Biomedical subjects

N M Grzywacz

Publications and source records attributed to N M Grzywacz.

At least 19 recordsLinked to original sources

PubMed2001

Occlusions contribute to scaling in natural images.

Spatial power spectra from natural images fall approximately as the square of spatial frequency, a property also called scale invariance (scaling). Various theories for visual receptive fields consider scale invariance key. Two hypotheses have been advanced in the literature for why natural images obey scale invariance. The first is that these images have luminance edges, whose spectra fall as frequency squared. The second is that scale invariance follows from natural images being essentially a collage of independent, constant-intensity regions, whose sizes follow a power-law distribution. Recently, an argument by example was made against the first hypothesis. Here we refute that argument and show that the first hypothesis is consistent with the scaling under a wide variety of distributions of sizes. There are two reasons for this: first, for every frequency, the log-log slope of the rotationally averaged power spectrum of an image is the weighted mean of the log-log slopes from the independent regions of the image formed by objects occluding one another. Second, the log-log slopes of the spectrum envelope for a constant-intensity region are 0 and -3 for frequencies corresponding to periods much larger and much smaller than the region's size, respectively. Therefore, it is not surprising that natural images have log-log slopes between -1.5 and -3, with a mean near -2.

Depth Perception↗

PubMed2000

Localization of choline acetyltransferase in the developing and adult turtle retinas.

Acetylcholine has important epigenetic roles in the developing retina. In this study, cells that expressed choline acetyltransferase (ChAT), the enzyme that synthesizes acetylcholine, were investigated in embryonic, postnatal, and adult turtle retinas by using immunofluorescence histochemistry. ChAT was present at stage 15 (S15) in cells near the vitreal surface. With the formation of the inner plexiform layer (IPL) at S18, ChAT-immunoreactive (-IR) cells were located in the inner nuclear layer (INL) and the ganglion cell layer (GCL). In the INL, presumed starburst amacrine cells were homogenous in appearance and formed a single row next to the IPL: This pattern was conserved until adulthood. In the GCL, however, there were multiple rows of ChAT-IR cells early in development, and this high density of labeled cells continued during the embryonic stages, until around birth. The high density of ChAT-IR cells in the GCL was due in part to a population of cells that expressed ChAT transiently. In postnatal stages and adult retinas, the presumed starburst amacrine ChAT-IR cells formed two mirror-like rows of homogenous cells on both borders of the IPL. Two cholinergic dendritic strata that were continuous with these cells were observed as early as S18, and their depths in the IPL were relatively stable throughout development. A third population of ChAT-IR cells was observed toward the middle of the INL around S25 and persisted into adulthood. Finally, cells in the outer nuclear layer (ONL) were ChAT-IR during the embryonic stages, were less immunoreactive during the postnatal stages, and were not immunoreactive in the adult retinas.

Animals↗

PubMed2000

Colocalization of choline acetyltransferase and gamma-aminobutyric acid in the developing and adult turtle retinas.

Acetylcholine and gamma -aminobutyric acid (GABA) are putative neurotransmitters in the adult vertebrate retina. In this study, cells that coexpress choline acetyltransferase (ChAT) and GABA or glutamic acid decarboxylase (GAD) were investigated in turtle retinas from stage 14 (S14) to adulthood by using a double-labeling immunofluorescence technique. ChAT immunoreactivity was observed at S15 and included not only the presumptive starburst cholinergic amacrine cells but also a population in the ganglion cell layer (GCL) that expressed ChAT transiently during the embryonic stages (see the accompanying paper: Nguyen et al. [2000] J. Comp. Neurol. 420:512-526).

Aging↗

PubMed2000

A Bayesian model of temporal frequency masking.

The data of Anderson and Burr [1985. Vision Research, 25, 1147-1154] on the temporal-frequency (TF) specificity of noise maskers indicate that the effect of TF masking is broad and varies across spatial frequency (SF) channels. One subtle but significant feature of the data is that the TF at which the effect of masking is maximal falls continuously as the test TF falls. This continuous shift is hard to reconcile with models of detection in the literature that relate detection to the most sensitive filter, without resorting to a large number of temporal filters. We developed a new model, which relies on only three temporal filters and posits that detection is the result of a threshold decision based on the compound Bayesian probability of all filter responses, not just the most sensitive filter.

