We recorded from 43 neurons in the S. gregaria brain, including 6 neurons with arborizations in the optic lobe, 31 CX neurons, and 5 descending neurons. We examined the responses for sensitivity to the angle of polarization of blue light presented from ventral direction. Eleven neurons showed polarization sensitivity during both directions of rotation of the polarizer and 9 additional neurons during one rotation direction.

Recordings were obtained from three neurons connecting the medulla of both hemispheres, termed intermedulla neurons (IM) and three neurons with more complex ramifications ipsilaterally in the lobula complex and contralaterally in the lobula complex and medulla. Those neurons were termed interlobular-medulla neurons (ILM). The IM neurons showed morphological similarities with two previously characterized IM neurons (Beetz et al. 2016), but because of distinct differences from the earlier characterized cells were termed IM3 and IM4. IM3- and IM4 neurons connected the right and left medulla and had additional beaded ramifications in the contralateral posterior slope of the central brain (Figs. 2a–c, 3a–c). The soma of IM3 was located near the accessory medulla anterior-medial to the medulla (Fig. 2c). Fine processes innervated a small sector near the equator in an outer layer of the ipsilateral medulla (Fig. 2c). The main neurite passed through the posterior optic commissure into the contralateral brain hemisphere and into the contralateral optic lobe (Fig. 2a, b). In the contralateral optic lobe, the neuron ramified widely in an outer layer of the medulla (Fig. 2b), possibly layer 4 as defined by Rosner et al. (2017). In the contralateral hemisphere of the central brain beaded side branches were given off into the posterior slope. Some of these extended into the ocellar root, the lateral accessory lobe, and the wedge (Fig. 2b). The neuron had a background activity of 11.7 ± 3.6 (SD) imp s−1 and was strongly inhibited by the blue light with a stationary angle of polarization of 0° (Fig. 2e), as well as during all other polarizer orientations. Correlation analysis revealed a significant dependence of the response to the angle of polarization, i.e., reduced inhibition at Φmax in both directions of polarizer rotation (Fig. 2d, f, Table 1). The preferred angle of polarization was at Φmax = 178° for clockwise and at Φmax = 148° for counterclockwise polarizer rotation, respectively (Fig. 2d). The rotation-direction dependent shift in tuning of the neuron was ΔΦmax = 30°.

Morphology and physiology of intermedulla neuron IM3. a–c Projection views of the Neurobiotin-labeled IM3 neuron. The neuron has ipsilateral ramifications in a small equatorial sector of an outer medulla (ME) layer. An axonal fiber crosses the brain midline in the posterior optic commissure (POC) and gives rise to beaded side branches in the posterior slope (PS), which partly extend into the wedge (WED), ocellar root (OR), and upper lateral accessory lobe (ULAL). The axon continues toward the contralateral optic lobe and gives rise to wide-field ramifications in an outer layer of the ME. Yellow arrowhead in c points at the soma near the accessory medulla. d Circular diagrams showing mean spiking activities (+ SD, black bars) plotted against polarization angle during 4 rotations in clockwise (left) and 3 rotations in counterclockwise (middle) direction. Red bars indicate preferred polarization angle (Φmax). Right diagram shows mean activities (+ SD) from all rotations with Φmax set at 0°. e Spike train (bottom trace) and mean spiking activity illustrating changes in activity when turning the light source (blue LED, polarizer at 0°) off and on. f Spike trains (bottom traces) and mean spike frequency (top, moving average with bin size of 0.5 s) during clockwise (0°–360°) and counterclockwise (360°–0°) rotation of the polarizer. Scale bars = 150 µm

