.

• The Retina and Translation of Light into Neural Signals (LO 6.3):

  • The retina consists of five types of neurons: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
  • Light passes through these layers to reach the receptors.
  • The retina is "inside-out," with receptors located behind other layers.
  • The fovea is a specialized area of the retina for high-acuity vision (fine details).
  • Amacrine and horizontal cells facilitate lateral communication across sensory channels.
  • The inside-out structure creates two problems: distortion of incoming light and the blind spot (where ganglion axons exit the eye. More than 60 kinds of retinal neurons have been identified, including about 30 different retinal ganglion cells

• Cone and Rod Vision (LO 6.4):

  • The duplexity theory posits that cones and rods mediate different types of vision.
  • Photopic vision (cones) is used in bright light for high acuity and color perception.
  • Scotopic vision (rods) is used in dim light for low acuity, but high sensitivity to detect light.
  • Species active in the day tend to have cone-only retinas, and species active only at night tend to have rod-only retinas. There is large difference in convergence between the two systems

• Spectral Sensitivity (LO 6.5):

  • Spectral sensitivity curves show how sensitive the visual system is to different wavelengths of light.
  • The photopic curve peaks at a different wavelength than the scotopic curve
  • The Purkinje effect describes the shift in color perception from cones to rods as light levels decrease. The shift is from reds/yellows to blues/greens in perception of relative brightness.

• Eye Movement (LO 6.6):

  • Involuntary fixational eye movements (tremor, drifts, saccades) are essential for maintaining clear vision.
  • Blocking these movements causes blurry vision or image loss.

• Visual Transduction (LO 6.7):

  • Visual transduction is the conversion of light energy into neural signals.
  • Rhodopsin, a light-sensitive pigment, plays a crucial role in rod-mediated vision.
  • Light causes a cascade of reactions that ultimately decrease the release of glutamate, transmitting the neural signal.
  • Rhodopsin is a G-protein-coupled receptor.

• Retina-Geniculate-Striate System (LO 6.8):

  • Most retinal ganglion cell axons project to the primary visual cortex (V1) via a pathway involving lateral geniculate nuclei.
  • This system is retinotopic; adjacent areas of the retina project to adjacent areas of the visual cortex.
  • The visual system receives input from both eyes, and the two visual fields are processed in the opposite hemisphere at the Primary Visual Cortex.
  • About 90% of the axons of the retinal ganglion cells become part of the retina-geniculate-striate pathways
  • Ipsilaterally signals come from the temporal hemiretina
  • Contralaterally signals come from the nasal hemiretina

• Retinotopic Organization (LO 6.9):

  • The disproportionately large representation of the fovea occupies about 25% of the promary visual cortex.

• The M and P Channels (LO 6.10):

  • Visual information is processed through parallel channels in the lateral geniculate nucleus:
    • Parvocellular (P) layers process color, fine detail and stationary or slow-moving objects, primarily from cones.
    • Magnocellular (M) layers process motion, primarily from rods.

• Contrast Enhancement (LO 6.11):

  • Mach bands enhance contrast at edges.

• Receptive Fields of Visual Neurons (LO 6.12):

  • David Hubel and Torsten Wiesel developed a method to map neurons' receptive fields.
  • The receptive field of a neuron is the region of the visual field that can influence the neuron's firing rate.
  • Receptive fields of retinal ganglion cells, lateral geniculate neurons, and layer IV neurons have on-center/off-surround and off-center/on-surround properties.
  • Most visual neurons are continually active.

• Receptive Fields of Primary Visual Cortex Neurons (LO 6.13):

  • Simple cells have rectangular receptive fields, respond best to oriented edges, and are monocular.
  • Complex cells also have rectangular receptive fields, but respond to oriented edges regardless of position, are faster-acting and binocular.

• Receptive Fields of Primary Visual Cortex Neurons (LO 6.14):

  • The borders between the on and off regions of simple cells are straight lines rather than circles
  • Many complex cells respond more robustly to the movement of a straight across their receptive fields in a particular direction.
  • If the preferred stimulation is applied through both eyes simultaneously, a binocular usually fires more robustly than is only one eye is stimulated -Most of the binocular cells display some degree of ocular dominance
  • Hubel and Wiesel concluded that the primary visual cortex was organized into functional vertical columns

• Changing concept of Visual Receptive Fields (LO 6.16):

  • Recent research reveals more complex receptive field properties than originally thought.
  • These properties include selectivity for specific features like orientation, motion, and direction of motion. Those cells have receptive fields that are sensitive to one or more of the following orientation, motion, and direction of motion
  • Retinal ganglion cells display about 20 to 40 distinct sorts of receptive fields
  • Contexts influence receptive-field properties; they are not static.

• Changing Concept of Visual Receptive Fields (LO 6.17):

  • Response of a visual cortex neuron depends not only on the stimuli in its receptive field, but on the larger scene in which these stimuli are imbedded

• Component and Opponent Processing (LO 6.18):

  • Component (trichromatic) theory proposes three types of color-sensitive cones.
  • Opponent-process theory proposes three opponent color channels (red-green, blue-yellow, black-white) for color processing in the visual system.
  • At all subsequent levels of the retina-geniculate-striate system, there are cells that respond in one direction to one color and in the opposite direction to its complementary color. For example increase when red, depreciate when blue.

• Color Constancy and the Retinex Theory (LO 6.19):

  • Edwin Land's demonstration showed that perceived color depends on image context rather than on light wavelengths alone.
  • Land proposed his retinex theory: color constancy emerges from analyzing the reflectance of objects compared with the reflectance of their surroundings.
  • The efficiency with which a surface absorbs wavelength does not change

• Cortical Mechanisms of Vision and Conscious Awareness (LO 6.20):

  • Secondary visual cortex and association areas analyze visual data.
  • These areas are highly interconnected.

• Damage to Primary Visual Cortex (LO 6.21):

  • Damage to the primary visual cortex results in a scotoma (blind area) in the contralateral visual field.
  • Completion phenomena occur – patients perceive missing parts of objects due to the brain's filling in process.

• Functional Areas of Secondary and Association Visual Cortex (LO 6.22):

  • In the macaque monkey, there are 24 areas of secondary visual cortex and 7 areas of association visual cortex. There are anatomical and organizational differences among these areas. Interconnections are reciprocal and are prodigiously interconnected Major flow of visual information to the cortex moves from the primary visual cortex to the various areas of secondary visual cortex to the areas of association cortex

• Dorsal and Ventral Streams (LO 6.23):

  • The dorsal stream extends primary visual cortex to dorsal prestriate cortex to the posterior parietal cortex. The ventral stream moves information from the primary visual cortex to the ventral prestriate cortex to the inferotemporal cortex.

• Prosopagnosia (LO 6.24):

  • Prosopagnosia is an inability to recognize faces.
  • While often associated with face recognition deficits, damage can impact object recognition in general. Diagnosis of prosopagnosia are usually associated with damage to either or both of the fusiform face area and the occipital

• Akinetopsia (LO 6.25):

  • Akinetopsia is a deficit in motion perception, an inability to perceive continuous smooth movement.
  • Damage to area MT (middle temporal) is often involved. 95% of the neurons of MT respond to specific directions of movement and little else. Can result from brain damage or high doses of anti depressants.