Biomedical subjects

Lee M Miller

Publications and source records attributed to Lee M Miller.

10 recordsLinked to original sources

PubMed2006

Functional connectivity of cortical networks involved in bimanual motor sequence learning.

Motor skill learning requires the involvement and integration of several cortical and subcortical regions. In this study, we focus on how the functional connectivity of cortical networks changes with the acquisition of a novel motor skill. Using functional magnetic resonance imaging, we measured the localized blood oxygenation level-dependent (BOLD) signal in cortical regions while subjects performed a bimanual serial reaction time task under 2 conditions: 1) explicitly learning a novel sequence (NOVEL) and 2) playing a previously learned sequence (LEARNED). To investigate stages of learning, each condition was further divided into nonoverlapping early and late conditions. Functional connectivity was measured using a task-specific low-frequency coherence analysis of the data. We show that within the cortical motor network, the sensorimotor cortex, premotor cortex, and supplementary motor area have significantly greater inter- and intrahemispheric coupling during the early NOVEL condition compared with the late NOVEL condition. Additionally, we observed greater connectivity between frontal regions and cortical motor regions in the early versus late NOVEL contrast. No changes in functional connectivity were observed in the LEARNED condition. These results demonstrate that the functional connectivity of the cortical motor network is modulated with practice and suggest that early skill learning is mediated by enhanced interregional coupling.

Adult↗

PubMed2005

The contribution of spike threshold to acoustic feature selectivity, spike information content, and information throughput.

Hypotheses of sensory coding range from the notion of nonlinear "feature detectors" to linear rate coding strategies. Here, we report that auditory neurons exhibit a novel trade-off in the relationship between sound selectivity and the information that can be communicated to a postsynaptic cell. Recordings from the cat inferior colliculus show that neurons with the lowest spike rates reliably signal the occurrence of stereotyped stimulus features, whereas those with high response rates exhibit lower selectivity. The highest information conveyed by individual action potentials comes from neurons with low spike rate and high selectivity. Surprisingly, spike information is inversely related to spike rates, following a trend similar to that of feature selectivity. Information per time interval, however, was proportional to measured spike rates. A neuronal model based on the spike threshold of the synaptic drive accurately accounts for this trade-off: higher thresholds enhance the spiking fidelity at the expense of limiting the total communicated information. Such a constraint on the specificity and throughput creates a continuum in the neural code with two extreme forms of information transfer that likely serve complementary roles in the representation of the auditory environment.

Acoustic Stimulation↗

PubMed2005

Measuring temporal dynamics of functional networks using phase spectrum of fMRI data.

We present a novel method to measure relative latencies between functionally connected regions using phase-delay of functional magnetic resonance imaging data. Derived from the phase component of coherency, this quantity estimates the linear delay between two time-series. In conjunction with coherence, derived from the magnitude component of coherency, phase-delay can be used to examine the temporal properties of functional networks. In this paper, we apply coherence and phase-delay methods to fMRI data in order to investigate dynamics of the motor network during task and rest periods. Using the supplementary motor area (SMA) as a reference region, we calculated relative latencies between the SMA and other regions within the motor network including the dorsal premotor cortex (PMd), primary motor cortex (M1), and posterior parietal cortex (PPC). During both the task and rest periods, we measured significant delays that were consistent across subjects. Specifically, we found significant delays between the SMA and the bilateral PMd, bilateral M1, and bilateral PPC during the task condition. During the rest condition, we found that the temporal dynamics of the network changed relative to the task period. No significant delays were measured between the SMA and the left PM and left M1; however, the right PM, right M1, and bilateral PPC were significantly delayed with respect to the SMA. Additionally, we observed significant map-wise differences in the dynamics of the network at task compared to the network at rest. These differences were observed in the interaction between the SMA and the left M1, left superior frontal gyrus, and left middle frontal gyrus. These temporal measurements are important in determining how regions within a network interact and provide valuable information about the sequence of cognitive processes within a network.

Adult↗

PubMed2005

Perceptual fusion and stimulus coincidence in the cross-modal integration of speech.

