Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 176-194).

The study of brain dynamics has traditionally focused study on well-established cortico-cortical, thalamo-cortical, and hippocampo-cortical pathways in the brain, neglecting other brain structures. The superior colliculus (SC) is a highly conserved midbrain structure that displays vast intrinsic, local and global patterns of anatomical connectivity, and therefore presents itself as an interesting alternative structure to study the fundamental principles of neural dynamics. Here, we report for the first time the analysis of both stimulus evoked and spontaneously generated SC neural dynamics across three spatial scales: dynamics intrinsic to the SC, local dynamical interaction with the neighboring inferior colliculus (IC), and large-scale dynamical interaction with the cortex. Within the SC, visual evoked neural dynamics was best characterized by the presence of temporally precise gamma oscillations in retinorecipient superficial SC layers following flash and grating stimuli. Local interareal dynamics in the midbrain were defined by the presence of subthreshold visually evoked activity in the IC that was driven by visual inputs from the SC. Finally, spontaneously generated SC activity is strongly governed by the state of cortical networks, with SC activity locked to prominent slow cortical and spindle oscillations. Collectively, this work provides evidence that nature of neural activity in the SC is strongly governed by both bottom-up sensory and top-down cortical inputs.

Brain development and function depend on intrinsic and extrinsic factors. In particular, the proper functioning of sensory systems can be altered according to the quality of extrinsic sensory information received during life. In this context, questions concerning neuroplasticity take on special relevance when considering that sensory experience has a big impact on the degree of plasticity of the brain. In this thesis, we have sought to understand how visual deprivation affects the development and maintenance of visual centers in the brain and the role of visual deprivation on plasticity throughout life. We have addressed this question by studying the retinocollicular projection, which is the neuronal pathway that connects the retina with a visual input processing center, the superior colliculus (SC). Unexpectedly, we found that in Syrian hamsters (Mesocricetus auratus) the size of receptive fields (RFs) of neurons in the SC is plastic in adult animals if they have been deprived of a minimum of visual experience when juveniles. Specifically, dark-reared (DR) hamsters refine SC RFs as do their normally-reared counterparts, but they lose RF refinement if they remain in the dark after their RFs get refined. We found that a well defined period and duration of visual experience can stabilize RF size in adulthood. Furthermore, we sought to investigate the mechanisms by which RF size is increased in adult DR hamsters. By testing the strength of intracollicular inhibition using electrophysiological and molecular techniques, we have found that visually-deprived animals have weaker inhibitory circuitry in their SC than normal animals. The quantity of GABA receptors and GABA containing neurons is decreased in the SC of adult DR animals. We propose that these results explain at least in part the RF enlargement we find in visually-deprived animals. Knowledge from this study provides general insight into sensory system plasticity in adulthood and new information about visual system development that is relevant for treatments of diseases.

The superior colliculus (SC) is a laminated oculomotor structure in the midbrain. In non-human primates SC has long been known to contain a retinotopically-organized map of visual stimulation in its superficial layers, which is aligned to a map of saccadic eye movements in the deeper layers. Microstimulation and electrophysiology experiments have shown that SC also plays a key role in covert visuospatial attention and suggest that attentional modulation also occurs in a retinotopic manner. Retinotopic organization of the visual field can be non-invasively mapped in humans using functional MRI with a technique called phase-encoded retinotopy. In this technique, rotating wedges and expanding rings of visual stimuli are used to map the polar angle and eccentricity dimensions of a polar coordinates system, respectively. A similar technique can also be used to map spatial attention by keeping the visual stimulus constant and cueing subjects to attend to apertures of rotating wedges and expanding rings within the stimulus. A previous study using fMRI has shown the polar angle representation of visual stimulation in human SC but was unable to find a representation of eccentricity. This work uses high-resolution fMRI along with special surface analysis techniques developed in our lab to demonstrate maps of both polar angle and eccentricity for visual stimulation. Moreover, visual attention is also shown to be topographically organized within SC and in registration with visual stimulation. Finally, in human visual cortex, fMRI is known to show activity for sustained spatial attention even in the absence of a significant visual stimulus, an attentional "base response". In this work, SC is shown to exhibit a similar sustained attention base response using a threshold-contrast detection paradigm. This base response was compared with a response for attention with visual stimulation. The peak amplitude of the base response occurred more deeply within SC tissue than the peak for attention with stimulation. It is proposed that this reflects the specific attentional enhancement of the deeper visuomotor neurons, which are hypothesized to be a direct neuronal correlate of the oculomotor theory of attention. / text

