Using spatiotemporal correlations to learn topographic maps for invariant object recognition

The retinal image of visual objects can vary drastically with changes of viewing angle. Nevertheless, our visual system is capable of recognizing objects fairly invariant of viewing angle. Under natural viewing conditions, different views of the same object tend to occur in temporal proximity, thereby generating temporal correlations in the sequence of retinal images. Such spatial and temporal stimulus correlations can be exploited for learning invariant representations. We propose a biologically plausible mechanism that implements this learning strategy using the principle of self-organizing maps. We developed a network of spiking neurons that uses spatiotemporal correlations in the inputs to map different views of objects onto a topographic representation. After learning, different views of the same object are represented in a connected neighborhood of neurons. Model neurons of a higher processing area that receive unspecific input from a local neighborhood in the map show view-invariant selectivities for visual objects. The findings suggest a functional relevance of cortical topographic maps.

Scale-invariance of receptive field properties in primary visual cortex

Background

Our visual system enables us to recognize visual objects across a wide range of spatial scales. The neural mechanisms underlying these abilities are still poorly understood. Size- or scale-independent representation of visual objects might be supported by processing in primary visual cortex (V1). Neurons in V1 are selective for spatial frequency and thus represent visual information in specific spatial wavebands. We tested whether different receptive field properties of neurons in V1 scale with preferred spatial wavelength. Specifically, we investigated the size of the area that enhances responses, i.e., the grating summation field, the size of the inhibitory surround, and the distance dependence of signal coupling, i.e., the linking field.

Results

We found that the sizes of both grating summation field and inhibitory surround increase with preferred spatial wavelength. For the summation field this increase, however, is not strictly linear. No evidence was found that size of the linking field depends on preferred spatial wavelength.

Conclusion

Our data show that some receptive field properties are related to preferred spatial wavelength. This speaks in favor of the hypothesis that processing in V1 supports scale-invariant aspects of visual performance. However, not all properties of receptive fields in V1 scale with preferred spatial wavelength. Spatial-wavelength independence of the linking field implies a constant spatial range of signal coupling between neurons with different preferred spatial wavelengths. This might be important for encoding extended broad-band visual features such as edges.

Inhomogeneous retino-cortical mapping is supported and stabilized with correlation-learning during self-motion

In primates, the area of primary visual cortex representing a fixed area of visual space decreases with increasing eccentricity. We identify visual situations to which this inhomogeneous retino-cortical mapping is well adapted and study their relevance during natural vision and development. We assume that cortical activations caused by stationary objects during self-motion along the direction of gaze travel on average with constant speed across the cortical surface, independent of retinal eccentricity. This is the case if the distribution of objects corresponds to an ellipsoid with the observer in its center. We apply the resulting flow field to train a simple network of pulse coding neurons with Hebbian learning and demonstrate that the density of learned receptive field centers is in close agreement with primate retino-cortical magnification. In addition, the model reproduces the increase of receptive field size and the decrease of its peak sensitivity with increasing eccentricity. Our results suggest that self-motion may have played an important role in the evolution of the visual system and that cortical magnification can be refined and stabilized by Hebbian learning mechanisms in ontogenesis under natural viewing conditions.

Coding the presence of visual objects in a recurrent neural network of visual cortex

Before we can recognize a visual object, our visual system has to segregate it from its background. This requires a fast mechanism for establishing the presence and location of objects independently of their identity. Recently, border-ownership neurons were recorded in monkey visual cortex which might be involved in this task [Zhou, H., Friedmann, H., von der Heydt, R., 2000. Coding of border ownership in monkey visual cortex. J. Neurosci. 20 (17), 6594-6611]. In order to explain the basic mechanisms required for fast coding of object presence, we have developed a neural network model of visual cortex consisting of three stages. Feed-forward and lateral connections support coding of Gestalt properties, including similarity, good continuation, and convexity. Neurons of the highest area respond to the presence of an object and encode its position, invariant of its form. Feedback connections to the lowest area facilitate orientation detectors activated by contours belonging to potential objects, and thus generate the experimentally observed border-ownership property. This feedback control acts fast and significantly improves the figure-ground segregation required for the consecutive task of object recognition.

Visual resolution with retinal implants estimated from recordings in cat visual cortex

We investigated cortical responses to electrical stimulation of the retina using epi- and sub-retinal electrodes of 20-100 microm diameter. Temporal and spatial resolutions were assessed by recordings from the visual cortex with arrays of microelectrodes and optical imaging. The estimated resolutions were approximately 40 ms and approximately 1 degrees of visual angle. This temporal resolution of 25 frames per second and spatial resolution of about 0.8 cm at about 1m and correspondingly 8 cm at 10 m distance seems sufficient for useful object recognition and visuo-motor behavior in many in- and out-door situations of daily life.

Spatiotemporal receptive field properties of epiretinally recorded spikes and local electroretinograms in cats

Background

Receptive fields of retinal neural signals of different origin can be determined from extracellular microelectrode recordings at the inner retinal surface. However, locations and types of neural processes generating the different signal components are difficult to separate and identify. We here report epiretinal receptive fields (RFs) from simultaneously recorded spikes and local electroretinograms (LERGs) using a semi-chronic multi-electrode in vivo recording technique in cats. Broadband recordings were filtered to yield LERG and multi unit as well as single unit spike signals. RFs were calculated from responses to multifocal pseudo-random spatiotemporal visual stimuli registered at the retinal surface by a 7-electrode array.

Results

LERGs exhibit spatially unimodal RFs always centered at the location of the electrode tip. Spike-RFs are either congruent with LERG-RFs (N = 26/61) or shifted distally (N = 35/61) but never proximally with respect to the optic disk. LERG-RFs appear at shorter latencies (11.9 ms ± 0.5 ms, N = 18) than those of spikes (18.6 ms ± 0.4 ms, N = 53). Furthermore, OFF-center spike-RFs precede and have shorter response rise times than ON-center spike-RFs. Our results indicate that displaced spike-RFs result from action potentials of ganglion cell axons passing the recording electrode en route to the optic disk while LERG-RFs are related to superimposed postsynaptic potentials of cells near the electrode tip.

Conclusion

Besides contributing to the understanding of retinal function we demonstrate the caveats that come with recordings from the retinal surface, i.e., the likelihood of recordings from mixed sets of retinal neurons. Implications for the design of an epiretinal visual implant are discussed.

