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L M Optican

Publications and source records attributed to L M Optican.

At least 19 recordsLinked to original sources
PubMed2000

Striate cortex in humans demonstrates the relationship between activation and variations in visual form.

Electrophysiologic and functional imaging studies have shown that the visual cortex produces differential responses to the presence or absence of structure within visual textures. To further define and characterize regions involved in the analysis of form, functional magnetic resonance imaging (fMRI) was used to detect changes in activation during the viewing of four levels of isodipole textures. The texture levels systematically differed in the density of visual features such as extended contours and blocks of solid color present within the images. A linear relationship between activation level and density of structure was observed in the striate cortex of human subjects. This finding suggests that a special subpopulation of striate cortical neurons participates in the ability to extract and process structural continuity within visual stimuli.

Adult↗
PubMed2000

Extent of compensation for variations in monkey saccadic eye movements.

We investigated and quantified the ability of the primate saccadic system to generate accurate eye movements in spite of naturally occurring variations in saccadic speed and trajectory. We show that the amplitude of a series of saccades directed to the same target is positively correlated to their peak speed, i.e., the faster the saccade, the bigger its amplitude. We demonstrate that this result cannot be simply accounted for by the main sequence, and that on average the saccadic system is able to compensate for only 61% of the variability in speed. Deviations from the average trajectory are also only partially compensated: the underlying mechanism, which tends to bring the eyes back toward the desired trajectory, underperforms for small movements and overperforms for large movements. We also demonstrate that the performance of this compensatory mechanism, and the metrics of saccades in general, do not depend on the presence of visual information during the movement. By showing that deviations from the desired behavior are corrected during the saccade, our results further support the hypothesis that the innervation signal that generates saccadic eye movements is not pre-programmed but rather is dynamically adjusted during the movement. However, the compensation for deviations from the desired behavior is only partial, and the underlying mechanisms have yet to be completely understood. Although none of the current models of the saccadic system can account for our results, some of them, if appropriately modified, probably could.

Adaptation, Physiological↗
PubMed1999

Model of the control of saccades by superior colliculus and cerebellum.

Experimental evidence indicates that the superior colliculus (SC) is important but neither necessary nor sufficient to produce accurate saccadic eye movements. Furthermore both clinical and experimental evidence points to the cerebellum as an indispensable component of the saccadic system. Accordingly, we have devised a new model of the saccadic system in which the characteristics of saccades are determined by the cooperation of two pathways, one through the SC and the other through the cerebellum. Both pathways are influenced by feedback information: the feedback determines the decay of activity for collicular neurons and the timing of the activation for cerebellar neurons. We have modeled three types of cells (burst, buildup, and fixation neurons) found in the intermediate layers of the superior colliculus. We propose that, from the point of view of motor execution, the burst neurons and the buildup neurons are not functionally distinct with both providing a directional drive to the brain stem circuitry. The fixation neurons determine the onset of the saccade by disfacilitating the omnipause neurons in the brain stem. Excluding noise-related variations, the ratio of the horizontal to the vertical components of the collicular drive is fixed throughout the saccade (i.e., its direction is fixed); the duration of the drive is such that it always would produce hypermetric movements. The cerebellum plays three roles: first, it provides an additional directional drive, which improves the acceleration of the eyes; second, it keeps track of the progress of the saccade toward the target; and third, it ends the saccade by choking off the collicular drive. The drive provided by the cerebellum can be adjusted in direction to exert a directional control over the saccadic trajectory. We propose here a control mechanism that incorporates a spatial displacement integrator in the cerebellum; under such conditions, we show that a partial directional control arises automatically. Our scheme preserves the advantages of several previous models of the saccadic system (e.g., the lack of a spatial-to-temporal transformation between the SC and the brain stem; the use of efference copy feedback to control the saccade), without incurring many of their drawbacks, and it accounts for a large amount of experimental data.

Action Potentials↗
PubMed1998

Cortical regions involved in visual texture perception: a fMRI study.

