Biomedical subjects

Burak Güçlü

Publications and source records attributed to Burak Güçlü.

15 recordsLinked to original sources

PubMed2005

Population-response model for vibrotactile spatial summation.

A computational model based on previous physiological and psychophysical data is presented for the human Pacinian (P) psychophysical channel. The model can predict the probability of detection in simple psychophysical tasks, and hence psychometric functions and thresholds. The model simulates stimulating variable and fixed glabrous skin sites with different-sized contactors and includes spatial variation of monkey P-fiber sensitivities. Therefore, it is especially suitable for studying spatial summation, i.e. the improvement of threshold with increasing contactor area. Selective contributions of neural integration (n.i.) and probability summation (p.s.) are also incorporated into the model. Model predictions are compared to psychophysical results of Gescheider et al. (2005). The performance of the model regarding the effects of contactor size is very good. In addition to predicting approximately 3 dB improvement of thresholds when the contactor area is doubled, the model also reveals nonlinear contributions of p.s. and n.i. Furthermore, the model asserts that thresholds are largely governed by neural integration when small contactors are used. These and other findings discussed in the article show that the presented model is a helpful tool for formulating testable hypotheses. Although the model can also simulate some temporal summation effects, simulation results do not conform well to previous data on temporal response properties. Thus, the model needs to be refined in that respect.

Cell Count↗

PubMed2005

Spatial summation in the tactile sensory system: probability summation and neural integration.

Psychophysical thresholds for the detection of a 300-Hz burst of vibration applied to the thenar eminence were measured for stimuli applied to the skin through 1.5 cm2 and through 0.05 cm2 contactors. Thresholds were approximately 13 dB lower when the area of the contactor was 1.5 cm2 than when it was 0.05 cm2. The difference between the thresholds measured with the large and small contactors was significantly reduced when only the lowest thresholds obtained in the testing sessions were considered. This result supports the hypothesis that one component of spatial summation in the P channel is probability summation. In addition, threshold measurements within a session were less variable when measured with the 1.5 cm2 contactor. We conclude that spatial summation in the P channel is a joint function of two processes that occur as the areal extent of the stimulus increases: probability summation in which the probability of exceeding the psychophysical detection threshold increases as the number of receptors of varying sensitivities increases, and neural integration in which neural activity originating from separate receptors is combined within the central nervous system rendering the channel more sensitive to the stimulus.

Adult↗

PubMed2005

Maximizing the entropy of histogram bar heights to explore neural activity: a simulation study on auditory and tactile fibers.

Neurophysiologists often use histograms to explore patterns of activity in neural spike trains. The bin size selected to construct a histogram is crucial: too large bin widths result in coarse histograms, too small bin widths expand unimportant detail. Peri-stimulus time (PST) histograms of simulated nerve fibers were studied in the current article. This class of histograms gives information about neural activity in the temporal domain and is a density estimate for the spike rate. Scott's rule based on modem statistical theory suggests that the optimal bin size is inversely proportional to the cube root of sample size. However, this estimate requires a priori knowledge about the density function. Moreover, there are no good algorithms for adaptive-mesh histograms, which have variable bin sizes to minimize estimation errors. Therefore, an unconventional technique is proposed here to help experimenters in practice. This novel method maximizes the entropy of histogram-bar heights to find the unique bin size, which generates the highest disorder in a histogram (i.e., the most complex histogram), and is useful as a starting point for neural data mining. Although the proposed method is ad hoc from a density-estimation point of view, it is simple, efficient and more helpful in the experimental setting where no prior statistical information on neural activity is available. The results of simulations based on the entropy method are also discussed in relation to Ellaway's cumulative-sum technique, which can detect subtle changes in neural activity in certain conditions.

Algorithms↗

PubMed2004

Influence of target size and luminance on the White-Todorovic effect.