Morphology and physiology of intermedulla neuron IM4. a-c Stacks of confocal images of the Neurobiotin-labeled IM4 neuron. a The neuron has ipsilateral ramifications in a small equatorial sector of an outer medulla (ME) layer. Yellow arrowhead points at the soma near the accessory medulla. b An axonal fiber crosses the brain midline in the posterior optic commissure (POC) and gives rise to beaded side branches in the posterior slope (PS, arrowheads). c The axon continues toward the contralateral optic lobe and gives rise to wide-field ramifications in an outer layer of the ME. d Circular diagrams showing mean spiking activities (+ SD, black bars) plotted against polarizer orientation during 10 rotations in clockwise (left) and counterclockwise (middle) direction. Red bars indicate preferred polarization angle (Φmax), red circles indicate background activity in the dark. Right diagram shows mean activities (+ SD) from all rotations with Φmax set at 0°. e Spike train (bottom trace) and mean spiking activity (top trace, moving average with bin size of 0.5 s) illustrating changes in activity when turning the light source (blue LED, polarizer set at 0°) off and on. f Spike trains (bottom traces) and mean spike frequency (top, moving average with bin size of 0.5 s) during clockwise (0°–360°) and counterclockwise (360°–0°) rotation of the polarizer. Scale bars = 200 µm

Two recordings were from IM4 neurons (Fig. 3). The IM4 neurons were morphologically highly similar to the IM3 neuron, but had less extensive ramifications in the central brain confined to the posterior slope (Fig. 3b) and showed a slight response distinct from that of IM3. Background activity was 12.4 ± 1.0 (SD) imp s−1 (Fig. 3) and 6.6 ± 0.0 (SD) imp s−1. In contrast to IM3, both IM4 neurons were excited by ventral blue light irrespective of polarizer orientation (Fig. 3d, e). One of the two neurons showed a weak but significant maximum excitation at particular angles of polarization in both directions of rotation with ΔΦmax = –33° (clockwise: Φmax = 91°, counterclockwise: Φmax = 124°; Fig. 3d, f). The second neuron showed a significant tuning to polarization angle only during clockwise rotation of the polarizer (Φmax = 129°; Table 1).

Three recordings were made from ILM neurons. The neurons were morphologically indistinguishable. Their somata were located in the cell body rind above the superior lateral protocerebrum, near the mushroom-body calyx (Fig. 4b inset). Fine branches innervated the outer and inner lobe of the ipsilateral lobula complex (Fig. 4c). Axonal fibers crossed the brain midline in the superior arch commissure along the dorsal face of the upper division of the central body (CBU, Fig. 4b). In the contralateral optic lobe, the neurons innervated the dorsal, outer and inner lobe of the lobula complex. A fiber bypassing the lobula complex bifurcated at the anterior inner margin, one collateral passing dorsally and the other one ventrally along the inner face of the medulla (Fig. 4a). Both collaterals gave rise to ramifications in an inner layer of the medulla, likely layer 9 and/or 10. No side branches were observed in the central brain (Fig. 4b). Background activities of the ILM neurons were 9.9 ± 2.0 (SD) imp s−1, 24.4 ± 6.6 (SD) imp s−1 and 18.2 ± 3.1 (SD) imp s−1. The neurons were transiently inhibited by switching on the blue light with stationary polarizer at 0° and showed a short phasic excitation following lights off (Fig. 4f). Two of the three ILM neurons showed preferred angles of polarization, i.e. minimum inhibition but no net excitation, during both directions of rotation of the polarizing filter (Fig. 4d, e; Table 1), whereas the third cell showed significant angle of polarization tuning only during clockwise rotation of the polarizer rotation (Table 1). The preferred polarizer angles differed in the three neurons suggesting that recordings originated from different individual cells of the same morphological type.

