Biomedical subjects

John L Semmlow

Publications and source records attributed to John L Semmlow.

9 recordsLinked to original sources

PubMed2006

Dry dissection of disparity divergence eye movements using independent component analysis.

Dry dissection, a concept developed by Lawrence Stark, includes a variety of techniques designed to isolate internal neural control components by using cleverly designed stimulus or measurement protocols. As envisioned by Stark, the concept applies only to motor systems that have multiple stimulus inputs and/or response behaviors. A new application of independent component analysis (ICA) can be used to extend the dry dissection concept to identify motor components from a single, isolated response. It is only necessary that multiple responses can be obtained to the same stimulus. This "ensemble ICA" technique is well suited to analyze various eye movement behaviors as even isolated motor systems often include multiple control processes. Here we apply ensemble ICA to vergence eye movements: the inward (convergence) or outward (divergence) turning of the eyes that allows us to view images at various distances. Previous studies concerning the dynamics of convergence and divergence eye movements have produced varied, sometimes contradictory, results: most studies report that convergence is considerably faster than divergence, but opposite results have also been reported. Experimental results have shown that the dynamics of divergence movements depend on the initial vergence position while those of convergence do not: divergence eye movements in response to targets initially near to the subject can attain peak velocities twice that of those driven by more distant targets. To determine the underlying cause of this behavior, ensemble ICA was applied to divergence responses. Results show that both convergence and divergence contain a sustained (step-like) and a transient (pulse-like) control component, but the amplitude of the transient component in divergence is dependent on initial stimulus position.

Adolescent↗

PubMed2006

Dynamic assessment of disparity vergence ramps.

Previous work has shown that the disparity vergence eye movement system responds to inward (i.e., convergent) ramp stimuli with both smooth and step-like behavior depending on target velocity. The responses to diverging ramp stimuli have not been previously studied, but convergence and divergence responses to other stimuli often show different behaviors. Converging and diverging 6 degrees/s ramps were presented to four subjects over a stimulus range of 2 degrees-20 degrees. Step-like behavior was seen in both convergence and divergence responses, but the dynamics was different. For divergent ramps, the peak velocity of each step-like movement decreased as the stimulus moved away from the subject, but no such trend was observed for convergence. The step-like behavior seen in divergence supports the hypothesis that the transient component is active in disparity divergence similar to the transient component proposed for convergent movements. However, the transient component in divergence may be dependent on stimulus position which is not the case for convergence.

Adolescent↗

PubMed2005

Short-term predictive changes in the dynamics of disparity vergence eye movements.

Repetitive stimulation of the disparity vergence system to large convergent step stimuli has been shown to increase the dynamics of subsequent responses to smaller step stimuli. Here we show that decreases in the dynamics of both disparity convergence and divergence eye movements can be induced using a frequently occurring small amplitude conditioning stimulus to modify responses to a larger, occasionally presented test stimulus. In one experiment, a simple conditioning stimulus consisting of repetitive 1 degrees step stimuli was used to modify the dynamic vergence response to an occasional 4 degrees step test stimulus. An experimental trial consisted of three phases: baseline, conditioning, and recovery. The baseline and recovery phases used only the 4 degrees test stimuli. The dynamic characteristics of the responses to test stimuli were quantified by measuring the magnitude of the peak velocity. A statistically significant change was observed between the dynamics of conditioned responses compared to baseline and recovery responses indicting modification by the conditioning stimuli. During recovery, the response dynamics returned to levels near baseline levels showing that the decrease in response dynamics was caused by the conditioning stimulus, not fatigue. Another experiment showed that the response dynamics to large stimuli could be decreased whereas the dynamics of small stimuli could be increased by the same intermediate conditioning stimulus. Other experiments suggest that the modifications are due to a predictive mechanism. The results indicate that the dynamics of disparity vergence eye movements are malleable and depend to some extent on the amplitude of preceding stimuli.

Adult↗

PubMed2005

Divergence eye movements are dependent on initial stimulus position.

Previous studies on the speed and latency of convergence and divergence eye movements have produced varied, sometimes contradictory, results. Four subjects were studied and tracked 4 degrees disparity step changes for convergence and divergence at different initial target positions. Here we report that the dynamics of divergence movements not only differ from convergence movement, but depend on the initial vergence position. Velocities of divergence eye movements in response to targets that were initially near to the subject were approximately twice that of responses to initially distant targets and also exhibited shorter temporal properties. Hence, while convergence responses are fairly similar irrespective of the initial position, divergence dynamic and temporal properties are dependent on the initial stimulus position. It is speculated that the differences observed in divergence may be the result of nonlinear properties of the extraocular muscles or a difference in the underlying neural controller potentially a difference in the magnitude of the fusion initiating component of divergence.

Adolescent↗

PubMed2004

Optical imaging of breast tumor through temporal log-slope difference mappings.

A novel optical temporal log-slope difference mapping approach is proposed for cancerous breast tumor detection. In this method, target tissues are illuminated by near-infrared (700-1000 nm) ultrashort laser pulses from various surface source points, and backscattered time-resolved light signals are collected at the same surface points. By analyzing the log-slopes of decaying signals over all points on the source-detection grid, a log-slope distribution on the surface is obtained. After administration of absorption contrast agents, the presence of cancerous tumors increases the decaying steepness of the transient signals. The mapping of log-slope difference between native tissue and absorption-enhanced cancerous tissue indicates the location and projection of tumors on the detection surface. In this paper, we examine this method in the detection of breast tumors in two model tissue phantoms through computer simulation. The first model has a spherical tumor of 6mm in diameter embedded at the tissue center. The second model is a large tissue phantom embedded with a non-centered spherical tumor 8mm in diameter. Monte Carlo methods were employed to simulate the light transport and signal measurement. It is shown that the tumor in both the tissue models will be accurately projected on the detection surface by the proposed log-slope difference mapping method. The image processing is very fast and does not require any inverse optimization in image reconstruction.

