Visual cortex neurons phase-lock selectively to subsets of LFP oscillations.

It is generally thought that apart from receptive field differences, such as preferred orientation and spatial frequency selectivity, primary visual cortex neurons are functionally similar to each other. However, the genetic diversity of cortical neurons plus the existence of inputs additional to those required to explain known receptive field properties might suggest otherwise. Here we report the existence of desynchronized states in anesthetized cat area 17 lasting up to 45 min, characterized by variable narrow-band local field potential (LFP) oscillations in the range 2-100 Hz and the absence of a synchronized 1/f frequency spectrum. During these periods, spontaneously active neurons phase-locked to variable subsets of LFP oscillations. Individual neurons often ignored frequencies that others phase-locked to. We suggest that these desynchronized periods may correspond to REM sleep-like episodes occurring under anesthesia. Frequency-selective codes may be used for signaling during these periods. Hence frequency-selective combination and frequency-labeled pathways may represent a previously unsuspected dimension of cortical organization. NEW & NOTEWORTHY We investigated spontaneous neuronal firing during periods of desynchronized local field potential (LFP) activity, resembling REM sleep, in anesthetized cats. During these periods, neurons synchronized their spikes to specific phases of multiple LFP frequency components, with some neurons ignoring frequencies that others were synchronized to. Some neurons fired at phase alignments of frequency pairs, thereby acting as phase coincidence detectors. These results suggest that internal brain signaling may use frequency combination codes to generate temporally structured spike trains.

How different feature spaces may be represented in cortical maps.

This paper explores how different high-dimensional feature spaces might be represented in cortical maps, subject to continuity and completeness constraints. Spaces explored included products of circular variables (such as orientation), products of linear dimensions encoding scalar features (such as spatial frequency) and products of binary features. Maps were generated using the Kohonen algorithm, with uniform or non-uniform stimulus distributions. A 2D retina was always assumed to be present. Simulations were run with and without annealing. For uniform input distributions, coverage uniformity (Swindale 1991 Biol. Cybern. 65 415-24) was used to measure how well the map was able to represent the feature space. For non-uniform distributions a weighted measure of coverage uniformity was calculated. Good coverage could be achieved for up to five or six cyclic variables but was substantially worse for a similar number of uniformly distributed scalar features. For annealed maps of multi-dimensional stimuli with Gaussian distributions, the distribution of receptive field centres and the distribution of total activity evoked on the cortex matched the stimulus distribution well. For annealed maps of non-uniformly distributed binary features there was an approximately linear relationship between the area of a map devoted to a specific feature and the probability of occurrence of the feature during development. Deviations from uniform retinotopy often led to improved coverage.

The auditory motion aftereffect: its tuning and specificity in the spatial and frequency domains.

In this paper, the auditory motion aftereffect (aMAE) was studied, using real moving sound as both the adapting and the test stimulus. The sound was generated by a loudspeaker mounted on a robot arm that was able to move quietly in three-dimensional space. A total of 7 subjects with normal hearing were tested in three experiments. The results from Experiment 1 showed a robust and reliable negative aMAE in all the subjects. After listening to a sound source moving repeatedly to the right, a stationary sound source was perceived to move to the left. The magnitude of the aMAE tended to increase with adapting velocity up to the highest velocity tested (20 degrees/sec). The aftereffect was largest when the adapting and the test stimuli had similar spatial location and frequency content. Offsetting the locations of the adapting and the test stimuli by 20 degrees reduced the size of the effect by about 50%. A similar decline occurred when the frequency of the adapting and the test stimuli differed by one octave. Our results suggest that the human auditory system possesses specialized mechanisms for detecting auditory motion in the spatial domain.

How many maps are there in visual cortex?

In addition to a topographic map of the retina, mammalian visual cortex contains superimposed, orderly periodic maps of features such as orientation, eye dominance, direction of motion and spatial frequency. There is evidence that these maps are overlaid so as to ensure that all combinations of the different parameters are represented as uniformly as possible across visual space. However, it is unknown to what extent geometrical factors limit the number of periodic maps which might simultaneously be present, given this constraint. This paper attempts to investigate the question by using a dimension reduction model to generate maps of simple, many- dimensional feature spaces onto a model two-dimensional cortex. The feature space included a model retina, plus N binary variables, corresponding to parameters such as ocular dominance or spatial frequency. The results suggest that geometrical factors do not sharply limit the ability of the cortex to represent combinations of parameters in spatially superimposed maps of similar periodicity. Considerations of uniform coverage suggest an upper limit of six or seven maps. A higher limit, of about nine or ten, may be imposed by the numbers of neurons (or minicolumns) available to represent each of 2(N) features within a given small region of cortex.

Visual cortex maps are optimized for uniform coverage.

Cat visual cortex contains a topographic map of visual space, plus superimposed, spatially periodic maps of ocular dominance, spatial frequency and orientation. It is hypothesized that the layout of these maps is determined by two constraints: continuity or smooth mapping of stimulus properties across the cortical surface, and coverage uniformity or uniform representation of combinations of map features over visual space. Here we use a quantitative measure of coverage uniformity (c') to test the hypothesis that cortical maps are optimized for coverage. When we perturbed the spatial relationships between ocular dominance, spatial frequency and orientation maps obtained in single regions of cortex, we found that cortical maps are at a local minimum for c'. This suggests that coverage optimization is an important organizing principle governing cortical map development.