Morphology and physiology of the interlobula-medulla neuron (ILM). a–c Stacks of confocal images of the neuron. c In the ipsilateral hemisphere, the neuron innervates the inner (ILO) and outer lobe (OLO) of the lobula complex. b Its axon crosses the brain midline in the superior arch commissure dorsally from the upper division of the central body (CBU). Inset shows cell body of the neuron near the calyx (CA) of the mushroom body. Yellow arrowheads in b and c point at the site where the cell body fiber joins the main neurite of the neuron. a In the contralateral optic lobe, the neuron targets the ILO, OLO, and dorsal lobe (DLO) of the lobula complex and has continuing fibers into the medulla (ME). d Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise and counterclockwise rotation of the polarizer. Red bars indicate preferred polarization angles, red circles indicate background activity in the dark. Right diagram shows mean activities (+ SD) from all rotations with Φmax set at 0°. e Spike trains (bottom traces) and mean spike frequency (top, moving average with bin size of 0.5 s) during clockwise (0°–360°) and counterclockwise (360°–0°) rotation of the polarizer. f Change in spike rate when switching the light source (blue LED, polarizer at 0°) off and on. Scale bars = 200 µm

The central complex (CX) is specifically targeted by polarization vision pathways from the DRAs of both eyes transmitting celestial polarization information (Homberg et al. 2003, 2011). Computations within the CX result in a compass-like representation of spatial directions in the protocerebral bridge (PB) of the CX (Heinze and Homberg 2007; Pegel et al. 2019; Zittrell et al. 2020). We were, therefore, interested in exploring the possibility that the CX might also serve as a site for evaluating non-celestial polarization information. In total, recordings were obtained from 31 neurons with arborizations in the CX, including 15 columnar neurons, 9 tangential neurons, and 7 pontine neurons.

Five CL1-type columnar neurons with arborizations in the protocerebral bridge (PB) and lower division of the central body (CBL) could be examined for responsiveness to polarized light from ventral direction (Table 1). Their somata were located in the pars intercerebralis. The neurons connected ipsilateral columns of the PB to specific columns of the CBL. Small axonal fibers extended to the lateral accessory lobe and innervated the gall with beaded ramifications (Fig. 5a). Background activities of the neurons ranged from 4.2 ± 2.5 (SD) imp s−1 to 12.3 ± 0.3 (SD) imp s−1. Two of the five CL1 neurons showed a directional response to the rotating polarizer, however only during one of the two turning directions. In one cell, Φmax during clockwise rotation was 124° (Fig. 5b), whereas no significant tuning to polarization angle was found when the polarizing filter was rotated counterclockwise (Fig. 5b). The second neuron showed significant angular tuning during counterclockwise rotation of the polarizer with a preferred angle of polarization at 114° (Table 1), but was insensitive to the angle of polarization during clockwise rotation of the polarizer (Table 1).

Tuning to polarization angle of light from ventral direction in a CL1 neuron of the central complex. a The neuron innervates column 4 in the right hemisphere of the protocerebral bridge (PB), the innermost column 1 in the right hemisphere of the lower division of the central body (CBL), and sends an axon to the gall (GA). b Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise and counterclockwise rotation of the polarizer. Red bar in the left diagram indicates preferred polarization angle, red circles indicate background activity in the dark. No significant tuning to polarization angle was observed during counterclockwise rotation of the polarizer. Scale bar = 100 µm

In five recordings CPU1a-type columnar neurons were examined for responsiveness to ventrally presented polarized light. The somata of the cells were located in the pars intercerebralis. The neurons connected an ipsilateral column of the PB to certain columns of the upper division of the central body (CBU). From there, an axon projected to wide beaded ramifications in the contralateral lateral accessory lobe (Fig. 6a). Background activities ranged from 19.4 ± 14.1 (SD) imp s−1 to 38.3 ± 5 (SD) imp s−1. The neurons were slightly excited by blue light from ventral direction with stationary angle of polarization of 0°. In two of the five CPU1 neurons, clockwise rotation of the polarizer revealed responses to the ventral polarized light stimulus. One cell had a preferred Φmax of 161° (Fig. 6b), whereas the preferred polarization angle of the second cell was at a Φmax of 116° (Table 1).

