Frog Accessory Optic System

Frogs need to stabilize their gaze relative to their visual background in order to detect moving small prey objects. Frogs may be on some floating or partly floating object in which the visual background will slowly shift and they need to determine if some object is really moving or if it is just an artifact of their own motion.

This gaze stabilization process is known as optokinetic nystagmus. This is usually tested by placing the frog in the center of a drum having vertically painted stripes. Normally the frog turns its head and eyes to match the motion of the stripes (slow phase). After a head turn of 10 to 15 degrees the head is snapped back towards the forward position (fast phase). Frogs are often tested in a drum 25 cm (one foot) in diameter with 2 cm wide stripes which is rotated at 8 degrees per second even though this small drum produces escape behavior in some frogs (Lazar - 1973).

The Accessory Optic Tract Region is Responsible for Optokinetic Nystagmus

As shown in figure 1 several brain regions receive inputs direct from the retina. Lesion tests by Lazar (1973) localized optokinetic nystagmus to the accessory optic system also called the basal optic complex. The accessory optic tract separates from from the main optic tract at the optic chiasm and travels caudally (tailward) to its nucleus called the basal optic root or accessory optic nucleus.

Figure 2 shows the actual lesioning done by Lazar. Row 1 - removal of one eye shows that optokinetic nystagmus detection is directional with the direction being from the side towards the nose. Row 2 - no nystagmus with a cut optic chiasm. Row 3, 5, 6 - after tectal and pretectal lesions the frog occasionally circled around in the drum following the stripes before stopping and allowing nystagmus to commence, Row 7 - destruction of the accessory optic nucleus (basal optic nucleus) abolishes the nystagmus but larger lesions also affect balance reflexes such that the frog did not compensate for unexpected externally applied motions and the head was chronically tilted to the side of the lesion. Row 13 - damage to the tectum and basal optic nucleus neuropil (the dendrites from the cell bodies located in the nucleus) also prevented nystagmus

Figure 1
Centers in the Frog Brain Which Get Inputs from the Retina.
1 - nucleus of Bellonci, 2 - lateral geniculate body, 3 - pretectal area, 4 - tectum, 5 - basal optic root, 6 - nucleus of the basal optic root. (Lazar - 1973)

Figure 2
Effects of Various Lesions on Optokinetic Nystagmus. (Lazar - 1973)

The basal optic complex receives most of its inputs from the contralateral retina (Montgomery, Fite, and Bengston - 1981). Yet it also receive inputs from some thalamic and tegmental regions as shown in figure 3 where the large dots represent the neural cell bodies which send projections to the boc (basal optic complex). The small dots represent fibers from the basal optic complex.Significantly, fibers from the basal optic complex project down to the spinal cord where they presumably trigger the neck muscles used for turning the head.

Figure 3
Non-Retinal Inputs to the Basal Optic Complex in the Frog *Rana pipiens*.
dv - dorsal ventrolateral thalamic nucleus, gc - tegmental griseum centrale, pt - posterior thalamic nucleus. Numbers correspond to brain slices shown in upper left corner. (Gruberg and Grasse - 1984)

Not surprisingly most of the neurons in the basal optic nucleus (root or complex) are direction sensitive with only a small number (9 of 51) being non-directional. These non-directional neurons are driven either by changes in illumination or by motion in any direction. Of the directional units approximately half are sensitive to motion in the horizontal direction and half are sensitive to motion in the vertical direction. The directional units can be further subdivided by their specific directional sensitivity. The horizontal neurons are sensitive to motion in either the leftward or rightward directions just as the vertical neurons are sensitive to motion in either the upward or downward directions (Gruberg and Grasse - 1984).

Figure 4 shows the response from one such directional neuron. This neural cell is sensitive to rightward motion in the horizontal direction with the length of each arrow being proportional to the firing rate. A striped pattern (8 degree period) was moved at a rate of 1 degree per second randomly in twelve directions with a half second between stimuli. The radius of the circle shows the average spontaneous firing rate.

The visual receptive fields in the basal optic nucleus range in size from 10 to 60 degrees and respond to moving stimuli in the range of 0.2 to 10 degrees per second (Gruberg and Grasse - 1984).

Figure 4
Many Neurons in the Basal Optic Nucleus are Sensitive to Direction from the frog *Rana pipiens (Gruberg and Grasse - 1984)*

The directional neurons are spontaneously active and have a wide range of responses to ambient lighting. Some respond when lights are turned on while others respond when the lights are turned off. Others respond to both the “on” and “off” conditions. This illumination level response also varies such that it could consist of a simple rate reduction or a brief burst of increased intensity followed by the rate reduction.

Figure 5 shows the responses of two of these neurons under different conditions. Left column (A) gives the response of the vertical motion neuron while the right column gives the response of the horizontal motion neuron. Top - light turned off then on, Middle Left - as indicated by the arrows a 10 degree black disk is moved up then down then up again. Middle Right - as indicated by the arrows the disk moved from the nose to the side (nasotemporal direction), then back, then to the side again. Bottom - activity in the absence of stimulus.(Gruberg and Grasse - 1984).

Figure 5
The Responses of Representative Directional Neurons from the Frog *Rana pipiens*.(Gruberg and Grasse - 1984)

The dashed line circle with the arrow pointing at it in top view of Figure 6 shows the location of the horizontal neuron of figure 5 which was lesioned by the electrode after it was recorded while the bottom view shows the location of the vertical neuron (arrow).

Figure 6
The Locations of the Neurons Shown in Figure 4..(Gruberg and Grasse - 1984)

While the basal optic nucleus is just caudal (tailward) of the hypothalamus it is not considered part of the hypothalamus proper. Yet its spontaneously active neurons make it very closely related since spontaneously active neurons generally represent some long term bias or baseline motivation level. What this means in terms of optokinetic nystagmus remains to be determined.

For some models of optokinetic nystagmus see Anastasio (1996 and 1997).

References

Anastasio, T.J. (1996). Random Walk Model of Fast-Phase Timing During Optokinetic Nystagmus. Biological Cybernetics 75:1-9

Anastasio, T.J. (1997). A Burst-Feedback Model of Fast-Phase Burst Generation During Nystagmus. Biological Cybernetics 76:139-152

Gruberg, E.R. and Grasse, K.L. (1984). Basal Optic Complex in the Frog (Rana pipiens); a Physiological and HRP Study. Journal of Neurophysiology 51:998-1010

Lazar, G. (1973). The Role of the Accessory Optic System in the Optokinetic Nystagmus of the Frog. Brain, Behavior, and Evolution 5:443-460

Montgomery, N., Fite, K.V., and Bengston, L. (1981) The Accessory Optic System of Rana pipiens: Neuroanatomical Connections and Intrinsic Organization. Journal of Comparative Neurobiology 203:595-612

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