Biology subjects

Juusola, M.

Publications and source records attributed to Juusola, M..

10 recordsLinked to original sources

bioRxiv2026

Associative learning reconfigures colour-guided behaviour in Drosophila larvae

Colour-guided behaviour can be innate or learned, yet how a numerically simple nervous system assigns value to spectral cues remains unresolved. We developed an automated differential-conditioning assay in which third-instar Drosophila larvae were trained with blue and green light, one of which was paired with electric shock. Wild-type larvae subsequently avoided the shock-paired colour, whereas naive controls showed no strong blue-green preference. This learned avoidance persisted when the relative intensity cue was transferred to the alternative colour, indicating that behavioural choice followed spectral identity rather than simply tracking the dimmer stimulus. Disruption of either the blue-sensitive Rh5 or green-sensitive Rh6 photoreceptor pathway significantly weakened the training-induced behavioural shift relative to wild type, with Rh5 mutants showing only a small residual shift. Complementary extracellular recordings confirmed light-evoked activity in the larval visual system, with robust responses to ultraviolet and broadband white illumination but smaller and more variable responses to blue and green light. Together, these results show that larvae can discriminate blue from green, demonstrating elementary colour vision; that associative experience reconfigures their colour-guided behaviour; and that both photoreceptor pathways contribute to this learned response. This paradigm separates learned spectral preference from baseline light preference and provides a tractable framework for linking defined sensory inputs to the circuit mechanisms of visual memory.

neuroscience↗

bioRxiv2026

A model of depth-dependent responses from neural superposition in fly compound eyes

Fly compound eyes pool signals from photoreceptors that sample the same region of visual space through neural superposition in the lamina. The optical axes of photoreceptors projecting to a single lamina cartridge are not perfectly parallel, but instead converge at a point a few millimeters in front of the eye. At short viewing distances (1--10mm) this leads to distance-dependent differences in receptive field overlap. We explored whether it was possible that flies could sense depth in this "personal space" purely from the geometry of neural superposition. To this end, we combined a computational model of fly eye optics with a disparity-tuned lamina model originally developed for stereoscopic prey capture in praying mantises, and simulated the responses of lamina monopolar cells to moving stimuli at different distances. Across variations in stimulus parameters and lamina models, we found that lamina neuron responses indeed contain a distance-dependent component, which can overall be summarised as an enhanced response due to temporally overlapping receptor responses at a critical distance of 3--4mm, the convergence distance of photoreceptor axes. Depending on the lamina model and stimulus size, either response amplitude or onset gradient, or both, exhibited this peak. We further show that changes in eye size systematically shift this preferred distance, such that larger flies had a peak response at a greater distance. Our results demonstrate that lamina cell responses may contain a robust, geometry-derived component that is specific to object distance and invariant to other stimulus properties. This suggests that neural superposition, beyond improving sensitivity, may function analogously to a light-field camera system that is effectively "focused" on a behaviorally relevant distance.

neuroscience↗

bioRxiv2025

Synaptic high-frequency jumping synchronises vision to high-speed behaviour

During high-speed behaviour, animals must synchronise perception and action despite rapid environmental and self-generated motion. How neural systems achieve such precision remains unclear. Here we show how the housefly (Musca domestica) maintains visual accuracy during fast motion. Using intracellular and photomechanical recordings during saccade-like stimulation, we traced information flow from photoreceptors to large monopolar cells (LMCs). Visual neurons achieved record-high information sampling ([~]2,500 bits{middle dot}s-1) and synaptic transmission ([~]4,100 bits{middle dot}s-1), far exceeding previous estimates. We identify a previously unknown mechanism - synaptic high-frequency jumping - in which photoreceptor-LMC synapses dynamically shift transmission toward higher frequencies during saccades, extending visual bandwidth to [~]1,000 Hz, effectively eliminating synaptic delays, and quadrupling classical flicker-fusion limits ([~]230 Hz). Behavioural experiments show flies respond synchronously within [~]13-20 ms, even before photoreceptor responses peak. A biophysically realistic model reveals how photomechanical-stochastic-refractory quantal sampling co-adapts with saccadic behaviour: through self-motion, flies efficiently translate image motion into temporally-precise, predictive high-speed vision.

neuroscience↗

bioRxiv2025

Drosophila larval Odd neurons process innate and learned information to regulate chemotaxis behavior