Sensitivity to polarization angle in two types of columnar neuron of the upper division of the central body (CBU). a, b Morphology and physiology of a type 1a columnar neuron of the CBU (CPU1a). The neuron has columnar ramifications in the protocerebral bridge (PB) and the CBU and sends an axon with beaded terminals into the lateral accessory lobe (LAL). b Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise and counterclockwise rotation of the polarizer. Red bar in b indicates preferred polarization angle. No tuning to polarization angle was found during counterclockwise polarizer rotation. Red circles indicate background activity in the dark. c–e Morphology and physiology of a type 5 columnar neuron of the CBU (CPU5). c The neuron has columnar ramifications in the PB and the CBU and in the upper unit of the contralateral nodulus (NO). d Spike trains (bottom traces) and mean spike frequency (top, moving average with bin size of 0.5 s) during clockwise (0°–360°) and counterclockwise (360° − 0°) rotation of the polarizer. e Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise and counterclockwise rotation of the polarizer. Red bars indicate preferred polarization angles. Throughout both rotation directions spiking activity of the neuron is reduced compared to background activity in the dark (red circles). Diagram on the right shows mean activities (+ SD) from all rotations with Φmax set at 0°. Scale bars = 200 µm (a), 100 µm (c)

Two recordings were from CPU5-type columnar neurons. Their somata were located in the pars intercerebralis. The neurons connected single columns of the PB with several columns in layer III of the CBU, and invaded dorsal layers of the upper unit of the contralateral nodulus (Fig. 6c). Background activities of the neurons were 6.7 ± 0.5 imp s−1 (Fig. 6c) and 15.5 ± 6.9 imp s−1. Spiking activity in both neurons was reduced during ventral light stimulation (Fig. 6e). One neuron was sensitive to the angle of polarization during both rotation directions, with a more pronounced response during clockwise than during counterclockwise rotation (clockwise: Φmax = 59°, counterclockwise: Φmax = 33°; Fig. 6d, e), and an angular difference in tuning between both rotation directions of ΔΦmax = 26°. The second neuron was insensitive to polarization angle during ventral stimulation (Table 1).

Recordings from three other types of columnar neurons showed no sensitivity to the polarization angle during stimulation from ventral direction (Table 1). These were a CPU2 neuron with axonal projections to both lateral accessory lobes, a CPU3 neuron without axonal projections to the lateral accessory lobe, and a double impalement of two CU neurons, likely CU1 or CU3. These neurons had columnar ramifications in the CBU and continuing processes innervating the anterior lip and both lateral accessory lobes.

Recordings were obtained from 9 tangential neurons of the CX including one neuron innervating the CBU, one neuron with ramifications in the CBL, and seven neurons innervating the PB. One neuron of the TUSLP4 subtype was studied (Fig. 7a–d). The soma of the cell was located ventrolateral to the calyx in the superior lateral protocerebrum. The main neurite of the cell passed through the anterior bundle toward the CBU. Fine, likely dendritic side branches were concentrated around the vertical lobe of the mushroom body in a wide mesh of various neuropils (Fig. 7a, b). The main fiber entered the central body through the second branch of the anterior bundle and gave rise to fan-like ramifications in layer IIb of the CBU (Fig. 7a, b). The background activity was 8.1 ± 5.4 (SD) imp s−1. The neuron was tuned to a polarization angle of Φmax = 18° during both directions of polarizer rotation (Fig. 7c, d; Table 1).

Sensitivity to polarization angle in two tangential neurons of the central body. a-d Morphology and physiology of a type 4 tangential neuron of the upper division of the central body (CBU) with soma near the superior lateral protocerebrum (TUSLP4). The neuron has wide ramifications in several brain areas, including the superior medial protocerebrum (SMP), the superior intermediate protocerebrum (SIP), the crepine (CRE), the anterior ventro-lateral protocerebrum (AVLP), and the superior clamp (SCL) encircling the vertical lobe of the mushroom body (asterisks in a and b). Its axon enters the central body through the anterior bundle and gives rise to beaded ramifications throughout layer IIa of the CBU. c Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise (left) and counterclockwise rotation of the polarizer (middle). Red bars indicate preferred polarization angles. Red circles indicate background activity in the dark. Diagram on the right shows mean activities (+ SD) from all rotations with Φmax set at 0°. d Spike trains during clockwise (0°–360°) and counterclockwise (360° − 0°) rotation of the polarizer. e Morphology and physiology of a TL2 neuron connecting the lateral bulb (LBU) and lower division of the central body (CBL). f Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise and counterclockwise rotation of the polarizer. Red bar indicates preferred polarization angle. Red circles indicate background activity in the dark. Scale bars = 100 µm