To determine visual areas of the human brain involved in elementary form processing, functional magnetic resonance imaging (fMRI) was used to measure regional responses to two types of achromatic textures. Healthy young adults were presented with 'random' textures which lacked spatial organization of the black and white pixels that make up the image, and 'correlated' textures in which the pixels were ordered to produce extended contours and rectangular blocks at multiple spatial scales. Relative to a fixation condition, random texture stimulation resulted in increased signal intensity primarily in the striate cortex, with slight involvement of the cuneus and middle occipital, lingual and fusiform gyri. Correlated texture stimulation also resulted in activation of these areas, yet the regional extent of this activation was significantly greater than that produced by random textures. Unlike random stimulation, correlated stimulation additionally resulted in middle temporal activation. Direct comparison of the two stimulation conditions revealed significant differences most consistently in the anterior fusiform gyrus, but also in striate, middle occipital, lingual and posterior temporal regions in subjects with robust activation patterns. While both random and correlated stimulation produced activation in similar areas of the occipital lobe, the increase in regional activation during the correlated condition suggests increased recruitment of neuronal populations occurs in response to textures containing visually salient features. This increased recruitment occurs within striate, extrastriate and temporal regions of the brain, also suggesting the presence of receptive field mechanisms in the ventral visual pathway that are sensitive to features produced by higher-order spatial correlations.

Adult↗
PubMed1998

Reversible inactivation of monkey superior colliculus. II. Maps of saccadic deficits.

Neurons in the superior colliculus (SC) are organized as maps of visual and motor space. The companion paper showed that muscimol injections into intermediate layers of the SC alter the trajectory of the movement and confirmed previously reported effects on latency, amplitude, and speed of saccades. In this paper we analyze the pattern of these deficits across the visual field by systematically comparing the magnitude of each deficit throughout a grid of targets covering a large fraction of the visual field. We also translate these deficits onto the SC map of the visual/movement fields to obtain a qualitative estimate of the extent of the deficit in the SC. We found a consistent pattern of substantially increased saccadic latency to targets in the contralateral visual hemifield, accompanied by slight and inconsistent increases and decreases for saccades to the ipsilateral hemifield. The initial and peak speed of saccades was reduced after the injection. The postinjection amplitude of the saccades were either hypometric or normometric, but rarely hypermetric. Although errors in the initial direction of the postinjection saccades were small, they consistently formed a simple pattern: an initial direction with minimal errors (a null direction) separating regions with clockwise and counterclockwise rotations of the initial direction. However, the null direction did not go through the center of the inactivated zone, as would be expected if the SC alone were determining saccade direction, e.g., with a population code. One hypothesis that can explain the misalignment of the null direction with the lesion site is that another system, acting in parallel with the SC, contributes to the determination of saccadic trajectory.

Animals↗
PubMed1998

Commutative saccadic generator is sufficient to control a 3-D ocular plant with pulleys.

One-dimensional models of oculomotor control rely on the fact that, when rotations around only one axis are considered, angular velocity is the derivative of orientation. However, when rotations around arbitrary axes [3-dimensional (3-D) rotations] are considered, this property does not hold, because 3-D rotations are noncommutative. The noncommutativity of rotations has prompted a long debate over whether or not the oculomotor system has to account for this property of rotations by employing noncommutative operators. Recently, Raphan presented a model of the ocular plant that incorporates the orbital pulleys discovered, and qualitatively modeled, by Miller and colleagues. Using one simulation, Raphan showed that the pulley model could produce realistic saccades even when the neural controller is commutative. However, no proof was offered that the good behavior of the Raphan-Miller pulley model holds for saccades different from those simulated. We demonstrate mathematically that the Raphan-Miller pulley model always produces movements that have an accurate dynamic behavior. This is possible because, if the pulleys are properly placed, the oculomotor plant (extraocular muscles, orbital pulleys, and eyeball) in a sense appears commutative to the neural controller. We demonstrate this finding by studying the effect that the pulleys have on the different components of the innervation signal provided by the brain to the extraocular muscles. Because the pulleys make the axes of action of the extraocular muscles dependent on eye orientation, the effect of the innervation signals varies correspondingly as a function of eye orientation. In particular, the Pulse of innervation, which in classical models of the saccadic system encoded eye velocity, here encodes a different signal, which is very close to the derivative of eye orientation. In contrast, the Step of innervation always encodes orientation, whether or not the plant contains pulleys. Thus the Step can be produced by simply integrating the Pulse. Particular care will be given to describing how the pulleys can have this differential effect on the Pulse and the Step. We will show that, if orbital pulleys are properly located, the neural control of saccades can be greatly simplified. Furthermore, the neural implementation of Listing's Law is simplified: eye orientation will lie in Listing's Plane as long as the Pulse is generated in that plane. These results also have implications for the surgical treatment of strabismus.