Primates, including humans, explore their visual environment with sequences of gaze fixations interrupted by saccadic eye movements that re-orient the fovea to objects of interest. This visual behavior is thought to involve two separate processes. First, the current foveal image is analyzed and the next object of interest is selected as a saccade target. Second, previously examined objects are retained to prevent their re-examination. Visual behavior has been studied successfully using the visual search paradigm, in which subjects locate a unique target stimulus from amongst multiple distracting stimuli. Models of visual search posit that the process of saccade target selection is guided by a visual salience map. This map receives both stimulus-driven and goal-directed inputs to form representations of visual objects, and a competition between those representations is played out to determine the next saccade target. Neurophysiological studies using nonhuman primates have suggested that the salience map is distributed across a network of brain areas that includes the midbrain superior colliculus (SC). These studies, however, have not ruled out the possibility that selective activity for a saccade target may instead be related to the preparation of the saccade. Moreover, not much is known about the selection of a saccade target beyond the first in a sequence of gaze fixations. Finally, the mechanisms underlying the process of saccade target retention are not well understood. In this thesis, I will investigate the role of the primate SC in visual behavior by recording the activity of single neurons while monkeys perform visual search tasks. The major findings will describe 1) how SC sensory-motor neurons instantiate the visual salience map; 2) how this salience map is dynamically updated so that saccade targets are retained; and 3) how multiple representations on this salience map are processed in parallel for saccade target selection. Given SC’s role in the control of visual behavior and its position within the network involved in cognitive processes, these findings have important implications for our understanding of the neural basis of human cognition and of its dysfunctions in disease states. / Thesis (Ph.D, Neuroscience Studies) -- Queen's University, 2010-12-22 09:52:20.143

Behavioural choices of animals as they acquire resources in the wild are well characterized by foraging theory; however, the neural mechanisms underlying these behaviours are not well understood. The goal of this thesis is to understand the brain mechanisms involved in selecting and executing such foraging behaviours. To do so, rhesus monkeys performed a novel visuosaccadic foraging task while we recorded the activity of single neurons in the intermediate layers of the superior colliculus (SCi). An important innovation of this task is that both target profitability – the measure of value in the simplest case of foraging theory – and saccade choice are measured separately. We hypothesized that target profitability is represented in the SCi in addition to its well characterized role in saccade planning and preparation. Visual Foraging Task: Monkeys harvested coloured dots representing prey items by fixating them for a pre-specified handling time. On each trial, multiple prey are presented, sharing identical physical attributes except that each was one of three colours. All prey of the same colour shared the same profitability [Profitability = reward magnitude (ml)/handling time (s)]. According to foraging theory, intake of reward is maximized if prey are selected in descending order of their profitability. Indeed, we found subjects gradually approached optimal efficiency. We computed an index of the relative subjective profitability of each prey colour, which compared the rank order with which monkeys chose prey of each colour. This subjective index of profitability was then compared to concomitant SC activity attributed to the prey item in the neuron’s response field (RF). First, we found that the amount of SC activity reflected the subjective profitability of the RF targets, and established that this effect was not simply a result of saccade goal planning. Second, profitability information remains dominant throughout the handling period until reward delivery, after which activity also became selective for upcoming saccades. Together, our results highlight the prominent role of target profitability in shaping SCi activity. We propose that profitability information in the SCi may play an important role in resolving competition between numerous target representations to choose the next saccade goal. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2011-02-21 15:37:43.468

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Psychology, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND HUMANITIES. / Vita. / Bibliography: leaves 133-142. / by Bjorn Hellmut Merker. / Ph.D.

