Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2005. / Includes bibliographical references. / In the world, objects and settings tend to co-occur. Cars usually appear on streets with other vehicles, not in kitchens next to refrigerators. The present studies provide evidence that the semantic consistency of an object and its setting is available in a glimpse and affects perception. Objects are perceived more accurately in typical rather than atypical settings and when they appear with related objects regardless of the setting. Backgrounds are perceived more accurately when they contain plausible rather than unlikely foreground objects. Objects and scenes are processed interactively, not in isolation. / by Jodi L. Davenport. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, February 2011. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / Inhibitory interneurons are thought to play a crucial role in several features of neocortical processing, including dynamics on the timescale of milliseconds. Their anatomical and physiological characteristics are diverse, suggesting that different types regulate distinct aspects of neocortical dynamics. Interneurons expressing parvalbumin (PV) and somatostatin (SOM) form two non-overlapping populations. Here, I describe computational, correlational (neurophysiological) and causal (optogenetic) studies testing the role of PV and SOM neurons in dynamic regulation of sensory processing. First, by combining extra- and intracellular recordings with optogenetic and sensory stimulation and pharmacology, we have shown that PV cells play a key role in the generation of neocortical gamma oscillations, confirming the predictions of prior theoretical and correlative studies. Following this experimental study, we used a biophysically plausible model, simulating thousands of neurons, to explore mechanisms by which these gamma oscillations shape sensory responses, and how such transformations impact signal relay to downstream neocortical areas. We found that the local increase in spike synchrony of sensory-driven responses, which occurs without decreasing spike rate, can be explained by pre- and post-stimulus inhibition acting on pyramidal and PV cells. This transformation led to increased activity downstream, constituting an increase in gain between the two regions. This putative benefit of PV-mediated inhibition for signal transmission is only realized if the strength and timing of inhibition in the downstream area is matched to the upstream source. Second, we tested the hypothesis that SOM cells impact a distinct form of dynamics, sensory adaptation, using intracellular recordings, optogenetics and sensory stimulation. In resting neocortex, we found that SOM cell activation generated inhibition in pyramidal neurons that matched that seen in in-vitro studies. Optical SOM cell activation also transformed sensory-driven responses, decreasing evoked activity. In adapted responses, optical SOM cell inactivation relieved the impact of sustained sensory input, leading to increased membrane potential and spike rate. In contrast, SOM cell inactivation had minimal impact on sensory responses in a non-adapted neocortex, supporting the prediction that this class of interneurons is only recruited when the network is in an activated state. These findings present a previously unappreciated mechanism controlling sensory adaptation. / by Ulf Knoblich. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 1995. / Includes bibliographical references (p. 83-89). / by Thomas M. Brashers-Krug. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 1997. / Includes bibliographical references. / by Alessandra Angelucci. / Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 117-122). / This thesis aims to elucidate shared mechanisms between retrieval in sentence processing and memory retrieval processes in nonlinguistic domains using an individual differences approach. Prior research in individual differences in sentence processing has provided conflicting evidence as to whether the same memory mechanisms operate in linguistic processing, potentially a quite specialized cognitive domain, and in other, more general areas of cognition (Just & Carpenter, 1992; Caplan & Waters, 1999). This question has been primarily addressed from the point of view of capacity-based theories of working memory (Baddeley, 1986). Under these theories, verbal working memory is either comprised of multiple components including separate components for syntactic and non-syntactic verbal processing, or is dependent on a unitary pool of resources shared across all verbal domains. However, recent memory research has suggested that the capacity-theory architecture may be incorrect. Instead of a three-part memory system composed of focal attention, working memory, and long-term memory, a better model of the memory system may be bipartite, comprising focal attention and long-term memory. In the bipartite theory, working memory is viewed as a set of mechanisms mediating between these two stores, and accurately describes empirical data (McElree, 2006). If the latter hypothesis is correct, then it follows that the bipartite system underlying sentence processing should rely on the same set of working memory mechanisms as in general memory processes. In particular, a number of empirical studies have shown that both general memory and sentence processing are subject to interference from contextually-relevant intervening elements. Such interference is thought to occur at retrieval (as opposed to encoding) both for general memory tasks (e.g., retrieving items from a list) and in sentence processing (e.g., retrieving elements in long-distance syntactic dependencies). However, no systematic attempts have been made to investigate whether this interference results from the same processing limitations. In Study 1, performance on a battery of memory and cognitive tasks is compared to performance on sentence processing tasks. One of the sentence processing tasks correlated with multiple measures likely to rely on general memory mechanisms involved in resolution of retrieval interference. However, low internal reliability of the language tasks in the first study was observed. In Study 2, a series of sentence processing tasks is examined in order to determine which tasks exhibit the highest internal reliability. The results indicate that syntactic complexity manipulations presented in null (isolated) contexts exhibit highest internal reliability and are good candidates for future studies investigating individual differences in sentence processing. Suggestions for future studies investigating shared resources between sentence processing tasks and general memory mechanism are then discussed, informed by the results from these studies. / by Melissa L. Troyer. