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At least 577 records · Page 32Linked to original sources
PubMed1996

Development of contrast sensitivity across the visual field in macaque monkeys (Macaca nemestrina).

Interpretation of measurements of visual performance in infants must be based on knowledge of the locus of highest sensitivity in the infant retina. While we know that adult contrast sensitivity and spatial resolution is highest at the fovea, recent anatomical data show that the infant fovea is relatively immature. We have studied that variation of contrast sensitivity across the visual field during development in infant monkeys in order to investigate the behavioral consequences of this immaturity. The results show that, unlike adults, the sensitivity of the infant foveal region is similar to that of the near periphery. Central contrast sensitivity and spatial resolution improve substantially relative to the periphery over the first 20-40 postnatal weeks. Thus, contrast sensitivity in the periphery is relatively mature in infants with respect to more central regions of the visual field. The maturation pattern seen behaviorally is consistent with physiological and anatomical maturation patterns in macaque monkey.

Aging↗
PubMed1994

Simulation of dynamic receptive fields in primary visual cortex.

A model network of spiking neurons with lateral connections was used to simulate short-term receptive field (RF) changes by removal of afferent input in the primary visual system. Several possible mechanisms for the dynamic RFs were explored and the simulation results were compared with experimental results obtained by Pettet and Gilbert [(1992) Proceedings of the National Academy of Science, U.S.A., 89, 8366-8370]. We found that appropriate input stimuli could induce a shift in the balance between modeled cortical lateral excitation and inhibition and in doing so cause RF expansion. Synaptic plasticity was neither necessary nor appropriate for short-term RF changes. An inhibition dominant network with neural adaptation successfully simulated Pettet and Gilbert's experiment of RF expansion and its reversibility induced by an "artificial scotoma". RF expansions induced by lesions were also explored with the model.

Action Potentials↗
PubMed2005

Evolution of the cerebellum as a neuronal machine for Bayesian state estimation.

The cerebellum evolved in association with the electric sense and vestibular sense of the earliest vertebrates. Accurate information provided by these sensory systems would have been essential for precise control of orienting behavior in predation. A simple model shows that individual spikes in electrosensory primary afferent neurons can be interpreted as measurements of prey location. Using this result, I construct a computational neural model in which the spatial distribution of spikes in a secondary electrosensory map forms a Monte Carlo approximation to the Bayesian posterior distribution of prey locations given the sense data. The neural circuit that emerges naturally to perform this task resembles the cerebellar-like hindbrain electrosensory filtering circuitry of sharks and other electrosensory vertebrates. The optimal filtering mechanism can be extended to handle dynamical targets observed from a dynamical platform; that is, to construct an optimal dynamical state estimator using spiking neurons. This may provide a generic model of cerebellar computation. Vertebrate motion-sensing neurons have specific fractional-order dynamical characteristics that allow Bayesian state estimators to be implemented elegantly and efficiently, using simple operations with asynchronous pulses, i.e. spikes. The computational neural models described in this paper represent a novel kind of particle filter, using spikes as particles. The models are specific and make testable predictions about computational mechanisms in cerebellar circuitry, while providing a plausible explanation of cerebellar contributions to aspects of motor control, perception and cognition.

Adaptation, Physiological↗
PubMed2006

Pattern generation using likelihood inference for cellular automata.

Cellular automata are discrete dynamical systems which evolve on a discrete grid. Recent studies have shown that cellular automata with relatively simple rules can produce highly complex patterns. We develop likelihood-based methods for estimating rules of cellular automata aimed at the re-generation of observed regular patterns. Under noisy data, our approach is equivalent to estimating the local map of a stochastic cellular automaton. Direct computations of the maximum likelihood estimates are possible for regular binary patterns. The likelihood formulation of the problem is congenial with the use of the minimum description length principle as a model selection tool. We illustrate our method with a series of examples using binary images.

Algorithms↗
PubMed1991

Characterization of electroencephalographic state in fetal baboons.

