Visual Cortical Mechanisms of Perceptual Grouping: Interacting Layers, Networks, Columns, and Maps

The visual cortex has a laminar organintion whose circuits form functional columns in cortical maps. How this laminar architecture supports visual percepts is not well understood. A neural model proposes how the laminar circuits of Vl and V2 generate perceptual groupings that maintain sensitivity to the contrasts and spatial organization of scenic cues.The model can decisively choose which groupings cohere and survive, even while balanced excitatory and inhibitory interactions preserve contrast-sensitive measures of local boundary likelihood or strength. In the model, excitatory inputs from LGN activate layers 4 and 6 of Vl. Layer 6 activates an on-center off-surround network of inputs to layer 4. Together these layer 4 inputs preserve analog sensitivity to LGN input contrast. Layer 4 cells excite pyramidal cells in layer 2/3 which activate monosynaptic long-range horizontal excitory connections between layer 2/3 pyramidal cells, and short-range disynaptic inhibitory connections mediated by smooth stellate cells. These interactions support inward perceptual grouping between two or more boundary inducetd, but not outward grouping from a single inducer. These lJO\UHlary signals feed back to layer 4 via the layer 6-to-4 on-center off-surround network. This folded feecdback joind cells in different layers into functional columnns while selecting winning groupings. Layer G in V1 also sends top-dlown signals to LGN using an on-center off-surround network, which suppresses LGN cells that do not receive feedback, while selecting, enhaneing, and synchronizing activity of those that do. The model is used to simulate psychophysical and neurophysiological data about perceptual grouping, including various Gestalt grouping laws.Air Force Office of Scientific Research (D0-0175), British Petroleum (BP 89A-1204); Defense Advanced Research Projects Agency and Office of Naval Research (N00014-92-J-4015); HNC Software (SC9-4-001), National Science Foundation (IRI-90-00530); Office of Naval Research (N00014-1-91-4100, N00014-95-1-0409, 0NR N00014-9510657

Cortical mechanisms for tinnitus in humans /

PhD ThesisThis work sought to characterise neurochemical and neurophysiological processes underlying tinnitus in humans. The first study involved invasive brain recordings from a neurosurgical patient, along with experimental manipulation of his tinnitus, to map the cortical system underlying his tinnitus. Widespread tinnitus-linked changes in low- and high-frequency oscillations were observed, along with inter-regional and cross-frequency patterns of communication. The second and third studies compared tinnitus patients to controls matched for age, sex and hearing loss, measuring auditory cortex spontaneous oscillations (with magnetoencephalography) and neurochemical concentrations (with magnetic resonance spectroscopy) respectively. Unlike in previous studies not controlled for hearing loss, there were no group differences in oscillatory activity attributable to tinnitus. However, there was a significant correlation between gamma oscillations (>30Hz) and hearing loss in the tinnitus group, and between delta oscillations (1-4Hz) and perceived tinnitus loudness. In the neurochemical study, tinnitus patients had significantly reduced GABA concentrations compared to matched controls, and within this group there was a positive correlation between choline concentration (potentially linked to acetylcholine and/or neuronal plasticity) and both hearing loss, and subjective tinnitus intensity and distress. In light of present and previous findings, tinnitus may be best explained by a predictive coding model of perception, which was tested in the final experiment. This directly controlled the three main quantities comprising predictive coding models, and found that delta/theta/alpha oscillations (1-12Hz) encoded the precision of predictions, beta oscillations (12-30Hz) encoded changes to predictions, and gamma oscillations represented surprise (unexpectedness of stimuli based on predictions). The work concludes with a predictive coding model of tinnitus that builds upon the present findings and settles unresolved paradoxes in the literature. In this, precursor processes (in varying combinations) synergise to increase the precision associated with spontaneous activity in the auditory pathway to the point where it overrides higher predictions of ‘silence’.Medical Research Council Wellcome Trust and the National Institutes of Healt

Cortical mechanisms of seeing and hearing speech

In face-to-face communication speech is perceived through eyes and ears. The talker's articulatory gestures are seen and the speech sounds are heard simultaneously. Whilst acoustic speech can be often understood without visual information, viewing articulatory gestures aids hearing substantially in noisy conditions. On the other hand, speech can be understood, to some extent, by solely viewing articulatory gestures (i.e., by speechreading). In this thesis, electroencephalography (EEG), magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) were utilized to disclose cortical mechanisms of seeing and hearing speech. One of the major challenges of modern cognitive neuroscience is to find out how the brain integrates inputs from different senses. In this thesis, integration of seen and heard speech was investigated using EEG and MEG. Multisensory interactions were found in the sensory-specific cortices at early latencies and in the multisensory regions at late latencies. Viewing other person's actions activate regions belonging to the human mirror neuron system (MNS) which are also activated when subjects themselves perform actions. Possibly, the human MNS enables simulation of other person's actions, which might be important also for speech recognition. In this thesis, it was demonstrated with MEG that seeing speech modulates activity in the mouth region of the primary somatosensory cortex (SI), suggesting that also the SI cortex is involved in simulation of other person's articulatory gestures during speechreading. The question whether there are speech-specific mechanisms in the human brain has been under scientific debate for decades. In this thesis, evidence for the speech-specific neural substrate in the left posterior superior temporal sulcus (STS) was obtained using fMRI. Activity in this region was found to be greater when subjects heard acoustic sine wave speech stimuli as speech than when they heard the same stimuli as non-speech.reviewe

