Motor Control of Rapid Eye Movements in Larval Zebrafish

Animals move the same body parts in diverse ways. How the central nervous system executes one action over related ones is poorly understood. To investigate this, I assessed the behavioural manifestation and neural control of saccadic eye rotations made by larval zebrafish, since these movements are simple and easy to investigate at a circuit level. I first classified the larva’s saccadic repertoire into 5 types, of which hunting specific convergent saccades and exploratory conjugate saccades were the main types used to orient vision. Convergent and conjugate saccades shared a nasal eye rotation, which had kinematic differences and similarities that suggested the rotation was made by overlapping but distinct populations of neurons between saccade types. I investigated this further, using two-photon Ca2+ imaging and selective circuit interventions to identify a circuit from rhombomere 5/6 to abducens internuclear neurons to motoneurons that was crucial to nasal eye rotations. Motoneurons had distinct activity patterns for convergent and conjugate saccades that were consistent with my behavioural observations and were explained largely by motoneuron kinematic tuning preferences. Surprisingly, some motoneurons also modulated activity according to saccade type independent of movement kinematics. In contrast, pre-synaptic internuclear neuron activity profiles were almost entirely explained by movement kinematics, but not neurons in rhombomere 5/6, which had mixed saccade type and kinematic encoding, like motoneurons. Regions exerting descending control on this circuit from the optic tectum and anterior pretectal nucleus had few neurons tuned to saccade kinematics compared to neurons selective for convergent saccades. My results suggest a transformation from encoding action type to encoding movement kinematics at successive circuit levels. This transformation was not monotonic or complete, and suggests that control of even simple, highly comparable, movements cannot be entirely described by a shared kinematic encoding scheme at a motor or premotor level

Nucleus Isthmi Is Required to Sustain Target Pursuit during Visually Guided Prey-Catching

Animals must frequently perform a sequence of behaviors to achieve a specific goal. However, the neural mechanisms that promote the continuation and completion of such action sequences are not well understood. Here, we characterize the anatomy, physiology, and function of the nucleus isthmi (NI), a cholinergic nucleus thought to modulate tectal-dependent, goal-directed behaviors. We find that the larval zebrafish NI establishes reciprocal connectivity with the optic tectum and identify two distinct types of isthmic projection neuron that either connect ipsilaterally to retinorecipient laminae of the tectum and pretectum or bilaterally to both tectal hemispheres. Laser ablation of NI caused highly specific deficits in tectally mediated loom-avoidance and prey-catching behavior. In the context of hunting, NI ablation did not affect prey detection or hunting initiation but resulted in larvae failing to sustain prey-tracking sequences and aborting their hunting routines. Moreover, calcium imaging revealed elevated neural activity in NI following onset of hunting behavior. We propose a model in which NI provides state-dependent feedback facilitation to the optic tectum and pretectum to potentiate neural activity and increase the probability of consecutive prey-tracking maneuvers during hunting sequences

Binocular eye movements in humans: from optimal binocularity to strabismus

Binocular vision and stereopsis is one of the major characteristics of frontal eyed species, such as humans. Binocular vision results from the projection of 3D-objects on corresponding area's of the retina's of the two eyes. An individual with. binocular vision perceives one single image of the object containing depth information (binocular vision). The minute differences between the' left and the right image (disparity) ideally give rise to stereoscopic vision. This entity of the binocular system has been used for artificial stereopsis in 3-D pictures and movies. The importance of binocular vision in daily life becomes clear when individuals with optimal binocular coordination are forced to use mostly one eye (e.g. because of an eye infection). Their loss of binocular visual information causes difficulties with simple tasks, like shaking hands. Individuals who have sub-optimal binocularity for a longer period can function very well due to adaptive mechanisms and the use of nonstereoscopic depth cues. Many people are unaware of their slight sub-optimal binocularity which only a stereopsis test reveals. People with low vision in one of their eyes, have less binocularity but the use of both eyes can still be very useful for the detection of large moving objects such as approaching cars. They usually have no impairments in their daily tasks. Individuals with vision in one eye only adapt mainly by making more head movements. With this adaptation they have no difficulties, even with tasks like driving a car