Retino-cortical information transmission achievable with a retina implant

Blind subjects with photoreceptor degeneration perceive phosphenes when their intact retinal ganglion cells are stimulated electrically. Is this approach suitable for transmitting enough information to the visual cortex for partially restoring vision? We stimulated the retina of anesthetized cats electrically and visually while recording the responses in the visual cortex. Transmission of retino-cortical information T was quantified by information theory. T was 20-160 bit/s (per stimulation and recording site) with random electrical or visual impulse stimulation at rates between 20 and 40 s-1. While increasing spatial density of independent electrical stimulation channels T did not saturate with 7 electrodes/mm2 retina. With seven electrodes up to 500 bit/s was transmitted to 15 cortical recording sites. Electrical stimulation basically employs temporal stimulus patterns. They are intimately linked with intensity/contrast information coded by the spike density of retinal ganglion cells. From the cortical information spread we estimated the spatial resolution as 0.5mm cortex corresponding to 0.5-1.0 degrees visual angle. If the human cortex can receive and decode the information transmitted by a retina implant, our quantitative results measured in cats suggest that visuo-motor coordination and object recognition in many in- and out-door situations will be possible.

Different types of signal coupling in the visual cortex related to neural mechanisms of associative processing and perception

The hypothesis of object representation by synchronization in the visual cortex has been supported by our recent experiments in monkeys. They demonstrated local synchrony among gamma activities (30-90 Hz) and their perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few mm, challenging the synchronization hypothesis for larger cortical object representations. The restriction is due to randomly changing phase relations among locally synchronized patches which, however, form continuous waves of gamma-activity, traveling across object representations. The phase continuity of these waves may support coding of object continuity. Interactions across still larger distances, measured among cortical areas in human data, involve amplitude envelopes of gamma signals. Based on models with spiking neurons we discuss potentially underlying mechanisms. Most important for gamma synchronization are local facilitatory connections with distance-dependent delays. They also explain the occurrence of gamma waves and the restriction of gamma-synchrony. Fast local feedback inhibition generates gamma oscillations and supports local synchrony, while slow shunting inhibitory feedback supports figure-ground segregation. Finally, dispersion in inter-areal far projections destroys coherence of gamma signals, but preserves their amplitude modulations. In conclusion, we propose that the hypothesis of associative processing by gamma synchronization be extended to more general forms of signal coupling.

Perception-related modulations of local field potential power and coherence in primary visual cortex of awake monkey during binocular rivalry

Cortical synchronization at gamma-frequencies (35-90 Hz) has been proposed to define the connectedness among the local parts of a perceived visual object. This hypothesis is still under debate. We tested it under conditions of binocular rivalry (BR), where a monkey perceived alternations among conflicting gratings presented singly to each eye at orthogonal orientations. We made multi-channel microelectrode recordings of multi-unit activity (MUA) and local field potentials (LFP) from striate cortex (V1) during BR while the monkey indicated his perception by pushing a lever. We analyzed spectral power and coherence of MUA and LFP over 4-90 Hz. As in previous work, coherence of gamma-signals in most pairs of recording locations strongly depended on grating orientation when stimuli were presented congruently in both eyes. With incongruent (rivalrous) stimulation LFP power was often consistently modulated in consonance with the perceptual state. This was not visible in MUA. These perception-related modulations of LFP occurred at low and medium frequencies (< 30 Hz), but not at gamma-frequencies. Perception-related modulations of LFP coherence were also restricted to the low-medium range. In conclusion, our results do not support the expectation that gamma-synchronization in V1 is related to the perceptual state during BR, but instead suggest a perception-related role of synchrony at low and medium frequencies.

Task-related coupling from high- to low-frequency signals among visual cortical areas in human subdural recordings

Cortical cooperativity during cognitive demands includes high- and low-frequency activities, which raises the question whether there are interdependencies between fast and slow processes and how they are reflected in electrical brain signals. We had the opportunity to record signals intracranially from occipital visual areas in an epileptic patient and quantified inter-areal signal coupling while the patient performed a visual delayed-match-to-sample task. We computed coherence, phase consistency and amplitude envelope correlation and we also determined inter-frequency coupling through correlation between low-frequency signal components and amplitude envelopes of high-frequency components. There was a pronounced task-related increase of correlation between gamma-band (28-70 Hz) signal envelopes from a superior (occipital) and low-frequency (0-3.5 Hz) signals from an inferior (occipital) visual area, lasting for approximately 1 s and possibly reflecting a short-term memory encoding process. The correlational delay between envelopes and low-frequency components was 40 ms. In contrast, coherence, phase consistency and envelope correlation showed event-, but no task-related changes of intra-areal and no changes of inter-areal coupling. Our data suggest a specific effect of gamma-activity in the superior onto low-frequency activity in the inferior area. We argue that temporal dispersion of conduction delays might prevent coherent transmission of high-frequency signals and thus account for the absence of gamma-coherence. As such dispersion is a general property of long-range projections, envelope-to-signal correlation possibly reflects a general neuronal mechanism. Hence, our method provides a powerful tool for detecting such inter-areal interactions not visible with conventional linear coupling measures.

A multi-channel correlation method detects traveling gamma-waves in monkey visual cortex

Correlations among simultaneously recorded signals are mostly analyzed pairwise and include temporal averaging. However, pairwise methods are not suitable for characterizing relationships among multiple channels for signals which vary temporally in an unpredictable way. Here we develop a time-resolved spatio-temporal correlation (STC) measure among simultaneously recorded signals. We demonstrate the capabilities of the method with artificial data sets and with multiple-channel recordings from striate cortex of awake monkeys. We concentrate on correlations in the gamma-frequency range (gamma: 30-90 Hz) because they were prominent in the analyzed recordings and gained high interest in the recent years due to their assumed role in associative processing, including perceptual binding. Former analyses of gamma-activities in visual cortex, using pairwise correlation methods, mostly revealed zero-delay correlation, indicating synchrony. In cat and monkey visual cortex this gamma-synchrony is restricted to 1.5-3.0 mm (half-height decline). However, our spatio-temporal correlation (STC)-method demonstrates for striate cortex from awake monkeys that gamma-synchrony is a local phenomenon of more global traveling plane waves that appear stimulus-induced at randomly varying orientations. These gamma-waves are coupled over much larger cortical distances (approximately 7 mm half-height decline) than the gamma-synchrony ranges obtained by pairwise correlation analyses from the same data. Our STC-method therefore suggests that the previously reported results of short-range and zero-delay correlations were often due to temporal averaging of traveling gamma-waves.

