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First published online August 31, 2007
Journal of Experimental Biology 210, 3277-3284 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.008425

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Neural mechanisms underlying target detection in a dragonfly centrifugal neuron

Bart R. H. Geurten^1,*, Karin Nordström^1,, Jordanna D. H. Sprayberry^2,, Douglas M. Bolzon¹ and David C. O'Carroll¹

¹ Discipline of Physiology, School of Molecular and Biomedical Science, The University of Adelaide, SA 5005, Australia
² Department of Zoology, University of Washington, Box 351800, Seattle, WA 98195, USA

View larger version (88K): [in this window] [in a new window]   Fig. 1. Characterization of CSTMD1. (A) Intracellular responses to targets of different sizes illustrate the extreme size selectivity of CSTMD1. While a 0.6x0.6° high-contrast target traversing the receptive field evokes a strong response (top trace), larger targets elicit lower spike frequencies (middle traces), and full screen gratings (bottom trace) give no response above spontaneous firing rates. The targets scanned the center of the receptive field at 26 deg. s–1, as indicated by arrows under each trace. (B) A physiologically recorded (from the left hemisphere) CSTMD1 receptive field shows excitation in the opposite hemisphere only. The false color plot was generated by scanning the entire monitor horizontally with a high-contrast 0.8x0.8° target 21 equidistant locations at 50 deg. s–1. Arrows indicate the strength of the directionality at each location as constructed by drifting targets in four directions across the stimulus display (see Materials and methods). Elevation values are positive above the equator, and azimuths negative to the left of the midline. (C) A reconstructed Lucifer Yellow fill of CSTMD1 shows massive arborizations (black) in the left hemisphere (recording side). As the soma is located in the opposite hemisphere, and the dendrites of this hemisphere are not beaded (inset III) unlike the arborizations on the left side (insets I and II), the right side most probably provides the input. A displayed mirror image projection of the neuron (red) shows how output arborizations (II) from one hemisphere co-localize with input dendrites from the other hemisphere (III), thus providing the opportunity for synaptic control of responses. Arrow indicates the recording site. Med, medulla; Ch, inner optic chiasm; Prot, protocerebrum; SOG, sub-oesophageal ganglion; L, lateral; D, dorsal; M, medial; V, ventral.

View larger version (9K): [in this window] [in a new window]   Fig. 2. Size tuning of the dragonfly CSTMD1. Responses to targets of different heights are shown averaged for the time the target traversed the receptive field from left to right (black circles) followed by scans from right to left (gray triangles). The target width was fixed at 0.8° and the targets scanned the receptive field at 50 deg. s–1. The dashed line indicates pre-stimulus spontaneous firing rates. Error bars denote standard error of the mean (N=3).

View larger version (7K): [in this window] [in a new window]   Fig. 3. The response of CSTMD1 to target contrast. Responses to targets (0.6x0.6°) of different contrast are averaged for the time during which they traversed the center of the receptive field at 26 deg. s–1. Error bars denote standard error of the mean (N=4).

View larger version (60K): [in this window] [in a new window]   Fig. 4. Velocity tuning to differently sized targets. (A) The false color plot shows the normalized modeled response to a small high-contrast target (0.8°, see inset) traversing a one-dimensional elementary motion detector (EMD) array at velocities between 5 and 2000 deg. s–1. (B) The false color plot shows the normalized modeled response to a wide high-contrast target (8°, see inset) traversing a one-dimensional EMD array. (C) The mean model output shows the response of the EMD array to targets of two widths (0.8° in red line and 8° wide in green line). (D) The graph shows the physiologically recorded response of dragonfly STMDs to target velocity as black targets traversed the center of the receptive field. Two sizes were used: 0.8x0.8° (red circles, solid line) and 8° wide x 0.8° high (green triangles, broken line). Error bars denote standard error of the mean (N=4, two of which were CSTMD1s).

View larger version (12K): [in this window] [in a new window]   Fig. 5. Velocity tuning to targets of different orientation. (A) The response of a single CSTMD1 to targets oriented along the direction of travel (inset) shows a shift in velocity tuning to higher velocities with wider targets. (B) When the targets are oriented perpendicular to the direction of travel (inset) they all show the same velocity optimum, but the response is reduced for larger targets.

View larger version (22K): [in this window] [in a new window]   Fig. 6. Spatial interactions between two moving targets. (A) The data trace (on the left) shows the response to two targets traversing the center of the receptive field (as indicated by horizontal bars under the trace) of CSTMD1 at 41 deg. s–1. The two 0.6x0.6° high-contrast targets were separated by 41° (measured from center to center). The histogram on the right shows the mean of three repeats from the same neuron, divided into 50 ms bins. (B) The response of CSTMD1 to two targets separated by 15°. (C) The response to two targets separated by 5°. (D) The response to two targets separated by 2°. (E) The response to two targets separated by 0.6° (in effect creating one elongated target).