Exploring behavioral circuits with holographic optogenetics and network imaging

Included works This thesis contains three previously published works: Semmelhack, Donovan, et al.; eLife 2014 Temizer, Donovan, et al.; Current Biology 2015 Thiele, Donovan, Baier; Neuron 2014 And one full manuscript, soon to be in the second round of review: Dal Maschio*, Donovan*, et al. *(Equal contributions) The work presented in these manuscripts is equivalent to a standard thesis

Toward a second-person neuroscience

LS & BT : equal contributions (shared first-authorship)Peer reviewedPreprin

Neuronal circuits underlying visual attention during naturalistic behaviour in zebrafish larvae

To survive, animals need to sustain behavioural responses towards specific environmental stimuli to achieve an overall goal. One example is the hunting behaviour of zebrafish larvae, which is characterised by a set of discrete visuomotor events that begin with prey detection, followed by target-directed swims and end with prey capture. Several studies have begun elucidating the neuronal circuits that govern prey detection and initiation of hunting routines, which are largely dependent on the midbrain optic tectum (OT). However, it is not known how the brain is able to sustain a behavioural routine directed towards a specific target, especially in complex environments containing distractors. In this study, I have discovered that the nucleus isthmus (NI), a midbrain nucleus implicated in visual attention in other vertebrates, is required for sustained tracking of prey during hunting routines in zebrafish larvae. NI neurons co-express cholinergic and glutamatergic markers and possess two types of axonal projection morphology. The first type targets the ipsilateral OT and AF7, a retinorecipient pretectal region involved in hunting. The second type projects bilaterally to the deep OT layers. Laser ablation of the NI followed by tracking of naturalistic hunting behaviour, revealed that while hunting initiation rates and motor kinematics were unaltered, ablated animals showed an elevated probability of aborting hunting routines midway. Moreover, 2-photon calcium imaging of tethered larvae during a closed-loop virtual reality hunting assay, showed that NI neurons are specifically active during hunting. These results suggest that the NI supports the maintenance of action sequences towards specific prey targets during hunting, most likely by modulating pretectal and tectal activity. This in turn supports its presence at the centre of an evolutionarily conserved circuit to control selective attention to ethologically relevant stimuli in the presence of competing distractors

Emotion word processing: evidence from electrophysiology, eye movements and decision making

