The Representation of Object Distance: Evidence from Neuroimaging and Neuropsychology

Perceived distance in two-dimensional (2D) images relies on monocular distance cues. Here, we examined the representation of perceived object distance using a continuous carry-over adaptation design for fMRI. The task was to look at photographs of objects and make a judgment as to whether or not the item belonged in the kitchen. Importantly, this task was orthogonal to the variable of interest: the object's perceived distance from the viewer. In Experiment 1, whole brain group analyses identified bilateral clusters in the superior occipital gyrus (approximately area V3/V3A) that showed parametric adaptation to relative changes in perceived distance. In Experiment 2, retinotopic analyses confirmed that area V3A/B reflected the greatest magnitude of response to monocular changes in perceived distance. In Experiment 3, we report that the functional activations overlap with the occipito-parietal lesions in a patient with impaired distance perception, showing that the same regions monitor implied (2D) and actual (three-dimensional) distance. These data suggest that distance information is automatically processed even when it is task-irrelevant and that this process relies on superior occipital areas in and around area V3A

Early and Late Stage Mechanisms for Vocalization Processing in the Human Auditory System

The human auditory system is able to rapidly process incoming acoustic information, actively filtering, categorizing, or suppressing different elements of the incoming acoustic stream. Vocalizations produced by other humans (conspecifics) likely represent the most ethologically-relevant sounds encountered by hearing individuals. Subtle acoustic characteristics of these vocalizations aid in determining the identity, emotional state, health, intent, etc. of the producer. The ability to assess vocalizations is likely subserved by a specialized network of structures and functional connections that are optimized for this stimulus class. Early elements of this network would show sensitivity to the most basic acoustic features of these sounds; later elements may show categorically-selective response patterns that represent high-level semantic organization of different classes of vocalizations. A combination of functional magnetic resonance imaging and electrophysiological studies were performed to investigate and describe some of the earlier and later stage mechanisms of conspecific vocalization processing in human auditory cortices. Using fMRI, cortical representations of harmonic signal content were found along the middle superior temporal gyri between primary auditory cortices along Heschl\u27s gyri and the superior temporal sulci, higher-order auditory regions. Additionally, electrophysiological findings also demonstrated a parametric response profile to harmonic signal content. Utilizing a novel class of vocalizations, human-mimicked versions of animal vocalizations, we demonstrated the presence of a left-lateralized cortical vocalization processing hierarchy to conspecific vocalizations, contrary to previous findings describing similar bilateral networks. This hierarchy originated near primary auditory cortices and was further supported by auditory evoked potential data that suggests differential temporal processing dynamics of conspecific human vocalizations versus those produced by other species. Taken together, these results suggest that there are auditory cortical networks that are highly optimized for processing utterances produced by the human vocal tract. Understanding the function and structure of these networks will be critical for advancing the development of novel communicative therapies and the design of future assistive hearing devices

Neural markers of suppression in impaired binocular vision

Even after conventional patching treatment, individuals with a history of amblyopia typically lack good stereo vision. This is often attributed to atypical suppression between the eyes, yet the specific mechanism is still unclear. Guided by computational models of binocular vision, we tested explicit predictions about how neural responses to contrast might differ in individuals with impaired binocular vision. Participants with a history of amblyopia (N = 25), and control participants with typical visual development (N = 19) took part in the study. Neural responses to different combinations of contrast in the left and right eyes, were measured using both electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). Stimuli were sinusoidal gratings with a spatial frequency of 3c/deg, flickering at 4 Hz. In the fMRI experiment, we also ran population receptive field and retinotopic mapping sequences, and a phase-encoded localiser stimulus, to identify voxels in primary visual cortex (V1) sensitive to the main stimulus. Neural responses in both modalities increased monotonically with stimulus contrast. When measured with EEG, responses were attenuated in the weaker eye, consistent with a fixed tonic suppression of that eye. When measured with fMRI, a low contrast stimulus in the weaker eye substantially reduced the response to a high contrast stimulus in the stronger eye. This effect was stronger than when the stimulus-eye pairings were reversed, consistent with unbalanced dynamic suppression between the eyes. Measuring neural responses using different methods leads to different conclusions about visual differences in individuals with impaired binocular vision. Both of the atypical suppression effects may relate to binocular perceptual deficits, e.g. in stereopsis, and we anticipate that these measures could be informative for monitoring the progress of treatments aimed at recovering binocular vision