Variants of a lightness effect described by [Todorovic's, D. (1997). Lightness and junctions. Perception, 26, 379] were studied to quantify the failure of lightness constancy as a function of target luminance and target size. Todorovic's effect is similar to White's effect. Simultaneous lightness contrast appears to operate selectively between stimuli belonging to the same perceptual group, and not between stimuli of equal proximity belonging to different perceptual groups. We found that mid-gray targets grouped with a white contextual stimulus were matched on average to a darker-than-veridical gray. Those grouped with a black contextual stimulus were matched on average veridically. This is consistent with 'anchoring' effects observed in simple two-stimulus displays. However, target luminance had an effect that was not captured by mid-level target luminance data or data averaged across target luminances. For both white and black contextual stimuli, light-gray targets were matched to a darker-than-veridical gray and the direction of this error shifted toward the lighter-than-veridical direction as the luminance of the target was lowered. The result was a constant difference between the perceived lightnesses of targets presented with white and black contextual stimuli. Target size had no effect on perceived lightness. These data imply that the Todorovic-White effect can be characterized as lightness assimilation rather than as lightness contrast. By accounting for compression as well as the Todorovic-White effect, assimilation is the more general explanation.

Adult↗

PubMed2004

Probability of stimulus detection in a model population of rapidly adapting fibers.

The goal of this study is to establish a link between somatosensory physiology and psychophysics at the probabilistic level. The model for a population of monkey rapidly adapting (RA) mechanoreceptive fibers by Güçlü and Bolanowski (2002) was used to study the probability of stimulus detection when a 40 Hz sinusoidal stimulation is applied with a constant contactor size (2 mm radius) on the terminal phalanx. In the model, the detection was assumed to be mediated by one or more active fibers. Two hypothetical receptive field organizations (uniformly random and gaussian) with varying average innervation densities were considered. At a given stimulus-contactor location, changing the stimulus amplitude generates sigmoid probability-of-detection curves for both receptive field organizations. The psychophysical results superimposed on these probability curves suggest that 5 to 10 active fibers may be required for detection. The effects of the contactor location on the probability of detection reflect the pattern of innervation in the model. However, the psychophysical data do not match with the predictions from the populations with uniform or gaussian distributed receptive field centers. This result may be due to some unknown mechanical factors along the terminal phalanx, or simply because a different receptive field organization is present. It has been reported that human observers can detect one single spike in an RA fiber. By considering the probability of stimulus detection across subjects and RA populations, this article proves that more than one active fiber is indeed required for detection.

Action Potentials↗

PubMed2003

Distribution of the intensity-characteristic parameters of cat rapidly adapting mechanoreceptive fibers.

Modeling population responses of nerve fibers requires statistical characterization of fiber-response properties. The rate/intensity characteristics of cat rapidly adapting (RA) fibers were fitted by four-parameter, piece-wise linear functions using nonlinear regression (n = 14; R2 > 0.958). The parameters were tested against the null hypothesis that they are log normally distributed. The test fail to reject this hypothesis (Kolmogorov-Smirnov p>0.477). However, a significant statistical difference was found between the specific lognormal distributions obtained from monkey (Johnson, J Neurophysiol 37: 48-72, 1974) and cat for all four parameters (Kolmogorov-Smirnov, p<0.0075, p<0.05, p<0.0001, p<0.00007). Although the stimulus contactor size was not the same in monkey and cat studies, the differences between monkey and cat fibers are attributed to anatomical differences in the glabrous sin of both species. Modeling studies suggest that the absolute firing thresholds of RA fibers have a right-skewed distribution because of the anatomical constraints present in both species' skin. Meissner corpuscles, which are the sensory end-organs of RA fibers, are likely to be found deeper in the skin within dermal papilla, therefore, the thresholds can be elevated. However, the thresholds are bounded at lower end, probably due to the epidermal junction that acts as a superficial mechanical barrier for these corpuscles.

Afferent Pathways↗

PubMed2003

Frequency responses of cat rapidly adapting mechanoreceptive fibers.