Long-term recordings of the electroencephalograms (EEGs) from the non-human fetal primate have been visually scored for state. Data were obtained from three fetuses using recently developed techniques for chronic instrumentation of the pregnant baboon. Seven days or more after surgery, nine chart records of 4-5 h in duration were analyzed. These records were made during an interval in gestation from 143 to 153 days (term = 175 to 185 days). Criteria for differentiation of EEG state were based on the presence (state 1) or absence (state 2) of trace alternant, which is the predominant characteristic of EEG activity during quiet sleep in human preterm and term infants. Two patterns of EEG activity were readily distinguished at both standard (30 mm/s) and compressed (30 mm/min) EEG chart speeds. On a minute-by-minute basis, there was an overall concurrence of 82.9% for EEG state when scored from compressed and standard EEG records. From the compressed records, state 1 was present on average 38.3% of the time, with a mean epoch duration of 15.1 min, while epochs of state 2 averaged 25.9 min. We conclude that at least two EEG states are present at this gestation in the fetal baboon. These two states can be reliably defined visually by scoring methods directly comparable to those used for EEG recognition of sleep states in the human newborn infant. To validate the conclusion that these fetal state assignments based on patterns of EEG activity correspond to sleep states, it will be necessary to determine if the oscillations of other physiological parameters are coherent with these patterns.

Animals↗
PubMed1990

On the neurophysiological organization of binocular vision.

The considerable mixing in the visual cortex, of signals from left and right eyes, provides an abundant population of binocularly activated neurons. Based on this and on the fact that cortical cells respond best to different ranges of retinal disparities, it has been proposed that these neurons form the physiological substrate of stereoscopic depth discrimination. We outline reasons here for addressing first the more fundamental issue of the rules of convergence in the visual cortex, for input from the two eyes. We show that most of this convergence may be described by a linear summation process. However, there is a nonlinear mechanism that maintains binocular interaction regardless of large differences in stimulus strength between the eyes. This finding suggests that a cell which appears to be dominated by one eye, when monocular tests are conducted, may respond equally under binocular conditions. In this case, binocular processing for all cortical cells could be uniform and independent of the ocular dominance values determined monocularly. With respect to a neural mechanism for the processing of information concerning different depths in space, we propose an alternative to the conventional notion. First, we identify fundamental problems with the current view. Second, we describe a procedure which allows us to distinguish between the conventional view and our alternative proposal. Standard receptive field mapping techniques are not adequate for determining phase-disparity relationships of the type we require. Therefore, we have employed a reverse correlation procedure which enables efficient and detailed mapping of receptive field structure. Third, we describe preliminary data concerning the physiological mechanism of stereoscopic depth discrimination.

Action Potentials↗
PubMed2001

Eye movements and familiarity effects in visual search.

Familiarity with the distractors around an unfamiliar target facilitates visual search. Three Experiments examined whether the effect occurs because fixations are (a) shorter and fewer, (b) shorter, but more abundant, (c) equally long, but fewer, or (d) longer, but fewer when distractors are familiar. Results indicated comparably long, but fewer fixations when distractors are familiar. Hence, the theory that unfamiliar distractors need longer processing is discounted. In a fourth Experiment, a gaze-contingent moving window paradigm was used to control peripheral processing. Results revealed a wider span of effective processing for familiar distractors. A hypothesis based on low-level physiological processes is introduced to account for the familiarity effect.

Adolescent↗
PubMed1996

Magnetic coil suppression of visual perception at an extracalcarine site.

Perception of extrafoveal visual targets can be suppressed by magnetic stimulation over the occipital lobes, but the site of interference for this and similar phenomena has not been well defined. We modified a previously used technique to determine the locus of magnetic activation. Using butterfly stimulus coils of different sizes and electric field profiles, we determined a scalp position of minimum threshold and a level of stimulator output that produced 50% error rates for each coil. Intersection of the corresponding electric field profiles in air and in a saline model head was similar and identified superficial occipital cortex rather than the primary visual area as the site of perceptual suppression. Less direct analyses involving the distribution of induced electric fields produced the same conclusion. These results suggest specific hypotheses about the effects of magnetic stimulation on visual physiology.

Attention↗
PubMed1998

Single units and conscious vision.

Figures that can be seen in more than one way are invaluable tools for the study of the neural basis of visual awareness, because such stimuli permit the dissociation of the neural responses that underlie what we perceive at any given time from those forming the sensory representation of a visual pattern. To study the former type of responses, monkeys were subjected to binocular rivalry, and the response of neurons in a number of different visual areas was studied while the animals reported their alternating percepts by pulling levers. Perception-related modulations of neural activity were found to occur to different extents in different cortical visual areas. The cells that were affected by suppression were almost exclusively binocular, and their proportion was found to increase in the higher processing stages of the visual system. The strongest correlations between neural activity and perception were observed in the visual areas of the temporal lobe. A strikingly large number of neurons in the early visual areas remained active during the perceptual suppression of the stimulus, a finding suggesting that conscious visual perception might be mediated by only a subset of the cells exhibiting stimulus selective responses. These physiological findings, together with a number of recent psychophysical studies, offer a new explanation of the phenomenon of binocular rivalry. Indeed, rivalry has long been considered to be closely linked with binocular fusion and stereopsis, and the sequences of dominance and suppression have been viewed as the result of competition between the two monocular channels. The physiological data presented here are incompatible with this interpretation. Rather than reflecting interocular competition, the rivalry is most probably between the two different central neural representations generated by the dichoptically presented stimuli. The mechanisms of rivalry are probably the same as, or very similar to, those underlying multistable perception in general, and further physiological studies might reveal much about the neural mechanisms of our perceptual organization.