Bayes Theorem↗

PubMed2000

Occlusions and their relationship with the distribution of contrasts in natural images.

An Information-Theory-like hypothesis recently proposed for early visual processing (the Minimal Local-Asperity hypothesis) accounts for the adaptive behavior with intensity of horizontal cells. It has been shown that for this to hold, the probability that a point is traversed by an occluding border must increase supralinearly (that is, with a positive second derivative) as a function of contrast. We test this condition by analyzing the distribution of contrasts and their relationship with occluding borders in natural images. We find that the distribution of contrasts in natural images falls exponentially as a function of contrast. Moreover, the probability that a point is traversed by an occluding border in natural images always rises with contrast until reaching one. This rise tends to be supralinear and addition of noise (at low intensities) increases the supralinearity, shifting the rising portion of the curve towards higher contrasts. These findings lend support to the Minimal Local-Asperity hypothesis, which proposes that one of the main roles of early retinal processing is to extract optimally edge, contrast, and luminance attributes from the visual world based on previous knowledge about natural images.

Contrast Sensitivity↗

PubMed2000

A bayesian model for the measurement of visual velocity.

Several models have been proposed for how the brain measures velocity from the output of motion-energy units. These models make some unrealistic assumptions such as the use of Gabor-shaped temporal filters, which are non causal, or flat spatial spectra, which are invalidated by existing data. We present a Bayesian model of velocity perception, which makes more realistic assumptions and allows the estimation of local retinal velocity regardless of the specific mathematical form of the spatial and temporal filters used. The model is consistent with several aspects of speed perception, such as the dependence of perceived speed on contrast.

Bayes Theorem↗

PubMed2000

Probabilistic motion estimation based on temporal coherence.

We develop a theory for the temporal integration of visual motion motivated by psychophysical experiments. The theory proposes that input data are temporally grouped and used to predict and estimate the motion flows in the image sequence. This temporal grouping can be considered a generalization of the data association techniques that engineers use to study motion sequences. Our temporal grouping theory is expressed in terms of the Bayesian generalization of standard Kalman filtering. To implement the theory, we derive a parallel network that shares some properties of cortical networks. Computer simulations of this network demonstrate that our theory qualitatively accounts for psychophysical experiments on motion occlusion and motion outliers. In deriving our theory, we assumed spatial factorizability of the probability distributions and made the approximation of updating the marginal distributions of velocity at each point. This allowed us to perform local computations and simplified our implementation. We argue that these approximations are suitable for the stimuli we are considering (for which spatial coherence effects are negligible).

Bayes Theorem↗

PubMed2000

The minimal local-asperity hypothesis of early retinal lateral inhibition.

Recently we found that the theories related to information theory existent in the literature cannot explain the behavior of the extent of the lateral inhibition mediated by retinal horizontal cells as a function of background light intensity. These theories can explain the fall of the extent from intermediate to high intensities, but not its rise from dim to intermediate intensities. We propose an alternate hypothesis that accounts for the extent's bell-shape behavior. This hypothesis proposes that the lateral-inhibition adaptation in the early retina is part of a system to extract several image attributes, such as occlusion borders and contrast. To do so, this system would use prior probabilistic knowledge about the biological processing and relevant statistics in natural images. A key novel statistic used here is the probability of the presence of an occlusion border as a function of local contrast. Using this probabilistic knowledge, the retina would optimize the spatial profile of lateral inhibition to minimize attribute-extraction error. The two significant errors that this minimization process must reduce are due to the quantal noise in photoreceptors and the straddling of occlusion borders by lateral inhibition.

Animals↗

PubMed2000

Stimulus configuration determines the detectability of motion signals in noise.