The frequency responses of 11 rapidly adapting (RA) fibers in cat were studied by representing the average firing rate as a function of sinusoidal stimulus amplitude and stimulus frequency. Specifically, rate-intensity functions at different stimulation frequencies were fitted by four-parameter (a0, a1, a2, a3), piece-wise linear functions using nonlinear regression (n = 59; R2 > 0.877). Rate-intensity functions at intermediate frequencies were found by linear interpolation. The result of this analysis is rate-amplitude-frequency functions plotted as two-dimensional surfaces. The surfaces consist of five regions separated and sufficiently defined by four space curves. At 14 different frequencies, the statistical distribution of each rate-intensity-function parameter could be approximated by a particular lognormal distribution (n = 56; R2 > 0.796). The Kolmogorov-Smirnov test fails to reject this hypothesis for each combination of frequency and parameter (56 tests; p > 0.39). Therefore, at a given frequency, the variation of the parameters can be represented by lognormal distributions with specific means and standard deviations. Responses of six RA fibers, which are different from the data-set used for modeling, were compared with the stochastic model at different frequencies. The parameters of those fibers were tested against the null hypotheses that they were sampled from the particular parameter distributions dictated by the model. The Kolmogorov-Smirnov test fails to reject all the hypotheses at the alpha = 0.05 level (44 tests). At the alpha = 0.10 level, only a few test parameters were found to be departing from the model (a0 and a1 at 5 Hz; a2 at 20 Hz; a2 and a3 at 50 Hz). The remaining test parameters could be accurately described by the model. Having confirmed the validity of the model, the logarithmic means and the logarithmic standard deviations of the lognormally distributed rate-intensity-function parameters were estimated in the frequency range of 4-200 Hz. The rate-amplitude-frequency surfaces sampled from the established stochastic model completely characterize the rate responses of RA fibers to sinusoidal stimuli and are superior to tuning curves which require selecting criterion responses. The current rate-response model is promising for future computational work, especially on population modeling.

Action Potentials↗

PubMed2003

Correlation of spatial event plots with simulated population responses of mechanoreceptive fibers.

The experimental setup for generating spatial event plots (SEPs) from single mechanoreceptive fibers of the skin was computationally simulated. The generic fibers used in the simulations were similar to the rapidly adapting fibers (RAs), and had variable refractoriness and receptive-field size. The speed, lateral shift, and the contact width of the drum scanned across the receptive field of the fiber are adjustable parameters. The stimulus patterns used on the drum mimicked stimuli used by several other investigators. These were dot patterns, grating patterns, and the letter "E". First, the effects of simulation parameters on the SEPs were studied. The simulation output confirms the results of physiological experiments that SEPs contain information on the spatiotemporal resolution of the fiber. The next series of simulations involved generating SEPs of fibers obtained from the same or varying spatial distributions of receptive fields. Three hypothetical distributions were used: homogeneous rectangular, uniformly random, and Gaussian. The momentary population response at each case was found using the technique by Johansson and Vallbo (Brain Res 184: 353-366, 1980). The population responses were not isomorphic images of the stimulus patterns due to the variations in field sizes and locations. However, every fiber, no matter which distribution it came from, generated almost identical SEPs given similar response properties. Furthermore, the SEPs looked like the outline of the stimulus. These observations show that SEPs do not contain information about the population response. Therefore, reconstructing the population response using SEPs can result in misleading conclusions on central-nervous-system processing and should be viewed cautiously when formulating psychophysical/physiological linking hypotheses.

Animals↗

PubMedDate not supplied

Modeling population responses of rapidly-adapting mechanoreceptive fibers.

The population response of rapidly-adapting (RA) fibers is one component of the physiological substrate of the sense of touch. Herein, we describe a computational scheme based on the population-response model by K.O. Johnson (J. Neurophysiol. 37: 48-72, 1974) which we extended by permitting the capability to include the spatial distributions of receptors in the glabrous skin linked to RA fibers. The hypothetical cases simulated were rectangular, uniformly random and proximo-distally Gaussian distributions. Each spatial organization produced qualitatively distinct population-response profiles that also varied due to stimulus parameters. The effects of stimulus amplitude, average innervation density and contactor-probe location were studied by considering various response measures: number of active fibers, summated firing rate and the average firing rate of a subset of the modeled population. The outcome of the measures were statistically compared among simulated anatomical distributions. The response is the same for rectangular and uniformly random distributions, both of which have a homogeneous innervation density. However, the Gaussian distribution produced statistically different responses when the measure was not averaged over the subset population which represented the receptive field of a higher-order neuron. These results indicate that, as well as stimulus parameters, the anatomical organization is a significant determinant of the population response. Therefore, reconstructing population activity for testing psychophysical hypotheses must presently be done with care until the organization of the receptors within the skin has been clarified.

