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Minah Suh

Publications and source records attributed to Minah Suh.

10 recordsLinked to original sources
PubMed2006

A quantitative synchronization model for smooth pursuit target tracking.

We propose a quantitative model for human smooth pursuit tracking of a continuously moving visual target which is based on synchronization of an internal expectancy model of the target position coupled to the retinal target signal. The model predictions are tested in a smooth circular pursuit eye tracking experiment with transient target blanking of variable duration. In subjects with a high tracking accuracy, the model accounts for smooth pursuit and repeatedly reproduces quantitatively characteristic patterns of the eye dynamics during target blanking. In its simplest form, the model has only one free parameter, a coupling constant. An extended model with a second parameter, a time delay or memory term, accounts for predictive smooth pursuit eye movements which advance the target. The model constitutes an example of synchronization of a complex biological system with perceived sensory signals.

Attention↗
PubMed2006

Increased oculomotor deficits during target blanking as an indicator of mild traumatic brain injury.

Given the susceptibility of cerebellar-cortical tracts to shearing injury from traumatic brain injury (TBI), we investigated impairment in the generation of predictive eye movements and its relationship to cognitive deficits in mild TBI patients using a smooth pursuit target-blanking paradigm. Compared to a target-tracking paradigm without blanking, this paradigm more greatly necessitates the generation of predictive eye movements, which are subserved by brain regions involved in cognitive processing. Mild TBI patients showed impaired prediction of target trajectories during target blanking, demonstrated by generation of saccades at earlier and more variable time points, as well as greater and more variable oculomotor error compared to controls. In addition, California Verbal Learning Test (CVLT-II) scores related to working memory, learning, and executive function were more highly correlated with oculomotor variability during target blanking than during target tracking. Our results suggest that a disruption of cerebellar-cortical connections in TBI may account for both oculomotor and cognitive impairment, and that measures of predictive eye movements during target blanking may be a sensitive metric of cognitive deficits after mild TBI.

Adolescent↗
PubMed2006

Intrinsic optical signal imaging of neocortical seizures: the 'epileptic dip'.

Focal neocortical seizures, induced by injection of 4-aminopyridine, were imaged in the rat neocortex using the intrinsic optical signal, with incident light at various wavelengths. We observed focal, reproducible and prolonged reflectance drops following seizure onset, regardless of wavelength, in the ictal onset zone. A persistent drop in light reflectance with incident orange light, which corresponds to a decrease in oxygenated hemoglobin, was observed. We describe this phenomenon as an 'epileptic dip' as it is reminiscent of the 'initial dip' observed using the intrinsic optical signal, and also with blood oxygen level-dependent functional magnetic resonance imaging, after normal sensory processing, although with much longer duration. This persistent ictal ischemia was confirmed by direct measurement of tissue oxygenation using oxygen-sensitive electrodes.

4-Aminopyridine↗
PubMed2006

Deficits in predictive smooth pursuit after mild traumatic brain injury.

Given that even mild traumatic brain injury (TBI) may produce extensive diffuse axonal injury (DAI), we hypothesized that mild TBI patients would show deficits in predictive smooth pursuit eye movements (SPEM), associated with impaired cognitive functions, as these processes are dependent on common white matter connectivity between multiple cerebral and cerebellar regions. The ability to predict target trajectories during SPEM was investigated in 21 mild TBI patients using a periodic sinusoidal paradigm. Compared to 26 control subjects, TBI patients demonstrated decreased target prediction. TBI patients also showed increased eye position error and variability of eye position, which correlated with decreased target prediction. In all subjects, average target prediction, eye position error and eye position variability correlated with scores related to attention and executive function on the California Verbal Learning Test (CVLT-II). However, there were no differences between TBI and control groups in average eye gain or intra-individual eye gain variability, or in performance on the Wechsler Abbreviated Scale of Intelligence (WASI), suggesting that the observed deficits did not result from general oculomotor impairment or reduced IQ. The correlation between SPEM performance and CVLT-II scores suggests that predictive SPEM may be a sensitive assay of cognitive functioning, including attention and executive function. This is the first report to our knowledge that TBI patients show impaired predictive SPEM and eye position variability, and that these impairments correlate with cognitive deficits.

Adolescent↗
PubMed2006

Blood volume and hemoglobin oxygenation response following electrical stimulation of human cortex.

Our understanding of perfusion-based human brain mapping techniques relies on a detailed knowledge of the relationship between neuronal activity and cerebrovascular hemodynamics. We performed optical imaging of intrinsic signals at wavelengths sensitive to total hemoglobin (Hbt; which correlate with cerebral blood volume (CBV)) and deoxygenated hemoglobin (Hbr) directly in humans during neurosurgical operations and investigated the optical signals associated with bipolar cortical stimulation at a range of amplitudes. Cortical stimulation elicited a rapid focal increase in Hbr (initial dip) in all subjects. An equally rapid increase in Hbt (<200 ms), with a slightly higher signal-to-noise ratio, was also highly localized for <2 s in spite of the non-columnar nature of the stimulus, after which the signal spread to adjacent gyri. A later decrease in Hbr (>3 s), which is relevant to the blood oxygen level dependent (BOLD) signal, was poorly localized. Increasing the stimulus amplitude elicited a linear increase in the area of the optical signal for Hbt and the initial dip but not the late decrease in Hbr, and a nonlinear increase in optical signal amplitude with a plateau effect for initial dip, Hbt and late decrease in Hbr.

Blood Volume↗
PubMed2006

Recent developments in oximetry and perfusion-based mapping techniques and their role in the surgical treatment of neocortical epilepsy.