We measured the detectability of moving signal dots in dynamic noise to determine whether local motion signals are preferentially combined along an axis parallel to the direction of motion. Observers were asked to detect a signal composed of three dots moving in a linear trajectory among dynamic noise dots. The signal dots were collinear and equally spaced in a configuration that was either parallel to or perpendicular to their trajectory. The probability of detecting the signal was measured as a function of noise density, over a range of signal dot spacings from 0.5 degrees to 5.0 degrees. At any given noise density, the signal in the parallel configuration was more detectable than that in the perpendicular configuration. Our four observers could tolerate 1.5-2.5 times more noise in the parallel configuration. This improvement is not due merely to temporal summation between consecutive dots in the parallel trajectory. Temporal summation functions measured on our observers indicate that the benefit from spatial coincidence of the dots lasts for no more than 50 ms, whereas the increased detectability of the parallel configuration is observed up to the largest temporal separations tested (210 ms). These results demonstrate that dots arranged parallel to the signal trajectory are more easily detected than those arranged perpendicularly. Moreover, this enhancement points to the existence of visual mechanisms that preferentially organize motion information parallel to the direction of motion.

Artifacts↗

PubMed1999

Spontaneous activity in developing turtle retinal ganglion cells: pharmacological studies.

Extracellular recordings were obtained from the ganglion cell (GC) layer during correlated spontaneous bursting activity (SBA) in the immature turtle retina. Pharmacological agents were bath-applied, and their effects on burst and correlation parameters were determined. SBA requires synaptic transmission. It was blocked in the presence of curare and mecamylamine, two cholinergic nicotinic antagonists, and enhanced with neostigmine, a cholinesterase inhibitor. SBA was profoundly inhibited during blockade of glutamatergic receptors with the broad spectrum antagonist kynurenate and it vanished with 6,7-dinitroquinoxaline-2-3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), two AMPA/kainate receptor antagonists. Blockade of NMDA receptors with D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) led only to a modest reduction in SBA. Blockade of GABAA receptors with bicuculline prolonged the duration of the bursts. Inhibition of GABA uptake with nipecotic acid led to a decrease in burst rate. Blockade of K+ channels with cesium (Cs+) and tetraethylammonium (TEA) led to a dramatic decrease in excitability. Burst propagation between neighboring GCs was reduced by K+ channel blockade. Gap junction blockade had no consistent effect on bursts or correlation parameters. None of these drugs had a strong effect on the refractory period between bursts. We conclude that correlated SBA in immature turtle GCs requires both cholinergic nicotinic and glutamatergic (mainly through AMPA/kainate receptors) synaptic transmission. GABAergic activity modulates the intensity and the duration of the bursts. Extracellular K+ is involved in lateral activity propagation and increases retinal excitability, which may be required for burst generation.

Action Potentials↗

PubMed1999

Local motion detectors cannot account for the detectability of an extended trajectory in noise.

Previous work has shown that a single dot moving in a consistent direction is easily detected among noise dots in Brownian motion (Watamaniuk et al., Vis Res 1995;35:65-77). In this study we calculated the predictions of a commonly-used psychophysical motion model for a motion trajectory in noise. This model assumes local motion energy detectors optimally tuned to the signal, followed by a decision stage that implements the maximum rule. We first show that local motion detectors do indeed explain the detectability of brief trajectories (100 ms) that fall within a single unit, but that they severely underestimate the detectability of extended trajectories that span multiple units. For instance, a 200 ms trajectory is approximately three times more detectable than two isolated 100 ms trajectories presented together within an equivalent temporal interval. This result suggests a nonlinear interaction among local motion units. This interaction is not restricted to linear trajectories because circular trajectories with curvatures larger than 1 degree are almost as detectable as linear trajectories. Our data are consistent with a flexible network that feeds forward excitation among units tuned to similar directions of motion.

Humans↗

PubMed1998

Necessity of acetylcholine for retinal directionally selective responses to drifting gratings in rabbit.

