Chapter 9: Sense and Perception Flashcards
What is blindsight?
patient can’t identify objects in blind area
but can accurately tell about changes in visual field
patient not conscious that they can register changes in visual field
What is the relationship between blindsight and perception?
NS must construct images from bits of information
the brain must bind it all together, whole image perception
selective awareness: we can only access some part of the information our brain is currently processing
What is the nature of sensation and perception?
the only input our brain receives from the “real” world is a series of action potentials originating from external energy that are transduced by our sensory receptors
this info is passed along sensory neurons that form specific pathways
how we end up perceiving one set of nerve impulses as vision and another set as hearing is unknown
How are sensory system diverse?
vision, touch, taste, etc.
lead to different perceptual experience
but each sensory system is organized similarly
What are sensory receptors?
specialized cells that transduce (convert) sensory energy into neural activity
each sensory system’s receptors are designed to respond only to a narrow band of energy
vision: light energy produces chemical energy
auditory: air pressure produces mechanical energy
somatosensory: mechanical energy
taste and olfaction: chemical molecules
What is the receptive field?
region of sensory space (e.g. skin surface) in which a stimulus modifies a receptor’s activity
what the open eye sees is the receptive field of that eye
each photoreceptor in the eye points in a slightly different direction, so each one has a unique receptive field
RF helps us sample sensory information and can locate events in space
What is the relationship between sensory receptors and the receptive field?
sensory receptor receptive field can contrast the information each receptor is providing
adjacent receptive field may overlap
the contrast between their responses to events help us localize sensations
this spatial dimension of sensory information produces cortical patterns and maps that form each person’s sensory reality
What is receptor density and sensitivity?
sensory receptors are not evenly distributed across the body or its organs
density is important for determining the sensitivity of a sensory system, visual receptors packed towards the center of our visual field, peripheral vision is poorer
differences in receptor density determine the special abilities of many animals
What are neural relays?
all receptors connect to the cortex through a sequence of intervening neurons
sensory information is modified at each stage in the relay, each region constructs different aspects of the sensory experience
neural relays allow sensory systems to interact
What is sensory coding?
after transduction, sensory information is encoded by action potentials that travel along peripheral nerves to the CNS
from there, action potential travel on nerve tracks within the CNS, each bundle carries the same kind of signal
presence of a stimulus can be encoded by an increase or a decrease in discharge rate (the amount of increase/decreases can encode stimulus intensity)
changes in the visual field can be encoded by activity in different neurons or different levels of discharge within a neuron
What is representation of sensory coding?
the neocortex represents the sensory field of each sensory modality as a spatially organized neural representation of the external world
topographic map is a spatially organized neural representation of the external world
in mammals, each sensory system has at least one primary cortical area
these may project to secondary areas
What is the topographic map in the sensorimotor cortex?
the size of its features represent the relative proportions of the parts of the human brain responsible for motor and somatosensory function
the features that are most exaggerated have the largest correlate representations in the brain
What is sensation?
registration of physical stimuli from the environment by the sensory organs
sensory impressions are influenced by context, emotions, and memory
What is perception?
subjective interpretation of sensations by the brain
our visual experience is not an objective reproduction of what is out there, rather it is a subjective construction of reality that is manufactured by the brain
What is the cornea?
clear outer covering
controls and focuses the entry of light into the eye
What is the iris?
opens and closes to allow in more or less light
the hole in the iris is the pupil
What is the lens?
focuses light
bends to accommodate near and far objects
What is the retina?
where light energy initiates neural activity
light-sensitive surface at the back of the eye, consists of neurons and photoreceptor cells
translates light into action potentials
discriminates wavelengths (colors)
works in a wide range of light intensities
What is the fovea?
region at the center of the retina that is specialized for high acuity
receptive field at the center of the eye’s visual field
What is the acuity across the visual field?
vision is better in the center of the visual field than at the margins, or periphery
letters at the periphery must be much larger than those in the center for us to see them as well
What is the blind spot?
