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Notes: Summary 1. Most movement fibers in the crayfish optic nerve show no response during the regular flickering of a stationary light with a flash duration of less than 50 msec when the flash frequency is between 4 and 20 Hz (“habituation”), whereas they do respond with short “off” burst when the flickering stops after a certain number of flashes (“dishabituation”, Fig. 2). 2. The dishabituated response to cessation of regular flicker is not a normal “off” response because the discharge appears about 50 ms after the “missing flash” which would have occurred (Fig. 2). This fact indicates that the dishabituation to cessation of the flicker is entrained to the periodicity of the flicker and locked to the time of the first missing flash (Fig. 3). The features of this “entrained response” are not affected by differences in light intensity or flash duration of the flicker train used for entrainment (Fig. 4). 3. The timing of the entrained response can be shifted by a test flash following the entraining flicker. The entrained response advances when the test flash is given earlier than the expected flash, whereas it lags when the flash is given later. The amount of shift in the response timing is equal to the change in temporal position of the test flash (Fig. 5). The response also advances for a test flash of lower light value (light intensity × flash duration) than that of the entraining flicker, whereas it lags for a flash of higher light value. The amount of shift is proportional to the logarithm of the light value (Figs. 6, 7). 4. It is proposed that the habituation during regular flicker and dishabituation, that is, the entrained response evoked by cessation of the flicker, are both essentially based upon a copying mechanism of the incoming light pattern and a comparison mechanism between the copy and a newly incoming flash. These mechanisms are part of the circuitry of movement fibers (Fig. 11). The accuracy of the periodicity of the entrained activity is better than 5% of the original. 5. The movement fibers also show habituation and dishabituation to stimulus flicker with double periodicities consisting of paired flashes, so that the copying circuit can extract and store more than one periodicity at the same time (Figs. 9, 10).

Notes: Summary 1. Five pairs of nonspiking giant interneurons (NGIs) were identified morphologically and functionally in the protocerebrum of the crayfish (Procambarus clarkii) brain: G1, G2, and G3 are contained in one cluster, G4 and G5 are separate. 2. None of the NGIs has a demonstrable axon, but all have thick and long dendritic processes. The dendritic processes extend across the midline of the brain and spread fine branches out over the ipsi- and contralateral halves of the protocerebrum (Figs. 2–6, 11). The dendritic processes of Gl, G2, and G3 are characteristically intertwined to form a cluster just above the protocerebral bridge (Figs. 4–6). 3. The primary neurite of each NGI links its dendritic process through the optic tract to its pear-shaped soma in the distal end of the optic tract (Figs. 2, 4). 4. Selectively stained branches of an NGI and a sustaining fiber (SF) appear to have contact points between them within and around the cluster of G1, G2, and G3 at the light microscope level (Fig-7). 5. Illumination of the contra- and ipsilateral eyes, respectively, elicits depolarizing and hyperpolarizing potentials without action potentials in each of G1, G2, and G3 (Fig. 8). The amplitude of these potentials depends on the intensity of illumination and on the area of the eye that is illuminated. Larger potentials are evoked when the posterior part of either eye is illuminated (Fig. 9). Depolarizing the NGIs (G1, G2, G3) results in an increase of the ipsilateral response and a decrease of the contralateral response. Hyperpolarizing the NGIs has the opposite effect (Fig. 8). 6. Depolarizing potentials in G1, G2, and G3 are preceded by action potentials in the contralateral SFs with receptive fields in the posterior part of the eye. Hyperpolarizing potentials in these cells are preceded by action potentials in homologous ipsilateral SFs (Fig. 10). 7. Illumination of the anterior part of either eye strongly depolarizes G4. Illumination of the posterior part of either eye slightly hyperpolarizes it. G5 is hyperpolarized on illumination of the posterior part of the ipsilateral eye (Fig. 11). Neither cell supports action potentials. 8. The structural and functional implications of the nonspiking giant interneurons are discussed, in relation to the motor neurons which mediate compensatory eye-stalk movements in response to visual and geotactic stimuli.