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references. / A fundamental goal in neuroscience is to understand mechanisms underlying the ability to create memories from sensory experience. While large structures such as the hippocampus are known to be critical for certain types of learning, memories are ultimately thought to be represented in sparsely distributed neuronal ensembles within these larger structures. Currently, there are few tools that allow for the identification and manipulation of these ensembles, which has limited our understanding of the molecular and cellular processes underlying learning and memory. We have previously reported that the activity-regulated transcription factor Npas4 is selectively induced in a sparse population of CA3 following contextual fear conditioning. Global knockout or selective deletion of Npas4 in CA3 both resulted in impaired contextual memory, and restoration of Npas4 in CA3 was sufficient to reverse the deficit in global knockout mice. Taking advantage of the critical role of Npas4 in contextual memory formation, we developed a set of novel molecular tools to gain access to cell populations activated by experience. Using this system, we identified and manipulated the properties of neurons activated by behavioral experience in a variety of neural circuits in mice, rats, and Drosophila. We believe that the tools developed in this thesis can provide a major advancement in the field, and will allow researchers to target any neural circuit activated by experience in a variety of species. / by Kartik Ramamoorthi. / Ph. D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2005. / Includes bibliographical references (leaves 93-97). / A fundamental consideration in understanding neuronal networks is determining what sets the unitary functional strength of a synaptic connection between two cells, and what meta-processes such mechanisms answer to. This question can be asked either mechanistically, by characterizing the molecular processes that regulate the synapse's transmission, or ontologically, by considering the impact of this transmission from the standpoint of neighboring synapses, the post synaptic cell itself, or the network of neurons in which it is situated. The work presented here practices both approaches, by identifying a new molecular mechanism to which the quantal size of excitatory synapses can be attributed, and then beginning to explore how quantal size might be influenced by network activity and architecture. Chapter 1 identifies a new molecular control point for specifying the quantal size of excitatory transmission in the mammalian brain, and provides the first known demonstration that such specification can be provided by a presynaptic process. It then details how this mechanism is regulated by network activity. / (cont.) Chapter 2 makes use of a new methodology for designing the physical structure of cultured networks in order to begin to perturb network parameters and explore the role of quantal size in a network context. Applying this methodology I demonstrate that the quantal size of excitatory synapses is scaled by neurons inversely with their number of synaptic connections, and identify a set of mechanisms by which network firing is maintained at a constant level when the number of active synaptic partners is scaled. Chapter 3 demonstrates progress towards a methodology capable of producing and detecting changes in the unitary strength of multiple synapses with respect to one another within a dendritic tree. Future efforts can hopefully make use of similar principles to directly visualize how heterosynaptic processes establish and maintain a contrast in the strengths of unitary synapses to enable representation by synaptically-based memory traces. / by Nathan R. Wilson. / Ph.D.

Thesis: Ph. D. in Neuroscience, Massachusetts Institute of Technology, Department of Brain and Cognitive Sciences, 2018. / Cataloged from PDF version of thesis. Page 176 blank. / Includes bibliographical references (pages 159-175). / Despite abundant evidence that dopamine modulates medial prefrontal cortex (mPFC) activity to mediate diverse behavioral functions, the precise circuit computations remain elusive. One potentially unifying theoretical model by which dopamine can modulate functions from working memory to schizophrenia is that dopamine serves to increase the signal-to-noise ratio in mPFC neurons, where neuronal activity conveying sensory information (signal) are amplified relative to spontaneous firing (noise). To connect theory to biology, we lack direct evidence for dopaminergic modulation of signal-to-noise in neuronal firing patterns in vivo and a mechanistic explanation of how such computations would be transmitted downstream to instruct specific behavioral functions. Here, we demonstrate that dopamine increases signal-to-noise ratio in mPFC neurons projecting to the dorsal periaqueductal gray (dPAG) during the processing of an aversive stimulus. First, using electrochemical approaches, we reveal the precise time course of tail pinch-evoked dopamine release in the mPFC. Second, we show that dopamine signaling in the mPFC biases behavioral responses to punishment-predictive stimuli, rather than reward-predictive cues. Third, in contrast to the well-characterized mPFC-NAc projection, we show that activation of mPFC-dPAG neurons is sufficient to drive place avoidance and defensive behaviors. Fourth, to determine the natural dynamics of individual mPFC neurons, we performed single-cell projection-defined microendoscopic calcium imaging to reveal a robust preferential excitation of mPFC-dPAG, but not mPFC-NAc, neurons to aversive stimuli. Finally, photostimulation of VTA dopamine terminals in the mPFC revealed an increase in signal-to-noise ratio in mPFC-dPAG neuronal activity during the processing of aversive, but not rewarding stimuli. Together, these data unveil the utility of dopamine in the mPFC to effectively filter sensory information in a valence-specific manner. / by Caitlin Miya Vander Weele. / Ph. D. in Neuroscience

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 1997. / Includes bibliographical references (p. 170-175). / by Raquel Olguin Jaakkola. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2004. / Includes bibliographical references. / How does the visual system achieve stable estimates of surface properties - such as reflectance and 3D shape - across changes in the illumination? Under arbitrary patterns of illumination this problem is ill-posed. However, in the real world, illumination is not arbitrary. Here I argue that the visual system exploits the statistical regularities of real-world illuminations to achieve stable estimates of shape and surface reflectance properties. Specifically, I suggest that the visual system derives measurements from specular reflections that are (i) diagnostic of surface properties and (ii) relatively well-conserved across real-world scenes. One consequence of the theory is that the visual system does not have to estimate and explicitly 'discount' illumination to recover shape and surface reflectance. In support of this idea, subjects are shown to be good at estimating surface reflectance and 3D shape without any context to specify the surrounding scene, as long as the illumination is realistic. However, when the pattern of illumination is unrealistic, shape and surface reflectance estimation degrade in predictable ways. Systematic manipulation of illumination statistics reveals some properties of illumination that are important for surface reflectance estimation. To understand 3D shape constancy, I discuss the way that 3D surface curvature distorts the reflected world. For the special case of mirrored surfaces, I show how populations of oriented linear filters can 'read' the pattern of distortions to recover 3D surface curvatures. Finally I show that this principle applies to cases other than perfect mirrors, and can predict both successes and failures of human shape constancy as the illumination changes. / by Roland W. Fleming. / Ph.D.