Thad Polk | University of Michigan (original) (raw)

Papers by Thad Polk

Academic radiology, Jan 13, 2018

Healthy aging is associated with pervasive declines in cognitive, motor, and sensory functioning.... more Healthy aging is associated with pervasive declines in cognitive, motor, and sensory functioning. There are, however, substantial individual differences in behavioral performance among older adults. Several lines of animal research link age-related reductions of gamma-aminobutyric acid (GABA), the brain's primary inhibitory neurotransmitter, to age-related cognitive, motor, and sensory decline. Our study used proton magnetic resonance spectroscopy (MRS) at 3T to explore whether occipital GABA declines with age in humans and whether individual differences in occipital GABA are linked to individual differences in fluid processing ability. We used a MEGA-PRESS sequence that combines frequency spectral editing with a point-resolved spectroscopy sequence to quantify GABA. Spectra were obtained from a 30 × 30 × 25mm voxel placed in the occipital cortex of 20 young adults (mean age 20.7 years) and 18 older adults (mean age 76.5 years). Participants also performed 11 fluid processing ta...

Journal of Consumer Research, 2006

Journal of Neuroscience, 2008

We present a functional MRI experiment investigating the neural basis of feature-based attention ... more We present a functional MRI experiment investigating the neural basis of feature-based attention in humans using the Stroop task. Cortical areas specifically involved in color processing and word reading were first identified in individual participants using independent tests. These areas were then probed during the Stroop task (in which participants must selectively attend to the font color of a word while ignoring the word itself). We found that activation in functionally defined color areas increased during the task relative to a neutral color-naming task while activation in functionally defined word areas decreased. These results are consistent with a biased competition model of feature-based attention in which the processing of attended features is enhanced and the processing of ignored features is suppressed.

NeuroImage, 2010

Functional localizers are routinely used in neuroimaging studies to test hypotheses about the fun... more Functional localizers are routinely used in neuroimaging studies to test hypotheses about the function of specific brain areas. The specific tasks and stimuli used to localize particular regions vary widely from study to study even when the same cortical region is targeted. Thus, it is important to ask whether task and stimulus changes lead to differences in localization or whether localization procedures are largely immune to differences in tasks and contrasting stimuli. We present two experiments and a literature review that explore whether face localizer tasks yield differential localization in the fusiform gyrus as a function of task and contrasting stimuli. We tested standard localization tasks-passive viewing, 1-back, and 2-back memory tests-and did not find differences in localization based on task. We did, however, find differences in the extent, strength and patterns/reliabilities of the activation in the fusiform gyrus based on comparison stimuli (faces vs. houses compared to faces vs. scrambled stimuli).

Journal of Cognitive Neuroscience, 2002

Functional magnetic resonance imaging (fMRI) was used to estimate neural activity while subjects ... more Functional magnetic resonance imaging (fMRI) was used to estimate neural activity while subjects viewed strings of consonants, digits, and shapes. An area on or near the left fusiform gyrus was found that responded significantly more to letters than digits. Similar results were obtained when consonants were used whose visual features were matched with the digits and when an active matching task was used, suggesting that the results cannot be easily attributed to artifacts of the stimuli or task. These results demonstrate that neural specialization in the human brain can extend to a category of stimuli that is culturally defined and that is acquired many years postnatally.

Cognitive Systems Research, 2002

We present a neural network model of verbal working memory which attempts to illustrate how a few... more We present a neural network model of verbal working memory which attempts to illustrate how a few simple assumptions about neural computation can shed light on cognitive phenomena associated with the serial recall of verbal material. We assume that neural representations are distributed, that neural connectivity is massively recurrent, and that synaptic efficacy is modified based on the correlation between pre-and post-synaptic activity (Hebbian learning). Together these assumptions give rise to emergent computational properties that are relevant to working memory, including short-term maintenance of information, time-based decay, and similarity-based interference. We instantiate these principles in a specific model of serial recall and show how it can both simulate and explain a number of standard cognitive phenomena associated with the task, including the effects of serial position, word length, articulatory suppression (and its interaction with word length), and phonological similarity.

