An oscillation theory of handwriting

Handwriting production is viewed as a constrained modulation of an underlying oscillatory process. Coupled oscillations in horizontal and vertical directions produce letter forms, and when superimposed on a rightward constant velocity horizontal sweep result in spatially separated letters. Modulation of the vertical oscillation is responsible for control of letter height. Modulation of the horizontal oscillation is responsible for control of corner shape through altering phase or amplitude. The vertical velocity zero crossing in the velocity space diagram is important from the standpoint of control. Changing the horizontal velocity value at this zero crossing controls corner shape. Changing the slope at this zero crossing controls writing slant. The corner shape and slant constraints completely determine the amplitude and phase relations between the two oscillations. This theory applies generally to a number of acceleration oscillation patterns such as sinusoidal rectangular and trapezoidal oscillations. The oscillation theory also provides an explanation for how handwriting might degenerate with speed. An implementation of the theory in the context of the spring muscle model is developed. Here sinusoidal oscillations arise from a purely mechanical source; orthogonal antagonistic spring pairs generate particular cycloids depending on the initial conditions. Modulating between cycloids can be achieved by changing the spring zero settings at the appropriate times. Frequency can be modulated either by shifting between coactivation and alternating activation of the antagonistic springs or by presuming variable spring constant springs. An acceleration and position measuring apparatus was developed for measurements of human handwriting. Measurements of human writing are consistent with the oscillation theory.     

Zero crossing rate of electromyograms during occupational work and endurance tests as predictors for work related myalgia in the shoulder/neck region

The relationship between electromyographic signs of fatigue (ESF) during work and occupation-related myalgia in the shoulder/neck region was investigated in a longitudinal study. Forty-three healthy female assembly workers were studied over 2 years. Measurements were performed at the start of the study with follow-up measurements after 1 and 2 years. The ESF were estimated as the zero crossing rate of electromyograms (EMG) detected during short test contractions performed during short breaks in normal work. As a complement, an endurance test using EMG records was performed and analysed with the zero crossing technique. The occurrence of shoulder/neck disorders was assessed by a clinical investigation and a questionnaire. No significant relationship between ESF during work in year 0 and deterioration of the disorder was seen. On the other hand, the absolute zero crossing rate and the time constant of the zero crossing decline from the endurance test showed a significant relationship with deterioration of the disorder. The ESF during work year 2, showed a significant relationship with disorder year 2, while the endurance test parameters year 2 did not. It was concluded that ESF during work was not a predictor of muscle injury, whereas it could be useful as a diagnostic tool.     

Responses of neurons in the primary auditory cortex of the cat to the auditory motion stimuli with variable interaural delay

Unit responses in the primary auditory cortex of anesthetized cats to stationary and apparently moving stimuli resulted from a static and dynamically varying interaural delay (ITD) were recorded. The static stimuli consisted of binaurally presented tones and clicks. The dynamic stimuli were produced by in-phase and out-of-phase binaurally presented click trains with time-varying ITD. Sensitivity to ITDs was mostly seen in responses of the neurons with low characteristic frequency (below 2.8 kHz). All cells sampled with static stimuli responded to simulated motion. A motion effect could take the form of a difference in response magnitude depending on the direction of stimulus motion and a shift in the ITD-function opposite the direction of motion. The magnitude of motion effects was influenced by the position of motion trajectory relative to the ITD-function. The greatest motion effect was produced by motion crossing the ITD-function slopes.     

The lens as a nonuniform spherical syncytium.

The effective intracellular resistivity Ri of the ocular lens is a measure of the coupling between cells. Since degradation of coupling may accompany cataracts, measurements of Ri are of considerable interest. Experimental results show that the lens is a nonuniform syncytium in which Ri is much higher in the nuclear region than in the cortex. A theory describing the lens as a radially nonuniform spherical syncytium is proposed, solved, and described as a simple equivalent circuit. The impedance of the lens is measured with new circuitry which permits the accurate application and measurement of current and voltage over a wide bandwidth without arbitrary compensation of unstable capacitances. The fit of the nonuniform theory to experimental data is satisfactory and the parameters determined are consistent with theoretical assumptions. In the outer region (cortex) of the lens Ri = 2.4 k omega-cm, probably as a consequence of differences in coupling and cytoplasmic resistivity. The radial resistivity of the cortex is some five times the circumferential resistivity, demonstrating a marked anisotropy in the preparation, probably reflecting the anisotropy in the orientation of lens fibers and distribution of gap junctions. Current can flow in the circumferential direction without crossing from fiber to fiber; current can flow in the radial direction only by crossing from fiber to fiber.     

Hemoglobin-carbon monoxide binding rate. Low temperature magneto-optical detection of spin-tunneling.

The spin-tunneling model of Hb--CO binding is used to calculate the binding rate at low temperature and high magnetic fields. The rate is calculated in second order perturbation theory assuming that spin-orbit coupling mediates the Hb iron electronic state change. The reaction which occurs at the crossing of the S = 2 and S = 0 energy vs. configuration coordinate curves is nonadiabatic, having a small electronic transition matrix element. Since detection of CO binding by polarized light in the Soret band makes it possible to observe hemes at specific orientation to the field direction, the rate is calculated for arbitrary heme orientation. Comparison with measurements at low temperature in zero field is made for spin quantization along the molecular crystal field direction.     

Direction Selectivity and Spatiotemporal Separability in Simple Cortical Cells

Simple cells in mammalian visual cortex are quasi-linear mechanisms whose behavior departs from true linearity in a very consistent manner. Empirical research on direction selectivity (DS) clearly illustrates these characteristics. A linear DS cell will be DS for all stimuli, whereas a linear non-DS cell will not be DS for any stimuli. However, many simple cells have opposite preferred directions for stimuli of reversed polarity, and some cells are DS for some stimuli (e.g., moving bars) but not for others (e.g., drifting gratings). Also, linear non-DS cells must have separable spatiotemporal receptive fields (RFs), and linear DS cells must have inseparable RFs. Yet many actual DS cells have separable RFs. Here we present a nonlinear model of simple-cell behavior that reproduces all of these empirical behaviors. The model is a variant of the current linear model, amended to include an interleaved nonlinearity (half-wave rectification) that allows it to mimic the (im)balance of push-pull mechanisms. We present simulation results showing that balanced push-pull mechanisms result in linear behavior, while imbalanced push-pull arrangements produce all of the incongruent DS-related behaviors that have been reported for simple cells.     

Firing coincidences between neighboring retinal ganglion cells: inside information or epiphenomenon?

Retinal ganglion cells often fire impulses in synchrony; is this synchronization an irrelevant by-product of processing shared inputs, or does it encode information? We examined the rate of occurrence of coincident impulses from pairs of ganglion cells responding to stimuli that varied along several dimensions. We find that coincidences convey little if any additional information about simple static stimuli beyond what could be determined from the firing rates of the two cells considered separately. In fact, at least one of the separate cells generally provided a better information channel than the coincidence rate, implying that under these conditions ganglion cells do not employ a strategy of encoding by coincidences.     

Learning receptive field properties of complex cells in V1

There are two distinct classes of cells in the primary visual cortex (V1): simple cells and complex cells. One defining feature of complex cells is their spatial phase invariance; they respond strongly to oriented grating stimuli with a preferred orientation but with a wide range of spatial phases. A classical model of complete spatial phase invariance in complex cells is the energy model, in which the responses are the sum of the squared outputs of two linear spatially phase-shifted filters. However, recent experimental studies have shown that complex cells have a diverse range of spatial phase invariance and only a subset can be characterized by the energy model. While several models have been proposed to explain how complex cells could learn to be selective to orientation but invariant to spatial phase, most existing models overlook many biologically important details. We propose a biologically plausible model for complex cells that learns to pool inputs from simple cells based on the presentation of natural scene stimuli. The model is a three-layer network with rate-based neurons that describes the activities of LGN cells (layer 1), V1 simple cells (layer 2), and V1 complex cells (layer 3). The first two layers implement a recently proposed simple cell model that is biologically plausible and accounts for many experimental phenomena. The neural dynamics of the complex cells is modeled as the integration of simple cells inputs along with response normalization. Connections between LGN and simple cells are learned using Hebbian and anti-Hebbian plasticity. Connections between simple and complex cells are learned using a modified version of the Bienenstock, Cooper, and Munro (BCM) rule. Our results demonstrate that the learning rule can describe a diversity of complex cells, similar to those observed experimentally.     

Effect of Noise in Intelligent Cellular Decision Making

Similar to intelligent multicellular neural networks controlling human brains, even single cells, surprisingly, are able to make intelligent decisions to classify several external stimuli or to associate them. This happens because of the fact that gene regulatory networks can perform as perceptrons, simple intelligent schemes known from studies on Artificial Intelligence. We study the role of genetic noise in intelligent decision making at the genetic level and show that noise can play a constructive role helping cells to make a proper decision. We show this using the example of a simple genetic classifier able to classify two external stimuli.     

Dipole tracing of visual evoked potentials in human brain

3D tracing of equivalent current dipoles (ECDs) of averaged human visual evoked potentials (VEP) by their distribution across a 34-electrode array was obtained under short presentation of pattern-onset stimuli (sets of 45 horizontal, vertical bars or crosses). Using a 2-dipole spherical three-layer model, we dynamically (step of 1 ms) localized dipoles in four healthy subjects. Dipole locations were matched to anatomical brain regions visualized in structural MRI. Best-fitting source parameters were superimposed on MR images of each subject to identify the anatomical structures giving rise to the surface patterns. It was found that during 50-300 ms following the onset of the stimuli, the ECDs in all subjects were localized in the occipital cortex and demonstrated reliable systematic shift in localization. Two local (1-2 cm3) zones of the preferable dipole attendance were found at 5-6 cm behind zero line: the first one was localized near the midline of the brain, whereas the other zone was situated in the right hemisphere at a distance of 6-7 cm from the first zone. Their localization and strength of activation were reliably different for crosses and lines and changed during VEP generation. Zones of relatively rare dipole attendance were found also. The data are discussed in relation to localization of initial and endpoint of ECDs trajectories, as well as with sensitivity of the visual cortex to line crossing and branching.     

Spread of directly evoked responses in the cat''s cerebral cortex

A study has been made of the electrical responses to direct stimulation of the exposed cerebral cortex of cats that had been immobilized with neuromuscular blocking drugs, and whose muscle and skin wounds had been locally anesthetized. The characteristics and spread of the first and second surface-negative responses are described. It was found that the first surface-negative response to weak stimuli decays linearly to zero at 3 to 6 mm. from the point of stimulation. Intermediate stimuli cause farther and non-linear spread: responses are re-initiated, or reinforced, at 6 to 10 mm.; and supramaximal stimulation produces reinforcement both at 5 and at 10 mm. The conduction velocity of these responses is uniform for linear spread (0.7 to 2.0 m./sec.), but reinforced responses occur 1 to 3 msec. earlier than would be expected for simple conduction. The phenomenon of re-initiation, or reinforcement, depends upon the excitatory state of the brain; circulation and previous stimulation are important factors. Connections outside the gyrus matter only in so far as they provide other sources of general excitation. It is concluded that two types of transmission: slow and fast, can lead to generation of similar surface-negative responses. The suggestion is made that the slowly conducted surface-negative potentials are due to direct or to synaptic excitation of pyramidal cells; while the responses with shortened latency are initiated synaptically on other pyramidal cells after fast conduction at about 10 m./sec. in tangential fibres.     