Adaptive decision-making emerges from the integration of innate preferences and learned experiences to guide behavior. Using Drosophila larvae as a model, we investigated the neural circuitry underlying olfactory processing, focusing on Odd neurons--a distinct neuronal population that receives innate valence signals from Kenyon cells (KCs) rather than lateral horn inputs. Through larval connectomics, trans-tango labeling, and detailed anatomical analyses, we found that Odd neurons integrate innate valence through dendro-dendritic connections with KCs and learned valence via inhibitory inputs from mushroom body output neurons (MBON-g1 and MBON-g2). Optogenetic silencing of Odd neurons disrupted larval chemotaxis and associative memory by selectively impairing memory retrieval, while sparing memory formation, and by altering the turn rate during navigation. These results suggest that Odd neurons serve as an integrative hub, converging innate olfactory cues with learned reinforcement signals to fine-tune navigational choices. This study illuminates how distinct neural pathways converge in the larval brain to shape behavior, offering novel insights into decision-making circuitry that may extend to more complex nervous systems.

neuroscience↗

bioRxiv2025

Bumblebee string-pulling skill spreads between colonies under open diffusion conditions

Socially-transmitted behavioural traits can, if they persist in a group of animals over time, give rise to locally-adapted phenotypes that can enhance survival. This capacity is widespread through the animal kingdom, and forms the foundation of cultural inheritance. While social learning is well-documented among insects, and particularly in social insects such as bumblebees, the extent to which such behaviours can spread beyond initial kin groups and persist over time remains largely unknown. String-pulling is a non-natural foraging behaviour where bees must manipulate a string to extract an out-of-reach artificial flower and collect a reward, and has previously shown to spread via social learning. However, this was demonstrated only in highly controlled paired-dyad settings, where interactions between bees were strictly limited. Here, we show that string-pulling can spread both within and between bumblebee colonies and persist over time, under previously-untested open diffusion conditions. These are of greater ecological validity compared with classical paired dyad paradigms, and involve the seeding of a manually-trained demonstrator into a group of naive conspecifics. From this single point of origin, string-pulling behaviour spread rapidly within original, primary colonies. Once the behaviour was established in the primary colonies, secondary colonies were introduced, and string-pulling was also acquired by these new foragers. Furthermore, string-pulling was acquired through individual trial-and-error learning by a small number of bees in control colonies, which lacked trained demonstrators. These results confirm and build upon previous findings in bumblebees, and contribute to a growing body of evidence suggesting that social learning enables animals to establish local behavioural adaptations in the absence of the computational power provided by large brains.

animal behavior and cognition↗

bioRxiv2021

High-speed imaging of light-induced photoreceptor microsaccades in compound eyes

Inside compound eyes, photoreceptors contract to light changes, sharpening retinal images of the moving world in time. Current methods to measure these so-called photoreceptor microsaccades in living insects are spatially limited and technically challenging. Here, we present goniometric high-speed deep pseudopupil (GHS-DPP) microscopy to assess how the rhabdomeric insect photoreceptors and their microsaccades are organised across the compound eyes. This method enables non-invasive rhabdomere orientation mapping, whilst their microsaccades can be locally light-activated, revealing the eyes underlying active sampling motifs. By comparing the microsaccades in wild-type Drosophilas open rhabdom eyes to spam-mutant eyes, reverted to an ancestral fused rhabdom state, and honeybees fused rhabdom eyes, we show how different eye types sample light information. These results show different ways compound eyes initiate the conversion of spatial light patterns in the environment into temporal neural signals and highlight how this active sampling can evolve with insects visual needs.

neuroscience↗

bioRxiv2021

Binocular Mirror-Symmetric Microsaccadic Sampling of Hyperacute 3D-Vision

Neural mechanisms behind stereopsis, which requires simultaneous disparity inputs from two eyes, have remained mysterious. Here we show how ultrafast mirror-symmetric photomechanical contractions in the frontal forward-facing left and right eye photoreceptors give Drosophila super-resolution 3D-vision. By interlinking multiscale in vivo assays with multiscale simulations, we reveal how these photoreceptor microsaccades - by verging, diverging and narrowing the eyes overlapping receptive fields - channel depth information, as phasic binocular image motion disparity signals in time. We further show how peripherally, outside stereopsis, microsaccadic sampling tracks a flying flys optic flow field to better resolve the world in motion. These results change our understanding of how insect compound eyes work and suggest a general dynamic stereo-information sampling strategy for animals, robots and sensors. Significance statementTo move efficiently, animals must continuously work out their x,y,z-positions in respect to real-world objects, and many animals have a pair of eyes to achieve this. How photoreceptors actively sample the eyes optical image disparity is not understood because this fundamental information-limiting step has not been investigated in vivo over the eyes whole sampling matrix. This integrative multiscale study will advance our current understanding of stereopsis from static image disparity comparison to a new morphodynamic active sampling theory. It shows how photomechanical photoreceptor microsaccades enable Drosophila super-resolution 3D-vision and proposes neural computations for accurately predicting these flies depth-perception dynamics, limits, and visual behaviors.