Algorithms↗
PubMed1997

Scaling effects in the perception of higher-order spatial correlations.

Human texture discrimination depends both on spatial-frequency content and on higher-order or multi-point correlations. Spatial-frequency discrimination exhibits a high degree of scale invariance over a range of several octaves, but the scaling behavior of sensitivity to higher-order correlation structure is unknown. We explored the scale dependence of texture discrimination for image ensembles which shared the same power spectrum, but differed in their higher-order correlations. Literally scaling the ensembles so that they occupy larger retinal regions results in discrimination performance that is largely independent of scale over a 3 octave range. Holding the display size constant and scaling the texture being sampled within the display over the same range produces performance that varies with scale appreciably. The ideal observer performance is computed, and the absolute efficiency is seen to be quite small, on the order of 10(-2)-10(-1). As the texture is scaled down, increasing the number of checks within the fixed display size, performance increases while the efficiency decreases. These dependencies remain when the stimulus onset asynchrony is increased from 50 to 500 msec. We created sets of textures which varied both in check number and correlation strength, for which ideal observer performance was equated. For the human observers, efficiency was significantly higher for textures with higher correlation strength, but fewer checks. These results are consistent with a model in which a fixed number of checks is processed in a scale-invariant manner, while the remainder of the display is processed much less efficiently.

Humans↗
PubMed1997

Detection, classification, and superposition resolution of action potentials in multiunit single-channel recordings by an on-line real-time neural network.

Determination of single-unit spike trains from multiunit recordings obtained during extracellular recording has been the focus of many studies over the last two decades. In multiunit recordings, superpositions can occur with high frequency if the firing rates of the neurons are high or correlated, making superposition resolution imperative for accurate spike train determination. In this work, a connectionist neural network (NN) was applied to the spike sorting challenge. A novel training scheme was developed which enabled the NN to resolve some superpositions using single-channel recordings. Simulated multiunit spike trains were constructed from templates and noise segments that were extracted from real extracellular recordings. The simulations were used to determine the performances of the NN and a simple matched template filter (MTF), which was used as a basis for comparison. The network performed as well as the MTF in identifying nonoverlapping spikes, and was significantly better in resolving superpositions and rejecting noise. An on-line, real-time implementation of the NN discriminator, using a high-speed digital signal processor mounted inside an IBM-PC, is now in use in six laboratories.

Action Potentials↗
PubMed1997

Model with distributed vectorial premotor bursters accounts for the component stretching of oblique saccades.

During oblique saccades, the durations of the horizontal and vertical components are stretched until they are approximately equal. Models of the saccadic system have been proposed that provide a mechanism for that stretching. However, they fail to simulate the pattern of activity recorded from premotor medium lead burst neurons (MLBNs) in the brain stem. A new model of the saccadic system is proposed that accounts for both the component stretching of oblique movements and the pattern of activity recorded in MLBNs. MLBNs that project to horizontal (or vertical) motoneurons actually have a wide span of on-directions (the direction associated with the largest discharge) around the cardinal direction. We infer from the wide span of their on-directions that, at the level of individual MLBNs, the vectorial signal present in spatially organized structures (e.g., the superior colliculus) is not decomposed into the separate horizontal and vertical components represented by the motoneurons. Nonetheless, all prior models of the saccadic system have decomposed the vectorial premotor command into horizontal and vertical commands at the level of the MLBNs. That decomposition was explicit, because individual MLBNs, with a sine- or cosine-shaped directional tuning curve, were used. We propose here that the decomposition into horizontal and vertical commands is carried out only at the level of the motoneurons. This decomposition is implicit, because no single MLBN encodes the horizontal or vertical command; the command only exists implicitly in the activity of the population of MLBNs. The new vectorial burster model correctly simulates the pattern of activity recorded in primate MLBNs, and the components of its oblique saccades are stretched. Two mechanisms contribute to this stretching: the distribution of MLBN tuning curves and the inhibition exerted by the contralateral population of MLBNs. In contrast, feedback control of the saccade contributes negligibly to the stretching. Even though the vectorial burster model predicts a component stretching, it is not constrained to produce perfectly straight oblique saccades because no trajectory control is implemented. The amount of curvature depends on the similarity of the horizontal and vertical systems (both neural and mechanical). In this model, stretching is interpreted simply as a side effect of the properties of the MLBNs' tuning curves. The distributed MLBNs of the vectorial burster model forces the general organization of the saccadic system to be reconsidered. We propose that a distributed architecture in which several different neural systems cooperate is needed.