region of the retina where axons forming the optic nerve leave the eye and where blood vessels enter and leave
has no photoreceptors
visual fields overlap: left eye can see the right eye’s blind spot, brain compensation
What are photoreceptors?
the retina’s photoreceptor cells convert light energy first into chemical energy and then into neural activity
light arriving at photoreceptor triggers a series of chemical reactions
leads to a change in membrane potential
leads to a change in the release of neurotransmitters onto nearby neurons
What are rods?
more numerous than cones
sensitive to low levels of light (dim light)
used mainly for night vision
one type of pigment only
What are cones?
highly responsive to bright light
specialized for color and high visual acuity
in the fovea only
three types of pigment
What are the three types of cone pigments?
absorb light over a range of frequencies
blue, or short wavelength
green, or middle wavelength
red, or long wavelength
there are approximately equal numbers of red and green cones, but fewer blue cones
What are the types of retinal neurons?
bipolar cell: receives input from photoreceptors into ganglion cells
horizontal cell: links photoreceptors and bipolar cells
amacrine cell: links bipolar cells and ganglion cells
retinal ganglion cell (RGC): gives rise to the optic nerve
What are the two types of ganglion cells?
magnocellular cell (M-cell)
parvocellular cell (P-cell)
What are magnocellular cells?
magno-, large
receives input primarily from rods
sensitive to light and moving stimuli
What are parvocellular cells?
parvo-, small
receives input primarily from cones
sensitive to color
What is the optic chiasm?
junction of the optic nerves from each eye
axons from the nasal (inside) half of each retina cross over to the opposite side of the brain
axons from the temporal (outer) half of each retina remain on the same side of the brain
information from the left visual field goes to the right side of the brain, information from the right visual field goes to the left side of the brain
What are the three routes to the visual brain?
two main pathways lead to the visual cortex in the occipital lobe
geniculostriate pathway for processing the object’s image (made by all P ganglion axons and some M ganglion axons)
tectopulvinar pathway for directing rapid eye movements (made by remaining M ganglion axons)
another smaller pathway tracks into the hypothalamus
What is the geniculostriate system?
projections from the retina to the lateral geniculate nucleus to layer IV of the primary visual cortex
bridges the thalamus (geniculate) and the striate cortex)
What is the striate cortex?
the primary visual cortex (V1) in the occipital lobe
shows striped (striations) when strained
two visual paths emerge from the striate cortex:
one route goes to vision-related region of the parietal lobe, one route goes to vision-related regions of the temporal lobe
What is the tectopulvinar system?
projections from the retina to the midbrain’s superior colliculus
to the pulvinar (thalamus)
to the parietal and temporal visual areas
What is the retinohypothalamic tract?
synapses in the tiny superachiasmatic nucleus in the hypothalamus
roles in regulating circadian rhythms and in the pupillary reflex
contains photosensitive RGCs
What is the dorsal visual stream?
visual processing pathway from V1 to the parietal lobe
guides movement relative to objects
the how pathway (how action is to be guided toward objects)
What is the ventral visual stream?
V1 to the temporal lobe
object identification and perceiving related movements
the what pathway (identifies an object)
What is the lateral geniculate nucleus?
thalamus
right LGN: input from right half of each retina
left LGN: input from left half of each retina
What are the six layers of the lateral geniculate nucleus?
projections from the eyes go to different layers
layers 1,4, and 6: input from contralateral retina
layers 2, 3, and 5: input from ipsilateral retina
layers 1 and 2: input from magnocellular cells
layers 3 to 6: input from parvocellular cells
What is the geniculostriate pathway?