neuroscience↗

bioRxiv2020

Selective human tau protein expression in different clock circuits of the Drosophila brain disrupts different aspects of sleep and circadian rhythms

Circadian behavioural deficits, such as increased daytime naps and reduced night-time sleep, are common in Alzheimers disease and other tauopathies. But it has remained unclear whether these circadian abnormalities arise from tau pathology in either the master pacemaker or downstream neurons. Here we study this question by selectively expressing different human tau proteins in specific Drosophila brain circuits and monitoring locomotor activity under light-dark (LD) and in "free-running" dark-dark (DD) conditions. We show that expressing human tau proteins in the fly brain recapitulates faithfully several behavioural changes found in tauopathies. We identify discrete neuronal subpopulations within the clock network as the primary target of distinct circadian behavioural disturbances in different environmental conditions. Specifically, we show that the PDF-positive pacemaker neurons are the main site for night-activity gain and -sleep loss, whereas the non-PDF clock-neurons are the main site of reduced intrinsic behavioural rhythmicity. Bioluminescence measurements revealed that the molecular clock is intact despite the behavioural arrhythmia. Our results establish that dysfunction in both the central clock- and afferent clock-neurons jointly contribute to the circadian locomotor activity rhythm disruption in Drosophila expressing human tau. Significance StatementThis study directly links in vivo human tau protein expression in region-specific Drosophila clock-neurons with the resulting sleep and circadian rhythm deficits to extract new knowledge of how Alzheimers disease and other tauopathies perturb the balance of activity and sleep. We anticipate that this novel approach will provide a useful general template for other studies of neurodegeneration in model organisms, seeking to dissect the impact of neurodegenerative disease on circadian behaviour, and further deepening our understanding of how the clock-neuron network works.

neuroscience↗

bioRxiv2018

Ca2+-activated K+ channels reduce network excitability, improving adaptability and energetics for transmitting and perceiving sensory information

Ca2+-activated K+ channels (BK and SK) are ubiquitous in synaptic circuits, but their role in network adaptation and sensory perception remains largely unknown. Using electrophysiological and behavioral assays and biophysical modelling, we discover how visual information transfer in mutants lacking the BK channel (dSlo-), SK channel (dSK-) or both (dSK-;;dSlo-) is shaped in the Drosophila R1-R6 photoreceptor-LMC circuits (R-LMC-R system) through synaptic feedforward-feedback interactions and reduced R1-R6 Shaker and Shab K+ conductances. This homeostatic compensation is specific for each mutant, leading to distinctive adaptive dynamics. We show how these dynamics inescapably increase the energy cost of information and distort the mutants motion perception, determining the true price and limits of homeostatic compensation in an in vivo genetic animal model. These results reveal why Ca2+-activated K+ channels reduce network excitability (energetics), improving neural adaptability for transmitting and perceiving sensory information.\n\nHighlightsO_LIHomeostatic plasticity in Drosophila dSlo-, dSK- and dSK-;;dSlo- null-mutants retains R1-R6 photoreceptors light information sampling while reducing other K+-conductances\nC_LIO_LIAs mutant R-LMC-R circuits rebalance synaptic loads homeostatically, R1-R6s become more depolarized, with dSK- and dSK-;;dSlo- responding faster and dSlo- slower, whilst LMC outputs oscillate, with dSK- responding faster and dSK-;;dSlo- and dSlo- slower than wild-type\nC_LIO_LIHomeostatic compensation in the mutant circuits impedes adaptation, increases the energy cost of visual information and distorts optomotor behavior\nC_LIO_LIHence, Ca2+-activated K+ channels improve adaptability and energetics for transmitting and perceiving sensory information\nC_LI

neuroscience↗

bioRxiv2016

Microsaccadic information sampling provides Drosophila hyperacute vision

Small fly eyes should not see fine image details. Because flies exhibit saccadic visual behaviors and their compound eyes have relatively few ommatidia (sampling points), their photoreceptors would be expected to generate blurry and coarse retinal images of the world. Here we demonstrate that Drosophila see the world far better than predicted from the classic theories. By using electrophysiological, optical and behavioral assays, we found that R1-R6 photoreceptors encoding capacity in time is maximized to fast high-contrast bursts, which resemble their light input during saccadic behaviors. Whilst over space, R1-R6s resolve moving objects at saccadic speeds beyond the predicted motion-blur-limit. Our results show how refractory phototransduction and rapid photomechanical photoreceptor contractions jointly sharpen retinal images of moving objects in space-time, enabling hyperacute vision, and explain how such microsaccadic information sampling exceeds the compound eyes optical limits. These discoveries elucidate how acuity depends upon photoreceptor function and eye movements.

neuroscience↗