| ISLE | Chapter 4
| Simple Cells
Simple Cells are V1 neurons that respond to stimuli with particular orientations to objects within their receptive field. Like cells in the lateral geniculate nucleus (LGN), they have clear excitatory and inhibitory regions. But unlike the LGN cells, they have orientation selectivity rather than center-surround visual fields. Hubel and Wiesel (1959) found that elongated stimuli that looked like bars seemed to be particularly effective stimuli for these cells. Indeed, they found that some cells wanted a dark bar on a light background and others that responded to a white bar on a dark background. The bars may also occur at varying angles of orientation, and these vary in a predictable pattern. This selective firing rate to the orientation shows the selectivity of the cell to orientation.
The preferred orientation of a simple cell is the stimulus orientation that produces the strongest response from the simple cell. Experiments using single-cell recordings demonstrate orienting tuning curves for any particular simple cell in V1. These orienting tuning curves are graphs that demonstrate the typical response of a simple cell to stimuli or different orientations. You can create such a curve with this activity. Simple cells respond best to a stimulus with a particular orientation, and then as the orientation gets larger or smaller, the response of the cell decreases. If the orientation is greatly off, the cell will not respond at all. Other cells have different preferred orientations. Thus, V1 indicates the orientation of lines in the visual world by having select cells respond to different angles of orientation.
In this activity, you can simulate recording from a simple cell. Use a dot to map the receptive field and bars to try to determine the optimal stimulus. Then you can rotate the stimulus and even rotate the preferred orientation of the receptive field to explore the characteristics of these cells.
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 single cell you are recording from.
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. The receptive field of the
cell you are recording from is responding to some part of this screen.
Click on or touch the screen to move the dot around, and see how the cell responds by looking at
the graph
to the right.
Stimulus Type: choose the stimulus you are using. You can chose
a Dot, a White Bar, or a Black Bar.
If the stimulus is either a white or black bar, you can change the following two features of the bar:
Tilt: change the orientation of the bar.
Bar Width: change how wide the bar is. Try making the bar wide enough to
fill the entire screen.
The adding of marks using the controls mentioned below only works when the stimulus is a dot:
Excite Resp (+): if the firing rate goes above the horizontal blue line, the
cell is said to be excited. You can place a green plus mark at that location by pressing this button.
Inhib Resp (-): if the firing rate goes below the horizontal blue line, the
cell is said to be inhibited. You can place a red minus sign at that location by pressing this button.
Remove Last Mark: press to remove the last mark, and then sequentially the
previous marks, if you added any marks by mistake.
Show Cell: after you have placed enough marks on the screen, you can show
the shape of this receptive field. The green shape is the excitatory area, and the red shape
indicates the inhibitory area.
When the receptive field is visible, the following two controls work:
Increase Angle: rotate the prefered orientation of the receptive
field in a counterclockwise direction.
Decrease Angle: rotate the prefered orientation of the receptive
field in a clockwise direction.
Pressing this button restores the settings to their default values. It also gets you a new cell, which might have a different receptive field.