E-Letter responses to:

reports: Stanley Heinze and Uwe Homberg Maplike Representation of Celestial E-Vector Orientations in the Brain of an Insect Science 2007; 315: 995-997 [Abstract] [Full text] [PDF]

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Published E-Letter responses:

Response to Talbot H. Waterman

Stanley Heinze, Uwe Homberg   (21 June 2007)

Animal Navigation and Polarization Patterns of Scattered Directional Sunlight in Nature

Talbot H. Waterman   (21 June 2007)

Response to Talbot H. Waterman 21 June 2007
Stanley Heinze Animal Physiology, Department of Biology, Philipps University, 35032 Marburg, Germany, Uwe Homberg Respond to this E-Letter: Re: Response to Talbot H. Waterman	Dr. Waterman raises the question of whether the polarotopy found in the central complex of the locust should be referred to as a compass rather than a map. When talking about a maplike representation of E-vector orientations, we did not refer to a map in the sense of a two-dimensional representation of locations, but to the concept of computational maps that is abundantly used in vertebrate systems (1). Like in the visual cortex of monkeys or in the tectum of barn owls, a specific feature of the sensory environment (edge orientation; interaural time differences; or, in our case, orientation of E-vectors) is topographically represented, i.e., mapped, within a specific brain area. We are aware that the systematic representation of E-vectors could be used as an internal compass, if issues like time compensation are solved by the animal, as discussed in our Report. The second question addresses the possibility of artifacts caused by using a rotating polarizer for stimulation of the neurons. Although the rate of E-vector rotation we used (20° per second) is much higher than changes of E-vectors in the sky over time due to the apparent movement of the sun, this is not the case as soon as the animal itself rotates around its vertical body axis. We propose that it is during these situations, i.e., when flight directions are changed actively or course deviations have to be compensated due to perturbations, when compass information is most needed by the animal. However, the proposed compass should also work during straight flight without rotating E-vectors. It has been shown by H. Vitzthum et al. (2) in recordings from central-complex neurons, including CPU1 neurons, that a sequence of stationary E-vectors results in the same E-vector tuning curve as a rotating stimulus. Therefore, a movement-specific component in the response is unlikely. The advantage of using rotating instead of stationary E-vectors lies in the higher angular resolution for determination of the preferred E-vector orientation, since more orientations can be tested during the recording. The third question: "Why are locust E-vector sensitive neurons receiving just zenith sky information?" is easy to answer. They don’t. These neurons do not just receive zenithal information. We used zenithal E-vectors as stimuli in the recordings due to the simple relation these E-vectors have with the solar azimuth (independent of the sun’s elevation). Nevertheless, Dr. Waterman is completely right when pointing out the problems of low or even undetectable degrees of polarization in the zenith during midday. However, the involved neurons have very large zenith-centered receptive fields, extending as far as 60° to either side of the solar meridian (3). This eliminates the problem of low degrees of polarization, but raises the question of how preferred E-vector orientations are distributed within the neurons’ receptive field. In the blue sky, the simple 90° relation between E-vector orientation and solar azimuth does not apply to eccentric parts of the receptive fields of these cells. This issue is currently being addressed in our lab and further recordings should show if and how the receptive fields of these POL-neurons are adapted to the E-vector distribution in the sky. Stanley Heinze and Uwe Homberg Animal Physiology, Department of Biology, Philipps University, 35032 Marburg, Germany. References 1. E. I. Knudsen, S. du Lac, S. Esterly, Annu. Rev. Neurosci. 10, 41 (1987). 2. H. Vitzthum, M. Müller, U. Homberg, J. Neurosci. 22, 1114 (2002). 3. U. Homberg, Naturwisssenschaften 91, 199 (2004).
Animal Navigation and Polarization Patterns of Scattered Directional Sunlight in Nature 21 June 2007
Talbot H. Waterman MCD Biology, P.O Box 280103, Yale University, 219 Prospect Street, New Haven, CT 06520-8103, USA Respond to this E-Letter: Re: Animal Navigation and Polarization Patterns of Scattered Directional Sunlight in Nature	S. Heinze and U. Homberg (1), reporting on desert locust central neuron responses to polarized light (16 Feb., p. 995), like Fenton’s recent notable account (2) of navigational neurons within rats’ hippocampus, provide prime evidence for the remarkable progress currently achieved in understanding animals’ navigation systems. Since about 2000, such progress, with some exceptions (3, 4), has been limited to terrestrial animals (5). Biologists’ subsequent research on sun-induced in situ underwater polarization patterns and on aquatic animal visual navigation has lagged or been handicapped by misunderstandings and errors concerning underwater optics (6). Closer to the point, Heinze and Homberg’s article (1), however welcome, does pose several questions. First, why does it refer both in title and text to maps rather than a compass? Maps usually represent relations between places and locations (e.g., 7), whereas the locust neurons reported sense directions, the traditional domain of a compass. Indeed, “place neurons” in maplike arrays were identified in the rat brain (2). A second question about the Report (1) is: Why is an E-vector continuously rotating much faster than the sky pattern used as stimulus? E-vector movement itself may affect the locust neuron responses reported. Other evidence from crustaceans shows that flashing, fixed, and moving E-vectors activate single optic neurons differently as do particular stimulus intensities (8-11). A third question about the Report (1) is: Why are locust E-vector sensitive neurons receiving just zenith sky information? That sky point is strongly polarized only around dawn and dusk when polarization there exceptionally can reach 85.5% (12). But at low and moderate latitudes, zenith sky polarization around midday is low or zero. In evaluating the Report (1), remember that despite its seeming esoteric nature, animal navigation is an essential component of much of mobile animal behavior. Similarly, the control of locomotion ranging from local foraging (13) to global migrations (14, 15) is a substantial factor in world ecosystems (16). Talbot H. Waterman MCD Biology, Post Office Box 280103, Yale University, 219 Prospect Street, New Haven, CT 06520-8103, USA. References 1. S. Heinze, U. Homberg, Science 315, 995 (2007). 2. A. A. Fenton, Science 315, 947 (2007). 3. S. Kleinlogel, J. N. Marshall, J. Exp. Biol. 209, 4262 (2006). 4. R. M. Glantz, J. P. Schroeter, J. Comp. Physiol. A 193, 371 (2007). 5. G.Horváth, D. Varjú, Polarized Light in Animal Vision (Springer, Berlin, 2006). 6. T. H. Waterman, Biol. Rev. 81, 111 (2006). 7. R. Wehner, S. Wehner, Ethol. Ecol Evol. 2, 27 (1990). 8. T. Yamaguchi, Y. Katagiri, K. Ochi, Biol. J. Okayama Univ. 17, 61 (1976). 9. K. Ochi, T. Yamaguchi, Biol. J. Okayama Univ. 17, 47 (1976). 10. T. H. Waterman, in Identified Neurons and Behavior of Arthropods, G. Hoyle, Ed. (Plenum Press, New York, 1977), pp. 371-385. 11. R. M. Glanz , J. Schroeter, J. Comp. Physiol. A 192, 905 (2006). 12. K. L. Coulson, Polarization and Intensity of Light in the Atmosphere (A. Deepak Publishing, Hampton, VA, 1988). 13. R. Wehner, in Neural Basis of Behavioural Adaptation, K. Schildberger, N. Elsner, Eds. (G. Fischer, Stuttgart, New York , 1994), pp. 103-143. 14. C. S. Baker et al., Nature 344, 238 (1990). 15. T. Alerstam, Science 313, 791 (2006). 16. A. Sugden, E. Pennisi, Eds. Special Section on Migration and Dispersal, Science 313, 775 (2006).