The Pigeon Retina

Feature extraction is also performed by the pigeon's retina. Maturana (1974) reported on pigeon GC responses. Recall that the frog is a sessile animal; it normally sits

FIGURE 6.2-3 Response sensitivity for a DS unit located in the frog's superficial, contralateral tectum to a 2°-diameter black spot moving at different velocities through its RF. The total response to eight back-and-forth movements of the spot is plotted for each of the four velocities used. Dotted line, 0.32°/s; small dashed line, 1.1°/s; solid line, 1.85°/s; long dashed line, 4°/s. Note that there is an optimum velocity; DS response falls off for 4°/s. (Redrawn from Reddy, 1977.)

FIGURE 6.2-3 Response sensitivity for a DS unit located in the frog's superficial, contralateral tectum to a 2°-diameter black spot moving at different velocities through its RF. The total response to eight back-and-forth movements of the spot is plotted for each of the four velocities used. Dotted line, 0.32°/s; small dashed line, 1.1°/s; solid line, 1.85°/s; long dashed line, 4°/s. Note that there is an optimum velocity; DS response falls off for 4°/s. (Redrawn from Reddy, 1977.)

and waits for prey to come to it. Pigeons fly, and any flying animal has additional burdens for its visual system. It must stabilize its flight (roll, pitch, and yaw), and it must avoid fixed and moving obstacles such as trees and other birds. Maturana reported six classes of pigeon optic nerve fiber:

1. Verticality detectors have RFs < 1°. They fire for stationary vertical edges in the RF, or for vertical edges moving through the RF. Rotation of edge 20° to 30° from vertical stops the response.

2. Moving horizontal edge detectors have RFs ~1/2° in diameter. To fire, they require vertical motion of a long, contrasting, horizontal boundary. Presumably they fire for downward motion, as well. Tilting the edge ±20° to 30° from horizontal suppresses the response. In order for a response to occur, the horizontal edge must be long enough to overlap the edges of the RF.

FIGURE 6.2-4 The sharply directional response of a cerebellar DS unit to 7°-diameter black (dashed lines) or white (solid lines) spots. (Redrawn from Reddy, 1977.)

3. General edge detectors (2 kinds): 3a has a large RF, 2° to 3° diameters; 3b has a small RF, < 1/2° diameter. Common properties:

• May respond to ON or OFF of general illumination with one or two spikes.

• Respond strongly with six to ten spikes to an edge moving across their RFs.

• Respond to a spot of light turned on or off or moved across their RF.

• Their RFs are uniformly of the ON/OFF type. Differential properties:

• Ganglion cells with large RFs do not "see" the details of a drawing (test object background), but they are able to respond to objects a few minutes in diameter that move within their RF against any background.

• Ganglion cells with small RFs can "see" background detail as well as small objects moving independently to the background.

4. Directional moving edge detectors have 0.5° RFs to 1° diameter, and the unit responds to ON/OFF of a small spot of light over the RF. These units respond preferentially to a contrasting edge moved through the RF in a preferred direction. There is little or no response to edge movement in d d

FIGURE 6.2-5 The distribution of preferred directions of 32 DS units recorded from the frog's brain. The numbers on the vectors indicate the number of DS units found with that particular preferred direction. (Test axes were at every 22.5°). Note that in the posterior hemicircle there are 22 out of 32 DS units. (Redrawn from Reddy, 1977.)

FIGURE 6.2-5 The distribution of preferred directions of 32 DS units recorded from the frog's brain. The numbers on the vectors indicate the number of DS units found with that particular preferred direction. (Test axes were at every 22.5°). Note that in the posterior hemicircle there are 22 out of 32 DS units. (Redrawn from Reddy, 1977.)

the anti-preferred direction, and response falls off as the direction of edge motion departs from the preferred direction, although no quantitative data was given.

5. Convex edge detectors have very small RFs, a few minutes of angle in diameter. They do not respond to moving straight edges, only to moving spots. There does not appear to be directional specificity.

6. Luminosity Detectors LDs fire at a rate proportional to the general illumination level. (Maturana gave no data on RFs for LDs.)

Was this article helpful?

0 0
Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment