Concurrent processing and complexity of temporally encoded neuronal messages in visual perception.

The intrinsic neuronal code that carries visual information and the perceptual mechanism for decoding that information are not known. However, multivariate statistics and information theory show that neurons in four visual areas simultaneously carry multiple, stimulus-related messages by utilizing multiplexed temporal codes. The complexity of these temporal messages increases progressively across the visual system, yet the temporal codes overlap in time. Thus, visual perception may depend on the concurrent processing of multiplexed temporal messages from all visual areas.

Irregularity distinguishes limb tremor in cervical dystonia from essential tremor.

INTRODUCTION: Patients with cervical dystonia (CD) often have limb tremor that is clinically indistinguishable from essential tremor (ET). Whether a common central mechanism underlies the tremor in these conditions is unknown. We addressed this issue by quantifying limb tremor in 19 patients with CD and 35 patients with ET. METHOD: Postural, resting and kinetic tremors were quantified (amplitude, mean frequency and regularity) using a three-axis accelerometer. RESULTS: The amplitude of limb tremor in ET was significantly higher than in CD, but the mean frequency was not significantly different between the groups. The cycle-to-cycle variability of the frequency (ie the tremor irregularity), however, was significantly greater (approximately 50%) in CD. Analysis of covariance excluded the possibility that the increased irregularity was related to the smaller amplitude of tremor in CD (ANCOVA: p = 0.007, F = 5.31). DISCUSSION: We propose that tremor in CD arises from oscillators with different dynamic characteristics, producing a more irregular output, whereas the tremor in ET arises from oscillators with similar dynamic characteristics, producing a more regular output. We suggest that variability of tremor is an important parameter for distinguishing tremor mechanisms. It is possible that changes in membrane kinetics based on the pattern of ion channel expression underlie the differences in tremor in some diseases.

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.

Age-related differences in visual perception: a PET study.

To assess age-related differences in cortical activation during form perception, two classes of visual textures were shown to young and older subjects undergoing positron emission tomography (PET). Subjects viewed even textures that were rich in rectangular blocks and extended contours and random textures that lacked these organized form elements. Within-group significant increases in regional cerebral blood flow (rCBF) during even stimulation relative to random stimulation in young subjects were seen in occipital, inferior and medial temporal regions, and cerebellum, and in older subjects, in posterior occipital and frontal regions. Group by texture type interactions revealed significantly smaller rCBF increases in older subjects relative to young in occipital and medial temporal regions. These results indicate that young subjects activate the occipitotemporal pathway during form perception, whereas older subjects activate occipital and frontal regions. The between-group differences suggest that age-related reorganization of cortical activation occur during early visual processes in humans.

Slow saccades in spinocerebellar degeneration.

Two patients with spinocerebellar degeneration made abnormally slow horizontal refixations. One patient produced quick phases of nystagmus with identical maximum velocities, suggesting her refixations were abnormal saccades and not voluntary pursuit movements. In response to double target jumps, neither patient showed an obligatory refractory period after each saccade; they responded to every target movement after one reaction time. Their slow refixations were not preprogrammed since they could be modified in flight. To reconcile these observations with normal saccadic behavior, we hypothesized a neural network that made saccades by driving the eyes to an orbital position rather than preprogramming a distance for movement. Computer simulation of this model produced both realistically appearing normal saccades and, when appropriately "lesioned" to simulate a loss of saccadic "burst" neurons in the pontine reticular formation, slow saccades that could be modified in flight.

Unilateral pursuit-induced congenital nystagmus.

We describe two patients with a lifelong history of oscillopsia only when following objects moving toward their left side. Neurologic examination was normal except for eye movements. The patients showed nystagmus during any tasks that required visual following toward the left (ie, smooth pursuit, optokinetic nystagmus, and vestibulo-ocular-reflex-suppression), but had no nystagmus during fixation of stationary targets or visual following tasks to the right. Eye-movement recordings showed waveforms during pursuit to the left that were typical of congenital nystagmus.

Head shaking and vestibulo-ocular reflex in congenital nystagmus.

The authors investigated the mechanisms underlying the head shaking shown by some patients with congenital nystagmus (CN). In order to improve visual function by head shaking, a patient with CN must have some visual acuity loss due to retinal image motion created by the nystagmus; an abnormal vestibulo-ocular reflex (VOR); and the head shaking must be correlated with the nystagmus. The authors measured the VOR gain (eye velocity/head velocity) and examined eye-head coordination in five patients with CN with various combinations of these three factors. One patient met all three criteria and was able to increase his acuity by shaking his head. Other patients who shook their heads either had no loss of visual acuity due to the nystagmus or had a normal VOR. In either case, head shaking was of no apparent visual benefit and may represent, instead, an associated pathologic tremor in the cephalomotor control system.

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.

Graphic analysis of paralytic strabismus with the Lancaster red-green test.

We used the Lancaster red-green test to monitor changes in ocular alignment in patients with paralytic strabismus. By inferring the position of the right eye and that of the left eye at many different positions of gaze and then plotting the data on a graph, one can derive a static eye position curve. The location of the curve relative to the line depicting normal ocular alignment ( orthophoria ) indicates whether there is an esodeviation or an exodeviation . The slope of a line drawn tangent to the curve indicates, for that particular point, whether the deviation is concomitant or not and which eye is relatively weak or restricted and by how much. This graphic technique provides a simple, sensitive, and quantitative measure of ocular alignment that may be especially useful for detecting subtle changes in the relative positions of the two eyes. This method may be a useful adjunct in the planning and evaluation of therapy for patients with paralytic strabismus.