P and M retinal ganglion cells send separate pathways to the thalamus
segregation continues in the striate cortex
left and right eyes also send separate pathways to the thalamus
pathways remain segregated in the striate cortex
How is separate visual input maintained?
after info is segregated in the LGN
it maintains this segregation in the primary visual cortex
pattern: alternating regions of the left and right eye
How is the tectopulvinar pathway made up of M cells?
formed by the axons of remaining M cells (some form the geniculostriate pathway along P cells)
M cells are sensitive to light but not color, sensitive to movement but not fine detail
M cells from the retina project to the superior colliculus (in the midbrain’s tectum)
function: orienting movements to detect and focus the eyes towards stimuli
superior colliculus sends connection to the pulvinar region of the thalamus
What are the medial and lateral pulvinar in the tectopulvinar pathway?
medial pulvinar: sends connections to the parietal lobe
lateral pulvinar: sends connections to the temporal lobe
provide information regarding location (where)
damage in this pathway prevents individuals from identifying the location of stimuli
What is the primary visual cortex (V1; striate cortex)?
striate cortex receives input from the lateral geniculate nucleus
What is the secondary visual cortex (V2 to V5, extrastriate cortex)?
visual cortical areas outside the striate cortex
each region processing specific features of visual input
What is the heterogenous layering in the V1 area?
Blob (V1): region in the visual cortex that contains color-sensitive neurons
interblob (V1): region that separates blobs, participates in the perception of form and motion
within the V1 region, visual input is segregated by info type: color, form, motion, and projected into the V2 region
What is the heterogenous layering in the V2 region?
thick and thin stripes mixed with pale zones
the thick stripes receive input from the movement-sensitive neurons in region V1
the thin stripes receive input from V1’s color-sensitive neurons
pale zones receive input from V1’s form-sensitive neurons
visual pathways proceed from V2 to other occipital/temporal regions
What is the visual field?
region of the visual world that is seen by the eyes
divided into left and right fields
information in the left visual field goes to the right hemisphere
information in the right visual field goes to the left hemisphere
What is the coding location in the retina?
each retinal ganglion cell (RGC) responds to stimulation on just a small circular patch of the retina, the cell’s receptive field
the receptive field represents the outer world as seen by a single cell
the visual field is composed of thousands of receptive fields
light falling on one place on the retina will activate one ganglion cell, and light falling on another place will activate a different ganglion cell
What is the relationship between the location in the LGN and region V1?
cells in the lateral geniculate nucleus (LGN) also have receptive fields
each LGN cell represents a particular place
projects to V1 (striate cortex), forming a topographic map, spatially guided visual representation
What is the topographic organization of region V1?
receptive fields of cells in the cortex are typically larger than those of retinal ganglion cells
more cortical tissue is devoted to cells in the fovea than in the periphery (higher acuity)
What is receptive-field hierarchy?
the receptive fields of many retinal ganglion cells combine to form the receptive field of a single LGN cell
the receptive fields of many LGN cells combine to form the receptive field of a single V1 cell
What is the neuronal activity associated with seeing shape?
place a straight line positioned at a 45 degree angle in front of the eye
the cell can respond to this stimulus either by increasing or decreasing its firing rate, cell could show excitation to one stimulus, inhibition to another, or no reaction at all
in either case, the cell is generating information about the line, cell’s response to a particular stimulus is selective
What are retinal ganglion cells?
respond only to the presence or absence of light, not to shape
concentric circle arrangement
center and surround (periphery)
What are on-center cells?
excited when light falls on the center portion of the receptive field, inhibited when light falls on the surround (periphery) of the receptive field
light across whole receptive field produces weak excitation
What are off-center cells?
excited when light falls on the surround of the receptive field, inhibited when light falls on the center of the receptive field
light across the whole receptive field produces weak inhibition
What are overlapping receptive fields?
neighboring retinal ganglion cells receive their inputs from an overlapping set of photoreceptors
a small spot of light shining on the retina is likely to produce activity in both on-center and off-center ganglion cells
What does processing look like in retinal ganglion cells?
luminance contrast: ganglion cells tell the brain about the amount of light hitting a certain spot on the retina relative to average amount of light falling on surrounding retinal regions
this allows input from RGCs to tell the brain about shape, this system emphasizes differences in luminance
How is shape processed in the primary visual cortex (V1)?
