Topic 3: The Visual Cortex and Beyond Flashcards
What occurs at the optic chiasm?
Some axons cross to the opposite side of the brain from the eye that they came from.
Result = right visual field goes to the left side; left visual field goes to the right side (contralateral processing)
What are visual fields determined by?
Where a person is fixating; Anything to the right of center vs. anything to the left of center
How are signals divided after partial crossover at the optic chiasm?
Approximately 90% of the signals will proceed to the lateral geniculate nucleus (LGN)
Other 10% proceeds to the superior colliculus
Thalamic nuclei; “Sensory relay station”
Lateral geniculate nucleus (LGN)
Midbrain (tectum) nuclei; Eye movements, orienting movements
Superior colliculus
Primary visual receiving area in the occipital lobe
Striate cortex; V1
Describe the pathway of visual signals to the occipital lobe.
Visual signals leave the back of both eyes as the optic nerves, which then meet at the optic chiasm.
At the optic chiasm, some axons cross to the opposite side of the brain from the eye that they came from.
After meeting, and partially crossing, at the optic chiasm, most signals will proceed to the lateral geniculate nucleus (LGN), while a smaller portion proceed to the superior colliculus
From LGN, signals travel to the occipital lobe (V1)
Describe what Hubel and Wiesel (1965) found regarding perceptual fields in the visual cortex.
Flashing spots of light on different places in the retina, found cells in V1 with receptive fields that, like the RGCs in the retina, have excitatory and inhibitory areas
Unlike the circular receptive fields in the retina, the areas are side-by-side
A neuron in the visual cortex that responds best to bars of a particular orientation
Simple cortical cells
Graph in which orientation (degrees) is plotted against firing rate (impulses per second)
Orientation tuning curve
Neurons in the visual cortex that respond best to moving bars with a particular orientation
Complex cortical cells
Neurons that respond best to lines of a specific length, that are moving in a particular direction
End-stopped cells
What are the three types of feature detectors?
Simple cortical cells
Complex cortical cells
End-stopped cells
Where does evidence that feature detectors in our visual cortex play a role in our perception come from?
Selective rearing experiments
Describe selective rearing experiments
Procedure in which animals are raised in special environments that contain only certain types/features of stimuli
Neurons that respond to that feature will become more prevalent
Neurons that respond to features that aren’t present will not develop
What characteristic of the nervous system do selective rearing experiments demonstrate?
Neural plasticity or experience-dependent plasticity; “Use it or lose it”
Horizontal/vertical orientations are perceived more easily than other orientations
Oblique effect
A map on a structure in the visual system that corresponds to locations on the retina
Retinotropic map
How do locations on the retina relate to locations in V1?
Locations adjacent to each other on the retina are represented by locations that are adjacent to each other on V1
What does it mean to say the fovea is “overrepresented” in V1?
More of the space on V1 is dedicated to the fovea than to the peripheral retina
Occurs when a disproportionately large area on the cortex is activated by stimulating a much smaller area on the receptor surface (e.g. overrepresentation of the fovea)
Cortical magnification
Describe the topographic organization of V1
Fovea = posterior
Periphery = anterior
Top of visual field = bottom of V1 (the opposite is true as well)
Describe the organization of V1 in terms of location, orientation, and ocular dominance columns.
Neurons in V1 have receptive fields at roughly the same location on the retina
Preferred orientation is also the same
Each orientation and each location will have one column for the left eye and one for the right eye
Name the areas of the extrastriate cortex and their functions.
V2: Initial processing of color, shape, motion; moves to V3, V4, or V5 depending on the processing required
V3: shapes of objects (particularly, when in motion)
V4: Color
V5: Motion
Areas of the occipital lobe aside from V1
Extrastriate cortex
Describe the role of the temporal lobe in terms of its visual stream function.
Ventral pathway/what pathway
Object recognition
Describe the role of the parietal lobe in terms of its visual stream function.
Dorsal pathway/where and how pathway
Location and direction
Describe the pathway of signals as they travel beyond the visual cortex.
Signals from the striate cortex (V1) travel to the extrastriate cortex (V2-V5)
Signals then leave the occipital lobe & continue through the ventral pathway (temporal lobe; what) and the dorsal pathway (parietal lobe; where)
What evidence do we have for separate what and where pathways?
Ablation monkeys
Temporal ablation = lose what
Parietal ablation = lose where/how
What are typical characteristics/symptoms of someone with damage to their ventral pathway?
Can’t tell you what they’re picking up
If the injury is exclusively to the ventral pathway, they will still be able to reach out appropriately for the object
Understand “how” to use the structural features of an object
What are typical characteristics/symptoms of someone with damage to their dorsal pathway?
If the injury is exclusively to the dorsal pathway, they will still be able to recognize objects
No longer understand “how” or “where” to use the structural features of an object
What are three characteristic symptoms of Balint syndrome?
Optic ataxia: Loss of visual guidance of hand and arm
Oculomotor apraxia: Difficulty performing voluntary eye movements
Simultagnosia: Inability to perceive more than one object at a time
Located at the apex of the ventral/”What?” pathway
Inferotemporal (IT) cortex
Why can neurons in the inferotemporal cortex respond to much more complex stimuli?
By the time we get here, the size of neuron’s receptive fields no longer correspond to just simple features; They are large enough now to encompass whole objects
Describe Charles Gross’ (1972) discovery of the function of the inferotemporal cortex
Was recording the firing of neurons in a monkey’s temporal lobe but was troubled when they found a neuron that didn’t respond to any simple stimuli
Put hand in front of the projector:
In response, the neuron fired with a burst of activity, so they determined that this neuron fired maximally to complex stimuli, and barely at all for simple ones
Further testing of the IT region found neurons in our temporal lobes that respond almost exclusively to faces, and not to non-face stimuli.
Provide three examples of regions towards the end of the temporal pathway that respond to incredibly specific complex stimuli
FFA: Fusiform face area
EBA: Extrastriate body area
PPA: Parahippocampal place area
Damage to which area causes prosopagnosia?
Fusiform face area (FFA)
Inability to recognize faces
Prosopagnosia
