Odour motion sensing enhances navigation of complex plumes.

Odour plumes in the wild are spatially complex and rapidly fluctuating structures carried by turbulent airflows1-4. To successfully navigate plumes in search of food and mates, insects must extract and integrate multiple features of the odour signal, including odour identity5, intensity6 and timing6-12. Effective navigation requires balancing these multiple streams of olfactory information and integrating them with other sensory inputs, including mechanosensory and visual cues9,12,13. Studies dating back a century have indicated that, of these many sensory inputs, the wind provides the main directional cue in turbulent plumes, leading to the longstanding model of insect odour navigation as odour-elicited upwind motion6,8-12,14,15. Here we show that Drosophila melanogaster shape their navigational decisions using an additional directional cue-the direction of motion of odours-which they detect using temporal correlations in the odour signal between their two antennae. Using a high-resolution virtual-reality paradigm to deliver spatiotemporally complex fictive odours to freely walking flies, we demonstrate that such odour-direction sensing involves algorithms analogous to those in visual-direction sensing16. Combining simulations, theory and experiments, we show that odour motion contains valuable directional information that is absent from the airflow alone, and that both Drosophila and virtual agents are aided by that information in navigating naturalistic plumes. The generality of our findings suggests that odour-direction sensing may exist throughout the animal kingdom and could improve olfactory robot navigation in uncertain environments.

Disparities in Care Among Gunshot Victims: A Nationwide Analysis.

INTRODUCTION: Given the well-known healthcare disparities most pronounced in racial and ethnic minorities, trauma healthcare in underrepresented patients should be examined, as in-hospital bias may influence the care rendered to patients. This study seeks to examine racial differences in outcomes and resource utilization among victims of gunshot wounds in the United States. METHODS: This is a retrospective review of the National Trauma Data Bank (NTDB) conducted from 2007 to 2017. The NTDB was queried for patients who suffered a gunshot wound not related to accidental injury or suicide. Patients were stratified according to race. The primary outcome for this study was mortality. Secondary outcomes included racial differences in resource utilization including air transport and discharge to rehabilitation centers. Univariate and multivariate analyses were used to compare differences in outcomes between the groups. RESULTS: A total of 250,675 patients were included in the analysis. After regression analysis, Black patients were noted to have greater odds of death compared to White patients (odds ratio [OR] 1.14, confidence interval [CI] 1.037-1.244; P = 0.006) and decreased odds of admission to the intensive care unit (ICU) (OR 0.76, CI 0.732-0.794; P < 0.001). Hispanic patients were significantly less likely to be discharged to rehabilitation centers (Hispanic: 0.78, CI 0.715-0.856; P < 0.001). Black patients had the shortest time to death (median time in minutes: White 49 interquartile range [IQR] [9-437] versus Black 24 IQR [7-205] versus Hispanic 39 IQR [8-379] versus Asian 60 [9-753], P < 0.001). CONCLUSIONS: As society carefully examines major institutions for implicit bias, healthcare should not be exempt. Greater mortality among Black patients, along with differences in other important outcome measures, demonstrate disparities that encourage further analysis of causes and solutions to these issues.

Mechanism for analogous illusory motion perception in flies and humans.

Visual motion detection is one of the most important computations performed by visual circuits. Yet, we perceive vivid illusory motion in stationary, periodic luminance gradients that contain no true motion. This illusion is shared by diverse vertebrate species, but theories proposed to explain this illusion have remained difficult to test. Here, we demonstrate that in the fruit fly Drosophila, the illusory motion percept is generated by unbalanced contributions of direction-selective neurons' responses to stationary edges. First, we found that flies, like humans, perceive sustained motion in the stationary gradients. The percept was abolished when the elementary motion detector neurons T4 and T5 were silenced. In vivo calcium imaging revealed that T4 and T5 neurons encode the location and polarity of stationary edges. Furthermore, our proposed mechanistic model allowed us to predictably manipulate both the magnitude and direction of the fly's illusory percept by selectively silencing either T4 or T5 neurons. Interestingly, human brains possess the same mechanistic ingredients that drive our model in flies. When we adapted human observers to moving light edges or dark edges, we could manipulate the magnitude and direction of their percepts as well, suggesting that mechanisms similar to the fly's may also underlie this illusion in humans. By taking a comparative approach that exploits Drosophila neurogenetics, our results provide a causal, mechanistic account for a long-known visual illusion. These results argue that this illusion arises from architectures for motion detection that are shared across phyla.