Early vision impairs tactile perception in the blind

Researchers have known for more than a century that crossing the hands can impair both tactile perception and the execution of appropriate finger movements. Sighted people find it more difficult to judge the temporal order when two tactile stimuli, one applied to either hand, are presented and their hands are crossed over the midline as compared to when they adopt a more typical uncrossed-hands posture. It has been argued that because of the dominant role of vision in motor planning and execution, tactile stimuli are remapped into externally defined coordinates (predominantly determined by visual inputs) that takes longer to achieve when external and body-centered codes (determined primarily by somatosensory/proprioceptive inputs) are in conflict and that involves both multisensory parietal and visual cortex. Here, we show that the performance of late, but not of congenitally, blind people was impaired by crossing the hands. Moreover, we provide the first empirical evidence for superior temporal order judgments (TOJs) for tactile stimuli in the congenitally blind. These findings suggest a critical role of childhood vision in modulating the perception of touch that may arise from the emergence of specific crossmodal links during development.     

Calcium-activated conductance in skate electroreceptors: current clamp experiments

When current clamped, skate electroreceptor epithelium produces large action potentials in response to stimuli that depolarize the lumenal faces of the receptor cells. With increasing stimulus strength these action potentials become prolonged. When the peak voltage exceeds about 140 mV the repolarizing phase is blocked until the end of the stimulus. Perfusion experiments show that the rising phase of the action potential results from an increase in calcium permeability in the lumenal membranes. Perfusion of the lumen with cobalt or with a zero calcium solution containing EGTA blocks the action potential. Perfusion of the lumen with a solution containing 10 mM Ca and 20 mM EGTA initially slows the repolarizing process at all voltages and lowers the potential at which it is blocked. With prolonged perfusion, repolarization is blocked at all voltages. When excitability is abolished by perfusion with cobalt, or with a zero calcium solution containing EGTA, no delayed rectification occurs. We suggest that repolarization during the action potential depends on an influx of calcium into the cytoplasm, and that the rate of repolarization depends on the magnitude of the inward calcium current. Increasingly large stimuli reduce the rate of repolarization by reducing the driving force for calcium, and then block repolarization by causing the lumenal membrane potential to exceed ECa. Changes in extracellular calcium affect repolarization in a manner consistent with the resulting change in ECa.     

Neural processing of imminent collision in humans

Detecting a looming object and its imminent collision is imperative to survival. For most humans, it is a fundamental aspect of daily activities such as driving, road crossing and participating in sport, yet little is known about how the brain both detects and responds to such stimuli. Here we use functional magnetic resonance imaging to assess neural response to looming stimuli in comparison with receding stimuli and motion-controlled static stimuli. We demonstrate for the first time that, in the human, the superior colliculus and the pulvinar nucleus of the thalamus respond to looming in addition to cortical regions associated with motor preparation. We also implicate the anterior insula in making timing computations for collision events.     

Independent component filters of natural images compared with simple cells in primary visual cortex.

Properties of the receptive fields of simple cells in macaque cortex were compared with properties of independent component filters generated by independent component analysis (ICA) on a large set of natural images. Histograms of spatial frequency bandwidth, orientation tuning bandwidth, aspect ratio and length of the receptive fields match well. This indicates that simple cells are well tuned to the expected statistics of natural stimuli. There is no match, however, in calculated and measured distributions for the peak of the spatial frequency response: the filters produced by ICA do not vary their spatial scale as much as simple cells do, but are fixed to scales close to the finest ones allowed by the sampling lattice. Possible ways to resolve this discrepancy are discussed.     

Conjoint control of hippocampal place cell firing by two visual stimuli. I. The effects of moving the stimuli on firing field positions

To better understand how hippocampal place cell activity is controlled by sensory stimuli, and to further elucidate the nature of the environmental representation provided by place cells, we have made recordings in the presence of two distinct visual stimuli under standard conditions and after several manipulations of these stimuli. In line with a great deal of earlier work, we find that place cell activity is constant when repeated recordings are made in the standard conditions in which the centers of the two stimuli, a black card and a white card, are separated by 135 degrees on the wall of a cylindrical recording chamber. Rotating the two stimuli by 45 degrees causes equal rotations of place cell firing fields. Removing either card and rotating the other card also causes fields to rotate equally, showing that the two stimuli are individually salient. Increasing or decreasing the card separation (card reconfiguration) causes a topological distortion of the representation of the cylinder floor such that field centers move relative to each other. We also found that either kind of reconfiguration induces a position-independent decrease in the intensity of place cell firing. We argue that these results are not compatible with either of two previously stated views of the place cell representation; namely, a nonspatial theory in which each place cell is tuned to an arbitrarily selected subset of available stimuli or a rigid map theory. We propose that our results imply that the representation is map-like but not rigid; it is capable of undergoing stretches without altering the local arrangement of firing fields.     