Visual resolution with epi-retinal electrical stimulation estimated from activation profiles in cat visual cortex

Blinds with receptor degeneration can perceive localized phosphenes in response to focal electrical epi-retinal stimuli. To avoid extensive basic stimulation tests in human patients, we developed techniques for estimating visual spatial resolution in anesthetized cats. Electrical epi-retinal and visual stimulation was combined with multiple-site retinal and cortical microelectrode recordings of local field potentials (LFPs) from visual areas 17 and 18. Classical visual receptive fields were characterized for retinal and cortical recording sites using multifocal visual stimulation combined with stimulus–response cross-correlation. We estimated visual spatial resolution from the size of the cortical activation profiles in response to single focal stimuli. For comparison, we determined activation profiles in response to visual stimuli at the same retinal location. Activation profiles were single peaked or multipeaked. In multipeaked profiles, the peak locations coincided with discontinuities in cortical retinotopy. Location and width of cortical activation profiles were distinct for retinal stimulation sites. On average, the activation profiles had a size of 1.28 ± 0.03 mm cortex. Projected to visual space this corresponds to a spatial resolution of 1.49 deg ± 0.04 deg visual angle. Best resolutions were 0.5 deg at low and medium stimulation currents corresponding to a visus of 1/30. Higher stimulation currents caused lower spatial, but higher temporal resolution (up to 70 stimuli/s). In analogy to the receptive-field concept in visual space, we defined and characterized electrical receptive fields. As our estimates of visual resolutions are conservative, we assume that a visual prosthesis will induce phosphenes at least at this resolution. This would enable visuomotor coordinations and object recognition in many indoor and outdoor situations of daily life.

Neural mechanisms of visual associative processing

This is a review of our work on multiple microelectrode recordings from the visual cortex of monkeys and subdural recordings from humans--related to the potential underlying neural mechanisms. The former hypothesis of object representation by synchronization in visual cortex (or more generally: of flexible associative processing) has been supported by our recent experiments in monkeys. They demonstrated local synchrony among rhythmic or stochastic gamma-activities (30-90 Hz) and perceptual modulation, according to the rules of figure-ground segregation. However, gamma-synchrony in primary visual cortex is restricted to few millimeters, challenging the synchronization hypothesis for larger cortical object representations. We found that the spatial restriction is due to gamma-waves, traveling in random directions across the object representations. It will be argued that phase continuity of these waves can support the coding of object continuity. Based on models with spiking neurons, potentially underlying neural mechanisms are proposed: (i) Fast inhibitory feedback loops can generate locally synchronized gamma-activities; (ii) Hebbian learning of lateral and feed forward connections with distance-dependent delays can explain the stabilization of cortical retinotopy, the limited size of synchronization, the occurrence of gamma-waves, and the larger receptive fields at successive levels; (iii) slow inhibitory feedback can support figure-ground segregation; (iv) temporal dispersion in far projections destroys coherence of fast signals but preserves slow amplitude modulations. In conclusion, it is proposed that the hypothesis of flexible associative processing by gamma-synchronization, including coherent representations of visual objects, has to be extended to more general forms of signal coupling.

Simultaneous mapping of binocular and monocular receptive fields in awake monkeys for calibrating eye alignment in a dichoptical setup

We developed a modified Wheatstone stereoscope for simultaneous dichoptical and binocular stimulation in awake monkey. We, therefore, extended the conventional two-screen Wheatstone stereoscope to a setup with an additional third screen viewed binocularly via semi-transparent mirrors. With a sparse noise stimulation we mapped classical receptive field (CRF) positions via each screen independently but simultaneously. This was done for multiple recording positions (16 electrodes) at once in primary visual cortex based on multiple unit spike activity (MUA) and local field potentials (LFP), respectively. The technique can be used to (1) quickly and simultaneously determine binocular as well as left and right eye CRFs, including ocular dominance characteristics (net recording time for the given examples: approximately 2 min), (2) precisely adjust dichoptical stimulation by evaluating offsets between monocular and binocular CRF positions (average spatial incongruency between binocular and left/right eye stimulation after calibration: approximately 0.025 degrees visual angle), and (3) investigate left and right eye interaction in forming binocular CRFs. Due to the precise adjustment of the dichoptical and the simultaneous binocular stimulation investigations on the basis of stereo vision can be done with appropriate eye vergence alignment matching normal binocular viewing conditions in awake animals.

Activation zones in cat visual cortex evoked by electrical retina stimulation

BACKGROUND

A retina implant for restoring simple basic visual perception in patients who are blind due to photoreceptor loss requires optimisation of stimulation parameters for obtaining high spatio-temporal resolution. We developed effective low-power epi-retinal stimulation and intracortical recording in semichronically prepared cats.

METHODS

Individually driveable fibre electrodes were inserted through a small scleral incision and positioned at the area centralis. Polyimide-platinum film electrodes were inserted via a corneal incision and fixed by instillation of perfluorocarbon liquid on the internal limiting membrane. For electrical stimulation we used short charge-balanced current impulses of 100-400 micro s duration and amplitudes ranging from 1 to 100 micro A. During stimulation we recorded multiple single-cell and population activities from areas 17 and 18. Recordings were stored digitally. Stimulus-response relations including response strength, cortical activation zones, information transmission, and electrical receptive fields were analysed off-line.

RESULTS

We found low-threshold activations with fibre electrodes and polyimide-platinum film electrodes in close mechanical contact to the retina. Retinal stimulation with bipolar charge-balanced impulses resulted in cortical activation zones corresponding to 1-5 degrees visual angle at paracentral locations dependent on the eccentricity of the retinal stimulation point. Retino-cortical transinformation analysis revealed 20-30 bits/s per electrode, corresponding to 10-15 four-level pictures/s. Electrical receptive fields had sizes of 1-3 degrees visual angle.

CONCLUSIONS

Coarse visuomotor coordination and navigation seems possible with retina implants.

Assessing the encoding of stimulus attributes with rapid sequences of stimulus events

In a preceding paper (M. Eger and R. Eckhorn, J. Comput. Neurosci., 2002) we have published a three step method for the quantification of transinformation in multi-input and -output neuronal systems. Here we present an extension that applies to rapid series of transient stimuli and thus, fills the gap between the discrete and continuous stimulation paradigm. While the three step method potentially captures all stimulus aspects, the present approach quantifies the discriminability of selected attributes of discrete stimuli and thus, assesses their encoding. Based on simulated and recorded data we investigate the performance of the implemented algorithm. Our approach is illustrated by analyses of neuronal population activity from the visual cortex of the cat, evoked by electrical stimuli of the retina.