A TL2 tangential neuron arborizing in the CBL was tested for polarization sensitivity from ventral direction. Its soma was located ventral to the lateral accessory lobe. The neuron innervated the lateral bulb. An axonal fiber passed via the isthmus tract to the CBL and gave rise to wide fan-like arborizations in layer 2 of the CBL (Fig. 7e). The neuron had a background spiking activity of 7.7 ± 2.0 (SD) imp s−1 and was inhibited by ventral illumination of the locust eyes. The neuron showed a weak polarization angle-dependent response when the polarizer was rotated clockwise (Φmax = 87°; Fig. 7f), but was insensitive to polarization angle when the polarizer was rotated in counterclockwise direction (Table 1).

Seven tangential neurons of the PB were studied, including 5 TB1-type neurons (Fig. 8a), one TB2-type neuron (Fig. 8c) and one TB3-type neuron. The TB1 neurons were of the subtype TB1b (two neurons), TB1c (one neuron; Fig. 8a) and TB1d (2 neurons) as characterized by Beetz et al. (2015). All studied TB neurons innervated the posterior optic tubercle and the PB. TB1 neurons have varicose ramifications in one column of the PB in each hemisphere, TB2 neurons, in the outermost column of one hemisphere and in the innermost columns of both hemispheres, and TB3 neurons only invade the ipsilateral hemisphere of the PB. TB1 neurons had background activities ranging from 5.3 ± 8.9 (SD) imp s−1 to 23.4 ± 4.5 imp s−1. All TB neurons were insensitive to polarization angle (Fig. 8, Table 1).

Sensitivity to polarization angle in two tangential neurons of the protocerebral bridge (PB). Both neurons connect the posterior optic tubercle (POTU) with the PB but differ in the innervated columns of the PB. Arrowheads in a and c point to soma position of the neurons. a Morphology of a TB1d tangential neuron with varicose ramifications in columns 5 in the ipsilateral and 4 in the contralateral hemisphere of the PB. b Circular plots from the neuron in a showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (left) and counterclockwise rotation of the polarizer (right). The neuron shows no angular preference to polarized light. Red circles indicate background activity in the dark. c Morphology of a TB2 tangential neuron with varicose ramifications in columns 1 and 8 in the ipsilateral and 1 in the contralateral hemisphere of the PB. d Circular plots from the neuron in c showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (left) and counterclockwise rotation of the polarizer (right). The neuron shows no angular preference. Red circles indicate background activity in the dark. Scale bars = 100 µm

Seven pontine neurons (2 POU1, 1 POU2, 4 POU3) were studied. Pontine neurons connect individual columns of the CBU that are 8 columns apart. Depending on the innervation layers POU1, 2 and 3 neurons have been distinguished (Heinze and Homberg 2008). The somata of the neurons were located in the pars intercerebralis. Background activity was low in the POU1 neurons (2.4 ± 1.7 and 0.0 ± 0.0 imp s−1), slightly higher in the POU2 neuron (8 ± 0.9 imp s−1), and highest in POU3 neurons (9.4 ± 4.5, 11.1 ± 4.5 and 11.9 ± 5.5 imp s−1). All POU neurons except for two POU3 neurons were slightly excited by the blue light irrespective of polarizer orientation (Fig. 9b). When presenting light through a stationary polarizer (angle of polarization 0°) the activity increase was phasic and decreased again over time. One POU1 neuron showed polarization angle dependent responses in both directions of polarizer rotation (clockwise: Φmax = 49°, counterclockwise: Φmax = 43°, ΔΦmax = 6°; Fig. 9a, b). The second POU1 neuron was sensitive to polarization angle only during counterclockwise rotation of the polarizer (Φmax = 84°; Table 1). The recordings from POU2 and POU3 subtypes showed no angle of polarization dependence in their responses to the rotating polarizer (Table 1).