Action Potentials↗

PubMedDate not supplied

End-to-end linkage (EEL) clustering algorithm: a study on the distribution of Meissner corpuscles in the skin.

A novel hierarchical clustering algorithm was applied to the distribution of Meissner corpuscles in the skin of mammals. This method, called end-to-end linkage (EEL), is useful for grouping data that consists of chain-like contingencies in the multivariable space. Unlike the traditional techniques which uncover hyperspherical clusters (e.g. single linkage), EEL considers the shortest distance between the predefined end pairs of the two clusters as an inter-group distance. This scheme allows characterizing the internal structure of data better than other hierarchical techniques. The anatomical data used in the case study is important for studying the sense of touch. The results show a substantial improvement over the traditional single-linkage method. On average, the number of correctly classified corpuscles is increased to twice the number identified by the single-linkage method. EEL can also be used for analyzing other sensory modalities where geometric relationships need to be explored. In addition, the report contains corpuscle density and epidermal-ridge width data obtained from several species.

Algorithms↗

PubMedDate not supplied

Tristate markov model for the firing statistics of rapidly-adapting mechanoreceptive fibers.

Rapidly-adapting (RA) mechanoreceptive fibers, which are associated with Meissner corpuscles, mediate one component of the neural information that contributes to the sense of touch. Responses of cat RA fibers subject to 40-Hz sinusoidal stimulation were modeled as a Markov process. Since an RA fiber generates one, two or no spikes in each cycle of the stimulus, the fiber's activity was considered to exist in one of these three possible states. By analyzing empirically generated spike trains, the probability of each state and the probabilities of transitions between the three states were found as a function of the average firing rate of the fiber. The average firing rate depends on the stimulus amplitude. In addition, the phase of each spike with respect to the stimulus cycle was represented by a Laplace distribution. Based on empirical data, the mean and the standard deviation of this distribution decrease as the stimulus amplitude is increased. The entire stochastic model was implemented on a computer to simulate the responses of RA fibers. The post-stimulus time, inter-spike interval and period histograms generated from the simulations match the histograms obtained from the empirical data well as quantified by relative errors. This temporal model can be combined with a population model for average rate to derive a spatio-temporal description of the responses of somatosensory afferents. The effects of changing the stimulation frequency are discussed.

Action Potentials↗

PubMedDate not supplied

Vibrotactile thresholds of the Non-Pacinian I channel: I. Methodological issues.

Thresholds of the Non-Pacinian I (NP I) channel were measured using a two-interval forced-choice paradigm, a technique independent of the subject's criterion. The studies were performed using the terminal phalanx of the human middle finger with a 40-Hz vibratory stimulus. Unlike most of the previous experiments performed in our laboratory, a contactor surround was not used. This was done to enable comparison with population models of mechanoreceptive fibers in the literature. Since the Pacinian (P) channel and NP I channel have similar vibrotactile thresholds at 40?Hz, a forward-masking procedure was used to elevate the thresholds of the P channel with respect to the NP I channel. While it has been established that the Pacinian fibers are entrained at high stimulus levels, the P channel can be perceptually masked using a 250-Hz stimulus presented prior to the 40-Hz test stimulus. The masking functions were found to be approximately linear on log-log axes and the threshold shifts were found to increase as the masking-stimulus levels increased. The results are discussed in relation to previous studies that were performed at various stimulation sites by using a contactor surround or not. A companion paper presents the variation of NP I-channel thresholds, measured using the methods described herein, and addresses the effects of stimulation along the proximo-distal axis of the phalanx. The companion paper also discusses the predictions of a computational model, recently proposed, in light of the empirical results presented.

Adaptation, Physiological↗

PubMedDate not supplied

Vibrotactile thresholds of the Non-Pacinian I channel: II. Predicting the effects of contactor location on the phalanx.