As we interact with the world, we must decide what to do next based on previously acquired and incoming information. The study of perceptual decision-making uses highly controlled sensory stimuli and exploits known properties of sensory and motor systems to understand the processes that occur between sensation and action. Even these relatively simple decisions invoke operations like inference, integration of evidence, attention, appropriate action selection, and the assignment of levels of belief or confidence. Thus, the neurobiology of perceptual decision-making offers a tractable way of studying mechanisms that play a role in higher cognitive function. The controlled nature of perceptual decision-making tasks allows an experimenter to infer the latent processes that give rise to a decision. For example, many decisions are well-described by a process of bounded evidence accumulation, in which sensory evidence is temporally integrated until a terminating threshold is exceeded. This thesis improves our understanding of how these latent processes are implemented at the level of neurobiology. After an introduction to perceptual decision-making in Chapter 1, Chapter 2 focuses on the behavioral observations that corroborate whether a subject’s decisions are governed by bounded evidence accumulation. Through simulations of multiple decision-making models, I show that several commonly accepted signatures of evidence accumulation are also predicted by models that do not posit evidence accumulation. I then dissect these models to uncover the features that underlie their mimicry of evidence accumulation. Using these insights, I designed a novel motion discrimination task that was able to better identify the decision strategies of human subjects. In Chapter 3, I explore how the accumulation of evidence is instantiated by populations of neurons in the lateral intraparietal area (LIP) of the macaque monkey. Recordings from single LIP neurons averaged over many decisions have provided support that LIP represents the accumulation of noisy evidence over time, giving rise to diffusion dynamics. However, this diffusion-like signal has yet to be observed directly because of the inability to record from many neurons simultaneously. I used a new generation of recording technology—neuropixels probes optimized for use in primates—to record simultaneously from hundreds of LIP neurons, elucidating this signal for the first time. Through a variety of analyses, I show that the population’s representation of this signal depends on a small subset of neurons that have response fields that overlap the choice targets. Finally, in Chapter 4, I discover a neural mechanism in the midbrain superior colliculus (SC) involved in terminating perceptual decisions. I show that trial-averaged activity in LIP and SC is qualitatively similar, but that single-trial dynamics in each area are distinct. Unlike LIP, SC fired large bursts of activity at the end of the decision, which were sometimes preceded by smaller bursts. Through simultaneous recordings, I uncover the aspects of the diffusion signal in LIP that are predictive of bursting in SC. These observations led me to hypothesize that bursts in SC are the product of a threshold computation involved in terminating the decision and generating the relevant motor response. I confirmed this hypothesis through focal inactivation of SC, which affected behavior and LIP activity in a way that is diagnostic of an impaired threshold mechanism. In total, this work improves our ability to identify the hidden, intermediate steps that underlie decisions and sheds light on their neural basis. All four chapters have been published or posted as separate manuscripts (Steinemann et al., 2022; Stine et al., 2020; Stine et al., 2022; Stine et al., 2019).

No description available.

Deal or no deal? Hold ‘em or fold ‘em? Buy, hold or sell? When faced with uncertainty, a wise decision-maker evaluates each option and chooses the one they deem most valuable. Scientists studying decision making processes have spent much theoretical and experimental effort formalizing a framework that captures how decision makers can maximize the amount of subjective value they accrue from such decisions. This thesis tested two hypotheses. The first was that subjective value guides our simplest and most common of motor actions similar to how it guides more deliberative economic decisions. The second was that subjective value is allocated across pre-motor regions of the brain to make our actions more efficient. To accomplish these goals, I adapted a paradigm used by behavioural economists for use in neurophysiological experiments in non-human primates. In our task, monkeys repeatedly make quick, orienting eye movements, known as saccades, to targets, which they learned through experience, had different values. In support of the hypothesis that subjective value influences simple motor actions, the speed with which monkeys responded, known as saccadic reaction time (SRT), and their saccadic choices to valued targets were highly correlated and therefore both acted as a behavioural measures of subjective value. Two complimentary results support the hypothesis that subjective value influences activity in the intermediate layers of the superior colliculus (SCi) – a well-studied brain region important to the planning and execution of saccades - to produce efficient actions. First, when saccades were elicited with microstimulation, we found that the timing and spatial allocation of pre-saccadic activity in the SC was shaped by subjective value. Second, the baseline preparatory activity and transient visual activity of SCi neurons prior to saccade generation was also influenced by subjective value. Our results can be incorporated into existing models of SC functioning that use dynamic neural field theory. I suggest that saccades of higher subjective value will result in higher activation of their associated neural field such that they will be more likely and more quickly selected. In summary, this thesis demonstrates that subjective value influences neural mechanisms, not only for deliberative decision making, but also for the efficient selection of simple motor actions. / Thesis (Ph.D, Neuroscience Studies) -- Queen's University, 2013-06-25 17:18:25.393