Computer Simulation↗
PubMed1996

Primate striate and prestriate cortical neurons during discrimination. I. simultaneous temporal encoding of information about color and pattern.

1. We recorded the responses of neurons in cortical areas V1, V2, and V4 to a set of 36 colored patterns while monkeys discriminated among the stimuli on the basis of their color or their pattern. In the discrimination task a colored square or a black and white pattern was presented foveally as a cue stimulus. The monkey was required to choose, by making a saccade, which of three peripheral targets had the same property as the cue. One of the peripheral targets was centered on the receptive field of the neuron, and the other two were positioned at equally distant points around the circumference of an imaginary circle centered on the cue and passing through the receptive field. 2. An examination of the responses to the stimuli showed that there was a complex interaction between the effects of color and of pattern on the neuronal responses. Because of these interactions, we tested sensitivity to color and pattern by sorting the responses to all stimuli according to the color or pattern of the stimulus. We found that the number of spikes in the responses was affected by only one or the other of the stimulus parameters, but that the temporal distribution of spikes was affected by both stimulus parameters. We quantified the relative sensitivities of each neuron to color and pattern by dividing the amount of information the neuron transmitted about color by the amount of information the neuron transmitted about pattern. The distributions of information ratios assuming a spike count code were broad, indicating that many neurons were sensitive to only one stimulus parameter or the other. In contrast, the distributions of information ratios assuming a wave-form code were narrow and centered near 1.0, indicating nearly equal sensitivities to both stimulus parameters. 3. In our initial experiments, it appeared that the color or pattern used as the cue for the discrimination task affected the responses of many neurons to stimuli on the receptive field. To determine whether the cue effect was due to simple visual interactions or to the cognitive requirements of the discrimination task, we performed a control experiment in which the cue was turned on 80 ms after the peripheral stimuli. For many of the neurons in the control experiment, an effect related to the cue appeared in the response before the cue had been turned on. Thus the effect we observed must have been due to visual interactions with the distractor targets, even though these were outside the neuron's classically defined receptive field. 4. We compared the rate at which color and pattern information developed in the response over time assuming either a spike count or a waveform code. The spike count code gained more of its information in the first 20ms of the response than did the waveform code, but thereafter the information carried by the spike count code developed more slowly and reached a lower asymptote than did the information carried by the waveform code. 5. The waveform codes carried nearly equal amounts of information about color and pattern, but the messages about these two parameters did not develop at the same rate in all areas. The messages about color and pattern developed at the same rate in area V1, but messages about color developed more slowly than did the messages about pattern in areas V2 and V4. 6. These results offer a neurophysiological basis for both the psychological separateness of color and pattern, and the binding of color and pattern into a unified percept. We propose that the separateness of color and form arises not by virtue of their being encoded by different populations of neurons, but by virtue of their being encoded by separable waveform codes in the responses of single neurons. We propose that the binding of color and form occurs by virtue of their codes being multiplexed on the same neurons.

Animals↗
PubMed1996

Primate striate and prestriate cortical neurons during discrimination. II. separable temporal codes for color and pattern.