Predicting Unplanned Intensive Care Unit Admission for Trauma Patients: The CRASH Score.

INTRODUCTION: Unplanned transfer of trauma patients to the intensive care unit (ICU) carries an associated increase in mortality, hospital length of stay, and cost. Trauma teams need to determine which patients necessitate ICU admission on presentation rather than waiting to intervene on deteriorating patients. This study sought to develop a novel Clinical Risk of Acute ICU Status during Hospitalization (CRASH) score to predict the risk of unplanned ICU admission. METHODS: The 2017 Trauma Quality Improvement Program database was queried for patients admitted to nonICU locations. The group was randomly divided into two equal sets (derivation and validation). Multiple logistic regression models were created to determine the risk of unplanned ICU admission using patient demographics, comorbidities, and injuries. The weighted average and relative impact of each independent predictor were used to derive a CRASH score. The score was validated using area under the curve. RESULTS: A total of 624,786 trauma patients were admitted to nonICU locations. From 312,393 patients in the derivation-set, 3769 (1.2%) had an unplanned ICU admission. A total of 24 independent predictors of unplanned ICU admission were identified and the CRASH score was derived with scores ranging from 0 to 32. The unplanned ICU admission rate increased steadily from 0.1% to 3.9% then 12.9% at scores of 0, 6, and 14, respectively. The area under the curve for was 0.78. CONCLUSIONS: The CRASH score is a novel and validated tool to predict unplanned ICU admission for trauma patients. This tool may help providers admit patients to the appropriate level of care or identify patients at-risk for decompensation.

The impact of coronavirus 2019 on trauma.

PURPOSE OF REVIEW: The relationship between trauma and the ongoing global coronavirus 2019 (COVID-19) pandemic is still largely unclear. This comprehensive review of recent studies examining overall trauma volumes, mechanisms of injury, and outcomes after trauma during the COVID-19 pandemic was performed to better understand the impact of the pandemic on trauma patients. RECENT FINDINGS: In the early stages of the pandemic, the overall volumes of patients seen in many major trauma centers had decreased; however, these rates largely returned to historical baselines after the cessation of stay-at-home orders. An increasing proportion of trauma patients were injured by penetrating mechanisms during the pandemic. Being a victim of interpersonal violence was an independent risk factor for COVID-19 infection. In two studies utilizing propensity score-matched analysis among trauma patients, COVID-19 infection was associated with a five- to sixfold increase in mortality risk as compared to uninfected patients. SUMMARY: Consequences of the COVID-19 pandemic include increased financial stressors, job loss, mental illness, and illegal drug use, all of which are known risk factors for trauma. This is particularly true among vulnerable patient populations such as racial minority groups and low socioeconomic status patients. To lessen the impact of COVID-19 on trauma patients, increased awareness of the problem and heightened emphasis on injury prevention must be made.

Drosophila Sidekick is required in developing photoreceptors to enable visual motion detection.

The assembly of functional neuronal circuits requires growth cones to extend in defined directions and recognize the correct synaptic partners. Homophilic adhesion between vertebrate Sidekick proteins promotes synapse formation between retinal neurons involved in visual motion detection. We show here that Drosophila Sidekick accumulates in specific synaptic layers of the developing motion detection circuit and is necessary for normal optomotor behavior. Sidekick is required in photoreceptors, but not in their target lamina neurons, to promote the alignment of lamina neurons into columns and subsequent sorting of photoreceptor axons into synaptic modules based on their precise spatial orientation. Sidekick is also localized to the dendrites of the direction-selective T4 and T5 cells, and is expressed in some of their presynaptic partners. In contrast to its vertebrate homologs, Sidekick is not essential for T4 and T5 to direct their dendrites to the appropriate layers or to receive synaptic contacts. These results illustrate a conserved requirement for Sidekick proteins in establishing visual motion detection circuits that is achieved through distinct cellular mechanisms in Drosophila and vertebrates.