V1 neurons receive input from multiple RGCs, V1 neurons have a much larger receptive field than RGCs, V1 neurons respond to stimuli more complex than light on or light off
cells behave like orientation detectors, excited by bars of light oriented in particular directions
What are simple cells in V1?
receptive field with a rectangular on-off arrangement
What are complex cells in V1?
maximally excited by bars of light moving in a particular direction through the receptive field
What are hypercomplex cells in V1?
like complex cells, maximally responsive to moving bars
also have a strong inhibitory area at one end of the receptive field
How does receptivity in V1 compare to RGC?
RGC responds maximally to spots of light (not orientation)
V1 input comes from ganglion cells aligned in a row
How is shape processed in the temporal cortex?
temporal cortex cells are maximally excited by complex visual stimuli (eg. faces or hands), very selective (eg. particular facial expressions, etc.)
complex features are necessary for activation of most neurons in area TE, include a combination of characteristics such as orientation, size, color, and texture
neurons with similar but not identical responsiveness to particular features tend to cluster in columns
What is stimulus equivalence?
recognizing that an object is the same across different viewing orientations
What is the process of seeing color?
subtractive color mixing
obtains entire range of colors by mixing only three colors of paint (red, blue, and yellow)
property of the cones in the retina
light of different wavelengths stimulates the three cone receptor types in different ways
ratio of activity of these three receptor types forms our impressions of colors
What is the trichromatic theory?
explanation of color vision based on the coding of the three primary colors red, green, and blue
the color we see is determined by the relative responses of the different cone types, all three types equally active results in use seeing white
can explain different types of color blindness: lacking one cone receptor type = color blindness
What are the limitations of the trichromatic theory?
four basic colors: red, green, yellow, and blue
cannot explain afterimages (red-green, blue-yellow)
What is opponent processing?
explanation of color vision that emphasizes the importance of the opposition of colors (red vs. green, blue vs. yellow)
opponent processing occurs in retinal ganglion cells, on-off and center-surround receptive fields, about 60% retinal ganglion cells
cells excited by red are inhibited by green (and vice versa)
What is the opponent-color contrast response?
the center of the receptive field is excitatory in some cells, inhibitory in other cells
stimulation to the periphery has the opposite effect, the center is responsive to one wavelength and the surround is responsive to another
What is the opponent processes in neurons in cortical region V4?
do not respond to particular wavelengths but are responsive to various perceived colors, center of the receptive field is excited by a certain color, and the surround is inhibited
may be important for color constancy. perceived color is constant relative to other colors, regardless of changes in illumination
What is the neuronal activity in the dorsal stream?
involved in processing visual information for action, the “how” stream
neurons in this area are silent to visual stimulation when a person is under anesthesia
some cells in this area process the visual appearance of an object to be grasped, these cells will fire even when a monkey watches another monkey picking an object
What is monocular blindness?
destruction of the retina or optic nerve of one eye, producing loss of sight in that eye
What is homonymous hemianopia?
blindness of an entire left or right visual field
caused by cuts in the optic track, LGN or V1
What is quadrantanopia?
blindness of one quadrant of the visual field
What is scotoma?
small blind spot in the visual field caused by a small lesion or migraines of the visual cortex
What is visual-form agnosia?
inability to recognize objects or drawings of objects
What is color agnosia (achromatopsia)?
inability to recognize colors
What is face agnosia (prosopagnosia)?
inability to recognize faces
What is optic ataxia?
deficit in the visual control of reaching and other movements
damage to the parietal cortex
retention of the ability to recognize objects normally
What are the characteristics of damage to the dorsal visual stream?
people with damage to the parietal cortex in the dorsal visual stream can see perfectly well
yet they cannot accurately guide their movements on the basis of visual information
What are the characteristics of damage to the ventral visual stream?
people with damage to the ventral stream cannot perceive objects because object perception is a ventral stream function
yet these same people can guide their movements to objects on the basis of visual information