Human speech perception is profoundly influenced by vision. Watching a speaker's mouth movements significantly improves comprehension, both for normal listeners in noisy environments and especially for the hearing impaired. A number of brain regions have been implicated in audiovisual speech tasks, but little evidence distinguishes them functionally. In an event-related functional magnetic resonance imaging study, we differentiate neural systems that evaluate cross-modal coincidence of the physical stimuli from those that mediate perceptual binding. Regions consistently involved in perceptual fusion per se included Heschl's gyrus, superior temporal sulcus, middle intraparietal sulcus, and inferior frontal gyrus. Successful fusion elicited activity biased toward the left hemisphere, although failed cross-modal binding recruited regions in both hemispheres. A broad network of other areas, including the superior colliculus, anterior insula, and anterior intraparietal sulcus, were more involved with evaluating the spatiotemporal correspondence of speech stimuli, regardless of a subject's perception. All of these showed greater activity to temporally offset stimuli than to audiovisually synchronous stimuli. Our results demonstrate how elements of the cross-modal speech integration network differ in their sensitivity to physical reality versus perceptual experience.

Acoustic Stimulation↗

PubMed2005

Coherence between fMRI time-series distinguishes two spatial working memory networks.

Widespread and distributed brain regions are thought to form networks that together support working memory. We recently demonstrated that different cortical areas maintain relatively different codes across a memory delay (Curtis et. al., J Neurosci, 2004; 24:3944-3952). The frontal eye fields (FEF), for example, were more active during the delay when the direction of the memory-guided saccade was known compared to when it was not known throughout the delay. Other areas showed the opposite pattern. Despite these task-dependent differences in regional activity, we could only assume but not address the functional interactions between the identified nodes of the putative network. Here, we use a bivariate technique, coherence, to formally characterize functional interactions between a seed region and other brain areas. We find that the type of representational codes that are being maintained in working memory biases frontal-parietal interactions. For example, coherence between FEF and other oculomotor areas was greater when a motor representation was an efficient strategy to bridge the delay period. However, coherence between the FEF and higher-order heteromodal areas, e.g., dorsolateral prefrontal cortex, was greater when a sensory representation must be maintained in working memory.

Adolescent↗

PubMed2005

Functional interactions between oculomotor regions during prosaccades and antisaccades.

Human behavior reflects a continual negotiation of automatic and directed actions. The oculomotor network is a well-characterized neural system in which to study this balance of behavioral control. For instance, saccades made toward and away from a flashed visual stimulus (prosaccades and antisaccades, respectively) are known to engage different cognitive processes. Brain regions important for such controlled execution include the presupplementary motor area (pre-SMA), frontal eye fields (FEF), and intraparietal sulcus (IPS). Recent work has emphasized various elements of this network but has not explored the functional interactions among regions. We used event-related fMRI to image human brain activity during performance of an interleaved pro/antisaccade task. Since traditional univariate statistics cannot address issues of functional connectivity, a multivariate technique is necessary. Coherence between fMRI time series of the pre-SMA with the FEF and IPS was used to measure functional interactions. The FEF, but not IPS, showed significant differential coherence between pro- and antisaccade trials with pre-SMA. These results suggest that the pre-SMA coordinates with FEF to maintain a controlled, preparatory set for task-appropriate oculomotor execution.

Adult↗

PubMed2005

Auditory thalamocortical transformation: structure and function.

Communicative, predatory, and reproductive behaviors rely on the auditory thalamocortical system, a key nexus that combines, transforms, and distributes virtually all acoustic information relevant to survival. The rules of connectivity for this complex network, both anatomically and functionally, are only beginning to be uncovered. Although the auditory thalamocortical system shares many features with other modalities, its connectivity and information processing principles differ from those of other modalities in many ways. Some physiological and anatomical bases for these differences are the subject of this review.

Animals↗

PubMed2004

Measuring interregional functional connectivity using coherence and partial coherence analyses of fMRI data.