Cortical Mechanisms Of Adaptation In Auditory Processing

Adaptation is computational strategy that underlies sensory nervous systems’ ability to accurately encode stimuli in various and dynamic contexts and shapes how animals perceive their environment. Many questions remain concerning how adaptation adjusts to particular stimulus features and its underlying mechanisms. In Chapter 2, we tested how neurons in the primary auditory cortex adapt to changes in stimulus temporal correlation. We used chronically implanted tetrodes to record neuronal spiking in rat primary auditory cortex during exposure to custom made dynamic random chord stimuli exhibiting different levels of temporal correlation. We estimated linear non-linear model for each neuron at each temporal correlation level, finding that neurons compensate for temporal correlation changes through gain-control adaptation. This experiment extends our understanding of how complex stimulus statistics are encoded in the auditory nervous system. In Chapter 3 and 4, we tested how interneurons are involved in adaptation by optogenetically suppressing parvalbumin-positive (PV) and somatostatin-positive (SOM) interneurons during tone train stimuli and using silicon probes to record neuronal spiking in mouse primary auditory cortex. In Chapter 3, we found that inhibition from both PVs and SOMs contributes to stimulus-specific adaptation (SSA) through different mechanisms. SOM inhibition was stimulus-specific, suppressing responses to standard tones more strongly than responses to deviant tones, and increasing with standard tone repetition. PVs amplified SSA because inhibition was similar for standard and deviant tones and PV mediated inhibition was insensitive to tone repetition. PVs and SOMs themselves exhibit SSA, and a Wilson-Cowan dynamic model identified that PVs and SOMs can directly contribute to SSA in pyramidal neurons. In Chapter 4, we tested how SOMs and PVs inhibition is modulated with the dynamics of adaptation and across frequency tuning, during exposure to single frequency tone trains across the neuron’s tuning curve. We found that the magnitude of SOM inhibition correlated with the magnitude of adaptive suppression, while PVs inhibition was largely insensitive to stimulus conditions. Together Chapters 3 and 4 implicate SOM inhibition in actively suppressing responses in a stimulus-specific manner while PV inhibition may passively enhance stimulus-specific suppression. These experiments inform the underlying principles and mechanisms of cortical sensory adaptation

Observations on Cortical Mechanisms for Object Recognition andsLearning

This paper sketches a hypothetical cortical architecture for visual 3D object recognition based on a recent computational model. The view-centered scheme relies on modules for learning from examples, such as Hyperbf-like networks. Such models capture a class of explanations we call Memory-Based Models (MBM) that contains sparse population coding, memory-based recognition, and codebooks of prototypes. Unlike the sigmoidal units of some artificial neural networks, the units of MBMs are consistent with the description of cortical neurons. We describe how an example of MBM may be realized in terms of cortical circuitry and biophysical mechanisms, consistent with psychophysical and physiological data

Cortical Mechanisms Specific to Explicit Visual Object Recognition

AbstractThe cortical mechanisms associated with conscious object recognition were studied using functional magnetic resonance imaging (fMRI). Participants were required to recognize pictures of masked objects that were presented very briefly, randomly and repeatedly. This design yielded a gradual accomplishment of successful recognition. Cortical activity in a ventrotemporal visual region was linearly correlated with perception of object identity. Therefore, although object recognition is rapid, awareness of an object's identity is not a discrete phenomenon but rather associated with gradually increasing cortical activity. Furthermore, the focus of the activity in the temporal cortex shifted anteriorly as subjects reported an increased knowledge regarding identity. The results presented here provide new insights into the processes underlying explicit object recognition, as well as the analysis that takes place immediately before and after recognition is possible

Cortical mechanisms of sensory learning and object recognition

Learning about the world through our senses constrains our ability to recognise our surroundings. Experience shapes perception. What is the neural basis for object recognition and how are learning-induced changes in recognition manifested in neural populations? We consider first the location of neurons that appear to be critical for object recognition, before describing what is known about their function. Two complementary processes of object recognition are considered: discrimination among diagnostic object features and generalization across non-diagnostic features. Neural plasticity appears to underlie the development of discrimination and generalization for a given set of features, though tracking these changes directly over the course of learning has remained an elusive task

Cortical Mechanisms of Human Pelvic Floor Muscle Synergies

The human pelvic floor is an anatomically, functionally, and morphologically complex region that is associated with many disorders such as chronic prostatitis/pelvic pain syndrome (CPPS), chronic low back pain, and urinary incontinence. The purpose of this dissertation was to explore the cortical mechanisms that underlie human pelvic floor muscle synergies. Our first original experiment involved the study of 20 healthy male controls who were instructed to perform a variety of muscle tasks presumed to be associated with pelvic floor muscle synergies. Surface electromyography (EMG) method was used to detect timing onsets, as well as activation patterns of the pelvic floor, gluteus maximus, and first dorsal interosseous muscles. Functional magnetic resonance imaging (fMRI) was used to measure blood oxygenation density levels (BOLD) in the brain while subjects performed various prime mover tasks. Our second original experiment involved another set of 10 healthy male subjects who were trained to perform a complex synergy breaking/decoupling task that was confirmed with EMG. They repeated the coupling motor task (gluteal activation) as well as the more complex motor decoupling task while being scanned with fMRI, so that BOLD signals could be compared. The first experiment revealed evidence of cortically facilitated synergy of the pelvic floor muscles and the second experiment revealed that complex motor tasks such as the breaking of a cortically facilitated muscle synergy involves BOLD signals in the brain known to be involved with interoception