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Spatial integration in polarization-sensitive interneurones of crickets: a survey of evidence, mechanisms and benefits

Thomas Labhart^*, Jürgen Petzold and Hansruedi Helbling

Zoologisches Institut der Universität, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

View larger version (28K): [in a new window]   Fig. 1. e-vector tuning and visual field properties of POL1-neurones. (A) e-vectors eliciting maximal spike frequency (max) in 142 POL1-neurones stimulated with a zenithal stimulus. In this circular histogram, max is given with respect to the long axis of the head (the straight line connecting 0° and 180°). The radial scale indicates the number of POL1-neurones found for any particular max. Note that there are three types of POL1-neurone tuned to approximately 10°, 60° and 130°. (B–D) Data are mapped on zenith projections of the upper visual hemisphere, i.e. the centre of each graph gives the zenith and the concentric circles represent parallels of latitude. (B) max orientation as a function of stimulus position in the three tuning types of POL1-neurone (upper, middle and lower graphs). The orientation of the bars indicates mean max values at their respective positions on the upper visual hemisphere. The width of the bars shows the number of measurements: thin, N=1; medium, N=2–5; thick, N>5. The numbers of POL1-neurones represented by each graph are 50, 48 and 44 (top to bottom). The max values are plotted with respect to the tangent to the parallel of latitude at the indicated positions (compare Fig.2a in) (Schwind and Horváth, 1993). Thus, max can be read directly from the graphs; specifically, max orientations that are parallel within the visual field appear also parallel in the graph. Note that, for each e-vector type of POL1-neurone, max is quite independent of stimulus position. (C) Sensitivity as a function of stimulus position in the three tuning types of POL1-neurone. The diameter of the filled circles indicates mean relative sensitivity at their respective positions on the upper visual hemisphere (reference sensitivity of 1.0 at position 60°/right). Numbers of measurements at each position range from 1 to 9 (for details, see Petzold, 2001). The numbers of POL1-neurones represented by each graph are 5, 8 and 9 (top to bottom). Data in A-C are from POL1-neurones in the left optic lobe receiving input from the cricket’s left eye. (D) Position and size of the visual fields of POL1-neurones. The visual fields (approximate range of 25% sensitivity; see text) are indicated by the grey areas for the POL1-neurones of both the right and the left optic lobe. r, right; l, left; a, anterior; p, posterior. Compiled from data in Petzold, 2001.

View larger version (40K): [in a new window]   Fig. 2. Anatomical and optical properties of the dorsal rim area of the eye. (A) Light micrograph of a tangential section through the dorsalmost part of the eye. Note the missing screening pigment and the enlarged, trapezoidal rhabdoms in the ommatidia of the dorsal rim area (DRA, top). The much smaller rhabdoms of the unspecialized dorsal ommatidia (DA, bottom) are densely surrounded by screening pigment. Scale bar, 10µm. (B) Three-dimensional representations of photoreceptor visual fields in the DRA (above) and in the unspecialized DA of the eye (below). The x and y directions are parallel and perpendicular, respectively, to the elongated DRA. (C) Comparison between visual field size and interommatidial angle of photoreceptors in the DRA. The T symbols indicate position and orientation of the retinulae within the dorsal rim area (compare enlarged cross-section through an ommatidum at the lower right; cells are numbered according to) (Burghause, 1979). Typically sized visual fields (in grey; acceptance angle 20°) of three adjacent ommatidia (in white) are overlaid on a schematic representation of a cross-section through part of the dorsal rim area. The angular distance between the rhabdoms (interommatidial angle ) is approximately 1°. Note that the visual fields overlap extensively. (A) After Labhart and Petzold, 1993; (B) after Labhart et al., 1984; (C) according to data by Blum and Labhart, 2000.