Perceptual grouping correlates with short synchronization in monkey prestriate cortex

Synchronization in the visual cortex at 35-80 Hz is assumed to support perceptual grouping. We tested this hypothesis in a figure-ground task in which a trained monkey indicated by a key whether he perceived a figure that was composed of the same blobs as the background distractors. The task was sufficiently difficult such that about 25% of responses were incorrect. We recorded population activity with 7 microelectrodes in prestriate cortex (V2). During a short period before the monkey's perceptual response, locations of figure-activated neurons showed increased synchronization (50-80 Hz) in correct compared to incorrect responses, while other signal measures were unrelated to perception. These are first indications that a short synchronous burst in V2 may support perceptual grouping.

Quantification of sensory information transmission using timeseries decorrelation techniques

To estimate the information transmitted across a neuronal sensory system one has to deal with serial dependence among consecutive samples of the stimulus and the response signal. Common methods usually require a huge amount of data, or are restricted to Gaussian stimuli. Here, we describe stimulus and response as stochastic processes, i.e. as sequences of random variables, in the same coordinate system. Stimulus-response pairs of these random variables must not be considered independently because otherwise the transinformation is overestimated. To account for the linear fraction of the serial dependence, we present two decorrelation techniques based on coordinate transformation. They provide a representation of the processes with uncorrelated random variables and yield a more precise estimate of the transinformation.

Classification of neural signals by a generalized correlation classifier based on radial basis functions

A common problem in neuroscience is to identify the features by which a set of measurements can be segregated into different classes, for example into different responses to sensory stimuli. A main difficulty is that the derived distributions are often high-dimensional and complex. Many multivariate analysis techniques, therefore, aim to find a simpler low-dimensional representation. Most of them either involve huge efforts in implementation and data handling or ignore important structures and relationships within the original data. We developed a dimension reduction method by means of radial basis functions (RBF), where only a system of linear equations has to be solved. We show that this approach can be regarded as an extension of a linear correlation-based classifier. The validity and reliability of this technique is demonstrated on artificial data sets. Its practical relevance is further confirmed by discriminating recordings from monkey visual cortex evoked by different stimuli.

A model-based approach for the analysis of neuronal information transmission in multi-input and -output systems

We present a new method to characterize multi-input and output neuronal systems using information theory. To obtain a lower bound of transinformation we take three steps: (1) Estimation of the deterministic response to isolate components carrying stimulus information. The deviation of the original response from the deterministic estimate is defined as noise. (2) Coordinate transformation using PCA yields an uncorrelated representation. (3) Partial transinformation values are calculated independently either by Shannon's formula assuming normality or based on density estimation for arbitrary distributions. We investigate the performance of the algorithms using simulated data and discuss suitable parameter settings. The approach allows to evaluate the degree to which stimulus features are encoded. Its potential is illustrated by analyses of neuronal activity in cat primary visual cortex evoked by electrical retina stimulation.

Physiologische Funktionsprüfungen von Retinaimplantaten an Tiermodellen

Retinal implants can--by electrical stimulation--create visual impressions in people with certain kinds of degenerative retinal diseases (e.g. Retinitis Pigmentosa). Electrically evoked potentials in the retina must be transferred into the visual cortex in an orderly manner, a prerequisite for any kind of form- and movement-perception. In the current developmental stage the difficult investigations are performed in various animal models: isolated retinae of intact chicken and of RCS-rats (a model for Retinitis Pigmentosa), as well as in anesthetised rabbits, pigs and cats with intact retinae. Our investigations show that spatially selective ganglion-cell responses can be recorded following focal electrical stimulation, in healthy and as well in degenerated retinae. Registration of activities in area 17 of the visual cortex demonstrate that electrical retinal stimulation can indeed activate it.

Neural mechanisms of visual feature grouping

The present work relates compositions of visual scenes to signals in visual cortex and to cortical circuit models in order to understand neural mechanisms of perceptual feature grouping. It starts from the hypothesis that synchronization and decoupling of cortical gamma-activities (35-90 Hz) define the relations among visual objects. Here we concentrate on synchronization related to two basic visual situations, (1.) static retinal stimulation during ocular fixation, and (2.) transient stimulation during sudden luminance modulations or shifts in object position. For testing the synchronization hypothesis we investigated signal correlations of multiple micro-electrode recordings in visual cortex areas V1 and V2 of behaving monkeys. Static retinal stimuli induce gamma-activities that are loosely phase-coupled among neighboring neural populations of an object's cortical representation. This can explain why synchronization, measured by spectral coherence, is restricted to few millimeters cortex. Such patches of gamma-synchronization become de-coupled across the representation of an object's contour, and thereby can code figure-ground segregation. Transient stimuli evoke synchronized volleys of stimulus-locked activities that are typically non-rhythmic and include low frequency components in addition to those in the gamma-range. It is argued why stimulus-induced and stimulus-locked phase-coupled activations are both appropriate for supporting perceptual feature grouping. Clues for basic neural mechanisms participating in feature grouping are provided by our biologically motivated simulations of synchronization in cortical structures. (1.) Local populations generate gamma-oscillations via feedback inhibition during states of static retinal stimulation. (2.) Bidirectional facilitatory connections serve for phase-coupling among neighboring neural populations. (3.) Spike transmission delays, increasing with cortical distance, can explain the restriction of gamma-coherence to patches of few millimeters cortex. (4.) The size of synchronization patches in one visual area (e.g., V1) can define the size of classical receptive fields at the consecutive level of visual processing (V2) if Hebbian learning is operative. This may explain the increase in receptive field size at consecutive levels of visual processing. In conclusion, our results and those of others are supportive for the hypothesis that phase-coupled gamma-signals can code feature grouping and object continuity. However, convincing experimental proofs showing directly the dependence of perceptual grouping on cortical phase-coupling are still lacking.