1. A model for retinal directional selectivity postulates that GABAergic inhibition of responses to motions in the null (anti-preferred) direction underlies this selectivity. An alternative model postulates that besides this inhibition, there exists an asymmetric, nicotinic acetylcholine (ACh) input from starburst amacrine cells. It is possible for the latter but not the former model that stimuli could exist such that nicotinic blockade eliminates directional selectivity. Such stimuli would drive the cholinergic but not the GABAergic system well. 2. So far, attempts to eliminate directional selectivity with nicotinic blockade have failed, but they always used isolated, moving bars as the stimulus. We confirmed this failure for On-Off directionally selective (DS) ganglion cells in our preparation of the rabbit's retina. 3. However, while recording from these cells, we discovered that nicotinic blockade eliminated directional selectivity to drifting, low spatial frequency sine- and square-wave gratings. 4. This effect was not just due to the smallness of the responses under nicotinic blockade. NMDA blockade caused even smaller responses, but no loss of directional selectivity. 5. This result is consistent with a two-asymmetric-pathways model of directional selectivity, but inconsistent with an asymmetric-GABA-only model. 6. We conclude that asymmetric nicotinic inputs extend the range of stimuli that can elicit directional selectivity to include moving textures, that is, those with multiple peaks in their spatial luminance profile.

Acetylcholine↗

PubMed1998

Toward a biophysically plausible bidirectional Hebbian rule.

Although the commonly used quadratic Hebbian-anti-Hebbian rules lead to successful models of plasticity and learning, they are inconsistent with neurophysiology. Other rules, more physiologically plausible, fail to specify the biological mechanism of bidirectionality and the biological mechanism that prevents synapses from changing from excitatory to inhibitory, and vice versa. We developed a synaptic bidirectional Hebbian rule that does not suffer from these problems. This rule was compared with physiological homosynaptic conditions in the hippocampus, with the results indicating the consistency of this rule with long-term potentiation (LTP) and long-term depression (LTD) phenomenologies. The phenomenologies considered included the reversible dynamics of LTP and LTD and the effects of N-methyl-D-aspartate blockers and phosphatase inhibitors.

Biophysical Phenomena↗

PubMed1998

Constraints on long range interactions mediating contour detection.

Contour detection may be mediated by lateral interactions between neighboring cortical neurons whose receptive fields have collinear axes of preferred orientation. This hypothesis was tested in psychophysical experiments and computer simulations using a contour detection task in which observers searched for groups of Gabor patches that followed spatially extended contour paths embedded in noise consisting of several hundred Gabor patches with random positions and orientations. The orientation-selective units in the simulated neural network were linked by facilitatory interconnections whose strength depended on the geometry (distance, curvature, change in curvature) of smooth curves connecting the orientation axes of units in a pairwise fashion. Psychophysical detection performance was much higher for contour signal groups that followed closed rather than open-ended paths. However, just two sudden changes in orientation of neighboring Gabor patch elements in closed-path contours reduced detection performance to the same levels obtained with open-ended contours. These psychophysical data agreed with the results of the neural network simulations. Furthermore, the simulations also accounted for previous findings that removal of a single Gabor patch element from a closed-path contour group significantly degraded detection performance. We conclude that closure alone is not sufficient to enhance the visibility of a contour. However, if a closed contour meets certain geometric constraints, then lateral interactions based on these constraints can generate facilitation that reverberates around the closed path, thereby enhancing the contour's visibility.

Form Perception↗

PubMed1998

A biophysical model for the developmental time course of retinal orientation selectivity.

A quantitative study of the time course of development of the percentage of orientationally selective and isotropic ganglion cells in turtle retina has recently been performed. This study revealed that as soon as ganglion cells start responding to light, a large percentage of them are selective to the orientations of moving visual stimuli. This percentage decreases with age to reach a minimum around hatching, increases dramatically after birth and finally, decreases again following the first month of life to reach adult level. Concomitantly, the percentage of cells responding isotropically to the orientation of elongated stimuli increases monotonically until about 30 days after birth, stabilizing afterwards. To account for both time courses, we propose a biophysical model implementing features ubiquitous to developing vertebrate retinas. These features include early dendritic and synaptic spatial polarization, dendritic growth, and waves of activity generated spontaneously or by visual stimulation sweeping across the inner plexiform layer (IPL). The model also assumes a physiologically plausible Hebbian rule, which includes long-term potentiation and depression. Computer simulations of this model yield good fits of the data. The quality of these fits confirms and extends results from an earlier model using computationally-simple mechanisms, which suggested that early dendritic polarization might be the seed for mature orientation selectivity.

