VERTICAL CROSS-SECTION - A computer-aided investigation of motion detection units in the fly

Figure 3. 2 Diagrammatic representation of tentative location of class Ila- in unit [ 79] .

(a) Horizontal cross-section through brain and optic lobes. Both members of a pair of mirror-image Ila-in units are shown. Arrows indicate direction of conduction.

lb) Vertical cross-section showing one Ila-in unit.

the beetle ¹¹walks¹¹ the globe he periodically must choose a right or left branching path. By counting the numbers of times the animal turns with and against the drum directions, a response measure is obtained. By using different combinations of spatial wavelengths and slits through which the moving drum was viewed, Reichardt [ 69] was able to develop a model for motion detection in the beetle in which he demonstrated that at least two ommatidia must take part in motion detection. Although the model accurately predicts certain phenomena found in Chlorophanus, such as the

stroboscopic effect, it is difficult to identify model components with known anatomical elements in the nervous system. The

experimental results of Reichardt's work showed that the

interommatidial angle is quite uniform over the eye and that only adjacent or next-to-adjacent ommatidia interact to produce a response to motion.

For other species, such as the housefly Musca, blowfly Calliphora and fruitfly Drosophila, the situation is not so clear cut. McCann and MacGinitie [ 59] showed that for Musca, due to the relatively wide half sensitivity angle of the ommatidia, the magnitude of the negative reaction (torque opposite to drum

rotation} was from 3 to 6 orders of magnitude less than the

positive reaction. They went on to show how previously reported positive responses to patterns of wavelengths small enough to

produce negative reactions were due to inaccuracies in the patterns

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used. It has also been shown that motion detection is primarily due to adjacent ommatidia for Musca, with little or no interaction between ommatidia more than 2 or 3 spacings apart [ 22], and

more importantly that responses to large field patterns were simple saturable summations of unit responses [ 22, 59]. The same workers found, as did Goetz [ 27] for Drosophila, that the effective sampling station derived from optomotor responses corresponded to the directly measured interommatidial angle.

Goetz also showed that for the best image from a sampling

station with a given light transmission there is an optimum ratio of interommatidial spacing (D 0) to sensitivity angle (a). The existence of this optimum depends on the fact that the total flux through an element, which is proportional to (a . D 0) 2

, must be greater than some threshold to excite neural activity [ 27]. For Drosophila the measured value· of (a/ D 0) was very close to optimal.

In other experiments on Drosophila, Goetz [ 28] considered both yaw torque and thrus·t measures of the optokinetic response.

He showed the two systems were independent and that the torque motion detection system must simultaneously excite and inhibit opposite sides of the motor output system while the thrust system must simultaneously excite or inhibit both sides. This implies at least four contralateral and four ipsilateral connections, each

capable of exciting or inhibiting one side of the motor system [ 28].

Goetz further showed this could be reduced to a minimum model with two contralateral and ipsilateral channels if common use were made of two orthogonal motion detection systems suitably

oriented with respect to the animal's long axis, Similar results were obtained for Musca.

In summary, each retinula cell in an ommatidium looks in a different direction [ 47]. However, each neuro-ommatidium (cartridge) has inputs from retinula cells whose visual axes are parallel [ 10]. This agrees with the optomotor response

experiments of McCann and MacGinitie [ 59], Goetz [ 27], Fermi and Reichardt [ 22], and others who show in essence that the effective sampling station for detection of motion is the ommatidial unit. Further, the information output by the

cartridge is equivalent to that represented by the total average flux entering an ommatidium [ 60].

A number of investigators have also examined the neural response to motion, both at the sub-system level and in more detail at the single cell level. Mc Cann and co-workers [ 60]

measured the electrical response of the locust ventral cord fibers to moving stripes. The response was found to be

essentially similar to the optomotor responses for this species and for Musca.

Single unit activity in response to visual stimuli has long been well established for both vertebrates and invertebrates. In

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the vertebrate retina, ganglion cells have been found which are sensitive to movement of a spot of light in certain directions.

The first demonstration of single units capable of coding optomotor responses in the visual system of Diptera was that of Bishop and Keehn [ 6, 7]. These investigators found units in the optic lobes of Musca, Calliphora, and Eucalliphora that were directionally sensitive and whose behavior as a

function of spatial wavelength of a striped drum, drum velocity, and light intensity was, . with one or two m inor exceptions, the

same as the optomotor response. The directionally sensitive units' response increased from a background rate of 5-15

spikes

I

sec to a maximum of 60-80 spikes

I

sec for motion in the preferred direction and decreased to 0-5 spikes/sec for motion in the null (opposite) direction (figure 3. 3). This is in contrast to the vertebrate motion detection units in which no background firing is observed. In the vertebrate, motion in the preferred direction simply causes a higher discharge rate than motion in the null direction [ 2]. The fact that pairs of oppositely directed motion detection units were occasionally

recorded simultaneously on the same electrode provides some clues as to the structural, and possibly also functional,

organization of the motion detection scheme. Collett and Blest [ 15] also report finding oppositely directed units close together in the optic lobe of the moth.

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Dalam dokumen A computer-aided investigation of motion detection units in the fly (Halaman 50-55)