Sensitivity to polarization angle in a POU1 pontine neuron of the upper division of the central body (CBU). a Stack of optical sections showing the morphology of the neuron. It has fine arborizations in outermost ipsilateral columns of the CBU and a second field of finely beaded processes in innermost contralateral columns. Yellow arrowhead indicates the position of the cell body. b Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (left) and counterclockwise rotation of the polarizer (right). Red circles indicate background activity in the dark. The neuron shows increased spiking activity during polarizer rotation in both directions. Red bars indicate preferred polarization angles. c Spike trains during clockwise (0°–360°) and counterclockwise (360° − 0°) rotation of the polarizer. Scale bar = 150 µm

A neuron with ramifications in several lateral protocerebral brain areas, termed interneuron of the lateral protocerebrum (ILP) was studied here for the first time (Fig. 10). The soma of the ILP neuron was located in the cell body rind between the antennal lobe and the anterior ventro-lateral protocerebrum (Fig. 10b). The neuron had wide, presumably dendritic, arborizations in the superior lateral protocerebrum, posterior lateral and ventro-lateral protocerebrum, and the epaulette in the ipsilateral hemisphere and sent an axonal process through the great commissure to wide ramifications concentrated in the posterior lateral protocerebrum, epaulette, and inferior clamp (Fig. 10a, b). The background activity of the ILP neuron was 24.5 ± 3.7 (SD) imp s−1. The neuron was markedly inhibited to 9.5 imp s−1 by ventrally presented blue light with a polarization angle of 0° (Fig. 10c) and, likewise, at all polarizer orientations during polarizer rotation (Fig. 10d). The neuron showed a significant response to polarization angle during both rotation directions. During clockwise rotation, the preferred Φmax was 71° (Fig. 10d), during counterclockwise rotation, Φmax was 98° (Fig. 10d), resulting in ΔΦmax of –27°.

Morphology and physiology of the ILP interneuron, connecting the lateral protocerebrum of both brain hemispheres. a, Anterior and b, posterior stack of optical sections illustrating the morphology of the neuron, its connection is marked by yellow asterisks. Its soma lies in the soma rind between the antennal lobe and the anterior ventro-lateral protocerebrum of the brain (yellow arrowhead). Ramifications in the ipsilateral brain hemisphere are concentrated in the superior lateral protocerebrum (SLP), the epaulette (EPA), and the posterior lateral (PLP) and posterior ventro-lateral protocerebrum (PVLP). An axonal fiber projects contralaterally and gives rise to varicose ramifications in the inferior clamp (ICL), PLP and EPA. c Spike trains (bottom trace) and mean spike frequency (top, moving average with bin size of 0.5 s) illustrating inhibition of spiking during ventral light illumination (polarization angle at 0°). d Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (left) and counterclockwise rotation of the polarizer (middle). Red bars indicate preferred polarization angles. Throughout both rotation directions spiking activity of the neuron is strongly reduced compared to background activity in the dark (25.7 imp s−1, not shown). Diagram on the right shows mean activities (+ SD) from all rotations with Φmax set at 0°. Scale bars = 200 µm

