| ISLE | Chapter 4
| Hypercolumns
Hubel and Wiesel found a vertical arrangement of cells that all responded to cells in the same orientation in the same retinal location. They called this single part of the cortex a column. Next, they noticed that adjacent columns responded to lines that were only tilted slightly different from each other. In fact, the columns formed an organized pattern according to their orientation. At a right angle to the direction that orientation changed, Hubel and Wiesel found that the brain alternated which eye the cells in V1 responsed to.
Thus, Hubel and Wiesel found both ocular dominance columns and orientation columns. Ocular dominance columns alternate systematically between left eye and right eye dominance. Orientation columns change systematically across orientations (Hubel & Wiesel, 1962). When ocular dominance columns and orientation columns are combined, they form something that Hubel and Wiesel called a hypercolumn. A hypercolumn is a 1 mm block of V1 containing both the ocular dominance and orientation columns for a particular region in visual space.
This view of the V1 persisted into the 1980s when researchers discovered that there was more to the story. Livingstone and Hubel (1984) discovered blobs and interblobs interspersed within V1 (also see Wong-Riley, 1979). Blobs are areas within V1 sensitive to color, whereas interblobs are areas sensitive to the orientation of an object. The interblob cells respond as the simple cells that we have described above. The blobs show color responses, and the layer 4B respond well to moving stimuli and stimuli of very low contrast.
Now, we need to integrate the blobs, interblobs, and layer 4B into the organization of the striate cortex that has been discussed. The orientational selectivity goes in one direction, and the ocular dominance goes at right angles. In each set of orientation columns for each eye, there are two blobs. Layer 4B runs throughout all of the cortex and cuts across all of the columns. This hypercolumn is one functional unit, processing all of the information from one region of the cortex. Adjacent hypercolumns process information from adjacent regions of the cortex, and it is these hypercolumns that make up the topographical map that was discussed above. What portions of the hypercolumns that are active at any point in time will indicate the features of what is stimulating that region of the retina.
In this activity, you can simulate stimulating a small part of the retina and see how simplified hypercolumns respond. Only the orientation columns will be examined in this simulation.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On the Illustration tab, you can stimulate a region of the retina and see the effect on the hypercolumns shown.
Below is a list of the ways that you can alter the model. The settings include the following:
Screen Area: The black area on the screen to the left is a screen that the
eye of your animal is seeing. Click on it to move the stimulus around.
In this area, there are four regions that will be processed
by separate hypercolumns. We will only be examining orientational selectivity here.
Show Hypercolumn Regions on Screen: You can show
where the different retinal regions for each hypercolumn is located.
Pressing this button restores the settings to their default values. It also gets you a new cell, which might have a different receptive field.