Visual Cortex Flashcards
Cerebral cortex lobes
Frontal, parietal, temporal, and occipital
Gray matter of cerebral cortex
Cell bodies
4mm thick
2200cm2
White matter
Underlying
2000 miles of axonal connections
Where is the striate cortex located
Occipital lobe
Name of straite cortex derived from
Gennari
-distinctive band formed by myelinated geniculate axons that synapse in layer 4B
Other names for the striate cortex
- primary visual cortex
- visual are 1
- V1
- Brodmaan area 17
Layers of striate cortex
Comprised of 6 layers
- layer 1 is the most superficial
- layer 6 the deepest
Striate cortex contains a representation of the
Entire visual field
Striate cortex is dominated by
The fovea input
- fovea comprises only 0.01% of the retinal area; yet is represented in at least 8% of striate cortex
- this cortical magnification of fovea vision may be due primarily to the increased area of cortex devoted to individual fovea ganglion cells, rather than solely to the high density of fovea ganglion cells
What is the cortical magnification of the fovea vision in the straite cortex due to
Increased are aof cortex developed to individual ganglion cells, rather than solely to the high density of fovea ganglion cells
Striate cortex projects to
Extra striate cortex
-region of visual cortex not distinguished by the line of gennari
Areas of extra striate cortex to study
- visual area 2 (V2)
- visual area 4 (V4)
- inferotermporal cortex (IT)
- middle temporal cortex (MT) or visual area 5 (V5)
How many areas are predominantly for processing visual info?
20 differnt areas
What are the two streams of cortical processing for visual information
Ventral processing stream
Dorsal processing stream
Ventral processing stream
- temporal pathway
- “what” system
- though to receive its predominant input from the parvo RGN pathway
Dorsal processing stream
- parietal pathway
- “where” system
- thought to receive its predominant input from the magno pathway
Are the two different visual systems pathways independent or do they communicate
There is significant communication between the two cortical processing streams
Other than the extra striate cortex, where does the striate cortex project
Major reciprocal projection to the LGN, as well as a projection to the pulvinar
-pulvinar-thalamic nucleus thought to be associated with visual attention, motion processing, and visually guided movement
Pulvinar-thalami nucleus
Thought to be associated with visual attention, motion processing, and visually guided movement
Projections to lower visual centers (LGM and pulvinar) from the straite cortex are from what layers
Deeper layers (layer 6) -those to extra striate cortex are from more superficial layers (2 or 3)
Information flow in striate
Flows back toward striate cortex from extrastriate cortex via reciprocal pathways
- analogous to the retinal centrifugal pathway and the reciprocal projection from striate cortex to the LGN
- feedback to the receptorve field properties of V1 cells
Center-surround of retinal ganglion cell receptive fields
Science is initially looked for similar cells in straite cortex, however, experiments did not reveal such cells
Hubel and Wiesel
- unexpectedly found neurons in cat striate cortex that were sensitive to elongated stimuli, such as bars and edges
- divided these cells into two general categories (simple and complex cells)
- later found comparable cells in monkey cortex
- Nobel prize
Most sensitive to an edge or bar of a specific orientation
Simple cells
- mus be specific width
- bar or edge must be properly positioned within the cells receptive field
Mapping of simple cells
Simple cells receptive fields can be mapped out with small spots of light, doing so reveals receptive fields that are divided into antagonistic excitatoy and inhibitory regions
May results from input of LGN neurons whose receptive fields lie along a straight line
Simple cell
The formation of increasingly complicated receptive field arrangements from less complicated arrangements is referred to as
Serial, or hierarchical, processing
Serial or hierarchical processing in retina
Also occurs in the retina where the comparatively simple receptive fields of distal retinal elements give rise to the spatially antagonistic receptive fields of bipolar cells
Respond best to an elongated stimulus of a specific orientation, but stimulus position is not critical
Complex cells
These are characterized by direction selectivity-for the cell to be stimulated, the stimulus must move in a specific direction. A stimulus moving in the opposite direction, even if of the proper orientation, does not elicit a response
Many complex cells
Excitatory and inhibitory regions of complex cells
Unlike simple cells, the receptive fields of complex cells cannot be divided into separate excitatory and inhibitory regions
Theory of complex cells
- initially thought that complex cell receptive fields are result of hierarchical processing. Complex cells do not manifest separate excitatory and inhibitory areas, so this doesn’t work
- spatially nonlinear LGN calls play a larger role in the formation of complex cell receptive fields than for simple cells
