103 Columns and Hypercolumns in V1

In 1958, Hubel and Wiesel discovered the organization of the visual cortex by cortical columns through a landmark experiment. In this study, they exposed a cat to a variety of visual orientations of light and recorded the activations from the visual cortex. They found that certain parts of the visual cortex responded to unique orientations of light, establishing the idea of the cortical column.

A cortical column is a cluster of several thousand neurons that respond to one unique visual orientation. Columnar organization is an important property of the visual cortex (V1) across two directions: depth vs surface.

• If you sample neural responses as you move through the depth, they stay approximately the same. This means that as you move down through cortex, you find neurons with the same orientation preferences.
• If you sample neural responses as you move across the surface, they change. For V1, this means that as you move across the cortex, you find neurons with different orientation preferences.

Orientation mapping is a result of observing the overall organization of all cortical columns in the visual cortex. It gives an overview of what regions are responsive to what visual stimuli. Cortical columns can either be color selective or non-selective, and each will be organized into a column of their own. For example, if there is a cortical column that is color selective for green, then it will be surrounded by other cortical columns that are selective for green, creating a blob. Likewise, non-selective columns will also cluster together. The mapping of these blobs displays a pinwheel formation, in which several distinct orientation clusters will surround a singularity at the center of the pinwheel. This singularity contains neurons that are responsive to a variety of visual orientations. These blobs are centered on ocular dominance columns.

Ocular dominance columns (ODC) are columns that respond selectively to either the left or the right eye. These are organized into vertical stripes in V1. The highest segregation in responsiveness between the left and right eye is located in Layer 4 of the cortex, where V1 receives input from the thalamus. The blobs that are located on the ODC will respond only to the eye that it corresponds to – for example, a blob located on a left eye ODC will only respond to orientations of light that come in from the left eye. ODCs are not orientation selective, only monocularly selective. These columns pass through the blobs at a right angle, while perpendicular to the surface of the V1.

Hypercolumns are 1mm blocks in the V1 that contain a full set of values for any given receptive field, which include both the ocular dominance columns and the cortical columns..

For more in depth explanations of the topics, check this link out