1. In the previous paper we reported our analysis of the responses of neurons in cortical areas V1, V2, and V4 to a set of stimuli that consisted of all 36 combinations of six colors and six patterns. Neurons in all three cortical areas simultaneously encoded information about both the color and pattern of the stimulus in the number and temporal distribution of spikes in their responses. To account for this ability, we propose that a neuron's response consists of separable temporal codes representing the color and pattern of the stimulus that are multiplexed together. 2. We used nonlinear regression to fit the model parameters to the data. We used the responses to 30 of the 36 stimuli as a training set to estimate the parameters of the model and the responses to the remaining 6 stimuli as a test set. After training, the model fitted the responses to stimuli in the training sets very well and predicted the responses to stimuli in the test sets. Thus neuronal responses to colored patterns contain separate temporal codes representing color and pattern. 3. After establishing the model parameters, we obtained the waveforms that represented each neuron's temporal codes for the six colors and six patterns of our stimulus set. We then proceeded with a series of analyses to determine whether these waveforms were viable candidates for neuronal codes. Cluster analysis revealed that there were only a few different classes of waveforms representing each color and pattern, and there were many neurons in each class. Further, neurons that used similar waveforms to represent one color or pattern also tended to use similar waveforms to represent other colors or patterns. The waveforms representing five of the six colors and three of the six patterns were similar in the two monkeys used in this study. 4. We compared the shapes of the code waveforms across cortical areas and found no differences among areas in the shapes of the waveforms representing four of the six colors. In contrast, we found that there were differences among areas in the shapes of the waveforms representing all six patterns. These results suggest that messages about color are encoded at an early level and are then propagated upward, but that messages about pattern are altered in each successive cortical area. 5. Our results offer a neurophysiological explanation for the psychophysical evidence that color and form are processed by different channels. We propose that the psychophysical channels for color and pattern arise from the separability of the temporal codes for color and pattern in the responses of single neurons. This hypothesis implies that psychophysical channels correspond to classes of temporal codes rather than to classes of neurons.

Animals↗
PubMed1996

Involuntary attentional shifts due to orientation differences.

We tested the ability of orientation differences to cause involuntary shifts of visual attention and found that these attentional shifts can occur in response to an orientation "pop-out" display. Texture-like cue stimuli consisting of discrete oriented bars, with either uniform orientation or containing a noninformative orthogonally oriented bar, were presented for a variable duration. Subsequent to or partially coincident with the cue stimulus was the target display of a localization or two-interval forced-choice task, followed by a mask display. Naive subjects consistently showed greater accuracy in trials with the target at the location of the orthogonal orientation compared with trials with uniformly oriented bars, with only 100 msec between the cue and mask onsets. Discriminating these orientations required a stimulus onset asynchrony (SOA) of 50-70 msec. The attentional facilitation is transient, in most cases absent with a cue-mask SOA of 250 msec [corrected]. These results suggest that the preattentive character of some texture discrimination tasks with SOAs of only 100 msec is vitiated by the involuntary attentional shifts that are caused by orientation differences.

Adolescent↗
PubMed1995

A field theory of saccade generation: temporal-to-spatial transform in the superior colliculus.

Recent models have placed the superior colliculus inside the local feedback loop that generates the pulse of innervation needed to make a saccade. Such closed-loop models need to take into account the different coordinate systems of visual and motor signals. This paper presents a computational model showing how the superior colliculus can bring the visual and motor information together in a common reference frame.

Animals↗
PubMed1994

Superior colliculus cell types and models of saccade generation.

Recent experiments on the cat and monkey have revealed several different cell types within the superior colliculus, including fixation, burst, and build up cells. During primate saccades, activity remains fixed at one location in burst cells, but spreads across the colliculus in build up cells. New models based on the activity of these cell types suggest their functional roles in saccade generation.

Animals↗
PubMed1993

Cluster method for analysis of transmitted information in multivariate neuronal data.

A new method for quantifying the transmitted information and channel capacity of high-dimensional data, based on cluster formation, is described. The method's ability to handle high-dimensional data allows for a complete measurement of information transmitted by neuronal data. It is computationally efficient in terms of both processing time and memory storage. Application of the method to the responses of a VI neuron shows that more information was transmitted about the pattern of stimuli than about their color.

Animals↗
PubMed1993

Visually induced cross-axis postsaccadic eye drift.