Behavioral and Brain Sciences, 1993

How do people make deductions? The orthodox view in psychology is that they use formal rules of i... more How do people make deductions? The orthodox view in psychology is that they use formal rules of inference like those of a "natural deduction" system. Deduction argues that their logical competence depends, not on formal rules, but on mental models. They construct models of the situation described by the premises, using their linguistic knowledge and their general knowledge. They try to formulate a conclusion based on these models that maintains semantic information, that expresses it parsimoniously, and that makes explicit something not directly stated by any premise. They then test the validity of the conclusion by searching for alternative models that might refute the conclusion. The theory also resolves long-standing puzzles about reasoning, including how nonmonotonic reasoning occurs in daily life. The book reports experiments on all the main domains of deduction, including inferences based on propositional connectives such as "if" and "or," inferences based on relations such as "in the same place as," inferences based on quantifiers such as "none," "any," and "only," and metalogical inferences based on assertions about the true and the false. Where the two theories make opposite predictions, the results confirm the model theory and run counter to the formal rule theories. Without exception, all of the experiments corroborate the two main predictions of the model theory: inferences requiring only one model are easier than those requiring multiple models, and erroneous conclusions are usually the result of constructing only one of the possible models of the premises.

Abstract: According to the dual route reading hypothesis, there are two distinct processes involv... more Abstract: According to the dual route reading hypothesis, there are two distinct processes involved in visual word recognition-a phonological route which is presumed to underlie our capacity to read pseudowords (eg, glimf) and a lexical route, assumed to be critical in reading exception words (eg, yacht). An apparent double dissociation between these two routes has been observed in patients with acquired alexias. In this study, we addressed whether such a processing distinction is honored in the intact brain. Previous studies ...

Abstract: Damage to prefrontal cortex (PFC) can lead to a variety of dissociable cognitive defici... more Abstract: Damage to prefrontal cortex (PFC) can lead to a variety of dissociable cognitive deficits (eg, perseveration, distractibility, disinhibition, working memory impairments, etc.). Although these deficits are quite diverse, they share a family resemblance and are often classified together as executive deficits or as a dysexecutive syndrome. A central challenge is to provide an explicit model of executive control and show how it can unify these seemingly disparate cognitive deficits. We present a simple and computationally explicit ...

Eighth International Conference on Cognitive Modeling, 2007

Representations in sensory cortices are organized topographically: auditory cortex is organized t... more Representations in sensory cortices are organized topographically: auditory cortex is organized tonotopically, somatosensory cortex is organized somatotopically, and visual cortex is organized retinotopically. Substantial progress has been made in understanding how topography develops at a neurocomputational level, particularly in the early and middle stages of processing in the visual system. We extend this work to investigate how higher-level semantic representations could develop based on topographic input from sensory ...

Abstract: "Soar is an architecture for general intelligence, which has been shown to be capa... more Abstract: "Soar is an architecture for general intelligence, which has been shown to be capable of supporting a wide variety of intelligent behavior involving problem-solving, learning, designing, planning, etc. Soar has also been put forth as a unified theory of human cognition. We provide support for this by presenting a theory of syllogistic reasoning based on Soar and some assumptions about subjects' knowledge and representation. The resulting theory (and system, Syl-Soar/S88) is plausible in its details and accounts for existing data quite well."

Cerebral Aging, 2001

There are substantial declines in behavioral measures of cognitive function with age, including d... more There are substantial declines in behavioral measures of cognitive function with age, including decreased function of executive processes and long-term memory. There is also evidence that, with age, there is a decrease in brain volume, particularly in the frontal cortex. When young and older adults perform cognitive tasks that depend heavily on frontal function, neuroimaging evidence indicates that older adults recruit additional brain regions in order to perform the tasks. This additional neural recruitment is termed "dedifferentiation," and can take multiple forms. This recruitment of additional neural tissue with age to perform cognitive tasks was not reflected in the behavioral literature, and suggests that there is more plasticity in the ability to organize brain function than was previously suspected. We review both behavioral and neuroscience perspectives on cognitive aging, and then connect the findings in the two areas. From this integration, we suggest important ...