View larger version (40K): [in a new window]   Fig. 3. Responses of a POL1-neurone (A) and a polarization-sensitive photoreceptor (PS10) of the dorsal rim area (B) to rotating e-vector orientation at very low light intensities. The dark-adapted cells were stimulated with a small-field blue stimulus (2° diameter, 443nm for A, 1° diameter, 440nm for B) positioned in the centre of their visual fields, and the e-vector orientation was rotated forwards and backwards by 360° (see ascending and descending line at the bottom of the graphs). Light intensity is indicated at the upper right of each response trace. Light intensity increases in steps of approximately 1 log unit from the bottom to the top traces. The bottom trace in A shows the threshold response of the POL1-neurone (half-maximal response). Note that the photoreceptor absorbs only approximately 1 photons-1 in a random fashion at the same light intensity (see quantum bumps in bottom trace of B), i.e. no response modulation can be detected. (A) The first 2s of the POL1-neurone traces show spontaneous activity in the dark. (For details of the electrophysiological recording technique, see Blum and Labhart, 2000; Petzold, 2001.)

View larger version (35K): [in a new window]   Fig. 4. (A) Response of a POL1-neurone to selective stimulation of different parts of the dorsal rim area (DRA). Left, stimulus situation; right, response traces. The dorsal part of the eye was illuminated with a slit of blue light (150µm wide, 443nm) stimulating a frontal, middle or posterior section of the dorsal rim area (which is represented schematically as in Fig.2C), and the e-vector orientation was rotated forwards and backwards by 360° (see ascending and descending line below the response traces). Note that the POL1-neurone responds to all three modes of stimulation, showing maximal activity at the same e-vector orientation (see vertical grey lines connecting the response traces). (For details of the electrophysiological recording technique, see Petzold, 2001.) (B) Neural integration in POL1-neurones. Interpretation of both the selective stimulation experiments as exemplified in A and the threshold stimulation experiments shown in Fig.3. (i) All three e-vector types of POL1-neurone (represented by the coloured, double-arrowed circles) receive input from a large number of ommatidia along the whole dorsal rim area. (ii) Each e-vector type receives input from ommatidia of appropriate orientation, as exemplified by the coloured T symbols in each box (compare enlarged cross-section through an ommatidum at the lower right).

View larger version (42K): [in a new window]   Fig. 5. (A) Polarization pattern of the sky for two elevations of the sun (filled circle; left, 55°; right, 5°). In this two-dimensional representation of the polarization pattern, the concentric circles indicate parallels of latitude of the celestial hemisphere with the zenith at the centre. e-vector orientations (indicated by the bars) are plotted with respect to the tangent to the parallel of latitude at the indicated positions (compare Fig.2a in) (Schwind and Horváth, 1993). In this representation of the polarization pattern, e-vector orientations can be read directly from the graph; specifically, e-vectors that appear parallel in the sky also appear parallel in the graph. The degree of polarization is indicated by the length and width of the bars. Note that e-vector orientation in this figure is mapped the same way as max in Fig.1B. (B–D) Responses of an opto-electronic model POL1-neurone to natural skylight polarization. Three examples for different cloud conditions are shown. Left, 180° fisheye photographs indicating actual sky conditions; (B) a loosely cloud-scattered sky, (C) an asymmetrically clouded sky and (D) a heavily clouded sky. The zenith is at the centre (see Z in B), and the horizon is at the circumference of the photographs. The black disks in the photograph are sun screens that shade the photograph and the model neurone from direct sun light. The small disk indicates the position of the sun, and the thin line extending from below the disk gives the symmetry line of the polarization pattern (compare with A). The circle in B indicates the outline of the 60° visual field of the model. The bright 8-shaped area near the antisolar horizon in C is caused by reflections of sunlight in the camera lens. Right, response curves of the model to the skies on the left obtained by scanning the sky through 360°. Ordinate, response in dimensionless logarithmic units (solid line) (see Labhart, 1999); abscissa, orientation of the model with respect to the polarization pattern. Dotted line in D shows the response on a larger scale (right ordinate) Since the model is tuned to horizontal e-vector orientations, the response maxima are indicators of the solar (0°) and the antisolar (180°) azimuth (compare with A). The response curves show that, even under cloudy skies, the maxima deviate only little from the solar or antisolar azimuth. (E) Performance of the model POL1-neurone with a large (60°) and a small (15° or 7.5°) visual field. The histograms indicate the deviation (error) of the antisolar response maximum from the antisolar azimuth under clear (upper graphs) and cloudy (lower graphs) skies. The ordinate indicates the number of occurrences (measurements). Note the inferior directional performance (larger errors) with small visual fields (compare the left and right lower graphs). The effective degree of polarization was 10% for this comparative study. (B-E were compiled from data in Labhart, 1999)