The Rabbit Retina

It has become clear from the foregoing sections that the vertebrate retina is more than a simple transducer that maps local light intensity on the retina to impulse frequencies on the optic nerve fibers. Clearly, individual GC fibers signal the presence of certain specific, spatiotemporal features of the retinal image, thus performing a kind of pattern recognition preprocessing, which is sent to the visual portions of the CNS. Understanding of these operations has been greatly enhanced by recording from single rabbit GC fibers done by Barlow and Hill (1963), Levick (1967), and Oyster and Barlow (1967).

Levick (1967) reported a total of eight different types of GC responses ("trigger features") found in the fovea (visual streak) of the rabbit's retina. Five of the types of GC responses found in the peripheral retina were also found in the central region; however, there they had smaller, oval RFs. In addition, three types of GC responses were found only in the fovea (6, 7, and 8 below).

The eight GC types were (1 and 2) concentric RF (ON-center and OFF-center subclasses); (3) large-field; (4 and 5) direction-sensitive to image motion (ON/OFF and ON subclasses); (6) orientation-selective (horizontal and vertical subclasses); (7) local edge detectors; (8) uniformity detectors (UDs). Complex tests using both projected beams of light and contrasting objects were used to try to clarify the unique properties of each class of rabbit GC. Descriptions of GC types 7 and 8, unique to the visual streak, follow.

The local edge detectors (LEDs) of the rabbit visual streak had RFs from 0.5° to 2° diameter. They responded to ON and OFF of a spot of light anywhere in the RF. About one third of these units exhibited true directional sensitivity of the image, having a preferred direction. The response characteristic of the streak LEDs was little affected by the background illumination level over a range of 0.08 to 300 cd/m2. If a small (< 1° diameter) dark spot was moved into the LED unit's RF center and stopped, the unit fired a burst. It fired a burst again when the spot was moved out of the RF in any direction. If the spot was moved through the RF at a constant speed (~3°/s) in any direction, there was little response. A small spot of light flashed in the center of the RF gave both ON and OFF bursts. A larger spot of light covering the entire RF flashed on and off gave negligible response. If a black-on-white, square-wave grating was moved over the entire RF at 1°/s, there was negligible response. If the surround was masked off, and the grating was moved at the same speed, there was a continuous brisk response, maximum for grating periods of 0.5° to 1°. The rabbit local edge detector is probably similar to the frog's net convexity detector GC, according to Levick (1967).

The UDs in the rabbit's visual streak constituted only 4 out of 154 visual streak GCs studied by Levick. However, they had consistent, curious properties. Their RFs were about 4° in diameter. Under background illumination of 7 cd/m2, the unit fired continuously at 10 to 20 spikes/Sec. Firing could be interrupted during (1) a flash of the entire field (to 50 cd/m2); (2) a flash of a spot over the RF; (3) a flash of an annulus surrounding the RF; (4) moving a white disk object into the RF; (5) moving a black disk object into the RF; (6) moving a square-wave grating over the field. No stimulus could increase the rate of firing. In some UDs, firing suppression was transient at the onset of the stimulus, then increased again.

Oyster and Barlow (1967) measured the preferred directions of 102 DS GC units from the rabbit's retina; 79 of the "on-off' subtype and 23 of the "on" subtype were examined. The distribution of preferred directions for both types of DS GCs were tightly clustered about certain axes. The on-off type units had four preferred directions clustered around the four principal axes of the eye; anterior, posterior, superior, and inferior. The means of the vector directions were all rotated clockwise a small amount from these principal axes, as shown in Figure 6.3-1A (Oyster and Barlow, 1967). The "on" type DS GCs, had their preferred directions clustered around three major axes, approximately 120° apart (see Figure 6.3-1B). If superior is 0°, anterior is 90°, and inferior is 180°, etc., the preferred directions are approximately at 110°, 205°, and 345°. Oyster and Barlow speculated that from the preferred directions of the discrete grouping of the two classes of DS GC units, they may be the sensors for a dynamic image stabilizing motor system in which they send signals to the CNS, which in turn activates motor neurons controlling specific extraocular muscles that move the eyeball so that the DS GC unit outputs are nulled.

FIGURE 6.3-1 (A) Approximate distribution of preferred directions from ON/OFF type DS units from the rabbit's optic nerve. The four major lobes are spaced about 90° apart. (B) Approximate distribution of preferred directions from ON-type DS units from the rabbit's optic nerve. The three lobes are spaced about 120° apart. (Based on Oyster and Barlow's, 1967, data.)

DS GC units have been found in just about all vertebrate retinas studied: amphibians, birds, reptiles, fish, and some mammals, but they are rare in cat and primate retinas (Kolb et al., 1999). As seen in the preceding section, DS units have also evolved in the optic lobes of arthropods' compound eyes. Since insect compound eyes are fixed on their heads, insect DS units are also probably involved with flight stabilization and prey capture.

Rabbit Nerve Anatomy
Superior
Rabbit Nerve Anatomy

FIGURE 6.3-1 (A) Approximate distribution of preferred directions from ON/OFF type DS units from the rabbit's optic nerve. The four major lobes are spaced about 90° apart. (B) Approximate distribution of preferred directions from ON-type DS units from the rabbit's optic nerve. The three lobes are spaced about 120° apart. (Based on Oyster and Barlow's, 1967, data.)

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