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Chapter 4. The Brain and Seeing. 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.
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Chapter 4 The Brain and Seeing
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.
Optic Nerve • The optic nerves of the two eyes converge at the optic chiasm. • Ipsilateralfibers from each eye project to the same side of the brain. • Contralateralfibers cross to the opposite side of the brain.
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.
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.
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.
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.
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.
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.
Structure of the LGN • Each layer of the LGN contains a retinotopic map that preserves the topography of the retina.
Field Properties of P and M LGN Cells • Some P cells are color opponent cellsthatrespond 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.
Field Properties of P and M LGN Cells • M cells respond vigorously to rapid, abrupt fluctuations of light intensity. • P cells respond sluggishly.
Field Properties of P and M LGN Cells • Reticular activating system is buried in the brain stem and governs animal’s general level of arousal. • Orderly arrangement of information within the LGN could help the next stage of visual processing, which takes place in the primary visual cortex.
Visual Cortex • Visual cortex is located in occipital lobe at the back of the cerebral hemispheres.
Visual Cortex • Scotomas are patches of blindness within the visual field. • Cortical magnification is distortion in which representation of the field’s center is highly exaggerated.
Retinal Maps in the Cortex • Each cortical cell responds to stimulation of the restricted area of the retina that constitutes that cell’s receptive field. • The receptive fields of cells in each hemisphere of the V1 form a topographic map of the contralateral visual field.
A single cortical cell response to bars of various orientations
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.
Functional Properties of Cortical Cells • Complex cell: precise location is not as important as long as stimulus remains properly oriented within its receptive field. • Hypercomplex cell: maximum response is to an appropriately oriented bar whose length and width “fits” the receptive field.
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.
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.
Functional Properties of Cortical Cells • Monocular segregationgives way to binocularintegration when cells in layer 4 send signals to other cortical layers immediately above them.
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.
Functional Properties of Cortical Cells • Some color-sensitive cells are concentrated in blobs–regions containing neurons that receive input from the lower-subdivision of layer 4. • Color’s effect depends on whether it falls within center or surround of cell’s receptive field.
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.
Duality of Visual Processing • 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.
P and M Cell Contributions to Vision • P pathway is associated with perception of color, spatial detail, and texture. • M pathway is associated with perception of motion and flicker.
Specialized Visual Areas in Human Vision • Achromatopsia (loss of color vision) • Prosopagnosia (inability to recognize faces) • Akinetopsia (impaired motion perception)
Relating Visual Perception to Neurophysiology • How do we know that V1 is critical to vision? • phosphenes • cortical blindness • transcranial magnetic stimulation
What Cortical Cells Do • A change in a feature such as orientation and direction of motion reduces the level of activity in a cortical cell. • Not knowing which stimulus feature or features have changed is known as the ambiguity problem.
What Cortical Cells Do • Adaptation: when stimulated intensely for a period of time cells become temporarily adapted and less responsive. • Tilt aftereffect is a temporary change in perceived orientation of lines.