Complex cells: neurons that are sensitive to the stimulus length
- these send stopped neurons were originally classified as hyper complex cells
- subsequent examination of visual cortex revealed this feature to be common to many cortical cells
- therefore, hypercomplex cells are generally not considered to be a separate category of cortical neurons
Hierarchical processing model holds that receptive fields of higher neurons are
Constructed from those of preceding neurons
-explains why stimulus parameters required to activate neurons become more specific at progressively higher levels of the visual system
Hierarchical processing of the cells
PR respond to diffuse light
Ganglion cells respond to light of certain diameter
Simple cells to a bar of light of a specific orientation
-complex cells to this same bar of light moving in the proper direction
The higher I up in the visual system, the more
Stringent are the requirements to drive a visual neuron
The concepts of parallel proecssing and hierarchical processing
Are compatible
Hierarchical processing occurs along each of the
Parallel pathways
Cortical neurons respond well to ____ and are selective for a particular
Sine0wave gratings
Spatial frequency
Most critical neurons are ____, meaning they receive input from both eyes
Binocular
-demonstrated by recording from a binocular cortical cell while stimulating one eye
Horopter
Line or surface containing all those points in space whose images fall on corresponding points of each eyes retina
What may mediate stereo-sis
Binocular cortical cells
Binocular cortical cells and stereo-sis
The cortical receptive fields do overlap at a critical distance form the eyes
- because the inputs from the two eyes are summed, a stimulus located at this critical distance maximally activates the cortical neuron
- the stimulus distance can thereby encode, presumably contributing to the physiological basis for stereo-sis
The majority of striate cortical neurons are binocular, however
Mose are dominated by one eye
Stimulation of a neuron through the dominant eye causes a stronger response than stimulation though the fellow eye
Ocular dominance
Laid out in a regular pattern of alternating right nad left ocular dominance bands, sometimes called ocular dominance slabs or columns
- the bands run though the substance of the cortex, perpendicular to its surface
- an electrode that penetrates the cortex perpendicular to its surface tends to encounter neurons dominated by the same eye
How is the striate cortex organized
According to its orientation as well as ocular dominance
A complete set of ocular dominance columns (both eyes) and orientation columns (all orientations)
Hypercolumn
-each hypercolumn has dimensions of approximately 1x 1mm
Orientation sensitivity is seen to be arranged in a ______ where each sector extending from the center of this represents a different orientation, when looking upon the surface of the cortex
Pinwheel
-with such an arrangement, an electrode penetrating the cortex parallel to its surface may encounter a systematic change n orientation as originally described by hubel and Wiesel
Development of standard cortical architecture and normal visual input early in life
Cortical architecture may be altered by environmental deprivation early in life
May occur in certain forms of anisometropia and strabismus
The parallel processing found in the RGN pathway continues
Continues into the striatecortex and beyond
-stringing striate reveals irregular pattern of blobs within its superficial layers, regular pattern of stripes in adjoining visual area 2
____ are rush with concentrially organized double color-opponent neurons that result from parvo input
Straite blobs
-optimally repsosnive to stimuli that manifest color contrast
Interblob region
Superficial region of striate cortex between blob
Also appears to receive substantial parvo input
Striate cortex architecture: magno and blobs
The magno pathway apparently bypasses blobs and the interblob regions, the parvo and magno pathways provide input to the ventral and the dorsal processing stream, respectively. Neither of the RNG pathways, however, feeds exclusively into one of the cortical processing streams
Lesion that destroys the striate cortex
A patient with blindsight has no conscious visions
Double opponent receptive field in the striate cortex
Red center and a green annulus is most responsive stimuli
Single opponent striate cortex
Most responsive to a red center and a dark annulus, not a green annulus
Magno and blob
Bypasses blood and interblob
Secondary to a lesion that has destroyed all of striate cortex, a patient with ______ has no conscious vision
Blindsight
Lesion to striate cortex and blindsight: under certain circumstances
The patient will respond to a visual stimulus while claiming that no stimulus is seen
-force choice methodology
Results of forced choice in blind with lesion at striate cortex
Performance as locating the stimulus is above chance
- visual information is presumably processed along pathways that bypass striate cortex, but may activate extrastriate cortex
- superior colliculus and pulvinar
- this may result in nonconscious vision-blindsight