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.

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.

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.

DYNAMICAL COMPLEXITY ANALYSIS OF SACCADIC EYE MOVEMENTS IN TWO DIFFERENT PSYCHOLOGICAL CONDITIONS.

Saccadic eye movements of a normal subject were assessed through semi-quantitative analysis algorithms based on linear and non-linear test application in order to highlight the dynamics type characterizing saccadic neural system behavior. These movements were recorded during a simple visually-guided saccade test and one with a cognitive load involving button pressing to show a decision. Following the application of specific computational tests, chaotic dynamical trend dominancy was mostly revealed with some differences between the two saccade recording conditions: auto-correlation time was increased from 170 to 240 by cognitive task superposition and the Hurst exponent was enhanced from 0.52 to 0.76, denoting more persistence in the dynamics of saccadic system during increased neural activity related to cognitive task.

EYE MOVEMENT RECORDING AND NONLINEAR DYNAMICS ANALYSIS - THE CASE OF SACCADES.

Evidence of a chaotic behavioral trend in eye movement dynamics was examined in the case of a saccadic temporal series collected from a healthy human subject. Saccades are highvelocity eye movements of very short duration, their recording being relatively accessible, so that the resulting data series could be studied computationally for understanding the neural processing in a motor system. The aim of this study was to assess the complexity degree in the eye movement dynamics. To do this we analyzed the saccadic temporal series recorded with an infrared camera eye tracker from a healthy human subject in a special experimental arrangement which provides continuous records of eye position, both saccades (eye shifting movements) and fixations (focusing over regions of interest, with rapid, small fluctuations). The semi-quantitative approach used in this paper in studying the eye functioning from the viewpoint of non-linear dynamics was accomplished by some computational tests (power spectrum, portrait in the state space and its fractal dimension, Hurst exponent and largest Lyapunov exponent) derived from chaos theory. A high complexity dynamical trend was found. Lyapunov largest exponent test suggested bi-stability of cellular membrane resting potential during saccadic experiment.

Applying saccade models to account for oscillations.

Saccadic oscillations are unwanted back-to-back saccades occurring one upon the other that produce a high-frequency oscillation of the eyes (usually 15-30 Hz). These may occur transiently in normal subjects, for example, around the orthogonal axis of a purely horizontal or vertical saccade, during combined saccade-vergence gaze shifts or during blinks. Some subjects may produce saccadic oscillations at will, usually with convergence. Pathological, involuntary saccadic oscillations such as flutter and opsoclonus are prominent in certain diseases. Our recent mathematical model of the premotor circuit for generating saccades includes brainstem burst neurons in the paramedian pontine reticular formation (PPRF), which show the physiological phenomenon of post-inhibitory rebound (PIR). This model makes saccadic oscillations because of the positive feedback among excitatory and inhibitory burst neurons. Here we review our recent findings and hypotheses and show how they may be reproduced using our lumped model of the saccadic premotor circuitry by reducing the inhibitory efficacy of omnipause neurons.

Temporal encoding of two-dimensional patterns by single units in primate inferior temporal cortex. II. Quantification of response waveform.

The purpose of this study was to describe how the responses of neurons in inferior temporal (IT) cortex represent visual stimuli. In the preceding paper we described the responses of IT neurons to a large set of two-dimensional black and white patterns. The responses to different stimuli showed temporal modulation of the spike trains. This paper develops a method for quantifying temporal modulation and shows that the stimulus determines the distribution over time, as well as the number, of spikes in a response. The responses were quantified using an orthogonal set of temporal waveforms called principal components. The principal components related to each neuron were extracted from all the responses of that neuron to all of the stimuli, regardless of which stimulus elicited which response. Each response was then projected onto the set of principal components to obtain a set of coefficients that quantified its temporal modulation. This decomposition produces coefficients that are uncorrelated with each other. Thus each coefficient could be tested individually, with univariate statistics, to determine whether its relation to the stimulus was nonrandom. The waveforms of the principal components are unconstrained and depend only on the responses from which they are derived; hence, they can assume any shape. Nonetheless, the 21 neurons we analyzed all had principal components that belonged to only one of two sets. The two sets could be characterized by their first principal component, which was either phasic or tonic. This suggests that these neurons may use as few as two different mechanisms in generating responses. The first principal component was highly correlated with spike count, and both were driven by the stimulus. Higher principal components were uncorrelated with spike count, yet some of them were also driven by the stimulus. Thus the principal components form a richer description of the stimulus-dependent aspects of a neuronal response than does spike count. Bootstrap tests showed that several principal components (usually 3 or 4) were determined by the stimulus. Since higher principal components were not correlated with the spike count, the stimulus must have determined the distribution of spikes in the response as well as their number. However, it is possible that the number and distribution of spikes are both determined by the same characteristics of the stimulus. In this case, the temporal modulation would be redundant, and a simple univariate measure would be sufficient to characterize the stimulus-response relationship.(ABSTRACT TRUNCATED AT 400 WORDS)