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Close Up on Primary Visual Cortex.docx

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University of Guelph
PSYC 2390
Lana Trick

Close Up on Primary Visual Cortex 10/4/2012 3:57:00 PM Close Up on the Primary Visual Cortex General Description: Purpose  A.k.a. V1, striate cortex, area 17 o Came from different disciplines o Stripped cortex  stain it and it looks striped (“striate”)  Size of credit card on the back of the head  Hit on back of head, axons put under stress, see flash of light  Site of conscious visual perception o 80% of cortex responds to visual stimuli Cortical Magnification  Feature analysis: image is being taken apart into sections and at each section you are looking at line orientation, depth and motion, colour  Spatial relations are largely reserved (retinotopic maps) o Neurons near each other in the cortex have receptive fields near each other in the retina  Some parts of retina are more largely represented in visual cortex than others (cortical magnification) o magnification of area that you are looking right at in visual cortex  Fovea (100% cones, no convergence), large amount of brain devoted to it, 0.1% of retina but 8% of area in visual cortex  To read letter in periphery and get the same amount of detail, must be bigger than letter in fovea  Been shown via brain imaging (PET scans, subtraction techniques, fMRI)  Provides extra neural processing needed for tasks such as reading that require high visual acuity  Good visual acuity is associated with sharp focusing of images on retina, small amount of convergence and large amount of brain area devoted to fovea Effect of Damage  Damage can cause blindness  Scotomas: blind spots o Bigger the area of cortex that is damaged, the bigger the scotoma o Don’t see black, just don’t see it  Cortical blindness and blightsight o No conscious awareness of what they are looking at o Cannot report what they are looking at o If you ask the question in the right way, they do know something / some information that is in the blind field  Behave in way that they are somehow aware of what is in what they cannot see  Body is doing the right thing but they are not consciously aware of it  Where is the flash of light? I’m blind I don’t see it *arm points to flash of light* o How can they do this?  Tecto-pulvinar system allows us to do focalized things  Even if strait information is not getting through, it is coming through back door via tecto-pulvinar system Feature Analysis in the Striate: Hubel and Wiesel  Won Nobel Prize for physiology in 1982  Cells just do not respond to light, respond to some patterns of light and not to others (very few images in environment are just lines) Types of Cells (feature detectors)  Cells that are orientation sensitive (simple cells) o Side by side, not in center surround configuration o Most receptive when image meets criterion, inhibitory and excitatory areas arranged side by side o Orientation tuning curve  Impulses per second vs. orientation (single cell recording)  peak is orientation that neuron responds best to o Exposed stimuli to cats that were looking at them and measuring activity of cells  Complex cells o Respond to specific combination of line orientation and movement (particular direction across retina)  Receptive fields are not indicated by pluses and minuses, rather area, if stimulated, elicits response  E.g. diagonal line moving right o Complex because responding to multiple components  End stop cell (hyper-complex cells) o Look for combination of motion, orientation and length or angles or moving to corners  E.g. most receptive to medium length vertical line moving right  Short vertical line moving right, wouldn’t respond as well  Short, horizontal line moving right, would respond worse Hypercolumns (Organization)  Location columns o As you go down perpendicular to visual field, all neurons have their receptive fields on same location on retina  Orientation columns o As you go down, cells are also looking for the same orientation o Gradually going through all orientations (180 degrees) in matter of small distance across cortex (adjacent cells)  Only for one part of image  Ocular dominance columns o 80% of neurons in cortex respond to stimulation of both eyes o Most neurons respond better to one eye than other o Have columns that are responsive to right eye and responsive to left eye  Along perpendicular track, all neurons are best responsive to the same eye (alternate)  Divided into orientation and left or right eye  Hypercolumn  all 3 types of columns combined into unit  Zeki addition: The Blobs and interblobs o Blobs: cells that respond to colour (when stained look like blobs) o Interblobs: do not respond to colour, all cells that are between the blobs  At each point in image, cells are asking: is there colour, are there diagonal lines, are there vertical lines etc. o Cortical representation of a stimulus does not have to resemble the stimulus, it just has to contain information that represents the stimulus Newer Story