Perceptual Consequences And Neural Mechanisms Of Auditory Adaptation

During everyday listening, we seldom hear sounds in isolation. Whether listening to someone speak over the hum of an air conditioner, or in the bustle of traffic, the things we hear are commonly embedded in background sounds. In the previous example, the acoustic backgrounds, or contexts, have very different statistics. The sound of an air conditioner is composed of sounds that are not very loud nor very quiet; as such, its volume does not vary much over time. On the other hand, the sound of traffic can vary greatly over time, from relatively quiet periods when not many cars are present to very loud sounds such as a car horn or truck engine. In this example, the dynamic range, or contrast, of the sound of the air conditioner is low, while the contrast of the sound of traffic is high. Changes in contrast pose a unique challenge to the auditory system, which is composed of neurons with limited dynamic range. In order to consistently encode changes in volume within these two contexts, auditory neurons adapt the sensitivity of their response to compensate for the change in contrast, a process known as contrast gain control. In this thesis, we test whether contrast gain control affects the way mice hear target sounds (Chapter 2), examine the neural substrates of this behavior (Chapter 3) and explore neural mechanisms for contrast gain control (Chapter 4). We found that stimulus contrast shapes perception and neural encoding of target sounds, that auditory cortex is necessary for this process, and identified a specific inhibitory cell-type that controls gain. Taken together, this body of work demonstrates that the things we hear are shaped by efficient neural encoding of incoming information, and points towards neural mechanisms that would allow targeted manipulation of the process of perception

Unsupervised learning of invariant object representation in primate visual cortex

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.Visual object recognition (categorization and identification) is one of the most fundamental cognitive functions for our survival. Our visual system has the remarkable ability to convey to us visual object and category information in a manner that is largely tolerant ("invariant") to the exact position, size, pose of the object, illumination, and clutter. The ventral visual stream in non-human primate has solved this problem. At the highest stage of the visual hierarchy, the inferior temporal cortex (IT), neurons have selectivity for objects and maintain that selectivity across variations in the images. A reasonably sized population of these tolerant neurons can support object recognition. However, we do not yet understand how IT neurons construct this neuronal tolerance. The aim of this thesis is to tackle this question and to examine the hypothesis that the ventral visual stream may leverage experience to build its neuronal tolerance. One potentially powerful idea is that time can act as an implicit teacher, in that each object's identity tends to remain temporally stable, thus different retinal images of the same object are temporally contiguous. In theory, the ventral stream could take advantage of this natural tendency and learn to associate together the neuronal representations of temporally contiguous retinal images to yield tolerant object selectivity in IT cortex. In this thesis, I report neuronal support for this hypothesis in IT of non-human primates. First, targeted alteration of temporally contiguous experience with object images at different retinal positions rapidly reshaped IT neurons' position tolerance. Second, similar temporal contiguity manipulation of experience with object images at different sizes similarly reshaped IT size tolerance. These instances of experience-induced effect were similar in magnitude, grew gradually stronger with increasing visual experience, and the size of the effect was large. Taken together, these studies show that unsupervised, temporally contiguous experience can reshape and build at least two types of IT tolerance, and that they can do so under a wide range of spatiotemporal regimes encountered during natural visual exploration. These results suggest that the ventral visual stream uses temporal contiguity visual experience with a general unsupervised tolerance learning (UTL) mechanism to build its invariant object representation.by Nuo Li.Ph.D

Uncertainty, generalization, and neural representation of relevant variables for decision making

Dissertation presented to obtain the Ph.D degree in Biology, Computational Biology.Understanding decision making in various contexts is fundamental to understanding human behavior. This thesis presents several studies that examine decision making from many different points of view using a variety of research tools.(...

Neural representation of complex motion in the primate cortex

This dissertation is concerned with how information about the environment is represented by neural activity in the primate brain. More specifically, it contains several studies that explore the representation of visual motion in the brains of humans and nonhuman primates through behavioral and physiological measures. The majority of this work is focused on the activity of individual neurons in the medial superior temporal area (MST) – a high-level, extrastriate area of the primate visual cortex. The first two studies provide an extensive review of the scientific literature on area MST. The area’s prominent role at the intersection of low-level, bottom-up, sensory processing and high-level, top-down mechanisms is highlighted. Furthermore, a specific article on how information about self-motion and object motion can be decoded from a population of MSTd neurons is reviewed in more detail. The third study describes a published and annotated dataset of MST neurons’ responses to a series of different motion stimuli. This dataset is analyzed using a variety of different analysis approaches in the fifth study. Classical tuning curve approaches confirm that MST neurons have large, but well-defined spatial receptive fields and are independently tuned for linear and spiral motion, as well as speed. We also confirm that the tuning for spiral motion is position invariant in a majority of MST neurons. A bias-free characterization of receptive field profiles based on a new stimulus that generates smooth, complex motion patterns turned out to be predictive of some of the tuning properties of MST neurons, but was generally less informative than similar approaches have been in earlier visual areas. The fifth study introduces a new motion stimulus that consists of hexgonal segments and presents an optimization algorithm for an adaptive online analysis of neurophysiological recordings. Preliminary physiological data and simulations show these tools to have a strong potential in characterizing the response functions of MST neurons. The final study describes a behavioral experiment with human subjects that explores how different stimulus features, such as size and contrast, affect motion perception and discusses what conclusions can be drawn from that about the representation of visual motion in the human brain. Together these studies highlight the visual motion processing pathway of the primate brain as an excellent model system for studying more complex relations of neural activity and external stimuli. Area MST in particular emerges as a gateway between perception, cognition, and action planning.2021-11-1

Dynamic and Integrative Properties of the Primary Visual Cortex

The ability to derive meaning from complex, ambiguous sensory input requires the integration of information over both space and time, as well as cognitive mechanisms to dynamically shape that integration. We have studied these processes in the primary visual cortex (V1), where neurons have been proposed to integrate visual inputs along a geometric pattern known as the association field (AF). We first used cortical reorganization as a model to investigate the role that a specific network of V1 connections, the long-range horizontal connections, might play in temporal and spatial integration across the AF. When retinal lesions ablate sensory information from portions of the visual field, V1 undergoes a process of reorganization mediated by compensatory changes in the network of horizontal collaterals. The reorganization accompanies the brain’s amazing ability to perceptually “fill-inâ€, or “seeâ€, the lost visual input. We developed a computational model to simulate cortical reorganization and perceptual fill-in mediated by a plexus of horizontal connections that encode the AF. The model reproduces the major features of the perceptual fill-in reported by human subjects with retinal lesions, and it suggests that V1 neurons, empowered by their horizontal connections, underlie both perceptual fill-in and normal integrative mechanisms that are crucial to our visual perception. These results motivated the second prong of our work, which was to experimentally study the normal integration of information in V1. Since psychophysical and physiological studies suggest that spatial interactions in V1 may be under cognitive control, we investigated the integrative properties of V1 neurons under different cognitive states. We performed extracellular recordings from single V1 neurons in macaques that were trained to perform a delayed-match-to-sample contour detection task. We found that the ability of V1 neurons to summate visual inputs from beyond the classical receptive field (cRF) imbues them with selectivity for complex contour shapes, and that neuronal shape selectivity in V1 changed dynamically according to the shapes monkeys were cued to detect. Over the population, V1 encoded subsets of the AF, predicted by the computational model, that shifted as a function of the monkeys’ expectations. These results support the major conclusions of the theoretical work; even more, they reveal a sophisticated mode of form processing, whereby the selectivity of the whole network in V1 is reshaped by cognitive state