Detailed understanding of neurovascular coupling during epilepsy is critical for the interpretation of various perfusion-based imaging techniques, such as positron emission tomography, single-photon-emission computed tomography, and functional magnetic resonance imaging, which are used to guide surgical therapy. We used high-resolution intrinsic signal- and voltage-sensitive dye imaging, as well as oxygen-sensitive electrodes, to map the precise spatiotemporal relationship between excitatory and inhibitory neuronal activity, cerebral blood volume, and oximetry during epilepsy. We observed a rapid focal decrease in tissue oxygenation and an increase in deoxygenated hemoglobin in association with both interictal and ictal events. This "epileptic dip" in oxygenation lasts several seconds following both interictal and ictal events, implying that for a period, cerebral blood flow is inadequate to meet metabolic demand. We also observed a rapid focal increase in cerebral blood volume that soon spread to adjacent nonepileptic gyri. Likewise, a diffuse decrease in deoxygenated hemoglobin, related to the blood oxygen level-dependent signal recorded with functional magnetic resonance imaging, spread to adjacent gyri and was poorly localized.

Animals↗
PubMed2006

Neurovascular coupling and oximetry during epileptic events.

Epilepsy is an abnormal brain state in which a large population of neurons is synchronously active, causing an enormous increase in metabolic demand. Recent investigations using highresolution imaging techniques, such as optical recording of intrinsic signals and voltagesensitive dyes, as well as measurements with oxygen-sensitive electrodes have elucidated the spatiotemporal relationship between neuronal activity, cerebral blood volume, and oximetry in vivo. A focal decrease in tissue oxygenation and a focal increase in deoxygenated hemoglobin occurs following both interictal and ictal events. This "epileptic dip" in oxygenation can persist for the duration of an ictal event, suggesting that cerebral blood flow is inadequate to meet metabolic demand. A rapid focal increase in cerebral blood flow and cerebral blood volume also accompanies epileptic events; however, this increase in perfusion soon (>2 s) spreads to a larger area of the cortex than the excitatory change in membrane potential. Investigations in humans during neurosurgical operations have confirmed the laboratory data derived from animal studies. These data not only have clinical implications for the interpretation of noninvasive imaging studies such as positron emission tomography, single-photon emission tomography, and functional magnetic resonance imaging but also provide a mechanism for the cognitive decline in patients with chronic epilepsy.

Animals↗
PubMed2005

Temporal dependence in uncoupling of blood volume and oxygenation during interictal epileptiform events in rat neocortex.

We investigated the dynamic spatiotemporal relationships of cerebral blood volume (CBV), deoxygenated hemoglobin (Hbr), and light scatter (LS) associated with interictal epileptiform events with multiwavelength optical recording of intrinsic signals and simultaneous field potential recording. Interictal spikes (IISs) were induced with iontophoresis of bicuculline methiodide in rat neocortex. Intrinsic signal changes appeared as early as 100 msec after the IIS at all wavelengths and could be appreciated after only a single IIS. Initially, the largest signal arose from a focal increase in deoxygenation, which lasted for approximately 2 sec, consistent with an "initial dip." An equally early focal increase in CBV had a smaller amplitude than the Hbr signal until >2 sec after the IIS, when its amplitude surpassed that of the Hbr signal but also spread to a larger, less focal area. The most spatially restricted and smallest amplitude signal was produced by LS. A later hyperoxygenation, or increase in blood oxygenation level-dependent signal, was often seen in the draining veins but inconsistently seen in the IIS focus. An inverted optical signal was recorded at all wavelengths from multiple regions in the surrounding cortex within 100 msec of the IIS. We therefore conclude that the IIS induces a rapid increase in metabolic demand, which cannot be met by a rapid, initially focal but small increase in CBV that results in a prolonged increase in Hbr (epileptic dip in oxygenated hemoglobin). The inverted optical signal in the surround arises from a decrease in CBV and a decrease in Hbr, likely resulting from a combination of shunting of CBV to the focus and decreased metabolic demand resulting from decreased neuronal activity, consistent with "surround inhibition."

Action Potentials↗
PubMed2002

Modeling cerebellar flocculus and paraflocculus involvement in complex predictive smooth eye pursuit in monkeys.

The role of flocculus and paraflocculus neurons in the cerebellar control of predictive eye movements was examined using two modeling techniques. The first study characterized the dependence of individual Purkinje-cell firing patterns on oculomotor output, visual input, and response timing using multilinear regression techniques. Interestingly, no dependence on visual input was detected. Purkinje cell firing was explained by sensitivities to eye position and eye velocity alone. However, complex responses occurred when sensitivity vectors pointed in different directions. For example, some neurons showed a preference for circular pursuit in a particular rotation direction. Responses also tended to lead the eye during predictable pursuit and to lag during unpredictable, visually driven pursuit. This suggests that flocculus and paraflocculus neurons played a stronger role during predictive pursuit than visually driven pursuit. A second modeling study demonstrated how the flocculus/paraflocculus system might generate predictive pursuit. A biologically realistic neural network was simulated based on the known anatomy and physiology of this cerebellar system. It included mossy and climbing fibers with realistic responses, Purkinje cells acting on well-characterized brain-stem circuits, and granule, Golgi, basket, and stellate cells with appropriate connections. The network was able to learn new pursuit trajectories based on long-term alterations in synaptic connectivity at parallel-to-Purkinje synapses. Interestingly, this model was able to generate predictive pursuit without visual input based only on eye-motion input. Thus, both models provide complementary evidence for the generation of nonvisual predictive control by flocculus and paraflocculus neurons.

Animals↗