Selective Computed Tomography (CT) Imaging is Superior to Liberal CT Imaging in the Hemodynamically Normal Pediatric Blunt Trauma Patient.

BACKGROUND: The optimal imaging strategy in hemodynamically stable pediatric blunt trauma remains to be defined. The purpose of this study was to determine the differences between selective and liberal computed tomography (CT) strategy in a pediatric trauma population with respect to radiation exposure and outcomes. METHODS: We performed a retrospective analysis of hemodynamically stable blunt pediatric trauma patients (≤16 y) who were admitted to a Level I trauma center between 2013-2016. Patients were stratified into selective and liberal imaging cohorts. Univariate and multivariate regression analyses were used to compare outcomes between the groups. Outcomes included radiation dose, hospital and ICU length of stay, complications and mortality. RESULTS: Of the 485 patients included, 176 underwent liberal and 309 selective CT imaging. The liberal cohort were more likely to be severely injured (ISS>15: 34.1 versus 8.4%, P< 0.001). The odds of exposure to a radiation dose of >15 mSv were higher with liberal scanning in patients with both ISS > 15 (OR 2.78, 95% CI 1.76-5.19, P< 0.001) and ISS ≤ 15 (OR 3.41, 95% CI 2.19-8.44, P < 0.001). Adjusted outcomes regarding mortality, ICU length of stay, and complications were similar between the cohorts. CONCLUSION: Selective CT imaging in hemodynamically stable blunt pediatric trauma patients was associated with reduced radiation exposure and similar outcomes when compared to a liberal CT strategy.

A Novel Abdominal Decompression Technique to Treat Compartment Syndrome After Burn Injury.

BACKGROUND: Prevalence of abdominal compartment syndrome (ACS) is estimated to be 4%-17% in severely burned patients. Although decompressive laparotomy can be lifesaving for ACS patients, severe complications are associated with this technique, especially in burn populations. This study outlines a new technique of releasing intraabdominal pressure without resorting to decompressive laparotomy. MATERIALS AND METHODS: Ten fresh tissue cadavers were studied; none of whom had had prior abdominal surgery. Using Veress needles, abdomens were insufflated to 30 mm Hg and subsequently connected to arterial pressure transducers. Two techniques were then used to incise fascia. First, large skin flaps were raised from a midline incision (n = 5). Second, small 2 cm cutdowns at the proximal and distal extent of midaxillary, subcostal, and inguinal incisional sites were made, followed by tunneling a subfascial plane using an aortic clamp with fascial incisions made through the grooves of a tunneled vein stripper (n = 5). Pressures were recorded in the sequence of incisions mentioned previously. RESULTS: The open midline flap technique decreased abdominal pressure from a mean pressure of 30 ± 1.8 mm Hg to 6.9 ± 5.0 mm Hg (P < 0.01). The minimally invasive technique decreased intraabdominal pressure from 30 ± 0.9 to 5.8 ± 5.2 mm Hg (P < 0.01). This technique significantly reduced intraabdominal pressure via extraperitoneal component separation and fascial release at the midaxillary, subxiphoid, and inguinal regions. CONCLUSIONS: This technique offers the benefit of reducing the morbidity, mortality, and complications associated with an open abdomen, which may be beneficial in the burn injury population.

The Impact of Seat Belt Use in Pregnancy on Injuries and Outcomes After Motor Vehicle Collisions.