Understanding functional connectivity within the brain is crucial to understanding neural function; even the simplest cognitive operations are supported by highly distributed neural circuits. We developed a novel method to measure task-related functional interactions between neural regions by applying coherence and partial coherence analyses to functional magnetic resonance imaging (fMRI) data. Coherence and partial coherence are spectral measures that estimate the linear time-invariant (LTI) relationship between time series. They can be used to generate maps of task-specific connectivity associated with seed regions of interest (ROIs). These maps may then be compared across tasks, revealing nodes with task-related changes of connectivity to the seed ROI. To validate the method, we applied it to an event-related fMRI data set acquired while subjects performed two sequence tapping tasks, one of which required more bimanual coordination. Areas showing increased functional connectivity with both tasks were the same as those showing increased activity. Furthermore, though there were no significant differences in mean activity between the two tasks, significant increases in interhemispheric coherence were found between the primary motor (M1) and premotor (PM) regions for the task requiring more bimanual coordination. This increase in interhemispheric connectivity is supported by other brain imaging techniques as well as patient studies.

Adolescent↗

PubMed2003

Naturalistic auditory contrast improves spectrotemporal coding in the cat inferior colliculus.

Statistical analysis of natural sounds and speech reveals logarithmically distributed spectrotemporal modulations that can cover several orders of magnitude. By contrast, most artificial stimuli used to probe auditory function, including pure tones and white noise, have linearly distributed amplitude fluctuations with a limited average dynamic range. Here we explore whether the operating range of the auditory system is physically matched to the statistical structure of natural sounds. We recorded single-unit and multi-unit neuronal activity from the central nucleus of the cat inferior colliculus (ICC) in response to dynamic spectrotemporal sound sequences to determine whether ICC neurons respond preferentially to linear or logarithmic spectrotemporal amplitudes. We varied the intensity, dynamic range, and contrast statistics of these sounds to mimic those of natural and artificial stimuli. ICC neurons exhibited monotonic and nonmonotonic contrast dependencies with increasing dynamic range that were independent of the stimulus intensity. Midbrain neurons had higher firing rates and higher receptive field energies and showed a net improvement in spectrotemporal encoding ability for logarithmic stimuli, with an increase in the mutual information rate of approximately 50% over linear amplitude sounds. This efficient use of logarithmic spectrotemporal modulations by auditory midbrain neurons reflects a neural adaptation to structural regularities in natural sounds and likely underlies human perceptual abilities.

Action Potentials↗

PubMed2002

Spectrotemporal receptive fields in the lemniscal auditory thalamus and cortex.

Receptive fields have been characterized independently in the lemniscal auditory thalamus and cortex, usually with spectrotemporally simple sounds tailored to a specific task. No studies have employed naturalistic stimuli to investigate the thalamocortical transformation in temporal, spectral, and aural domains simultaneously and under identical conditions. We recorded simultaneously in the ventral division of the medial geniculate body (MGBv) and in primary auditory cortex (AI) of the ketamine-anesthetized cat. Spectrotemporal receptive fields (STRFs) of single units (n = 387) were derived by reverse-correlation with a broadband and dynamically varying stimulus, the dynamic ripple. Spectral integration, as measured by excitatory bandwidth and spectral modulation preference, was similar across both stations (mean Q(1/e) thalamus = 5.8, cortex = 5.4; upper cutoff of spectral modulation transfer function, thalamus = 1.30 cycles/octave, cortex = 1.37 cycles/octave). Temporal modulation rates slowed by a factor of two from thalamus to cortex (mean preferred rate, thalamus = 32.4 Hz, cortex = 16.6 Hz; upper cutoff of temporal modulation transfer function, thalamus = 62.9 Hz, cortex = 37.4 Hz). We found no correlation between spectral and temporal integration properties, suggesting that the excitatory-inhibitory interactions underlying preference in each domain are largely independent. A small number of neurons in each station had highly asymmetric STRFs, evidence of frequency sweep selectivity, but the population showed no directional bias. Binaural preferences differed in their relative proportions, most notably an increased prevalence of excitatory contralateral-only cells in cortex (40%) versus thalamus (23%), indicating a reorganization of this parameter. By comparing simultaneously along multiple stimulus dimensions in both stations, these observations establish the global characteristics of the thalamocortical receptive field transformation.

Acoustic Stimulation↗