Brain mechanism of selective listening reflected by event-related potentials

Stimulus selection during selective listening on the basis of simple physical stimulus features is reflected by an event-related potential (ERP) component called the processing negativity (PN). PN has been proposed to indicate a matching or comparison process between the physical features of the stimulus and an ‘attentional trace’, an actively formed and maintained temporary neuronal representation of the features defining the relevant stimuli. According to this theory, the smaller is the difference between the eliciting stimulus and that represented by the attentional trace, the longer time is the stimulus processed, and thus the larger in amplitude and longer in duration is the PN elicited. The relevant stimuli, perfectly matching with the attentional trace, and therefore eliciting the largest and longest-duration PN, are selected for further processing. In the present study, the relevant and irrelevant stimuli differed in pitch, and the magnitude of this pitch separation was varied between different stimulus blocks. The results support the afore-mentioned matching or comparison hypothesis of selective attention by showing that PN is not elicited only by the relevant stimuli but even by irrelevant stimuli, and further that the latter PN is larger in amplitude and longer in duration the more similar the irrelevant stimuli are to the relevant stimuli. This PN, however, was smaller than that to the relevant stimuli even for very small separations, reflecting high accuracy of the discrimination function of the attentional trace mechanism proposed to underly selective listening. The termination of the PN to the irrelevant stimuli was followed by a positivity which thus partly explained the difference (Nd) between the ERPs to the relevant and irrelevant stimuli.     

Using gap‐crossing capacity to evaluate functional connectivity of two Atlantic rainforest birds and their response to fragmentation

Abstract One of the main consequences of habitat loss and fragmentation is the increase in patch isolation and the consequent decrease in landscape connectivity. In this context, species persistence depends on their responses to this new landscape configuration, particularly on their capacity to move through the interhabitat matrix. Here, we aimed first to determine gap‐crossing probabilities related to different gap widths for two forest birds (Thamnophilus caerulescens, Thamnophilidae; and Basileuterus culicivorus, Parulidae) from the Brazilian Atlantic rainforest. These values were defined with a playback technique and then used in analyses based on graph theory to determine functional connections among forest patches. Both species were capable of crossing forest gaps between patches, and these movements were related to gap width. The probability of crossing 40 m gaps was 50% for both species. This probability falls to 10% when the gaps are 60 m (for B. culicivorus) or 80 m (for T. caerulescens). Actually, birds responded to stimulation about two times more distant inside forest trials (control) than in gap‐crossing trials. Models that included gap‐crossing capacity improved the explanatory power of species abundance variation in comparison to strictly structural models based merely on patch area and distance measurements. These results highlighted that even very simple functional connectivity measurements related to gap‐crossing capacity can improve the understanding of the effect of habitat fragmentation on bird occurrence and abundance.     

Spatiotemporal elements of macaque v1 receptive fields

Neurons in primary visual cortex (V1) are commonly classified as simple or complex based upon their sensitivity to the sign of stimulus contrast. The responses of both cell types can be described by a general model in which the outputs of a set of linear filters are nonlinearly combined. We estimated the model for a population of V1 neurons by analyzing the mean and covariance of the spatiotemporal distribution of random bar stimuli that were associated with spikes. This analysis reveals an unsuspected richness of neuronal computation within V1. Specifically, simple and complex cell responses are best described using more linear filters than the one or two found in standard models. Many filters revealed by the model contribute suppressive signals that appear to have a predominantly divisive influence on neuronal firing. Suppressive signals are especially potent in direction-selective cells, where they reduce responses to stimuli moving in the nonpreferred direction.