BACKGROUND: Seat belt use during motor vehicle collisions (MVCs) has been shown to alter adults' intra-abdominal injury patterns, although the effect of seat belt use in pregnant women is unclear. The objective of this study was to determine the impact of seat belt use in pregnancy on injuries and outcomes after MVCs. METHODS: Patients injured by MVCs were identified from the National Trauma Data Bank (2007-2014). The exclusion criteria were transfer from an outside hospital, male or unspecified sex, missing restraint data, and nonchildbearing age. Demographics, clinical/injury data, pregnancy status, seat belt use, and outcomes were collected. Study groups were dichotomized by pregnancy status with subgroup analysis by seat belt use. Univariate/multivariate analyses compared outcomes and determined predictors of seat belt use. RESULTS: After exclusions, 162,964 women were included, of which 680 (<1%) were pregnant. Intra-abdominal injuries during pregnancy did not vary according to seat belt use (P > 0.05). Unrestrained pregnant women were more injured (Injury Severity Score: 13 versus 7, P < 0.001), more likely to need emergent operation (14% versus 10%, P < 0.001), and had a longer hospital stay (6 versus 4 d, P = 0.012) than restrained counterparts. On multivariate analysis among pregnant women, seat belt use was associated with age ≥25 y (odds ratio: 2.033, P = 0.001). The lack of restraint use was associated with the position in the passenger seat (odds ratio: 0.521, P = 0.001). CONCLUSIONS: Seat belt use in pregnancy does not alter abdominal injury patterns but is associated with lower injury severity, reduced need for emergent surgery, and shortened hospital stay. Public health interventions emphasizing the importance of seat belts could be focused on younger patients and vehicle passengers to reach the high-risk pregnant subset.

Processing properties of ON and OFF pathways for Drosophila motion detection.

The algorithms and neural circuits that process spatio-temporal changes in luminance to extract visual motion cues have been the focus of intense research. An influential model, the Hassenstein-Reichardt correlator, relies on differential temporal filtering of two spatially separated input channels, delaying one input signal with respect to the other. Motion in a particular direction causes these delayed and non-delayed luminance signals to arrive simultaneously at a subsequent processing step in the brain; these signals are then nonlinearly amplified to produce a direction-selective response. Recent work in Drosophila has identified two parallel pathways that selectively respond to either moving light or dark edges. Each of these pathways requires two critical processing steps to be applied to incoming signals: differential delay between the spatial input channels, and distinct processing of brightness increment and decrement signals. Here we demonstrate, using in vivo patch-clamp recordings, that four medulla neurons implement these two processing steps. The neurons Mi1 and Tm3 respond selectively to brightness increments, with the response of Mi1 delayed relative to Tm3. Conversely, Tm1 and Tm2 respond selectively to brightness decrements, with the response of Tm1 delayed compared with Tm2. Remarkably, constraining Hassenstein-Reichardt correlator models using these measurements produces outputs consistent with previously measured properties of motion detectors, including temporal frequency tuning and specificity for light versus dark edges. We propose that Mi1 and Tm3 perform critical processing of the delayed and non-delayed input channels of the correlator responsible for the detection of light edges, while Tm1 and Tm2 play analogous roles in the detection of moving dark edges. Our data show that specific medulla neurons possess response properties that allow them to implement the algorithmic steps that precede the correlative operation in the Hassenstein-Reichardt correlator, revealing elements of the long-sought neural substrates of motion detection in the fly.

A minimal synaptic model for direction selective neurons in Drosophila.

Visual motion estimation is a canonical neural computation. In Drosophila, recent advances have identified anatomic and functional circuitry underlying direction-selective computations. Models with varying levels of abstraction have been proposed to explain specific experimental results but have rarely been compared across experiments. Here we use the wealth of available anatomical and physiological data to construct a minimal, biophysically inspired synaptic model for Drosophila's first-order direction-selective T4 cells. We show how this model relates mathematically to classical models of motion detection, including the Hassenstein-Reichardt correlator model. We used numerical simulation to test how well this synaptic model could reproduce measurements of T4 cells across many datasets and stimulus modalities. These comparisons include responses to sinusoid gratings, to apparent motion stimuli, to stochastic stimuli, and to natural scenes. Without fine-tuning this model, it sufficed to reproduce many, but not all, response properties of T4 cells. Since this model is flexible and based on straightforward biophysical properties, it provides an extensible framework for developing a mechanistic understanding of T4 neural response properties. Moreover, it can be used to assess the sufficiency of simple biophysical mechanisms to describe features of the direction-selective computation and identify where our understanding must be improved.

