The insect central complex Daniel B. Turner-Evans, Vivek Jayaraman  Current Biology  Volume 26, Issue 11, Pages R453-R457 (June 2016) DOI: 10.1016/j.cub.2016.04.006 Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 1 Similarity of the anatomy of the central complex across species. (A) Overview of the central complex and support structures: the protocerebral bridge (PB), fan-shaped body (FB) or central body upper (CBU), ellipsoid body (EB) or central body lower (CBL), noduli (NO), and lateral accessory lobe (LAL) in the locust (left, modified with permission from Pfeiffer and Homberg, 2014 © by Annual Reviews, http://www.annualreviews.org; top, from M. Müller et al. 1997 with permission of Springer) and cockroach (bottom, with permission from R. Loesel et al. 2002). Several key anatomical features of the central complex are conserved across insects and crustaceans. (B) A columnar PB–FB–LAL neuron (in red) from the locust (top, from Heinze and Homberg, 2007, reprinted with permission of AAAS) and fruit fly (bottom, courtesy T. Wolff and Y. Aso). Current Biology 2016 26, R453-R457DOI: (10.1016/j.cub.2016.04.006) Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 2 Several orienting and adaptive navigation behaviors in insects require the central complex. (A) Intracellular recordings from the PB–FB–LAL neurons of the locust show tuning to different orientations of polarized light E-vectors across the protocerebral bridge. These neurons thus form a map of polarization directions across the structure. Recording shown is from neuron in Figure 1B. (From Heinze and Homberg, 2007; reprinted with permission from AAAS.) (B) When a cockroach’s antennae come in contact with a wall (0°), the animal turns away (thick black traces). When lesions are made to the central complex, the roach no longer reliably turns directly away from the wall (thin purple traces) (Adapted with permission from the Journal of Experimental Biology, Harley and Ritzmann, 2010.) (C) Sample neural recording from the cockroach central complex showing activity (orange trace) that precedes and predicts turns (blue trace). (Adapted from Martin et al. 2015.) (D) In a task called the Buridan paradigm, fruit flies are allowed to walk back and forth between two vertical landmarks. When they disappear, and another landmark transiently appears to the side, flies alter their course. When this landmark also disappears, the flies turn back towards the remembered position of the now invisible landmark that was their previous destination. Flies with specific ellipsoid body defects do not demonstrate this short-term orientation memory. (Reprinted from Neuser et al. copyright 2008.) (E) Modeled on the Morris water maze for rodents, this task places fruit flies inside a visual arena on an inhospitably hot floor with a single cool spot, whose position is locked relative to the visual scene. Flies learn to use surrounding visual landmarks to find the safe spot after several training trials in which both the spot and the visual scene are rotated together. In the “probe trial”, there is no cool spot, but flies selectively explore the quadrant containing its expected location. When specific ellipsoid body neurons are silenced, flies are unable to recall the location of the cool spot in the probe trial. (Reprinted Ofstad et al. copyright 2011.) (D,E) With permission from Macmillan Publishers Ltd. Current Biology 2016 26, R453-R457DOI: (10.1016/j.cub.2016.04.006) Copyright © 2016 Elsevier Ltd Terms and Conditions

Figure 3 Possible ring-attractor-like network in the central complex. (A) In the fly, visual and motor information is integrated by a population of neurons with dendrites that project to single wedges of the ellipsoid body (see Figure 3D, left, for two examples of such neurons). The dendritic activity of this population is localized to a single bump that moves around the ellipsoid body in concert with the fly’s turns. The activity bump may serve as a compass needle, giving the fly its heading. (With permission from Macmillan Publishers Ltd: Nature, Seelig and Jayaraman copyright 2015.) (B) Schematic of a sample ring attractor network, which consists of nodes (gray circles) topologically arranged in a ring with distance-dependent connection strengths between them (red and green colored lines depict weights of the connections). The connectivity of such a compass-like network localizes activity into a single bump, which moves around the ring depending on an animal’s rotational movements, much as has been observed in the central complex (see, for example, panel A). (C) Schematic of fly central complex that displays a ubiquitous feature of the region: the organization of most of its substructures into stereotypical layers and columns. Many neurons innervating the protocerebral bridge send projections to specific columns of the fan shaped body or sectors of the ellipsoid body, but no known columnar neurons directly connect the latter two structures. Color- and hatching-matched compartments depict connection rules obeyed by some of the neuron classes that connect subsets of these structures, for example, top row in panel D. Protocerebral bridge columns in gray denote areas untouched by these classes of neurons, but see panel E and bottom row in panel D for examples of other types of neurons. Compartmentalization is reduced in the noduli and gall, which receive projections from neurons innervating the contralateral protocerebral bridge, and appears to be entirely lost in the lateral accessory lobe, a likely output structure along with the posterior slope (not shown). External inputs from many regions, including the bulb and protocerebrum (neither shown), influence the activity of the central complex. (D) Sample neurons that link the protocerebral bridge to the ellipsoid body in stereotyped ways may provide some of the recurrent connections necessary to create a ring-attractor-like network within the central complex. (E) Sample of an interneuron that selectively innervates a single structure (here, the protocerebral bridge). Large asterisks denote putative outputs; small asterisk indicates that the arbor on far side of cell body is often sparse. Such neurons likely also play an important role in shaping the activity dynamics of the network. Current Biology 2016 26, R453-R457DOI: (10.1016/j.cub.2016.04.006) Copyright © 2016 Elsevier Ltd Terms and Conditions