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CHAPTER 10
Vision and
visual perception
Form Vision
Pathway to the Brain
• Basic pathway:
– Rods or cones  bipolar cells ganglion cells
–  optic nerve optic chiasm optic tracts (L and R)
–  lateral geniculate nuclei of thalamus
– Optic radiations leave LGN  occipital cortex
– Several areas of occipital cortex of processing:
• Areas 17, 18 and 19
Two kinds of pathways
• Ambient pathway:
– Spatial vision pathway
– Where things are, not what they are
– Magnocelluar pathway
• Focal vision
– Object or form vision pathway
– What things are, but not where they are
– Parvocellular pathway
• Damage to either or both pathways: Cortical blindness
– Damage to brain, not eyes or even optic tract
– Typically lose form vision first, then all of it
– Very rare to just lose ambient pathway
The Perception of
Movement and distance
• Visual information follows two routes from the retina
through the brain:
– Parvocellular system : Focal or form vision
– Magnocellular system: spatial vision or ambient pathway
• Parvocellular ganglion cells: located mostly in the fovea.
– These cells have circular receptive fields
– Receptive fields are small and color opponent
– These smaller receptive fields are best for discrimination of fine detail
and color.
– Thus this is the form pathway
Perception of Objects,
Color, and Movement
• Magnocellular ganglion cells
– have a larger circular receptive fields
– for example: Brightness opponent
• respond only briefly to stimulation.
• specialized for brightness contrast and for movement
– This is the spatial pathway.
• Parvocellular and magnocellular pathways travel to
– The lateral geniculate nucleus or LGN
– Then to the primary visual cortex, also known as V1.
– From there they begin to travel different paths
The Perception of Objects,
Color, and Movement
• But: Two pathways are highly interconnected
• Form pathway:
– Parvocellular system = ventral stream that flows
from the visual cortex into the temporal lobes
– Ventral stream = what of visual processing
• Spatial pathway:
– magnocellular system = dorsal stream from the
visual cortex to the parietal lobes.
– Dorsal stream = where of visual processing
Further processing
of form vision
• Beyond V1: the ventral stream
– Passes through V2 and into V4,
– Mostly concerned with color perception
– Fine vision….important to make fine perceptual distinctions
– Remember, this is form vision.
• Then projects to the inferior temporal cortex,
– Lower boundary of the temporal lobe.
– This area shows a remarkable specialization for object
recognition.
– Must link visual information to auditory information: connect
verbal label to object form!
Further processing of
spatial vision
• Dorsal stream (magnocellular/spatial) proceeds to
– V2 through to V5, also known as Medial temporal or MT
• It is on the middle temporal gyrus of the temporal lobe
– Neurons there have strong directional sensitivity
– Contributes to the perception of movement and distance
• The dorsal stream then travels to posterior parietal
cortex
– located just behind the somatosensory cortex.
– Primary role: locate objects in space
– Significant behavioral implications of this function VERY
important
Visual pathways rely
on inhibition
• A retinotopic map exists in the visual cortex
– adjacent retinal receptors activate adjacent cells in the visual cortex.
• For example: Form vision is able to
–
–
–
–
detect an object’s boundaries and features (such as texture)
Differs from spatial location (that is spatial vision!).
Relies much more heavily on stimulation of cones
Relies on lateral inhibition to detect “edges”
• Lateral inhibition,
– Each neuron’s activity inhibits the activity of its neighbors
– In turn, its activity is inhibited by them.
– The inhibition is delivered by horizontal cells to nearby synapses of
receptors with bipolar cells.
Form Vision and inhibition
• In on center cells
– light in the center increases firing
– light in the off surround reduces firing below resting levels.
• Other ganglion cells have an off center and an on
surround function.
– Work the opposite
– Off center: light in surround increases firing; center reduces firing
– On surround: light in surround increases firing
• Why: think of pixels: which pixel is triggered helps build the
visual image in the visual field
Form Vision:
Many specialized neurons
• Simple cells
– respond to a line or an edge that is at a specific orientation
– May also respond to specific place in the visual field on the retina
– This would be like an individual pixel being turned on or off.
• Complex cells:
– continue to respond when a line or edge moves to a different
location.
– Activating a “series of pixels”
– Allows processing of movement and change
Form Vision
• According to Spatial Frequency Theory
– Visual cortical cells “do” a Fourier frequency analysis of the
luminosity variations in a scene.
– Visual cortical cells can detect not just edges but gradations of
change.
– Can stimulate individual “pixels”,
– Can stimulate individual or groups of pixels at various degrees
– Can stimulate a moving series or group of pixels
• Allows the brain to detect movement, shape, color, etc.
Perception of Objects,
Color, and Movement
• Modular processing
– Segregation of the various components of processing into separate
locations.
– Processing of form is conducted in modules or groupings
• Hierarchical processing:
– Lower levels of the nervous system analyze their information and
pass the results on to the next higher level for further analysis.
– We process visual information in layers
• Maybe: Distributed vs. localized processing:
– Some neuroscientists reject modular notion,
– Argue instead that any visual function is instead distributed
– E.g., occurs across a relatively wide area of the brain and may occur
simultaneously
Cortical damage:
several specific forms
• Agnosias: inability to process sensory information
– Object agnosia: impaired ability to recognize objects.
– Prosopagnosia : inability to visually recognize familiar faces.
– Both object agnosia and prosopagnosia are caused by damage to
the inferior temporal cortex (part of the ventral stream).
• Fusiform face area
– Face-responsive neurons are often intermingled with
object-responsive neurons,
– Part of the fusiform gyrus on the underside of the temporal
lobe is critical for face recognition
– Hence, the name! Facial recognition is innate!
Other Vision disorders
• Blindness is not just “being blind”
– Depends on type of damage
– Depends on degree of damage
• Most often: damage to focal or form vision
– Cannot see individual shapes, colors or refined vision
– Can still move around in environment
• Movement agnosia: inability to perceive movement.
– Very debilitating
– More critical to be able to perceive distance and movement
• Typically, damage form vision or all of vision
– Very rare to have form vision but no spatial vision
– Loss of spatial vision = lack of ability to move in environment
Other Vision disorders
• Blindsight: Patients blind due to damage to V1
– Can locate and track the movement of objects; discriminate colors,
– But say they are guessing, unaware of abilities.
• Color agnosia: loss of the ability to perceive colors
– Depends on color constancy
– Color constancy: critical behavior:
• The ability to recognize the “natural color “ of object despite
of illuminating wavelength
• If could not, objects would seem to change colors as the sun
shifted position through the day or as we went indoors into
artificial light
Sensory neglect
• Function of posterior parietal cortex:
– combines input from visual, auditory, and somatosensory areas
– helps the individual locate objects in space
– Helps orient the body in the environment.
• Damage here impairs several abilities:
– reaching for objects
– often produces sensory neglect
– patient ignores visual, touch, and auditory stimulation on the side
opposite the injury.
• Neglect not due to any defect in visual processing
– Due to deficit in attention TO visual stimuli.
Where is all this processing
finally put together?
• Many researchers have wondered:
– where is all the information about a visual object brought back together?.
– Must put visual, auditory, kinesthetic information together to form “concept” of an
object, event or activity
• Ultimate understanding of an object appears to occur in part of superior
temporal gyrus
– Area receives input from both visual and kinesthetic neural streams
• Two other areas also important:
– Parietal cortex: where damage causes sensory neglect.
– Frontal areas where both visual streams eventually converge.
• Most likely: redundant processing involving all three areas
• Why such redundancy for our perceptual systems?