Shotgun Wounds: Nationwide Trends in Epidemiology, Injury Patterns, and Outcomes from US Trauma Centers.

BACKGROUND: Shotguns represent a distinct form of ballistic injury because of projectile scatter and variable penetration. Due in part to their rarity, existing literature on shotgun injuries is scarce. OBJECTIVE: This study defined the epidemiology, injury patterns, and outcomes after shotgun wounds at a national level. METHODS: Patients with shotgun injury were identified from the National Trauma Data Bank (2007-2014). Transferred patients and those with missing procedure data were excluded. Demographics, injury data, and outcomes were collected and analyzed. Categorical variables are presented as number (percentage) and continuous variables as median (interquartile range). RESULTS: Shotgun wounds comprised 9% of all firearm injuries. After exclusions, 11,292 patients with shotgun injury were included. The median age was 29 years (21-43) and most were male (n = 9887, 88%). Most injuries occurred in the South (n = 4092, 36%) and among white patients (n = 4945, 44%). The median Injury Severity Score was 9 (3-16). Overall in-hospital mortality was 14% (n = 1341), with 669 patients (7%) dying in the emergency department. Assault was the most common injury intent (n = 6762, 60%), followed by accidental (n = 2081, 19%) and self-inflicted (n = 1954, 17%). The lower and upper extremities were the most commonly affected body regions (n = 4071, 36% and n = 3422, 30%, respectively), while the head was the most severely injured (median Abbreviated Injury Scale score 4 [2-5]). CONCLUSIONS: In the United States, shotgun wounds are an infrequent mechanism of injury. Shotgun wounds as a result of interpersonal violence far outweigh self-inflicted and accidental injuries. White men in their 20s in the southern parts of the country are most commonly affected and thereby delineate the high-risk patient population for injury by this mechanism at a national level.

Nonoperative management of penetrating abdominal solid organ injuries in children.

BACKGROUND: Nonoperative management (NOM) of penetrating solid organ injuries (SOI) has not been well described in the pediatric population. The objective of this study was to characterize the epidemiology, injury patterns, and factors associated with trial and failure of NOM. METHODS: This is a retrospective cohort analysis of the National Trauma Data Bank for the period of 2007-2014. The study population included patients ≤18 y with penetrating injury to the liver, spleen, or kidney. NOM was defined as no operative intervention (exploratory laparotomy or operation involving the liver, spleen, or kidney) < 4 h of emergency department arrival. Failed NOM was defined as operative intervention ≥4 h after emergency department arrival. Multivariate logistic regression explored clinical factors potentially associated with trial and failure of NOM. RESULTS: Of 943,000 pediatric trauma patients included in the National Trauma Data Bank, 3005 (0.32%) met our inclusion criteria. Median age was 17.0 y; 88.8% were male. Gunshot wounds (GSW) accounted for 71.7% of injury mechanisms and stab wounds accounted for the remaining 28.3%. Median injury severity score was 9 (interquartile range: 5-13). Two thousand one hundred and twenty-one (70.6%) patients sustained kidney injury, 1210 (40.3%) liver injury, and 159 (5.3%) splenic injury. NOM was pursued in 615 (20.5%) patients. Factors significantly associated with immediate operative intervention included GSW, hypotension, and associated hollow viscus injury. Failed NOM was identified in 175 patients (28.5%). Factors significantly associated with failed NOM included GSW, high-grade SOI, and associated hollow viscus injury. Overall mortality was 26 (0.9%). CONCLUSIONS: NOM can be safe in a carefully selected group of pediatric patients with penetrating SOI. Future prospective studies are warranted to validate its feasibility.

Factors affecting the caloric and protein intake over time in critically ill trauma patients.