Aging is associated with widespread alterations in cerebral white matter (WM). Most prior studies... more Aging is associated with widespread alterations in cerebral white matter (WM). Most prior studies of age differences in WM have used diffusion tensor imaging (DTI), but typical DTI metrics (e.g., fractional anisotropy; FA) can reflect multiple neurobiological features, making interpretation challenging. Here, we used fixel-based analysis (FBA) to investigate age-related WM differences observed using DTI in a sample of 45 older and 25 younger healthy adults. Age- related FA differences were widespread but were strongly associated with differences in multifiber complexity (CX), suggesting that they reflected differences in crossing fibers in addition to structural differences in individual fiber segments. FBA also revealed a frontolimbic locus of age-related effects and provided insights into distinct microstructural changes underlying them. Specifically, age differences in fiber density were prominent in fornix, bilateral anterior internal capsule, forceps minor, body of the corpus c...

2019 Conference on Cognitive Computational Neuroscience, 2019

Human aging is characterized by losses in spatial cognition as well as reductions in distinctiven... more Human aging is characterized by losses in spatial cognition as well as reductions in distinctiveness of categoryspecific fMRI activation patterns. One mechanism linking theses two phenomena could be that broader neural tuning functions lead to more signal confusions when tuning-based representations of walking direction are read out. To test this idea, we developed a novel method that allowed us to investigate changes in fMRI-measured pattern similarity while participants navigated in different directions in a virtual spatial navigation task. We expected that adjacent directions are represented more similarly within direction sensitive brain areas, reflecting a tuning-function-like signal. Importantly, heightened similarity might lead downstream areas to become more likely to confuse neighboring directions. We therefore analyzed predictions of a decoder trained on these representations, asking (1) whether decoder confusions between two directions increased proportionally to their angular similarity, (2) and how this differs between age groups. Evidence for tuning-function-like signals was found in the retrosplenial complex and primary visual cortex. Significant age differences in tuning width, however, were only found in the primary visual cortex. Our findings introduce a novel approach to measure tuning specificity using fMRI and suggest broader visual direction tuning in older adults might underlie age-related spatial navigation impairments.

Age-related neural dedifferentiation – a decline in the distinctiveness of neural representations... more Age-related neural dedifferentiation – a decline in the distinctiveness of neural representations in the aging brain–has been associated with age-related declines in cognitive abilities. But why does neural distinctiveness decline with age? Based on prior work in non-human primates and more recent work in humans, we hypothesized that the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) declines with age and is associated with neural dedifferentiation in older adults. To test this hypothesis, we used magnetic resonance spectroscopy (MRS) to measure GABA and functional MRI (fMRI) to measure neural distinctiveness in the ventral visual cortex in a set of older and younger participants. Relative to younger adults, older adults exhibited lower GABA levels and less distinct activation patterns for faces and houses in the ventral visual cortex. Furthermore, individual differences in GABA within older adults positively predicted individual differences in neural distinctiveness. Th...

Neural activation patterns in the ventral visual cortex in response to different categories of vi... more Neural activation patterns in the ventral visual cortex in response to different categories of visual stimuli (e.g., faces vs. houses) are less selective, or distinctive, in older adults than in younger adults, a phenomenon known as age-related neural dedifferentiation. Previous work in animals suggests that age-related reductions of the inhibitory neurotransmitter, gamma aminobutyric acid (GABA), may play a role in this age-related decline in neural distinctiveness. In this study, we investigated whether neural dedifferentiation extends to auditory cortex and whether individual differences in GABA are associated with individual differences in neural distinctiveness in humans. 20 healthy young adults (ages 18-29) and 23 healthy older adults (over 65) completed a functional magnetic resonance imaging (fMRI) scan, during which neural activity was estimated while they listened to foreign speech and music. GABA levels in the auditory, ventrovisual and sensorimotor cortex were estimated ...