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Education
September 2008 - December 2011
September 2008 - December 2013
September 2003 - June 2007
Publications
Publications (40)
The functional organization of human auditory cortex has not yet been characterized beyond a rudimentary level of detail. Here, we use functional MRI to measure the microstructure of orthogonal tonotopic and periodotopic gradients forming complete auditory field maps (AFMs) in human core and belt auditory cortex. These AFMs show clear homologies to...
One of the fundamental properties of the mammalian brain is that sensory regions of cortex are formed of multiple, functionally specialized cortical field maps (CFMs). Each CFM comprises two orthogonal topographical representations, reflecting two essential aspects of sensory space. In auditory cortex, auditory field maps (AFMs) are defined by the...
Are ectopic responses in lesion projection zones (LPZs) the result of long-term reorganization (plasticity) or short-term filling-in (adaptation)? We used field-standard travelling-wave and cutting-edge population receptive field (pRF) model functional MRI visual field mapping techniques with 4 types of flickering checkerboard stimuli under photopi...
Does visual working memory represent a fixed number of objects, or is capacity reduced as object complexity increases? We measured accuracy in detecting changes between sample and test displays and found that capacity estimates dropped as complexity increased. However, these apparent capacity reductions were strongly correlated with increases in sa...
Brain-derived neurotrophic factor (BDNF) is the most abundant neurotrophin in the brain, influencing neural development, plasticity, and repair (Chen et al., 2004; Thoenen, 1995). The BDNF gene contains a single-nucleotide polymorphism (SNP) called Val 66 Met. The Met allele interferes with intracellular BDNF-trafficking, decreases activity-depende...
Human sensory systems are organized into processing hierarchies within cortex, such that incoming sensory information is analyzed and compiled into our vivid sensory experiences. Computations that are common to these sensory systems include the abilities to maintain enhanced focus on particular aspects of incoming sensory information (i.e., attenti...
One of the fundamental properties of mammalian brains is that sensory regions of cortex are organized into multiple, functionally specialized cortical field maps (CFMs). An individual CFM is composed of two orthogonal topographical representations, reflecting two essential aspects of a sensory feature space. Each CFM is thought to subserve a specif...
The cortical hierarchy of the human visual system has been shown to be organized around retinal spatial coordinates throughout much of low- and mid-level visual processing. These regions contain visual field maps (VFMs) that each follows the organization of the retina, with neighboring aspects of the visual field processed in neighboring cortical l...
This chapter reviews the differences in specific structural and functional characteristics of human visual cortex among young adults, healthy aging adults, and patients with dementia, with a primary focus on those with Alzheimer's disease (AD). Such visual cortex changes have been shown to underlie many of the behavioral deficits that develop in he...
This paper describes a conformal geometry based approach to objectively quantify the retinotopic organization of selected regions within the early human visual areas. The approach proposes using the Beltrami coefficient to measure the angle distortion of the mapping between the visual field and the V1 visual area. By treating the retinotopic mappin...
The cortical organization of the human auditory system has been incompletely measured. While we know some of the features and locations of low-level processing in cortex, we were not until recently able to localize individual auditory field maps (AFMs). The key insight to measuring AFMs was recently discovered: two orthogonal acoustic dimensions, t...
Significance
We use functional MRI to investigate the cortical effects on V1, V2, V3, hV4, and VO-1 when humans’ eyes have adapted to low-light vision. We show that populations of neurons with receptive fields interacting with the central rod scotoma are silenced because of lack of stimulation, shift their locations ectopically, and/or scale their...
The ultimate goal of many fields of neuroscience research is to harness the ability for the human
brain to reorganize, as an understanding of how to induce plasticity in cortex could foster the
development of treatments of such devastating conditions as paralysis, neurodegenerative
disease and stroke. The specifics of the timing and types of reorga...
Functional magnetic resonance imaging (fMRI) has been widely used to measure the retinotopic organization of early visual cortex in the human brain. Previous studies have identified multiple visual field maps (VFMs) based on statistical analysis of fMRI signals, but the resulting geometry has not been fully characterized with mathematical models. H...
Although several studies have suggested that cortical alterations underlie such age-related visual deficits as decreased acuity, little is known about what changes actually occur in visual cortex during healthy aging. Two recent studies showed changes in primary visual cortex (V1) during normal aging; however, no studies have characterized the effe...
A primary organizing principle of visual cortical organization is the visual field map: neurons with visual receptive fields next to one another in visual space are located next to one another in cortex, forming one complete representation of visual space. Similarly, we have shown that auditory cortex is organized into auditory field maps: neurons...
Visual working memory (VWM) is the ability to maintain visual information in a readily available and easily updated state. Converging evidence has revealed that VWM capacity is limited by the number of maintained objects, which is about 3 - 4 for the average human. Recent work suggests that VWM capacity is also limited by the resolution required to...