BACKGROUND: Major trauma leads to increased nutritional requirements. However, little is known about the actual amount of calories and protein administered and the factors affecting the intake over time in critically ill trauma patients. METHODS: Prospective study including 100 trauma patients admitted to the Los Angeles County + University of Southern California Medical Center intensive care unit between March 2014 and October 2014. Inclusion criteria were age > 16 y, surgery at admission, and no oral nutrition. The caloric and protein intake was recorded, and requirements were calculated daily for 28 d. The nutritional intake and the impact of clinical factors on the intake over time were assessed using mixed model analysis. RESULTS: The caloric and protein intake significantly increased over time, but the median intake did not meet the median calculated requirements at any time. Multivariable analysis revealed a smaller increase of the nutritional intake over time in patients with an injury severity score > 45, whereas penetrating injury and laparotomy were associated with a higher increase of the intake. Body mass index scores ≥ 30 kg/m2, traumatic brain injury, and gastrointestinal tract injuries were associated with a smaller increase of the caloric intake over time. CONCLUSIONS: The median nutritional intake did not meet the median calculated requirements over time. A smaller increase of the nutritional intake over time was found in patients with a higher injury burden, whereas penetrating injury and laparotomy were associated with a higher increase of the intake. Higher body mass index scores, traumatic brain injury, and gastrointestinal tract injuries were associated with a smaller increase of the caloric intake over time. These clinical factors can help to adjust the nutritional support in critically ill trauma patients.

Balanced blood product transfusion during liver transplantation.

INTRODUCTION: This study was conducted to determine whether an intra-operative ratio of at least 1:1:2 of fresh frozen plasma (FFP):platelets (PLTs):packed red blood cells (pRBCs) improves outcomes in orthotopic liver transplantation (OLT). METHODS: A single-center, retrospective study of deceased donor OLT recipients (MELD ≥15) requiring intra-operative pRBC transfusion (years 2013-2016). Patients were grouped into those receiving an intra-operative ratio of ≥1:1:2 of FFP:PLTs:pRBCs vs ratios <1:1:2. RESULTS: Patients in ≥1:1:2 group (n = 150) and patients in <1:1:2 group (n = 80) were matched for baseline characteristics (P > .05). Patients in the ≥1:1:2 group had lower pRBC and intra-operative blood product requirements (11 ± 0.5 vs 19 ± 1.4 units, P < .001, and 33 ± 1.3 vs 43 ± 3.3 units, P = .006, respectively), improved 1-month mortality (0 vs 8%, P = .002), improved 1-year survival (P = .004), less intra-operative cardiac arrest (3% vs 10%, P = .03), and shorter operating room time (389 ± 7.2 vs 431 ± 17.2 minutes, P = .03). After multivariate adjustment for baseline and intra-operative variables, balanced blood product transfusion (BBPT) was significantly associated with less intra-operative pRBC transfusion (95% confidence interval: 0.60-0.72). CONCLUSION: Balanced blood product transfusion is associated with reduced transfusion requirements in OLT.

Walking Drosophila align with the e-vector of linearly polarized light through directed modulation of angular acceleration.

Understanding the mechanisms that link sensory stimuli to animal behavior is a central challenge in neuroscience. The quantitative description of behavioral responses to defined stimuli has led to a rich understanding of different behavioral strategies in many species. One important navigational cue perceived by many vertebrates and insects is the e-vector orientation of linearly polarized light. Drosophila manifests an innate orientation response to this cue ('polarotaxis'), aligning its body axis with the e-vector field. We have established a population-based behavioral paradigm for the genetic dissection of neural circuits guiding polarotaxis to both celestial as well as reflected polarized stimuli. However, the behavioral mechanisms by which flies align with a linearly polarized stimulus remain unknown. Here, we present a detailed quantitative description of Drosophila polarotaxis, systematically measuring behavioral parameters that are modulated by the stimulus. We show that angular acceleration is modulated during alignment, and this single parameter may be sufficient for alignment. Furthermore, using monocular deprivation, we show that each eye is necessary for modulating turns in the ipsilateral direction. This analysis lays the foundation for understanding how neural circuits guide these important visual behaviors.