Contour decouples gamma activity across texture representation in monkey striate cortex

Previous work on figure-ground coding in monkey V1 revealed enhanced spike rates within an object's surface representation, synchronization of gamma oscillations (gamma = 35-90 Hz) in object and background regions, but no decrease in signal correlation across the representation of a contour. The latter observation seems to contradict previous statements on the role of gamma-synchronization for scene segmentation. We re-examine these findings by analyzing different coupling measures and frequency ranges of population activities potentially contributing to figure-ground segregation. Multiple unit activity (MUA) and local field potentials (LFPs) were recorded by parallel mu-electrodes in monkey V1 during stimulation by a grating in which an object was defined by a shifted rectangle. In contradiction to the conclusions in previous work, we find strong decoupling of population activity between figure and ground representations compared to the situation in which the object is absent. In particular, coherence of late gamma-LFPs is strongly reduced, while reduction is absent during the early epochs of high-amplitude transients for LFP- and MUA-coherence at all frequencies, and at low frequencies also in the subsequent epochs. Our results of decoupling in late LFP gamma-components among figure and ground representations suggest that these signals may support figure-ground segregation.

Lateral spike conduction velocity in the visual cortex affects spatial range of synchronization and receptive field size without visual experience: a learning model with spiking neurons

Classical receptive fields (cRF) increase in size from the retina to higher visual centers. The present work shows how temporal properties, in particular lateral spike velocity and spike input correlation, can affect cRF size and position without visual experience. We demonstrate how these properties are related to the spatial range of cortical synchronization if Hebbian learning dominates early development. For this, a largely reduced model of two successive levels of the visual cortex is developed (e.g., areas V1 and V2). It consists of retinotopic networks of spiking neurons with constant spike velocity in lateral connections. Feedforward connections between level 1 and 2 are additive and determine cRF size and shape, while lateral connections within level 1 are modulatory and affect the cortical range of synchronization. Input during development is mimicked by spike trains with spatially homogeneous properties and a confined temporal correlation width. During learning, the homogeneous lateral coupling shrinks to limited coupling structures defining synchronization and related association fields (AF). The size of level-1 synchronization fields determines the lateral coupling range of developing level-1-to-2 connections and, thus, the size of level-2 cRFs, even if the feedforward connections have distance-independent delays. AFs and cRFs increase with spike velocity in the lateral network and temporal correlation width of the input. Our results suggest that AF size of V1 and cRF size of V2 neurons are confined during learning by the temporal width of input correlations and the spike velocity in lateral connections without the need of visual experience. During learning from visual experience, a similar influence of AF size on the cRF size may be operative at successive levels of processing, including other parts of the visual system.

Amplitude envelope correlation detects coupling among incoherent brain signals

Cognitive processing involves gamma-activation over broad cortical regions. Phase coupling of these activities has rarely been reported for areas far apart. Other forms of coupling are generally not detected by conventional measures. Here, we use amplitude envelope correlation (AEC), which can detect signal coupling without phase coherence, even among different frequencies. We apply it to subdural recordings from humans performing a visual delayed match-to-sample task and systematically compare it with spectral amplitude and coherence. The different measures often show divergent results. In particular, AEC reveals y-coupling completely missed by coherence. We argue that coherence and AEC are adapted to different cortical mechanisms of short- and long-range interactions, respectively.

Fast oscillations display sharper orientation tuning than slower components of the same recordings in striate cortex of the awake monkey

We wanted to know whether fast oscillations ( approximately 30-80 Hz) in striate cortex of awake monkeys show sharper orientation selectivity than (i) slower components, including spike rate modulations, and (ii) broad-band signals of the same recordings. As fast oscillations are probably of cortical origin this may further clarify whether cortical network mechanisms are substantially involved in generating orientation selectivity. We recorded multi unit activity (MUA) and local field potentials (LFP, 1-140 Hz) by the same microelectrodes from upper layers of macaque striate cortex during visual stimulation with grating textures of different orientations. An orientation index (OI) was derived from the cortical responses in three frequency ranges (low, 0-11.7 Hz; medium, 11.7-31.3 Hz; and fast oscillations, 31.3-62.5 Hz) and for the broad-band LFP and MUA power. (i) Both LFP and MUA fast oscillations reveal a higher orientation index than signal components in the low and medium frequency ranges. (ii) For MUA the orientation index was significantly higher with fast oscillations than for the lower frequency ranges and the initial broad-band transient responses. (iii) LFPs show a significantly higher orientation index only for the fast oscillations during sustained activation compared with their broad-band power during the transient responses. Thus, our main result is the sharper orientation tuning of fast oscillations in spike activities of local populations compared with slower components of the same broad-band recordings. As fast oscillations occur synchronized in the awake monkey's striate cortex we assume that they have enhanced probability of activating successive stages of visual processing and hence contribute to the perception of orientation.

Functional coupling shows stronger stimulus dependency for fast oscillations than for low-frequency components in striate cortex of awake monkey

It has been argued that coupling among the neural signals activated by a visual object supports binding of local features into a coherent object perception. During visual stimulation by a grating texture we studied functional coupling by calculating spectral coherence among pairs of signals recorded in the striate cortex of awake monkeys. Multiple unit activity (MUA) and local field potentials (LFP, 1-140 Hz) were extracted from seven parallel broad band recordings. Spectral coherence was dominated by high-frequency oscillations in the range 35-50 Hz and often by additional low-frequency components (0-12 Hz). Functional coupling among separate cortical sites was more stimulus specific for MUA than for LFP: MUA coherence at high and low frequencies depended highly significantly on: (i) the similarity of the preferred orientations at the two sites - the more similar the higher the coherence; (ii) the orientation of the stimulus grating - with highest coherence at half angle between the preferred orientations at the two sites; (iii) cortical distance - coherence decreases to noise levels at approximately 3 mm (MUA) and 6 mm (LFP). Coherence of fast oscillations did not depend on the degree of coaxiality of the orientation-sensitive receptive fields, whereas low frequencies showed significant dependency. This indicates that different frequency components can engage different coupling networks in the striate cortex which probably support different coding tasks. Changes in average oscillation frequency with stimulus orientation were highly significant for fast oscillations while there was no dependency for low frequencies. Finally, stimulus-related spectral power and coherence of fast oscillations were considerably higher than of low frequency components. Fast oscillations may therefore contribute more to feature binding and coding of object continuity than low-frequency components, at least for texture surfaces as analysed here.