The search for organizing principles of visual processing in cortex has proven long and fruitful, demonstrating specific types of organization arising on multiple scales (e.g., magnocellular / parvo-cellular pathways [1] and ocular dominance columns [2]). One of the more important larger scale organizing principles of visual cortical organization i...
Aging often results in reduced visual acuity from changes in both the eye and neural circuits [1-4]. In normally aging subjects, primary visual cortex has been shown to have reduced responses to visual stimulation [5]. It is not known, however, to what extent aging affects visual field repre-sentations and population receptive sizes in human primar...
Game theory has been useful for understanding
risk-taking and cooperative behavior. However, in studies of the
neural basis of decision-making during games of conflict, subjects
typically play against opponents with predetermined strategies.
The present study introduces a neurobiologically plausible model
of action selection and neuromodulation, wh...
Introduction. Recent work demonstrates that there may be specific genetic profiles that predict the capacity for neural plasticity. Polymorphisms in the BDNF gene correlate with memory capacity and synaptic plasticity (Egan et al, 2003; Kleim et al, 2006), and may predict capacity for visual-motor learning and motor map plasticity (Pearson-Fuhrhop...
Knowledge of the normal organization of visual field map clusters allows us to study potential reorganization within visual cortex under conditions that lead to a disruption of the normal visual inputs. Here we exploit the dynamic nature of visuomotor regions in posterior parietal cortex to examine cortical functional plasticity induced by a comple...
Aging typically results in reduced visual acuity, both from changes within the eye and from acquired neural deficits. It is not known, however, to what extent aging affects visual field map organization in human cortex. In addition, patients with Alzheimer's disease (AD) often present with visual deficits as one of their earliest complaints. It is...
Game theory has been useful for understanding risk-taking, cooperation, and social behavior. However, in studies of the neural basis of decision-making during games of conflict, subjects typically play against an opponent with a predetermined strategy [1-3]. In the present study, human subjects played Hawk-Dove games against a neural agent, both si...
Introduction: The phenomenon of perceptual filling-in, where a region of the retina does not transmit visual information, yet we perceive visual information based on what information is available from nearby retinal locations, has been studied extensively at the blind spot (e.g., Ramachandran, 1992) and after inducing “artificial scotomas” (retina-...
Introduction: One of the more important larger scale organizing principles of visual cortical organization is the visual field map: neurons whose visual receptive fields lie next to one another in visual space are located next to one another in cortex. As increasing numbers of visual field maps have been defined in human visual cortex, one question...
Introduction: In this study, we exploit the dynamic nature of posterior parietal cortex to examine cortical functional plasticity induced by a complete reversal of visual input in normal adult humans. Using retinotopic fMRI measurements, we have previously demonstrated changes within the spatial representations of multiple parietal visual field map...
Introduction: Visual working memory (VWM) capacity has been shown to be limited by the number of items one can hold in memory and the resolution at which those items are represented. The number limit appears to be subserved by cortex in inferior interparietal sulcus (IPS), while the resolution limit seems to be subserved by superior IPS and part of...
Introduction: Primate visual cortex can be partitioned into distinct visual areas which serve separate perceptual functions. One important organizing principle within many visual areas is a topographical map of visual space, also called a visual field map. The organization within these maps follows the organization of the retina; hence, retinotopic...
Are resources in visual working memory allocated in a continuous or a discrete fashion? On one hand, flexible resource models suggest that capacity is determined by a central resource pool that can be flexibly divided such that items of greater complexity receive a larger share of resources. On the other hand, if capacity in working memory is defin...
INTRODUCTION. The rod and cone photoreceptors of the retina are organized such that the central fovea contains no rod photoreceptors. It is generally accepted that this pattern of organization continues through the input into the early retinotopic visual areas in human occipital cortex. Thus, signals from both rod and cone photoreceptors travel fro...
Stratton (Psych. Rev., 1897) first described visuomotor adaptation to altered visual input by wearing inverting prism spectacles. A number of studies (e.g., Miyauchi et al., J. Physio., 2004) have tried to confirm his findings and further examine the question of how responses in visual cortex change during this adaptation process. There is evidence...
Several studies have examined the role of primate parietal cortex in visuomotor tasks involving grasping, reaching, and saccadic eye movements (e.g., Hagler et al., Neuroimage, 2007). One especially useful way to study that role is to measure how motor systems adapt to alterations in visual input. Neuroimaging measurements in humans have begun to r...
Crowding refers to the phenomenon in which nearby distractors impede target processing. This effect is reduced as target-distractor distance increases, and it is eliminated entirely at a distance that is labeled the critical spacing point. Attention, distractor preview, and popout are each known to facilitate processing in crowded displays. Eight e...