Drosophila Melanogaster has been shown to exhibit short-term orientation memory by fixating on orientations toward previously displayed visual landmarks. However, the fixation behavior varies and is often mixed with other types of movement. Therefore, carefully designed statistical measures are required in order to properly describe the characteristics of the fixation behavior and to quantify the orientation memory exhibited by the fruit flies. To this end, we propose a set of analytical methods. First, we defined the deviation angle which is used to quantify the deviation of the fruit fly's heading from the landmark positions. The deviation angle is defined based on the fruit fly's perspective and is able to reveal more task-relevant movement patterns than the commonly used definition which is based on the “observer's perspective.” We further introduce a temporal deviation angle plot which visually presents the complex movement pattern as a function of time. Next, we define the fixation index which tolerates fluctuation in the movement and performs better in quantifying the level of fixation behavior, or the orientation memory, than the conventional method.

Visual Abstract

Hyperphosphorylated and truncated tau variants are enriched in neuropathological aggregates in diseases known as tauopathies. However, whether the interaction of these posttranslational modifications affects tau toxicity as a whole remains unresolved. By expressing human tau with disease-related Ser/Thr residues to simulate hyperphosphorylation, we show that despite severe neurodegeneration in full-length tau, with the truncation at Asp421, the toxicity is ameliorated. Cytological and biochemical analyses reveal that hyperphosphorylated full-length tau distributes in the soma, the axon, and the axonal terminal without evident distinction, whereas the Asp421-truncated version is mostly restricted from the axonal terminal. This discrepancy is correlated with the fact that fly expressing hyperphosphorylated full-length tau, but not Asp421-cleaved one, develops axonopathy lesions, including axonal spheroids and aberrant actin accumulations. The reduced presence of hyperphosphorylated tau in the axonal terminal is corroborated with the observation that flies expressing Asp421-truncated variants showed less motor deficit, suggesting synaptic function is preserved. The Asp421 cleavage of tau is a proteolytic product commonly found in the neurofibrillary tangles. Our finding suggests the coordination of different posttranslational modifications on tau may have an unexpected impact on the protein subcellular localization and cytotoxicity, which may be valuable when considering tau for therapeutic purposes.

The Buridan’s paradigm is a behavioral task designed for testing visuomotor responses or phototaxis in fruit fly Drosophila melanogaster. In the task, a wing-shortened fruit fly freely moves on a round platform surrounded by a 360° white screen with two vertical black stripes placed at 0° and 180°. A normal fly will tend to approach the stripes one at a time and move back and forth between them. A variety of tasks developed based on the Buridan’s paradigm were designed to test other cognitive functions such as visual spatial memory. Although the movement patterns and the behavioral preferences of the flies in the Buridan’s or similar tasks have been extensively studies a few decades ago, the protocol and experimental settings are markedly different from what are used today. We revisited the Buridan’s paradigm and systematically investigated the approach behavior of fruit flies under different stimulus settings. While early studies revealed an edge-fixation behavior for a wide stripe in the initial visuomotor responses, we did not discover such tendency in the Buridan’s paradigm when observing a longer-term behavior up to minutes, a memory-task relevant time scale. Instead, we observed robust negative photoaxis in which the flies approached the central part of the dark stripes of all sizes. In addition, we found that stripes of 20°-30° width yielded the best performance of approach. We further varied the luminance of the stripes and the background screen, and discovered that the performance depended on the luminance ratio between the stripes and the screen. Our study provided useful information for designing and optimizing the Buridan’s paradigm and other behavioral tasks that utilize the approach behavior.

15Spatial orientation plays a crucial role in animal navigation. Recent studies of tethered 16Drosophila melanogaster (fruit fly) in a virtual reality setting showed that the 17 orientation is encoded in the form of an activity bump, i.e. localized neural activity, in 18 the torus-shaped ellipsoid body (EB). Moreover, a fly can maintain working memory of 19 its orientation with a stable and persistent activity bump in the absence of any visual 20 cue, and update the memory in accordance with changes of the body orientation by 21shifting the location of the bump. Although the neural circuit that is responsible for 22shifting the bump has been extensively studied lately, how the nervous system shifts 23 the bump while maintains its stability and persistence is poorly understood. We 24 investigated this question using free moving fruit flies in a spatial orientation memory 25 task, and manipulated two EB subsystems, the P circuit, which has been suggested 26 for the stabilization function, and the C circuit, which has been suggested for the 27 updating function but was largely overlooked. We discovered that overactivating either 28 circuit produced distinct behavioral deficits, confirming that the two circuits play 29 important but different roles in the orientation working memory. Furthermore, 30 suppressing either circuit disrupted the memory, suggesting that the C or P circuit 31 alone is not sufficient to maintain the orientation working memory. We reproduced the 32 observations with a spiking neural network model of EB and demonstrated that spatial 33 orientation working memory requires coordinated activation of the stabilizing and 34 updating neural processes in different movement modes. 35 36 Keywords 37 Central complex, ellipsoid body, spatial orientation memory, working memory, Buridan's 38 paradigm, Drosophila melanogaster 39 40 Introduction 41Maintaining spatial orientation is a crucial cognitive capability required for animal 42 navigation [1,2], and understanding the detailed neural mechanisms of spatial orientation 43 is of great interest to researchers in the fields of neurobiology [3][4][5] or neuromorphic 44 engineering [6][7][8]. In recent years, significant progress has been made in identifying the 45 neural circuits that support spatial orientation [9] in the central complex of Drosophila 46 melanogaster [10,11]. The central complex has long been associated with short-term 47Recently, a model of the EB-PB circuits proposed in Su et al., (2017) was built strictly 64 based on connectomic data and provided a detailed picture of the neural circuit 65 interactions underlying spatial orientation and its working memory [15]. The model 66suggests the involvement of two sets of coupled circuits that connect the EB and PB. One 67 set that consists of EIP (or E-PG) and PEI neurons forms symmetric recurrent connections, 68 and the other set that consists of EIP and PEN (or P-EN) forms asymmetric recurrent 69 4 connections [20,21]. The symmetric circuit, named the C circuit in this paper, forms an 70 attract...