Dynamical adaptation in photoreceptors.

Adaptation is at the heart of sensation and nowhere is it more salient than in early visual processing. Light adaptation in photoreceptors is doubly dynamical: it depends upon the temporal structure of the input and it affects the temporal structure of the response. We introduce a non-linear dynamical adaptation model of photoreceptors. It is simple enough that it can be solved exactly and simulated with ease; analytical and numerical approaches combined provide both intuition on the behavior of dynamical adaptation and quantitative results to be compared with data. Yet the model is rich enough to capture intricate phenomenology. First, we show that it reproduces the known phenomenology of light response and short-term adaptation. Second, we present new recordings and demonstrate that the model reproduces cone response with great precision. Third, we derive a number of predictions on the response of photoreceptors to sophisticated stimuli such as periodic inputs, various forms of flickering inputs, and natural inputs. In particular, we demonstrate that photoreceptors undergo rapid adaptation of response gain and time scale, over ∼ 300[Formula: see text] ms-i. e., over the time scale of the response itself-and we confirm this prediction with data. For natural inputs, this fast adaptation can modulate the response gain more than tenfold and is hence physiologically relevant.

Optimization in Visual Motion Estimation.

Sighted animals use visual signals to discern directional motion in their environment. Motion is not directly detected by visual neurons, and it must instead be computed from light signals that vary over space and time. This makes visual motion estimation a near universal neural computation, and decades of research have revealed much about the algorithms and mechanisms that generate directional signals. The idea that sensory systems are optimized for performance in natural environments has deeply impacted this research. In this article, we review the many ways that optimization has been used to quantitatively model visual motion estimation and reveal its underlying principles. We emphasize that no single optimization theory has dominated the literature. Instead, researchers have adeptly incorporated different computational demands and biological constraints that are pertinent to the specific brain system and animal model under study. The successes and failures of the resulting optimization models have thereby provided insights into how computational demands and biological constraints together shape neural computation.

Broken time reversal symmetry in visual motion detection.

Our intuition suggests that when a movie is played in reverse, our perception of motion in the reversed movie will be perfectly inverted compared to the original. This intuition is also reflected in many classical theoretical and practical models of motion detection. However, here we demonstrate that this symmetry of motion perception upon time reversal is often broken in real visual systems. In this work, we designed a set of visual stimuli to investigate how stimulus symmetries affect time reversal symmetry breaking in the fruit fly Drosophila 's well-studied optomotor rotation behavior. We discovered a suite of new stimuli with a wide variety of different properties that can lead to broken time reversal symmetries in fly behavioral responses. We then trained neural network models to predict the velocity of scenes with both natural and artificial contrast distributions. Training with naturalistic contrast distributions yielded models that break time reversal symmetry, even when the training data was time reversal symmetric. We show analytically and numerically that the breaking of time reversal symmetry in the model responses can arise from contrast asymmetry in the training data, but can also arise from other features of the contrast distribution. Furthermore, shallower neural network models can exhibit stronger symmetry breaking than deeper ones, suggesting that less flexible neural networks promote some forms of time reversal symmetry breaking. Overall, these results reveal a surprising feature of biological motion detectors and suggest that it could arise from constrained optimization in natural environments. Significance: In neuroscience, symmetries can tell us about the computations being performed by a circuit. In vision, for instance, one might expect that when a movie is played backward, one's motion percepts should all be reversed. Exact perceptual reversal would indicate a time reversal symmetry, but surprisingly, real visual systems break this symmetry. In this research, we designed visual stimuli to probe different symmetries in motion detection and identify features that lead to symmetry breaking in motion percepts. We discovered that symmetry breaking in motion detection depends strongly on both the detector's architecture and how it is optimized. Interestingly, we find analytically and in simulations that time reversal symmetries are broken in systems optimized to perform with natural inputs.