A resting state network in the motor control circuit of the basal ganglia

<p>Abstract</p> <p>Background</p> <p>In the absence of overt stimuli, the brain shows correlated fluctuations in functionally related brain regions. Approximately ten largely independent resting state networks (RSNs) showing this behaviour have been documented to date. Recent studies have reported the existence of an RSN in the basal ganglia - albeit inconsistently and without the means to interpret its function. Using two large study groups with different resting state conditions and MR protocols, the reproducibility of the network across subjects, behavioural conditions and acquisition parameters is assessed. Independent Component Analysis (ICA), combined with novel analyses of temporal features, is applied to establish the basis of signal fluctuations in the network and its relation to other RSNs. Reference to prior probabilistic diffusion tractography work is used to identify the basal ganglia circuit to which these fluctuations correspond.</p> <p>Results</p> <p>An RSN is identified in the basal ganglia and thalamus, comprising the pallidum, putamen, subthalamic nucleus and substantia nigra, with a projection also to the supplementary motor area. Participating nuclei and thalamo-cortical connection probabilities allow this network to be identified as the motor control circuit of the basal ganglia. The network was reproducibly identified across subjects, behavioural conditions (fixation, eyes closed), field strength and echo-planar imaging parameters. It shows a frequency peak at 0.025 ± 0.007 Hz and is most similar in spectral composition to the Default Mode (DM), a network of regions that is more active at rest than during task processing. Frequency features allow the network to be classified as an RSN rather than a physiological artefact. Fluctuations in this RSN are correlated with those in the task-positive fronto-parietal network and anticorrelated with those in the DM, whose hemodynamic response it anticipates.</p> <p>Conclusion</p> <p>Although the basal ganglia RSN has not been reported in most ICA-based studies using a similar methodology, we demonstrate that it is reproducible across subjects, common resting state conditions and imaging parameters, and show that it corresponds with the motor control circuit. This characterisation of the basal ganglia network opens a potential means to investigate the motor-related neuropathologies in which the basal ganglia are involved.</p

Assessing the impact of emotion in dual pathway models of sensory processing.

In our daily environment, we are constantly encountering an endless stream of information which we must be able to sort and prioritize. Some of the features that influence this are the emotional nature of stimuli and the emotional context of events. Emotional information is often given preferential access to neurocognitive resources, including within sensory processing systems. Interestingly, both auditory and visual systems are divided into dual processing streams; a ventral object identity/perception stream and a dorsal object location/action stream. While effects of emotion on the ventral streams are relatively well defined, its effect on dorsal stream processes remains unclear. The present thesis aimed to investigate the impact of emotion on sensory systems within a dual pathway framework of sensory processing. Study I investigated the role of emotion during auditory localization. While undergoing fMRI, participants indicated the location of an emotional or non-emotional sound within an auditory virtual environment. This revealed that the neurocognitive structures displaying activation modulated by emotion were not the same as those modulated by sound location. Emotion was represented in regions associated with the putative auditory ‘what’ but not ‘where’ stream. Study II examined the impact of emotion on ostensibly similar localization behaviours mediated differentially by the dorsal versus ventral visual processing stream. Ventrally-mediated behaviours were demonstrated to be impacted by the emotional context of a trial, while dorsally-mediated behaviours were not. For Study III, a motion-aftereffect paradigm was used to investigate the impact of emotion on visual area V5/MT+. This area, traditionally believed to be involved in dorsal stream processing, has a number of characteristics similar to a ventral stream structure. It was discovered that V5/MT+ activity was modulated both by presence of perceptual motion and emotional content of an image. In addition, this region displayed patterns of functional connectivity with the amygdala that were significantly modulated by emotion. Together, these results suggest that emotional information modulates neural processing within ventral sensory processing streams, but not dorsal processing streams. These findings are discussed with respect to current models of emotional and sensory processing, including amygdala connections to sensory cortices and emotional effects on cognition and behaviour

