Receptive Fields
A receptive field refers to the portion of the visual field to which a neuron responds or is receptive. To understand this concept, you need some background about what types of cells exist in the visual system.
The retina contains millions of cells that are sensitive to light. One type is called the center-surround cell. It has two receptive fields. One is circular and located in the center, while the other is a surrounding ring-shaped field. Each receptive field is either an “ON” field or an “OFF” field. The “ON” receptive field produces an increase in neural activity (i.e., the cell fires action potentials) when light is present, while “OFF” receptive fields inhibit neural activity (i.e., action potential firing is reduced) when light is present. Some cells are “ON-center OFF-surround” and produce maximum neural activity when light hits the center, and no light reaches the surround. But when “ON” area and “OFF” area both stimulated by light, the resulting net output is little to no change in neural activity. When neither area receives light, there is also little to no change in neural activity. These properties combined make these cells good at detecting edges. Anywhere that you see an edge of an object there is usually a change in light level. Center-surround cells respond the most in areas of the visual field where there is contrast and a sharp difference in the amount of light.
“ON-center OFF-surround” Cell
In the first part of this lab activity, you will explore how neural activity changes as a function of cell type. In the first part, you see maximal neural activity (i.e., increased action potentials indicated by fast moving black vertical lines) when you click on an “ON-center OFF-surround” cell in which the center is indicated by a white circle (i.e., light hits the center) while the surround is black indicating no light hitting the surround. When you click on a receptive field with the opposite pattern, black center indicating no light and white surround indicating light hitting the surrounding area you will see that this cell returns to baseline firing. Cells with all white (light hitting center and surround) and cells with all black (no light hitting center or surround) also result in baseline firing (i.e., reduced action potentials indicated by slow moving black vertical lines).
“OFF-center ON-surround” Cell
In the second part of this activity, you see a demonstration of an “OFF-center ON-surround” cell in which maximum neural activity is induced when no light hits the center but light does hit the surround. Reduced baseline activity is obtained in three conditions: when no light hits the center or surround, when light hits both center and surround, and when light hits the center and no light hits the surround.
Summary
The stimulus in this lab activity is light. The variable being manipulated is where the light falls in a receptive field, an area of the visual field that causes a neuron to increase its rate of action potential firing. The light stimulus that causes the most firing in an “ON-center OFF-surround” cell is light falling in the center with no light falling on the surround. All other patterns induce baseline or reduced neural activity. In an “OFF-center ON-surround” cell, the stimulus producing the most firing will be when light falls in the surround and no light falls in the center. All other patterns will result in lower baseline neural activity.