370: Primary Visual Cortex Pt2, Objects

1. Organization: Retinotopic Map
1. VI shows a retinotopic map like the LGN
2. Stimulating adjacent areas on the retina causes neurons at adjacent points in VI to fire
2. Orientation Columns
1. Neurons within columns fire maximally to the same orientation of stimuli
2. Adjacent columns change preference in an orderly fashion
1. If one cell cares about a specific orientation then the next cell is slightly rotated and the next is more rotated until you get 180 degrees
3. 1 Millimeter across the cortex represents entire range of orientation
3. Cortical Magnification Factor
1. Stimulating the fovea activates a very large area of cortex relative to the size of the fovea
2. Fovea accounts for only .01% of retina, but signals from fovea account for 8% to 10% of the visual cortex.  Why?
3. How do we know this?
1. Positron Emission Tomography (PET)
1. person injected with a harmless radioactive tracer which moves through the bloodstream 
2. Monitoring the radioactivity measure blood flow changes in blood flow show changes in brain activity
3. Not really used very much anymore because of the radiactive element
2. Functional Magnetic Resonance Imaging
1. Hemoglobin in the blood carries oxygen with a ferrous molecule that is magnetic
2. Neurons change the strength of the magnetic field in the blood
3. fMRI detects areas of activity by detecting changes in magnetic response
3. Subtraction Method
1. Brain activity (baseline) is determined by:
1. Measuring activity in a control state
2. Measuring activity in a stimulation state
3. Subtracting the control activity from the stimulation activity
4. Color Blobs
1. Inserted in between the orientation columns and location columns are another type of column called color blobs
2. Involved in processing of color information (don't know what they are doing just know they are involved)
5. Ocular Dominance Columns
1. Rost VI neurons respond preferentially to one eye, although most respond to some degree to both eyes
2. Neurons with the same preference are organized into columns
3. As one moves across the surface of the cortex, preference changes from right to left
6. Development of VI
1. Animals reared in specific environment
1. Limited number of stimuli present
2. Neural plasticity would result in lack of ability to see characteristics that are unavailable in the environment
1. Development of orientation selectivity
3. Shows that neurons need environmental stimulation to develop fully

Objects

1. Feature Detectors
1. Psychophysical method
1. Selective adaptation
1. Neurons can fatigue if stimulated for too long
2. Selectivity means that only those neurons that are sensitive to a specific type of stimuli will fatigue
2. Psychophysiological method
1. Selective rearing