on the Striate 10/4/2012 3:57:00 PM Newer Story on the Striate: Spatial Frequency Analysis  Take info from image apart in different levels of detail Background Concepts: Selective Adaptation: if neurons fire long enough, they will get tired or adapt  Causes firing rate to decrease and neuron fires less when that stimulus is immediately presented again  Selective adaptation to orientation tests with adapting stimulus as vertical grating o Causes large increase in contrast threshold for vertically oriented test grating (contrast had to be increased in order for people to see the bars) o Nearly the same graphs for increase in contrast threshold vs. orientation and impulse per second vs. orientation  Supports selective adaptation theory Contrast and How Contrast is Measured  Grating stimulus o Have contours (bright lines and dark lines  alternating bars) o How big of a difference is there in contrast?  High contrast  big difference in bright and dark lines  Big difference between intensities in graph  Low contrast  little difference in bright and dark lines  Little difference between intensities in graph  Measuring contrast o Look at intensity of bright part (e.g. 10) and look at intensity of dark part (e.g. 2) o Contrast = (high intensity – low intensity) / (high intensity + low intensity)  Contrast (high contrast) = (10-2) / (10+2) = 0.6667 o For low contrast bright part is 4, dark part is 2  Contrast = (4-2) / (4+2) = 0.333 o Higher contrast  higher value of contrast Selective Reading and Feature Detectors:  Selective rearing  animal is reared in an environment that contains only certain stimuli, then neurons that respond to these stimuli will become more prevalent o Supports neural plasticity or experience dependent plasticity (response properties of neurons can be shaped by experience) o Occurs over longer time scale and strongest in young animals when visual system still developing o Animals seem blind to orientations that they were not reared to (Blakemore and Cooper)  Neurons of orientation not reared to were “lost” because hadn’t been used (use it or lose it) Different Types of Grating  Square wave o Sharp distinctive edges  Sine wave o Smoother edges Spatial Frequency  What is it? o How much detail there is  Number of contours (lines) per unit area o Fine spatial frequency  lots of detail  high spatial frequency o Course spatial frequency  less detail  low spatial frequency  How is spatial frequency measured? o Spatial cycles per degree visual angle  Number of contours per degree visual angle o What is a cycle?  Dark line plus a light line o What is a degree visual angle?  How big of area covered on back of eye  360 degrees  circle  Thumb at arms length, area covered by your eye is 2 degrees visual angle (“rule of thumb”)  If thumb covers up 1 bar, that’s 2 degrees for 1 bar and 1 bar is half (0.5) of a cycle therefore spatial frequency is…  Spatial cycles / degree angle = 0.5/2 = 1 cycle per degree Spatial Frequency and Contrast are Two Different Concepts. 1. Low spatial frequency and high spatial frequency can have the same contrast. 2. Low contrast and high contrast can have same spatial frequency. Fourier’s Insight: Fourier analysis and Fourier synthesis, how is it used?  Any complicated problem can be considered the sum of many simple problems  Take information out of image and store in a small place (CD) o Each image has large amount of information stored into CD o To reactivate image, open up small space (CD)  Decomposing image into sine waves and store sine waves (Fourier Analysis), sine waves  “small space” o Different sine waves for spatial frequencies, orientation, contrasts of image  Take mathematical description of image (sine wave, Fournier Analysis done to image) and put it back together again (Fournier Synthesis)  Is the brain doing Fourier analysis? Is the visual cortex performing Fourier Analysis? o Simple cells, complex cells, end stop cells o Taking image apart at different levels of detail  Low level of detail (general outline of city)  Medium level of detail (individual buildings)  Fine level of detail (windows)  Normally, we see high, medium and low level spatial frequency levels  Squint eyes, get rid of high spatial frequency information thus image looks more course  Look more attractive in low spatial frequency levels Spatial Frequency Perception:  We are not equally sensitive to all spatial frequencies o Can see smaller differences in brightness, can see low contrast things How do you measure sensitivity to spatial frequency?  Measuring the contrast sensitivity function (CSF) o Use contrast to find our how sensitive we are to spatial frequency o Measure contrast threshold  Increase contrast just enough to see bars  (High - low) / (high + low) = contrast sensitivity o Convert to contrast sensitivity  Sensitivity = 1 / threshold o If you were to graph threshold contrast (log scale) vs. spatial frequency (cycles/degree)  see picture  Most sensitive in middle range Factors that Affect Sensitivity to
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