Animals↗

PubMed1997

Possible roles of spontaneous waves and dendritic growth for retinal receptive field development.

Several models of cortical development postulate that a Hebbian process fed by spontaneous activity amplifies orientation biases occurring randomly in early wiring, to form orientation selectivity. These models are not applicable to the development of retinal orientation selectivity, since they neglect the polarization of the retina's poorly branched early dendritic trees and the wavelike organization of the retina's early noise. There is now evidence that dendritic polarization and spontaneous waves are key in the development of retinal receptive fields. When models of cortical development are modified to take these factors into account, one obtains a model of retinal development in which early dendritic polarization is the seed of orientation selectivity, while the spatial extent of spontaneous waves controls the spatial profile of receptive fields and their tendency to be isotropic.

Dendrites↗

PubMed1996

Influence of spontaneous activity and visual experience on developing retinal receptive fields.

BACKGROUND: The role played by early neural activity in shaping retinal functions has not yet been established. In the developing vertebrate retina, ganglion cells fire spontaneous bursts of action potentials before the onset of visual experience. This spontaneous bursting disappears shortly after birth or eye opening. In the present study, we have investigated whether the outgrowth of receptive fields in turtle retinal ganglion cells is affected by early spontaneous bursting or by early visual experience. RESULTS: Ganglion cells normally stop bursting spontaneously 2-4 weeks post-hatching, the time when receptive-field areas reach adult size. When turtles are reared in the dark, the spontaneous bursting persists. Concomitantly, receptive-field areas expand to more than twice those observed in normal adults. To test whether chronic blockade of spontaneous bursting inhibits the expansion of developing receptive-field areas, we have exposed the retina to curare, a nicotinic cholinergic antagonist, because spontaneous bursting by ganglion cells requires acetylcholine. Curare was released from Elvax, a slow-release polymer that was implanted in the eye. When spontaneous bursting was chronically blocked with curare in hatchlings, dark-induced expansion of receptive fields was abolished. Moreover, receptive fields of ganglion cells exposed to curare in hatchlings reared in normal light and dark cycles were smaller than normal. CONCLUSIONS: These results strongly suggest that early, acetylcholine-dependent spontaneous bursts of activity control the outgrowth of receptive-field areas in retinal ganglion cells. The onset of visual experience induces the disappearance of the immature spontaneous bursts, resulting in the stabilization of receptive-field areas to their mature size.

Animals↗

PubMed1995

Detecting a trajectory embedded in random-direction motion noise.

Human observers can easily detect a signal dot moving, in apparent motion, on a trajectory embedded in a background of random-direction motion noise. A high detection rate is possible even though the spatial and temporal characteristics (step size and frame rate) of the signal are identical to the noise, making the signal indistinguishable from the noise on the basis of a single pair of frames. The success rate for detecting the signal dot was as high as 90% when the probability of mismatch from frame-to-frame, based on nearest-neighbor matching, was 0.3. Control experiments showed that trajectory detection is not based on detecting a "string" of collinear dots, i.e. a stationary position cue. Nor is a trajectory detected because it produces stronger signals in single independent motion detectors. For one thing, trajectory detection improves with increases in duration, up to 250-400 msec, a duration longer than the integration typically associated with a single motion detector. For another, the signal dot need not travel in a straight line to be detectable. The signal dot was as reliably detected when it changed its direction a small amount (about 30 deg or less) each frame. Consistent with this, circular paths of sufficiently low curvature were as detectable as straight trajectories. Our data suggest that trajectory motion is highly detectable in motion noise because the component local motion signals are enhanced when motion detectors with similar directional tuning are stimulated in a sequence along their preferred direction.

Cues↗