A degree of confusion currently exists regarding how the emotionality of a textual stimulus influences its processing. Despite a wealth of research recently being conducted in the area, heterogeneity of stimuli used and methodologies utilized prevented general conclusion from being confidently drawn. This thesis aimed to clarify understanding of cognitive processes associated with emotional textual stimuli by employing well controlled stimuli in a range of simple but innovative paradigms. Emotion words used in this thesis were defined by their valence and arousal ratings. The questions asked here concerned early stages of processing of emotional words, the attention capturing properties of such words, any spill-over effects which would impact the processing of neutral text presented subsequently to the emotional material, and the effect of emotional words on higher cognitive processes such as attitude formation. The first experiment (Chapter 2) manipulated the emotionality of words (positive, negative, neutral) and their frequency (HF – high frequency, LF – low frequency) while ERPs were recorded. An emotion x frequency interaction was found, with emotional LF words responded to fastest, but only positive LF words responded to fastest. Negative HF words were also associated with a large N1 component. Chapter 3 investigated the attention-capturing properties of positive and negative words presented above and below a central fixation cross. The only significant effects appeared when a positive word was presented in the top condition, and a negative word in the bottom condition. Here saccade latencies were longer and there were a fewer number of errors made. Chapter 4 reports an eye tracking study which examined the effect of target words’ emotion (positive, negative, neutral) and their frequency (HF, LF). The pattern of results, produced in a variety of fixation time measurements such as first fixation duration and single fixation duration, was similar to those reported in Chapter 2. The existence of any spill-over effect of emotion onto subsequently presented neutral text was examined in a number of ways. Chapter 5 describes priming with emotional primes and neutral targets but no effect of emotion was found. Chapter 6 employed the same design as Chapter 4 but presented positive, negative or neutral sentences in the middle of neutral paragraphs. It was found that the positive sentences were read fastest, but the neutral sentences following the negative sentences were read faster than those following neutral sentences. Chapters 7 and 8 employed a version of the Velten mood-induction tool to examine the effect of mood when reading emotional text. Chapter 7 was a replication of Chapter 4 with 4 participant groups: positive, negative and neutral mood. While the neutral group showed similar results to those produced in Chapter 4, the positive group only fixated on the positive HF words faster, the negative group showed a frequency effect within each emotional word type, but within HF words positive words were viewed for less time than neutral words. Chapter 8 had participants read 4 product reviews and then afterwards rate each of the products on a set of semantic differentials. This was a 3 (mood: positive, negative, neutral) x 2 (message type: positive negative) x 2 (word type: positive negative). There was no effect of mood but positive messages were read quicker when they contained positive words and negative messages were read quicker when they contained negative words. Participants were asked to recommend each product to individuals in either a prevention in a promotion focus. When the focus was prevention there were additive effects of message and word type, but when the focus was positive there was an interaction, with the positive message conveyed using negative words being rated highest. The same pattern also emerged in the series of semantic differentials. Possible mechanisms to account for these findings are discussed, including many incarnations of McGinnies’s (1949) perceptual defense theory. Future studies should possibly aim to combine the current knowledge with motivational, goal-orientated models such as Higgins’s (1998) theory of regulatory focus

Two-photon all-optical interrogation of mouse barrel cortex during sensory discrimination

The neocortex supports a rich repertoire of cognitive and behavioural functions, yet the rules, or neural ‘codes’, that determine how patterns of cortical activity drive perceptual processes remain enigmatic. Experimental neuroscientists study these codes through measuring and manipulating neuronal activity in awake behaving subjects, which allows links to be identified between patterns of neural activity and ongoing behaviour functions. In this thesis, I detail the application of novel optical techniques for simultaneously recording and manipulating neurons with cellular resolution to examine how tactile signals are processed in sparse neuronal ensembles in mouse somatosensory ‘barrel’ cortex. To do this, I designed a whisker-based perceptual decision-making task for head-fixed mice, that allows precise control over sensory input and interpretable readout of perceptual choice. Through several complementary experimental approaches, I show that task performance is exquisitely coupled to barrel cortical activity. Using two- photon calcium imaging to simultaneously record from populations of barrel cortex neurons, I demonstrate that different subpopulations of neurons in layer 2/3 (L2/3) show selectivity for contralateral and ipsilateral whisker input during behaviour. To directly test whether these stimulus-tuned groups of neurons differentially impact perceptual decision-making I performed patterned photostimulation experiments to selectively activate these functionally defined sets of neurons and assessed the resulting impact on behaviour and the local cortical network in layer 2/3. In contrast with the expected results, stimulation of sensory-coding neurons appeared to have little perceptual impact on task performance. However, activation of non- stimulus coding neurons did drive decision biases. These results challenge the conventional view that strongly sensory responsive neurons carry more perceptual weight than non-responsive sensory neurons during perceptual decision-making. Furthermore, patterned photostimulation revealed and imposed potent surround suppression in L2/3, which points to strong lateral inhibition playing a dominant role in shaping spatiotemporally sparse activity patterns. These results showcase the utility of combined patterned photostimulation methods and population calcium imaging for revealing and testing neural circuit function during sensorimotor behaviour and provide new perspectives on sensory coding in barrel cortex