Animals↗
PubMed1993

Stereo-motion cooperation and the use of motion disparity in the visual perception of 3-D structure.

When an observer views a moving scene binocularly, both motion parallax and binocular disparity provide depth information. In Experiments 1A-1C, we measured sensitivity to surface curvature when these depth cues were available either individually or simultaneously. When the depth cues yielded comparable sensitivity to surface curvature, we found that curvature detection was easier with the cues present simultaneously, rather than individually. For 2 of the 6 subjects, this effect was stronger when the component of frontal translation of the surface was vertical, rather than horizontal. No such anisotropy was found for the 4 other subjects. If a moving object is observed binocularly, the patterns of optic flow are different on the left and right retinae. We have suggested elsewhere (Cornilleau-Pérès & Droulez, in press) that this motion disparity might be used as a visual cue for the perception of a 3-D structure. Our model consisted in deriving binocular disparity from the left and right distributions of vertical velocities, rather than from luminous intensities, as has been done in classical studies on stereoscopic vision. The model led to some predictions concerning the detection of surface curvature from motion disparity in the presence or absence of intensity-based disparity (classically termed binocular disparity). In a second set of experiments, we attempted to test these predictions, and we failed to validate our theoretical scheme from a physiological point of view.

Acceleration↗
PubMed2003

Brain responses for the subconscious recognition of faces.

We investigated the event-related responses following subthreshold and suprathreshold stimulation with facial and non-facial figures using magnetoencephalography (MEG) and EEG recordings to clarify the physiological nature of subconscious perception. Event-related magnetic fields and potentials were recorded from the right hemisphere in eight healthy subjects. Three types of stimulus, i.e., facial image (Face), letters of the alphabet (Letters) and random patterns of dots (Dots), with different presentation periods, subthreshold (16 ms), intermediate (32 ms) and suprathreshold (48 ms) were visually presented in a random order. A psychological discrimination task using the same stimuli was also employed. Clear MEG and EEG responses were recorded for all the stimuli, but the amplitude of the responses was largest for Face and smallest for Dots even in the subthreshold stimulation. The equivalent current dipoles (ECDs) for Face were located around the fusiform gyrus, although the correlation coefficients for ECDs were low under subthreshold and intermediate conditions. The ECDs for Letters and Dots were not estimated with reliable correlation coefficients. The results from the psychological task correlated with the dominancy of face recognition. Face perception was processed differently in the subthreshold condition as well as suprathreshold condition. The subconscious recognition of face might be processed around the fusiform gyrus.

Adult↗
PubMed1991

Graduate and post-graduate medical education with the synchronous systems model.

Behavioral sciences have an important new role, particularly in medicine and prevention (Revans, 1990; Starr, 1982). A decade ago when the ideology of medical care shifted from a biomedical to a biopsychosocial model, behavioral and social factors ascended in importance in modern health concerns (see Engel, 1977). Now, a living systems model called Synchronous Systems (Jasnoski & Schwartz, 1985) incorporates environmental (including the social and physical) along with physiological and psychological factors into its structure. The environment has been gaining recognition as an important determinant in health, disease, and wellness (Daniel, 1990; Demick & Wapner, 1990; Lawton, 1990; Stokols, 1990). The phrase "Synchronous Systems" departs from the traditional linear thought patterns to depict contemporaneous occurrences. Synchrony refers to simultaneous experiences or events, with an emphasis on healthy, congruent process and function. Positive, beneficial process in human functioning emphasizes prevention as found in public health efforts but refers also to dysfunctional and recuperative foundation of medicine as a departure from the healthy norm. These process and functional dynamics in the Synchronous Systems Model derive conceptually from systems theory, specifically from control and cybernetic theories (Cannon, 1932; Weiner, 1948). The dynamic complexity of health in the whole human system, which also incorporates the environment, requires synthesis of knowledge from the biological, social, behavioral, and ecological sciences. This article provides the fundamentals of the structural and functional synthesis possible in the Synchronous Systems Model. Its direct application to multidisciplinary research is also presented along with its specific application to medical education, policy, research, and service delivery.