Functional Magnetic Resonance Imaging

"Functional Magnetic Resonance Imaging - Advanced Neuroimaging Applications" is a concise book on applied methods of fMRI used in assessment of cognitive functions in brain and neuropsychological evaluation using motor-sensory activities, language, orthographic disabilities in children. The book will serve the purpose of applied neuropsychological evaluation methods in neuropsychological research projects, as well as relatively experienced psychologists and neuroscientists. Chapters are arranged in the order of basic concepts of fMRI and physiological basis of fMRI after event-related stimulus in first two chapters followed by new concepts of fMRI applied in constraint-induced movement therapy; reliability analysis; refractory SMA epilepsy; consciousness states; rule-guided behavioral analysis; orthographic frequency neighbor analysis for phonological activation; and quantitative multimodal spectroscopic fMRI to evaluate different neuropsychological states

Development of Low-Frequency Repetitive Transcranial Magnetic Stimulation as a Tool to Modulate Visual Disorders: Insights from Neuroimaging

Repetitive transcranial magnetic stimulation (rTMS) has become a popular neuromodulation technique, increasingly employed to manage several neurological and psychological conditions. Despite its popular use, the underlying mechanisms of rTMS remain largely unknown, particularly at the visual cortex. Moreover, the application of rTMS to modulate visual-related disorders is under-investigated. The goal of the present research was to address these issues. I employ a multitude of neuroimaging techniques to gain further insight into neural mechanisms underlying low-frequency (1 Hz) rTMS to the visual cortex. In addition, I begin to develop and refine clinical low-frequency rTMS protocols applicable to visual disorders as an alternative therapy where other treatment options are unsuccessful or where there are simply no existing therapies. One such visual disorder that can benefit from rTMS treatment is the perception of visual hallucinations that can occur following visual pathway damage in otherwise cognitively healthy individuals. In Chapters 23, I investigate the potential of multiday low-frequency rTMS to the visual cortex to alleviate continuous and disruptive visual hallucinations consequent to occipital injury. Combining rTMS with magnetic resonance imaging techniques reveals functional and structural cortical changes that lead to the perception of visual hallucinations; and rTMS successfully attenuates these anomalous visual perceptions. In Chapters 45, I compare the effects of alternative doses of low-frequency rTMS to the visual cortex on neurotransmitter levels and intrinsic functional connectivity to gain insight into rTMS mechanisms and establish the most effective protocol. Differential dose-dependent effects are observed on neurotransmitter levels and functional connectivity that suggest the choice of protocol critically depends on the neurophysiological target. Collectively, this work provides a basic framework for the use of low-frequency rTMS and neuroimaging in clinical application for visual disorders

Medical image enhancement

Each image acquired from a medical imaging system is often part of a two-dimensional (2-D) image set whose total presents a three-dimensional (3-D) object for diagnosis. Unfortunately, sometimes these images are of poor quality. These distortions cause an inadequate object-of-interest presentation, which can result in inaccurate image analysis. Blurring is considered a serious problem. Therefore, “deblurring” an image to obtain better quality is an important issue in medical image processing. In our research, the image is initially decomposed. Contrast improvement is achieved by modifying the coefficients obtained from the decomposed image. Small coefficient values represent subtle details and are amplified to improve the visibility of the corresponding details. The stronger image density variations make a major contribution to the overall dynamic range, and have large coefficient values. These values can be reduced without much information loss