Table 2.4 Command neuron hypothesis.
At first glance the giant axon of giant lateral interneuron in crayfish, controlling the escape tail-flip in response to mechanosensory stimulation, serves as a CN (Wiersma & Ikeda 1964). The idea that a neuron triggers a behavior was challenging and sparked intense debate among neuroethologists. Kupfermann and Weiss (1978) pointed out that a CN must fulfill two conditions: its excitation is not only necessary but also sufficient to activate the behavior. This strong definition can be examined: electrically exciting that neuron should be sufficient to elicit the corresponding behavior; removing it should abolish the behavioral response to the peripheral stimulus. For experimental studies, the best candidate of a CN is the reticulospinal Mauthner cell in teleost fish that, in response to vibratory stimulation, triggers the fast-body-bend escape reaction. However, quantitative investigations showed that a Mauthner cell did not fulfill the “double-condition” (Eaton 2001); a command neuron that does it, convincingly, remains to be discovered. If several neurons are involved in a command function, we use the term “command system.”
In fact, a releasing system—relying on attention and motivation—may take advantage of adequate receptor cells and assemblies of feature-sensitive/selective interneurons. This system translates the information of a sign-stimulus into a command, which is appropriate to activate the corresponding motor system (Figure 2.2).
Scent-coding by specialized receptor cells in insects
Scent perception in many insects involves receptor cells best tuned to chemical sign-stimuli. In the silk moth, Bombyx mori, scent hairs on the male’s antennae are equipped with “olfactory specialist receptor cells.” These respond maximally to a female’s key pheromone bombykol, as mentioned above (Figures 2.6 and 2.7a). A bombykol receptor is also somewhat sensitive to chemically related compounds, however, at 10- to 1000-fold higher concentrations (Kaissling 2014).
Figure 2.6 Male silk moth in alerted position, with combed antennae elevated (top). (Courtesy of R.A. Steinbrecht.) Bottom: schematic section through a scent hair with pore tubuli and two scent receptor cells. Arrangement for recording impulses from bombykol receptor. (Modified after R.A. Steinbrecht).
Figure 2.7 Principles of scent detection in moths by specialist receptor cells and interneurons. (a) In male silk moths a receptor channel is specialized for female’s sex pheromone bombykol. (b) In male nun moths a receptor channel is specialized for the sex pheromone (+)-disparlure. (c) In male gypsy moths two types of receptor channel are specialized for two pheromone compounds: (+)-disparlure stimulates interneurons, whereas (−)-disparlure inhibits their response to (+)-disparlure. (d) In male leaf-roller moths the concurrent excitatory influences of the two receptor channels specialized for the two sex pheromone stereoisomers (Z)-11-tetradecenyl-acetate and (E)-11-tetradecenyl-acetate are essential to activate interneurons. (Compiled from data in Hansen 1984; Kaissling 2014.) Note that a behavioral response requires activation of many specialist receptor cells and corresponding interneurons.
Other examples of such narrow-band olfactory specialists are the meat receptor in Necrophorus beetles, the rotten-meat receptor in blow-flies Calliphora erythrocephala, and the grass receptor in the locust Locusta migratoria. Grass receptors respond to chemically related components of fresh grass, such as hexenol, hexenal, and hexenic acid (Kafka 1970).
In leaf-roller moth species, as mentioned above, the males have two types of specialized receptor cells, each one tuned to a different pheromone, both emitted by the conspecific female in a characteristic proportion (e.g., Figure 2.7d), which minimizes the risk of mating with males of inadequate species.
Another way of species separation are interspecific inhibitors. Females of the nun moth, Lymantria monacha, and the gypsy moth, L. dispar, produce the male-attracting compound (+)-disparlure (Figure 2.7b, c). However, the female nun moth also produces (−)-disparlure, which stimulates a specialist receptor cell in the male gypsy moth to inhibit its behavioral response to (+)-disparlure (Figure 2.7c) (Hansen 1984). This kind of species separation would function in one direction, since a male nun moth could be attracted by (+)-disparlure of the female nun or gypsy moth. Presumably, female gypsy moths are less attractive to male nun moths because of the high emission concentrations of (+)-disparlure (Figure 2.7c ).
In addition to specialist receptors there are “generalist receptor cells.” These—showing partly overlapping response spectra—respond differently to various odor compounds. Such cells may be suitable to distinguish odorous substances by individual experience.
Visual feature detection in amphibians: a multimethodological analysis
Summertime in a forest, evening is falling, twilight—a toad Bufo bufo sits in front of its shelter. Suddenly, a moving milliped shows up. The toad orients its head and body toward it, watchfully creeps up, fixates it binocularly, and snaps. In the catching sequence, the releasing system of each behavioral component decides “what is it?” The releasing systems differ in determining “where is it?” and, in case of prey, “how can it be caught?”, e.g., by stalking or pouncing, tongue-flipping, or jaw-grasping. Key factors of these releasing systems involve prey-selective neurons in the midbrain optic tectum. How was this discovered?
Toward a features-relating-algorithm as a principle in configurational perceptio n
How does prey look to toads? Using dummies, an experimental procedure enables one to measure a toad’s prey-catching activity in response to the visual feature in question and to change it, while other stimulus parameters are held constant. This allows one to evaluate the effect of this change on prey-category formation (Ewert 1974, 1984; see also Further Reading, Movie A2).
Since toads hunt moving objects, it makes sense to change a visual variable related to movement. Figure 2.8A shows that stepwise extending a moving rectangular black stripe against white background parallel to the movement direction raises the prey value within limits, e.g., resembling woodlice, millipedes, or worms (chart p). Extending a stripe crosswise to the movement direction progressively reduces the prey value and signals threat (chart c). Toward squares of different sizes, the influences of both features p and c interact, thus yielding a preferred square of approximately 10 mm edge length, e.g., simulating bugs (chart s). A large moving square of about 100 mm edge length, like a shadow from an airborne predator, elicits escape.
Figure 2.8 Prey-feature analysis in common toads. The configurational paradigm involves three types of moving rectangular black stimuli: stripes extended parallel (p) or crosswise (c) to the movement direction, and squares (s) of different edge lengths. A) Prey-catching activity, RB (mean values, n = 15 toads (Ewert 1984). B) Activity, RN, of prey-selective tectal T5.2 neurons (mean values, n = 18 neurons) in awake toads being pharmacologically immobilized (required for stable neuron recordings through the entire stimulus program within a perimeter device); deg: degrees of visual angle (Wietersheim v. & Ewert 1978; cit. Ewert 1984.).
In reminiscence of the term key-stimulus, the “key” is inherent in an algorithm—calculation