In five recordings descending neurons were labeled. These included 3 contralaterally descending neurons from the PI(2) group (one PI(2):5 and two PI(2):6 neurons), morphologically described first by Williams (1975) and two ipsilateral descending neurons termed IDN1 and IDN2. The PI(2):5 neuron did not show polarization angle-dependent modulation during rotation of the polarizer (Table 1). The somata of the two PI(2):6 neurons were located in the posterior pars intercerebralis. The neurons had wide arborizations in the ipsilateral posterior slope (Fig. 11a). A secondary area of slightly beaded processes was in the contralateral antennal mechanosensory and motor center (Fig. 11a). The background activities of the neurons were 11.1 ± 2.9 (SD) imp s−1 (Fig. 11c), and 11.2 ± 10 (SD) imp s−1. The neurons showed a phasic increase in spiking activity after the end of ventral light stimulation (Fig. 11b). Both recordings from PI(2):6 showed an angle of polarization dependent modulation of spiking rate (Fig. 11b, c), consisting of strongest excitation at Φmax but no inhibition at Φmin (Fig. 11c). The preferred angle of polarization (Φmax) in one recording was at 96° during clockwise rotation (Table 1) and at 119° during counterclockwise rotation (ΔΦmax = − 23°; Table 1). The second cell had a preferred polarizer orientation at 56° during clockwise rotation and at 80° during counterclockwise rotation of the polarizer (Fig. 11c) resulting in a ΔΦmax of − 24°.

Morphology and physiology of a PI(2):6 descending neuron of the brain. a Stack of optical sections showing the morphology of the neuron. It has wide ramifications in the ipsilateral posterior slope (PS) and superior lateral protocerebrum (SLP), finely beaded processes in the contralateral antennal mechanosensory and motor center (AMMC), and an axonal fiber descending via the contralateral circumesophageal connective. Yellow arrowhead indicates the cell body of the neuron. b Spike trains (bottom traces) and mean spike frequency (top, moving average with bin size of 0.5 s) illustrating changes in spiking activity when ventral light (polarization angle at 0°) is turned off and on (upper part) and during clockwise and counterclockwise rotation of the polarizer (bottom). c Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (left) and counterclockwise rotation of the polarizer (middle). The neuron shows increased spiking activity during polarizer rotation in both directions. Red bars indicate preferred polarization angles. Diagram on the right shows mean activities (+ SD) from all rotations with Φmax set at 0°. Red circles indicate background activity in the dark. Scale bar = 100 µm

The ipsilaterally descending neurons IDN1 and IDN2 had a highly similar cell body position in the posterior lateral pars intercerebralis but differed in morphology of their dendritic side branches in the brain (Figs. 12, 13). Whereas both neurons had fine arborizations in the inferior clamp, posterior slope, and the antennal mechanosensory and motor center, the ramifications of IDN2 extended into several adjacent brain areas and differed in the pattern of side branches from those of IDN1. The background activity of IDN1 (30.6 ± 0.0 (SD) imp s−1) was substantially higher than that of IDN2 (10.8 ± 3.9 imp s−1). Spiking activity in IDN1 was markedly inhibited by ventral blue light (Fig. 12b) while inhibition was more moderate in IDN2 (Fig. 13b,c). Neural activity of IDN1 was significantly modulated by polarization angle during rotation of the polarizer in both direction (Fig. 12b), resulting in Φmax values of 158° during clockwise rotation and 151° during counterclockwise rotation of the polarizer and a Δ Φmax of 7°. In contrast, no polarization angle dependent modulation of activity occurred in IDN2 (Fig. 13b).

Morphology and physiology of the ipsilaterally descending neuron 1 (IDN1) of the brain. a Composite image of several stacks of optical sections showing the morphology of IDN1. The neuron has wide ramifications in the ipsilateral posterior slope (PS), inferior clamp (ICL), posterior lateral protocerebrum (PLP), and wedge (WED) and an axonal fiber descending via the ipsilateral circumesophageal connective. White arrowheads point to a fiber of a second colabeled neuron. Inset shows sparse beaded side branches in the antennal mechanosensory and motor center (AMMC, yellow arrowheads). b Circular plots showing mean spiking activity (+ SD, black bars) plotted in 10° bins during clockwise rotation (top left) and counterclockwise rotation of the polarizer (top right). Red circles indicate background activity in the dark. Spiking activity of the neuron, inhibited throughout polarizer rotation, shows polarization-angle dependent modulation during polarizer rotation in both directions. Red bars indicate preferred polarization angles. Bottom diagram shows mean activities (+ SD) from all rotations with Φmax set at 0°. Scale bars = 200 µm (a