Breast Neoplasms↗

PubMed2004

Magnetic resonance imaging study of the effects of age and accommodation on the human lens cross-sectional area.

PURPOSE: To evaluate the effect of age and accommodation on lens cross-sectional area (CSA). METHODS: High-resolution magnetic resonance images of the eye were acquired from 25 subjects ranging in age from 22 to 50 years during accommodation and with accommodation at rest. The images were analyzed to obtain the total lens CSA and the CSAs of the anterior and posterior portions of the lens. RESULTS: The total lens CSA and the CSA of the anterior portion increased with age in both accommodative states. With accommodation, the CSA was larger in these portions of the lens; however, this difference decreased with age. Conversely, the CSA of the posterior portion of the lens remained statistically independent of both age and accommodative state. CONCLUSIONS: This preliminary study documents, in vivo, that the lens grows with age. This growth appears to be confined to the anterior portion. A quite unexpected finding is that both the total lens CSA and the CSA of the anterior portion are greater during accommodation when zonular tension is minimized. This accommodative change in CSA, which decreases with age, may be due to compression of the lens material during relaxed accommodation when zonular tension is greatest. That both age and accommodative changes in CSA appear to be limited to the anterior portion of the lens may be related to properties of the anterior capsule and lens material, the position of the zonular attachments, and the location of the fetal nucleus.

Accommodation, Ocular↗

PubMed2004

Scheimpflug and high-resolution magnetic resonance imaging of the anterior segment: a comparative study.

High-resolution imaging with a camera system built on the Scheimpflug principle has been used to characterize the geometry of the anterior segment of the adult human eye as a function of aging and accommodative state but is critically dependent on algorithms for correction of distortion. High-resolution magnetic resonance imaging (MRI), in contrast, provides lower-resolution information about the adult eye but is undistorted. To test the accuracy of the Scheimpflug correction methods used by Cook and Koretz [J. Opt. Soc. Am. A 15, 1473 (1998)]; [Appl. Opt. 30, 2088 (1991)], data on anterior chamber and segment lengths, as well as lens thickness and anterior and posterior curvatures, were compared with corresponding MRI data for adults aged 18-50 at 0 diopter accommodation. Excellent statistical agreement was found between the MRI and the Scheimpflug data sets with the exception of the posterior lens radius of curvature, which is less well defined than the other measurements in the Scheimpflug images. The considerable agreement between data obtained with MR and Scheimpflug imaging, two different yet complementary in vivo imaging techniques, validates the Scheimpflug correction algorithms of Cook and Koretz and suggests the capability of directly integrating information from both. A third, equivalent, data set obtained with a Scheimpflug-style camera system differs considerably from both Scheimpflug and MRI results in magnitude and age dependence, with negative implications for this alternative method and its correction procedures.

Adult↗

PubMed2002

Components of disparity vergence eye movements: application of independent component analysis.

The "dual mode" theory for the control of disparity vergence eye movements states that two control components, a preprogrammed "transient" component and a feedback-controlled "sustained" component, mediate the motor response. Although prior experimental work has isolated and studied the transient component, little is known of the sustained component's contribution to the dynamic vergence response. The timing between the two components and their relative magnitudes are of interest as they relate to the strategies used by the brain to coordinate and control the two components. Modeling studies provide an estimate of component magnitudes, but cannot uniquely identify component timing nor can the provide detailed information on component dynamics. Here, an eigenvector analysis is applied to a multivariate data set consisting of multiple responses to a step stimulus to confirm the presence of two major components in the vergence response. Next, a new application of independent component analysis is used to estimate the activation patterns of the two components. Results from five subjects show that the sustained component is activated concurrently with the transient component, dominates the later portion of the response, and maintains final position.

Algorithms↗

PubMed2002

Adaptive modification of disparity vergence components: an independent component analysis study.

PURPOSE: Although a disparity vergence stimulus produces a smooth exponential-like response, considerable experimental evidence indicates that it is the product of at least two motor components: a pulselike transient component and a steplike sustained component. Recently, a new application of independent component analysis (ICA) has been used to decompose the vergence step response into these underlying components. Other recent experiments have shown that the vergence system is capable of rapidly modifying its dynamic characteristics (short-term adaptation) when exposed to specially designed "adapting" stimuli. Adapted responses were characterized by faster dynamics, often featuring large overshoots. In this study, ICA was used to examine changes in the underlying components produced by dynamic adaptation. METHODS: Disparity vergence eye movements in response to identical step stimuli were obtained from four subjects both in normal (baseline) conditions and after modification by adaptive training stimuli. ICA-based component decomposition was applied to vergence step-response data sets in both normal and adapted conditions to estimate, and compare activation patterns of the two underlying components. RESULTS: An eigenvector analysis indicated that both normal and adapted vergence responses contained two major components. ICA analysis showed that the enhanced dynamics seen in adapted responses was due to an increase in pulse component amplitude. In addition, the step component of adapted responses often showed double-step behavior in the later portion of the response. Finally, the magnitude of adaptation appeared to be related to the unadapted response dynamics. CONCLUSIONS: The adaptive process does not evoke additional components, but modifies the two components that are present under normal conditions. Double steps seen in the step component were attributed to an interaction between pulse and step neural mechanisms. The generation of an enhanced pulse component interfered with the production of the step component. Under this scenario, the reduced initial-step component was then compensated by the generation of a second-step component, probably mediated by an internal feedback mechanism.

Adaptation, Ocular↗