The firing-rate-based population model for rapidly-adapting (RA) mechanoreceptive fibers by Güçlü and Bolanowski is extended by including temporal-response properties of RA fibers. This representation allows for the generation of action-potential (spike) times for each fiber when a sinusoidal, steady-state stimulus is applied onto the skin. Signal detection theory was used to predict human psychophysical thresholds. Specifically, the effects of sensorineural innervation pattern, stimulus-contactor location and selected decision rules on the model predictions were studied. The predicted thresholds were lowest when the decision rule was one spike and highest when many active fibers were required for detection. These predictions were empirically tested by measuring vibrotactile thresholds of the Non-Pacinian I (NP I) channel, which required the special techniques discussed in the preceding article. Although the model predicted thresholds to decrease distally due to the known innervation density which is higher distally, the thresholds of the NP I psychophysical channel were found to be approximately constant (20-25 dB re 1 microm peak amplitude) from the proximal site on the terminal phalanx to the most distal portion. Interestingly, the mechanical impedance of the skin was found not to be constant along the proximo-distal axis. This latter result implies that the space-invariant mechanical attenuation function used in the model may not be valid at every location on the fingertip. Because of this, the discrepancy between the model's predictions and the psychophysical results may be reconciled.

Action Potentials↗

PubMedDate not supplied

Ovoid geometry of the Pacinian corpuscle is not the determining factor for mechanical excitation.

Static displacements in Pacinian corpuscles (PCs) were measured using video microscopy. Mechanical stimuli of 10-40 microm steps were applied to the PC capsule surfaces using cylindrical contacts with different diameters. Displacements parallel to the stimulation axis were measured at various locations in the focal plane of the optical setup. In contrast to previous data in the literature, the displacements within the corpuscle were found to be linearly related to the indentation amplitude. Displacements decreased as a function of lamella depth, with a more negative slope close to the surface and less negative slope at deeper locations. The experimental data were compared to the predictions of a previous mechanical model, and to the results of two new models: (1) elastic semi-infinite continuum model; (2) ovoid isotropic finite-element model. Although the previous model did not specify displacement boundary conditions, it predicted the current experimental results well. On the other hand, the experimental displacements were found to be smaller than those predicted by the semi-infinite continuum and finite-element models. However, both semi-infinite continuum and finite-element models yielded close results, which show that the three-dimensional ovoid geometry of the corpuscle is not the primary factor for determining the displacements in physiological conditions. Furthermore, simulations with the finite-element model using a wide range of material properties yielded similar results. This supports the hypothesis that a homogeneous isotropic model for the PC cannot predict experimental results. The modeling analyses suggest that the experimental results are largely affected by the displacement of the incompressible interlamellar fluid and the layered structure of the corpuscle.

Animals↗

PubMedDate not supplied

Constrained optimization of Drude's equations eliminates effects of confounding molecules for the polarimetric measurement of glucose.

Common confounding factors for polarimetric concentration measurements include additional optical rotations from unknown optically active molecules, linear birefringence of the medium, and path length variability. We show that by approximating Drude's equation and taking several measurements from the same sample at different wavelengths, the error due to confounding rotations in the measurements can theoretically be canceled. The analysis is developed with regard to glucose sensing in aqueous humor. First, we show that the optical rotatory dispersions of the known molecules in bovine aqueous humor could be represented by Drude's equations. Then, the total optical rotation is approximated by a function combining Drude's equations for the major contributors in the sample, i.e., glucose, glutamine, fructose, and phenylalanine. The concentration-related unknown coefficients in the approximating function are found by constrained nonlinear optimization of the function at different wavelengths. This technique is tested on a published data set and four alterations of those data: (1) concentrations randomly varied within narrow limits, (2) similar to alteration 1 but with significantly elevated glucose concentration, (3) similar to alteration 1 but with significantly decreased glucose concentration, and (4) concentrations randomly varied within wider limits than alteration 1. The method produces very accurate glucose-concentration estimates in all of these data sets. The relative error was smaller than 1% in all except the low-glucose sample (1.4%). This method may prove useful in noninvasive glucose measurement in humans.

Algorithms↗