Cortical synchronization suggests neural principles of visual feature grouping

Compositions of visual scenes are related here to neural signals in visual cortex and to cortical circuit models to understand neural mechanisms of perceptual feature grouping. Starting from the hypothesis that synchronization and decoupling of cortical gamma-activities (35-90 Hz) define the relations among visual objects, we concentrate on synchronization related to (1) static retinal stimulation during ocular fixation, and (2) transient stimulation by sudden shifts in object position. The synchronization hypothesis has been tested by analyzing signal correlations in visual cortex of monkeys with the following results: Static retinal stimuli induce loosely phase-coupled gamma-activities among neurons of an object's cortical representation. Patches of gamma-synchronization become decoupled across the representation of an object's contour, and thereby can code figure-ground segregation. Transient stimuli evoke synchronized volleys of stimulus-locked activities that are typically non-rhythmic and include low frequency components in addition to those in the gamma-range. It is argued that stimulus-induced and stimulus-locked synchronizations may play different roles in perceptual feature grouping.

Visual stimulation elicits locked and induced gamma oscillations in monkey intracortical- and EEG-potentials, but not in human EEG

Stimulus-related fast oscillations in the gamma-range (30-100 Hz) were clearly demonstrated with microelectrode recordings in visual cortex of awake monkeys, and they were also reported for recordings of human electroencephalograms (EEG). However, the presence of stimulus-related gamma-modulation in human EEG has repeatedly been disputed. To clarify this dispute, we recorded the scalp EEG of man and monkey as well as intracortical field potentials (LFP) from monkey primary visual cortex (V1) during identical visual stimulation (large-field sinusoidal gratings, which proved to induce the largest gamma-amplitudes in monkey V1 and V2). We found a strong stimulus-related increase of gamma-oscillations in monkey LFP and EEG, but no modulation of gamma-activity in human EEG. In contrast to previous results, gamma-oscillations in the monkey were strongly phase-locked to stimulus onsets in early response periods (80-160 ms) and became gradually independent in later periods. Our negative result on gamma-modulation in human subjects contradicts several published findings. We conclude from our results that visually evoked gamma-modulations in humans EEG are not as accessible as in the monkey.

Phase correlation among rhythms present at different frequencies: spectral methods, application to microelectrode recordings from visual cortex and functional implications

In classical EEG analysis rhythms with different frequencies occurring at separable regions and states of the brain are analysed. Rhythms in different frequency bands have often been assumed to be independent and their occurrence was interpreted as a sign of different functional operations. Independence has scarcely been proved because of conceptual and computational difficulties. It is, on the other hand, probable that different rhythmic brain processes are coupled because of the broad recurrent connectivity among brain structures. We, therefore, set out to find interactions among rhythmic signals at different frequencies. We were particularly interested in interactions between lower frequency bands and gamma-activities (30-90 Hz), because the latter have been analysed in our laboratory in great detail and had properties suggesting their involvement in perceptual feature linking. Fast oscillations occurred synchronized in a stimulus-specific way in the visual cortex of cat and monkey. Their presence was often accompanied by lower frequency components at considerable power. Such multiple spectral peaks are known from many cortical and subcortical structures. Despite their well known occurrence, coupling among different frequencies has not been established, apart from harmonic components. For the present investigation we extended existing analytical tools to detect non-linear correlations among signal pairs at any frequency (including incommensurate ones). These methods were applied to multiple microelectrode recordings from visual cortical areas 17 and 18 of anesthetized cats and V1 of awake monkeys. In particular, we assessed non-linear correlations by means of higher order spectral analysis of multi-unit spike activities (MUA) and local slow wave field potentials (LFP, 1-120 Hz) recorded with microelectrodes. Non-linear correlations among signal components at different frequencies were investigated in the following steps. First, the frequency content of short (approximately 250 ms) sliding window signal epochs was analyzed for simultaneously occurring rhythms of significant power at different frequencies. This was done by a newly developed method derived from the trispectrum using separate averaging of the products of short-epoch power spectra for any possible combination of frequency pairs. Second, non-linear (quadratic) phase coupling between different frequencies was assessed by the methods of bispectrum and bicoherence. We found phase correlations at different frequencies in the visual cortex of the cat and monkey. These couplings were significant in about 60% of the investigated MUA and LFP recordings, including several cases of coupling among incommensurate (i.e. non-harmonic) frequencies. Significant phase correlations were present: (1) within the gamma-frequency range; (2) between gamma- and low frequency ranges (1-30 Hz, including alpha- and beta-rhythms); and (3) within the low frequency range. Phase correlations depended, in most cases, on specific visual stimulation. We discuss the possible functional significance of phase correlations among high and low frequencies by including proposals from previous work about potential roles of single-frequency rhythms of the EEG. Our suggestions include: (1) visual feature linking across different temporal and spatial scales provided by coherent oscillations at high and low frequencies; (2) linking of visual cortical representations (high frequencies) to subcortical centers (low frequencies) like the thalamus and hippocampus; and (3) temporal segmentation of the sustained stream of incoming visual information into separate frames at different temporal resolutions in order to prevent perceptual smearing due to shifting retinal images. These proposals are, at present, merely speculative. However, they can, in principle, be proved by microelectrode recordings from trained behaving animals.

Parallel processing by a homogeneous group of coupled model neurons can enhance, reduce and generate signal correlations

Correlated activities have been proposed as correlates of flexible association and assembly coding. We addressed the basic question of how signal correlations on parallel pathways are enhanced, reduced and generated by homogeneous groups of coupled neurons, and how this depends on the input activities and their interactions with internal coupling processes. For this we simulated a fully connected group of identical impulse-coded neurons with dynamic input and threshold processes and additive or multiplicative lateral coupling. Input signals were Gaussian white noise (GWN), completely independent or partially correlated on a subgroup of the parallel inputs. We show that in states of high average spike rates input-output correlations were weak while the network could generate correlated activities of stochastic, oscillatory and rhythmic bursting types depending exclusively on lateral coupling strength. In states of low average spike rates input-output correlations were high and the network could effectively enhance or reduce differences in spatial correlation applied to its parallel inputs. The correlation differences were more pronounced with multiplicative lateral coupling than with the additive interactions commonly used. As the different modes of correlation processing emerged already by global changes in the average spike rate and lateral coupling strength, we assume that in real cortical circuits changes in correlational processing may also be induced by unspecific modulations of activation and lateral coupling.

Stimulus-dependent modulations of correlated high-frequency oscillations in cat visual cortex

The hypothesis that correlated neural activity is involved in the cortical representation of visual stimuli was examined by recording multi-unit activity and local field potentials from neurons with non-overlapping receptive fields in areas 17 and 18. Using coherence functions, correlations of oscillatory patterns (35-100 Hz) of neural signals were investigated under three stimulus conditions: (i) a whole field grating or a long bar moving across both receptive fields; (ii) masking the region between both receptive fields while stimulating the remaining visual field; and (iii) two separate stimuli simultaneously moving in opposite directions. Coherences of oscillations were found to be significantly higher in the first stimulus condition than in the other two conditions. Since different visual stimuli were reflected in the coherence of neural activity, we concluded that correlated neural activity is a potential candidate for coding of sensory information.

