The Brain and Seeing

1
Chapter 4

• The Brain and Seeing

2
Optic Nerve

• The optic nerve of each eye is a bundle of more
  than a million fibers carrying information from
  the eyes to various processing nerves in the
  brain.
• All neural processing of visual information
  within the brain depends on the optic nerves for
  input data.

3
Optic Nerve

• The optic nerves of the two eyes converge at the
  optic chiasm.
• Ipsilateral fibers from each eye project to the
  same side of the brain.
• Contralateral fibers cross to the opposite side
  of the brain.

4
Optic Nerve

• Optic tracts are combinations of crossed fibers
  from one eye with uncrossed fibers of other eye.
• Optic tracts run from the chiasm to structures
  deeper in the brain.

5
Visual Pathways
6
Superior Colliculus

• Cells in the superior colliculus have poorly
  defined ON-OFF regions and respond to any visual
  stimulus.
• Cells in the superior colliculus are involved in
  controlling eye movements.

7
Superior Colliculus

• Superior colliculus contains multisensory cells
  that respond when visual and auditory stimuli
  occur together in space.
• Multisensory cells in the superior colliculus
  enable organism to detect location of weak
  environmental stimuli.

8
Lateral Geniculate Nucleus

• Lateral geniculate nucleus (LGN) is involved in
  more detailed analysis of visual stimuli.
• LGN is a multilayered structure the number of
  layers varies by species.

9
Structure of the LGN

• First two layers of the LGN are the magnocellular
  layers.
• Layers 3, 4, 5, and 6 of the LGN are the
  parvocellular layers.

10
Structure of the LGN

• Fibers of the optic tract become segregated when
  they reach the LGN.
• Contralateral fibers contact layers 1, 4, and 6
  of the LGN.
• Ipsilateral fibers contact layers 2, 3, and 5.
• Each layer of the LGN contains a retinotopic map
  that preserves the topography of the retina.

11
Field Properties of P and M LGN Cells

• Some P cells are color opponent cells that
  respond most strongly when their center and
  surround are stimulated by different colors.
• P cells analyze spatial information at a finer
  level of detail than do M cells.

12
Visual Cortex

• Visual cortex is located in occipital lobe at the
  back of the cerebral hemispheres.

13
Visual Cortex

• Scotomas are patches of blindness within the
  visual field.

14
Visual Cortex

• Cortical magnification is distortion in which
  representation of the fields center is highly
  exaggerated.

15
Retinal Maps in the Cortex

• Each cortical cell responds to stimulation of the
  restricted area of the retina that constitutes
  that cells receptive field.
• The receptive fields of cells in each hemisphere
  of the V1 form a topographic map of the
  contralateral visual field.

16
A single cortical cell response to bars of
various orientations
17
Functional Properties of Cortical Cells

• Orientation selectivity cortical cell will
  respond only if the orientation of an edge or
  line falls within a narrow range.
• Simple cell there is simple relationship between
  the receptive field layout of a cortical cell and
  its preferred stimulus.

18
Functional Properties of Cortical Cells

• Complex cell precise location is not as
  important as long as stimulus remains properly
  oriented within its receptive field.

19
Functional Properties of Cortical Cells

• Hypercomplex cell maximum response is to an
  appropriately oriented bar whose length and width
  fits the receptive field.

20
Functional Properties of Cortical Cells

• Oblique effect horizontal and vertical lines can
  be detected more easily and identified more
  rapidly than can obliquely oriented lines.
• Most people experience the oblique effect, but
  those with astigmatism may show the opposite
  tendency and see oblique lines more clearly.

21
Functional Properties of Cortical Cells

• Direction selectivity one cell might respond
  when a vertical contour moved from left to right
  but be unresponsive if contour moved in the
  opposite direction.
• A different cell might respond only to a contour
  moving in the opposite direction.
• Motion aftereffect

22
Functional Properties of Cortical Cells

• Monocular segregation gives way to binocular
  integration when cells in layer 4 send signals to
  other cortical layers immediately above them.

23
Functional Properties of Cortical Cells

• Ocular dominance is when one cell responds more
  vigorously to stimulation of one eye than to the
  other.
• Any cell that can be excited through both eyes
  regardless of its ocular dominance is called a
  binocular cell.

24
Columns and Hypercolumns
25
Visual Processing Beyond Area V1

• Neural information from visual area V1 is
  distributed over a number of pathways to higher
  visual areas of the brain.
• Every cortical region receiving input from
  another region also sends feedback connections
  back to that other region.
• Parietal stream, or dorsal stream, consists of
  visual areas laid out along a trajectory leading
  from occipital to parietal brain regions.
• Temporal stream, or ventral stream, comprises a
  network of visual areas spanning the occipital
  and temporal lobes.

26
P and M Cell Contributions to Vision
27
Specialized Visual Areas in Human Vision

• Achromatopsia (loss of color vision)

28
Specialized Visual Areas in Human Vision

• Prosopagnosia (inability to recognize faces)

29
Specialized Visual Areas in Human Vision

• Akinetopsia
• (inability to see motion)