1. It has been previously shown that, if a visual pattern is transiently moved just after every saccade, it is possible to induce horizontal, postsaccadic, ocular drift after horizontal saccades that persists in the dark. In this study we show that horizontal ocular drift can also be created after vertical saccades. Five human subjects viewed binocularly the interior of a full-field hemisphere filled with a random-dot pattern. They were encouraged to make frequent vertical saccades. During training, eye movements were recorded by the electrooculogram. A computer detected the end of every saccade and immediately moved the pattern to the left after up saccades and right after down saccades. The motion was exponential, its amplitude was 25% of the vertical component of the antecedent saccade, its time constant was 50 ms. Before and after 2-3 h of training, movements of both eyes were measured by the eye-coil/magnetic-field method while subjects were instructed to make vertical saccades in the dark, in the presence of the movable adapting pattern, and between stationary targets for calibration. 2. After training (approximately 20,000 saccades) all subjects developed a zero-latency, exponential ocular drift to the left after up saccades and to the right after down saccades. The amplitude of the horizontal drift, expressed as a percentage of the vertical component of the preceding saccade, was 2.7% in the dark. This rose to 10.2% in the presence of the movable adapting stimulus. The latter rise is not due to visual following systems but to a zero-latency increase in initial drift velocity. 3. The horizontal drifts were usually unequal between the two eyes, indicating the presence of disconjugate movements. We measured intrasaccadic disconjugate horizontal movements of all subjects. In agreement with studies by others of saccades in the light, we measured a divergence during up saccades (1.3 degrees) and a convergence for down (0.4 degrees), but in this case for spontaneous saccades in the dark. After training, these values increased for saccades in the dark but decreased in the light in the presence of the adapting stimulus. These changes were largely idiosyncratic and statistically significant in only a few subjects. 4. The cross-axis postsaccadic drifts were separated into their conjugate and disconjugate components. The disconjugate components were small and idiosyncratic, and the means were small for saccades in the dark. The only consistent trend was in the presence of the adapting stimulus where up saccades were often followed by convergence.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
PubMed1992

Role of inferior temporal neurons in visual memory. I. Temporal encoding of information about visual images, recalled images, and behavioral context.

1. Lesions of the inferior temporal (IT) cortex selectively hamper monkeys in tasks requiring visual memory. A system that recognizes images must be able to encode a current stimulus, recall the code of a previous stimulus, compare the codes of the two stimuli, and make a decision on the basis of the outcome of the comparison. Therefore, IT neurons must be involved in at least one of these processes. To determine the specific role of IT neurons in visual memory, we measured the information conveyed in the neuronal responses about current patterns, recalled patterns, and behavioral context. 2. Two monkeys were trained to perform a sequential matching task using a set of 32 black and white Walsh patterns. In the course of an experiment, each pattern was presented repeatedly in sample, match, and nonmatch behavioral contexts. While the monkeys were performing the task, we recorded the activity of 76 neurons from area TE of IT. The neuronal responses to the stimuli were converted to spike density functions, and the resultant waveforms were quantified using their principal components. The relationships between the responses and the stimuli were studied using analysis of variance and information theory. 3. The analysis of variance was applied to the neuronal response waveforms using the context (sample, match, or nonmatch) and the patterns of the stimuli as independent variables and the spike count or the coefficients of the principal components as the dependent variables. We found that the waveforms of most neurons were significantly modulated by both the pattern and the context of the stimulus presentation. 4. We also analyzed the stimulus-response relationships using information theory. The input codes were based on the pattern and context of the stimuli, and the output codes were based on the spike count or the principal components of the responses. The neuronal response waveforms were found to convey significant amounts of information about both the pattern and context of the stimuli. Transmitted information was greatest when the response of a neuron was interpreted as a message about the combination of pattern and context. Nevertheless, there was information about context independent of pattern and vice versa. 5. We also used information theory to determine whether the neuronal responses to the second, or test, stimulus conveyed information about the pattern of the first, or sample, stimulus. The input codes were based on the patterns of the sample stimuli, and the output codes were based on the responses to the nonmatch test stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Analysis of Variance↗