Curriculum↗
PubMed2005

Efficient learning by combining confidence-rated classifiers to incorporate unlabeled medical data.

In this paper, we propose a new dynamic learning framework that requires a small amount of labeled data in the beginning, then incrementally discovers informative unlabeled data to be hand-labeled and incorporates them into the training set to improve learning performance. This approach has great potential to reduce the training expense in many medical image analysis applications. The main contributions lie in a new strategy to combine confidence-rated classifiers learned on different feature sets and a robust way to evaluate the "informativeness" of each unlabeled example. Our framework is applied to the problem of classifying microscopic cell images. The experimental results show that 1) our strategy is more effective than simply multiplying the predicted probabilities, 2) the error rate of high-confidence predictions is much lower than the average error rate, and 3) hand-labeling informative examples with low-confidence predictions improves performance efficiently and the performance difference from hand-labeling all unlabeled data is very small.

Algorithms↗
PubMed1983

Portal architecture: a differential guide to fatty infiltration of the liver on computed tomography.

Fatty infiltration of the liver, like fatty lesions elsewhere in the body, typically appears as low-density areas on computed tomography (CT). Fatty infiltration of the liver should be considered in the differential diagnosis of homogeneous low-density hepatic lesions regardless of distribution or size. As a physiologic rather than anatomic change, fatty infiltration of the liver characteristically leaves the portal venous architecture unaltered. Although fatty infiltration of the liver typically presents with complete or near complete involvement, other unusual patterns may be encountered. In these atypical cases, recognition of the normal portal structures on CT aids in correct diagnosis.

Fatty Liver↗
PubMed1983

A duration matching method for the measurement of jitter in single fibre EMG.

A new microprocessor method for on-line measurement of jitter in SFEMG is described. The method is based on a duration matching procedure: automatic detection of the repetitive occurrence of two single fibre potentials with stable durations and time-locked to one another. During the investigation of a motor unit a potential pair is accepted or rejected for the determination of jitter on the basis of the potential durations and the interpotential interval; the time criteria are set by the first potential pair measured. The measuring device consists of a microprocessor and an analog interface; it is small and can be connected to any EMG equipment. Compared with the methods previously described the duration matching method (a simple pattern recognition procedure) has the following advantages: the measurement during activation of several motor units is faster and more reliable; the 'technical jitter' is smaller, which allows a more accurate determination of the normal range. Consequently, the method yields values of physiological jitter which are significantly smaller than those previously described.

Computers, Analog↗
PubMed2006

Perceptual filling-in from the edge of the blind spot.

Looking at the world with one eye, we do not notice a scotoma in the receptor-free area of the visual field where the optic nerve leaves the eye. Rather we perceive the brightness, color, and texture of the adjacent area as if they were actually there. The mechanisms underlying this kind of perceptual filling-in remain controversial. To better understand these processes, we determined the minimum region around the blind spot that needs to be stimulated for filling-in by carefully mapping the blind spot and presenting individually fitted stimulus frames of different width around it. Uniform filling-in was observed with frame widths as narrow as 0.05 degrees visual angle for color and 0.2 degrees for texture. Filling-in was incomplete, when the frame was no longer contiguous with the blind spot border due to an eye movement. These results are consistent with the idea that perceptual filling-in of the blind spot depends on local processes generated at the physiological edge of the cortical representation.

Color Perception↗
PubMed2003

DMBT1, a regulator of mucosal homeostasis through the linking of mucosal defense and regeneration?

DMBT1 (deleted in malignant brain tumor 1), which encodes a large scavenger receptor cysteine rich (SRCR) B protein, has been proposed to be a tumor suppressor gene, due to the high frequency of its homozygous deletion and the lack of expression in a variety of cancers. However, studies on its physiological functions and its relationship with tumorigenesis are still at an initial stage. Two mucosal defense-related molecules, gp-340 and salivary agglutinin, have been identified to be alternatively spliced products of DMBT1, which suggests that DMBT1 is a pattern recognition receptor in innate immunity. Meanwhile, results from immunohistochemical staining and studies at the cellular level, began to associate DMBT1 with a proliferation to differentiation switching process in gastrointestinal epithelial cells. Together with its up-regulation in inflammation, these findings suggest that DMBT1 might be a local regulator of homeostasis, possibly through linking mucosal inflammation to the modulation of epithelial regeneration, and whose abnormality is a frequent cause of malignancy.

Agglutinins↗