Inhibition of sustained gamma oscillations (35-80 Hz) by fast transient responses in cat visual cortex

Interactions between stimulus-induced oscillations (35-80 Hz) and stimulus-locked nonoscillatory responses were investigated in the visual cortex areas 17 and 18 of anaesthetized cats. A single square-wave luminance grating was used as a visual stimulus during simultaneous recordings from up to seven electrodes. The stimulus movement consisted of a superposition of a smooth movement with a sequence of dynamically changing accelerations. Responses of local groups of neurons at each electrode were studied on the basis of multiple unit activity and local slow field potentials (13-120 Hz). Oscillatory and stimulus-locked components were extracted from multiple unit activity and local slow field potentials and quantified by a combination of temporal and spectral correlation methods. We found fast stimulus-locked components primarily evoked by sudden stimulus accelerations, whereas oscillatory components (35-80 Hz) were induced during slow smooth movements. Oscillations were gradually reduced in amplitude and finally fully suppressed with increasing amplitudes of fast stimulus-locked components. It is argued that suppression of oscillations is necessary to prevent confusion during sequential processing of stationary and fast changing retinal images.

Different rules of spatial summation from beyond the receptive field for spike rates and oscillation amplitudes in cat visual cortex

We measured spike rates in parallel with visually induced oscillations of multi-unit activity (MUA) and local field potentials (LFP) from cortical areas 17 and 18 of anesthetized cats. Variations in the three response types were systematically correlated with stimulus size and placement. Oscillation amplitudes of both MUA and LFP were on average low with stimuli covering just the receptive field and they increased progressively with larger stimuli, whereas average spike rates rather decreased monotonically with stimulus sizes beyond the receptive field (area 18) or reached a plateau with stimuli in the far surround (area 17). Thus, spike rates and oscillation amplitudes follow different rules of spatial summation. Since the spatial spread of the synchronized components of oscillations roughly matches the horizontal divergence zone of the pyramidal cells' axonal collaterals in area 17 and 18, the interconnected system of neighbouring columns seems to constitute a functional unit, within which the oscillations could exert their functional role.

Synchronous high-frequency oscillations in cat area 18

The present study extends knowledge of the basic properties of correlated oscillatory activity patterns in the visual cortex of anaesthetized cats. Recordings with multiple electrodes were performed in area 18 and the correlations of multi-unit activity in the frequency range 35-80 Hz were determined using the coherence function. Statistical analysis revealed that the multi-unit correlations depended on the cortical distance between the recording sites, the orientation selectivity of the neurons and their cortical layer. On average, correlations dropped to chance level within several millimetres and were higher in lower than in upper cortical layers. Similar results were found by analysing the correlations of oscillatory patterns in local field potentials recorded from the same electrodes. Correlations of neurons with similar orientation preferences were higher than those of neurons with different orientation preferences. Comparison to a matched sample from area 17 showed that the correlations in areas 18 and 17 depended on similar properties of the neurons. The dependences of correlated oscillations resembled the known pattern and specificity of intra-areal fibre connections, suggesting that the correlations were intracortically established. Since correlations were specifically and not randomly related to the response properties of cortical neurons and were prominent in a visual area other than area 17, the findings suggest that correlated oscillatory activity provides a potential neural code supporting sensory information processing.

Stimulus-specific fast oscillations at zero phase between visual areas V1 and V2 of awake monkey

Synchronization of fast cortical oscillations (35-90 Hz) has been proposed as a basis of sensory integration. This hypothesis requires stimulus specific oscillations that occur synchronously in different cortical areas of awake animals. Here, we demonstrate the presence of, and phase-locking between, high amplitude stimulus specific oscillations (50-90 Hz) in striate (V1) and extra striate (V2) visual cortex of an awake monkey. Oscillations of multiple unit spikes and local field potentials occurred with an average V1-V2 phase difference near zero. This finding was unexpected because V1 and V2 are thought to be serially arranged in the primate's visual processing stream. However, near zero-phase synchronization among cortical areas might enable fast and effective communication via the many reciprocal cortico-cortical connections for processes such as sensory integration.

Oscillatory and non-oscillatory synchronizations in the visual cortex and their possible roles in associations of visual features

It was postulated that the perceived association of visual features is based on the synchronization of those neural signals that are activated by a coherent visual object. Two types of synchronized cortical signals were found by us in cat and monkey visual cortex, and were proposed as candidates for feature association: (1) stimulus-locked signals, evoked by transient retinal stimulation, and typically non-rhythmic; (2) oscillatory signals, induced by sustained stimuli, and typically not locked in their oscillation phases to stimulus events. Both types of signals can occur synchronously in those neurons that are activated by a common stimulus. Synchronized activities were found in paired recordings within vertical cortex columns, in separate columns of the same cortical area, and even between different cortical areas or hemispheres. The average phase difference between such common oscillatory events was typically close to zero (< 1 msec mean +/- 2 msec S.D.). For the dependence of synchronization from stimulus and receptive field properties, a preliminary 'rule' can be given: the coherence of fast oscillations in separate cortical assemblies depends inversely on the 'coding distance' between the assemblies' RF properties, but directly on the degree of overlap between the assemblies' respective coding properties and the features of a common stimulus. This means that oscillatory events in any two assemblies, in the same or in different cortical areas or hemispheres, are more closely correlated the more similar are their receptive field properties, and the better a common stimulus activates the assemblies simultaneously. Our results can explain some neural mechanisms of perceptual feature-linking, including mutual enhancement among similar, spatially and temporally dispersed features, definitions of spatial and temporal continuity, scene segmentation, and figure-ground discrimination. We further propose that mutual enhancement and synchronization of cell activities are general principles of temporal coding by assemblies, that are also used within and among other sensory modalities as well as between cortical sensory and motor systems.

A new method for the insertion of multiple microprobes into neural and muscular tissue, including fiber electrodes, fine wires, needles and microsensors

We developed a new method for the insertion of thin-shaft probes into neural and muscular tissue. Axial forces for driving the probes into tissue and radial forces against buckling are both provided by a stretched elastic rubber tube in which the probe is guided outside the tissue. Various geometric arrangements of arrays with independently advanceable probes are possible. Prototypes with 7 linearly aligned fiber electrodes and computer-controlled positioning motors were successfully used in single- and multiple-unit recordings from the visual system of awake monkeys (Eckhorn et al., 1993). The method is suitable in a wide range of applications, including insertion of fine microprobe fibers and wire electrodes into brain and muscle through the skin or dura, provided that the tips of the probes are sharp and hard enough.

Single neurons are differently involved in stimulus-specific oscillations in cat visual cortex

Synchronised oscillatory population events (35-80 Hz; 60-300 ms) can be induced in the visual cortex of cats by specific visual stimulation. The oscillatory events are most prominent in local slow wave field potentials (LFP) and multiple unit spikes (MUA). We investigated how and when single cortical neurons are involved in such oscillatory population events. Simultaneous recordings of single cell spikes, LFP and MUA were made with up to seven microelectrodes. Three states of single cell participation in oscillations were distinguished in spike triggered averages of LFP or MUA from the same electrode: (1) Rhythmic states were characterised by the presence of rhythmicity in single cell spike patterns (35-80 Hz). These rhythms were correlated with LFP and MUA oscillations. (2) Lock-in states lacked rhythmic components in single cell spike patterns, while spikes were phase-coupled with LFP or MUA oscillations. (3) During non-participation states LFP or MUA oscillations were present, but single cell spike trains were neither rhythmic nor phase coupled to these oscillations. Stimulus manipulations (from "optimal" to "suboptimal" for the generation of oscillations) often led to systematic transitions between these states (from rhythmic to lock-in to non-participation). Single cell spike coupling was generally associated with negative peaks in LFP oscillations, irrespective of the cortical separation of single cell and population signals (0-6 mm). Our results suggest that oscillatory cortical population activities are not only supported by local and distant neurons with rhythmic spike patterns, but also by those with irregular patterns in which some spikes occur phase-locked to oscillatory events.

Construction of concepts by the nervous system: from neurons to cognition

Neurophysiological studies have recently identified a pattern of synchronized slow-wave activity in the visual cortex which characteristically encompasses groups of neurons activated by similar or closely related stimulus attributes. This slow-wave activity appears to tag clusters of neurons to form aggregates representing in their totality more complex, higher-order stimulus attributes across disparate positions in the cortical representation. The notion is advanced that the function of these aggregates is analogous to that ascribed to the subsymbolic computational level in connectionist networks. On this basis, the argument is presented that the synchronized cortical activity is an important aspect of the construction of symbolic representation by the nervous system and, thus, a step from neural information processing to the symbolic processes stipulated by classical cognitivism.

High frequency (60-90 Hz) oscillations in primary visual cortex of awake monkey

It has been proposed that synchronized oscillations play a key role in perceptual feature linking and sensory integration. This idea was supported by the discovery of strongly synchronized stimulus-specific oscillations in the visual cortex of anaesthetized cats. The 'synchronization hypothesis' was controversial because in the visual cortex of awake monkeys either only weak or no oscillations were found. We have now recorded high amplitude synchronized oscillation at the level of spike activity and local field potential from the primary visual cortex of an awake monkey. The dominant frequencies (70-80 Hz) were considerably higher than those observed previously in cats and monkeys (30-50 Hz). However, stimulus specificities of the oscillations were comparable to and amplitudes even higher than those in cats.

The RF-cinematogram. A cross-correlation technique for mapping several visual receptive fields at once

We present a spike-triggered averaging method capable of mapping the visual receptive fields of several neurons simultaneously. The stimulation is general and the mapping proceeds automatically without the need to match the stimulation to the cells' preference for position, orientation, direction, etc. The maps are spatiotemporal; receptive field (RF) structures are quantitatively determined in three dimensions: the two dimensions of visuotopic space, and time. The method presented is one of a family of "reverse correlation" or "spike-triggered averaging" techniques (DeBoer and Kuyper 1968) capable of revealing linear aspects of stimulus-response coupling. The formal relationship of these methods to stimulus-response cross-correlation is shown. The analysis is extended to provide some second-order axis-of-motion information ("direction marks"). The stimulus is a constantly illuminated, randomly jumping bright or dark spot, not an elongated bar. Spot diameters between one-third to 1 x RF width are effective. The method ascertains for each recorded action potential or "spike" the prior visual field position of the spot. The average or most probable spot positions define the receptive field spatially. Repeating the process for a succession of times prior to observed spikes defines the field temporally, presented here as a succession of spatial maps. We term this portrayal a receptive field cinematogram, RFc or ciné. The RFc reveals and economically portrays the spread of excitability and suppression across the receptive field, culminating in the generation of a spike. RFcs for LGN neurons and for simple cells recorded in cat cortical areas 17 and 18 are presented and interpreted in terms of classic ON/OFF regions. The availability of temporal information permits the separation of an excitatory exit response, generated when a moving bright spot leaves an OFF region, from an excitatory entrance response occurring when a bright spot enters an ON region, because these responses occur at different times (exit responses earlier). Spike emission remains coupled to (cross-correlated with) stimulus events over time periods as long as 96 ms, implying that some stimulus drive or afferent visual input is delayed by as much as 96 ms more than other input. This is a striking instance of temporal dispersion in the visual system. In some cells, said to be "spatiotemporally inseparable", the delay (latency) varies systematically across the visual field; i.e., the place for optimal stimulation varies with the time prior to spike emission. In these cells, the RFc shows receptive field structures which move across the visual field over trajectories equal to approximately twice the total conventional RF width.(ABSTRACT TRUNCATED AT 400 WORDS)

Complementary global maps for orientation coding in upper and lower layers of the cat striate cortex and their possible functions

Preferred stimulus orientations of striate cortical cells in the cat were analyzed for possible isotropic or anisotropic distributions. We separated the data twice, into central (0-5 degrees) vs. peripheral subgroups and into upper vs. lower layer cells. In the central group, absolute orientations were counted; in the periphery, a radial test was adopted by normalizing the preferred stimulus orientation of a cell to the line connecting the receptive field center to the retinal center. We found that in the center, vertical and horizontal orientations are overrepresented. In the periphery, the histograms show complementary anisotropies for upper and lower layers, favoring a map for radial orientation detection in upper layers and a more concentric map for orientation detection in lower layers. These results are possibly related to the probabilities of different optic flow fields on the retina under natural conditions of stimulation. They are discussed as possible neuronal structures supporting figure-ground discrimination, the distinction of self motion from object motion, and the location of objects in three-dimensional space.