Automated analysis of normal and glaucomatous optic nerve head topography images.

PURPOSE: To classify images of optic nerve head (ONH) topography obtained by scanning laser ophthalmoscopy as normal or glaucomatous without prior manual outlining of the optic disc. METHODS: The shape of the ONH was modeled by a smooth two-dimensional surface with a shape described by 10 free parameters. Parameters were adjusted by least-squares fitting to give the best fit of the model to the image. These parameters, plus others derived from the image using the model as a basis, were used to discriminate between normal and abnormal images. The method was tested by applying it to ONH topography images, obtained with the Heidelberg Retina Tomograph, from 100 normal volunteers and 100 patients with glaucomatous visual field damage. RESULTS: Many of the parameters derived from the fits differed significantly between normal and glaucomatous ONH images. They included the degree of surface curvature of the disc region surrounding the cup, the steepness of the cup walls, the goodness-of-fit of the model to the image in the cup region, and measures of cup width and cup depth. The statistics of the parameters were analyzed and were used to construct a classifier that gave the probability, P(G), that each image came from the glaucoma population. Images were classified as abnormal if P(G) > 0.5. The probabilities assigned to each image were in most cases close to 0 (normal) or 1 (abnormal). Eighty-seven percent of the sample was confidently classified with P(G) < 0.3 or P(G) > 0.7. Within this group, the overall classification accuracy was 92%. The overall accuracy of the method (the mean of sensitivity and specificity, which were similar) in the whole sample was 89%. CONCLUSIONS: ONH images can be classified objectively and dependably by an automated procedure that does not require prior manual outlining of disc boundaries.

Can Hebbian volume learning explain discontinuities in cortical maps?

It has recently been shown that orientation and retinotopic position, both of which are mapped in primary visual cortex, can show correlated jumps (Das & Gilbert, 1997). This is not consistent with maps generated by Kohonen's algorithm (Kohonen, 1982), where changes in mapped variables tend to be anticorrelated. We show that it is possible to obtain correlated jumps by introducing a Hebbian component (Hebb, 1949) into Kohonen's algorithm. This correspondents to a volume learning mechanism where synaptic facilitation depends not only on the spread of a signal from a maximally active neuron but also requires postsynaptic activity at a synapse. The maps generated by this algorithm show discontinuities across which both orientation and retinotopic position change rapidly, but these regions, which include the orientation singularities, are also aligned with the edges of ocular dominance columns, and this is not a realistic feature of cortical maps. We conclude that cortical maps are better modeled by standard, non-Hebbian volume learning, perhaps coupled with some other mechanism (e.g., that of Ernst, Pawelzik, Tsodyks, & Sejnowski, 1999) to produce receptive field shifts.

Receptive field and orientation scatter studied by tetrode recordings in cat area 17.

The receptive-field positions and orientation preferences of neurons occupying the same tangential location in visual cortex are thought to be similar but to have an associated random scatter. However, previous estimates of this scatter may have been inflated by the use of subjective plotting methods, sequential recording of single units, and residual eye movements. Here we report measurements of receptive-field position and orientation scatter in cat area 17 made with tetrodes, which were able to simultaneously isolate and record up to 11 nearby neurons (ensembles). We studied 355 units at 72 sites with moving light and dark bars. Receptive-field sizes and positions were estimated by least-squares fitting of Gaussians to response profiles. We found that receptive-field position scatter was about half of the ensemble average receptive-field size. We confirmed previous estimates of orientation scatter, but calculations suggested that much of it may be accounted for by anatomical scatter in the positions of recorded neurons relative to the tetrode in a smooth map. Orientation tuning width was positively correlated with the degree of orientation scatter. Scatter was not independent in the two eyes: deviations from the local mean for both preferred orientation and receptive-field position were correlated although a significant amount of residual inter-ocular orientation and receptive-field position scatter was present. We conclude that cortical maps of orientation and receptive-field position are more ordered than was previously thought, and that random scatter in receptive-field positions makes a relatively small contribution to cortical point image size.

Cortical organization: modules, polymaps and mosaics.

Recent studies of functional maps in the mammalian visual cortex fail to support the widespread belief that the cortex contains millimetre-sized modules; instead, they reveal a more fluid arrangement in which several separate maps are superimposed, with relatively weak geometric linkages and no common modular subunit.

Orientation tuning curves: empirical description and estimation of parameters.

This paper compares the ability of some simple model functions to describe orientation tuning curves obtained in extracellular single-unit recordings from area 17 of the cat visual cortex. It also investigates the relationships between three methods currently used to estimate preferred orientation from tuning curve data: (a) least-squares curve fitting, (b) the vector sum method and (c) the Fourier transform method (Wörgötter and Eysel 1987). The results show that the best fitting model function for single-unit orientation tuning curves is a von Mises circular function with a variable degree of skewness. However, other functions, such as a wrapped Gaussian, fit the data nearly as well. A cosine function provides a poor description of tuning curves in almost all instances. It is demonstrated that the vector sum and Fourier methods of determining preferred orientation are equivalent, and identical to calculating a least-square fit of a cosine function to the data. Least-squares fitting of a better model function, such as a von Mises function or a wrapped Gaussian, is therefore likely to be a better method for estimating preferred orientation. Monte-Carlo simulations confirmed this, although for broad orientation tuning curves sampled at 45 degree intervals, as is typical in optical recording experiments, all the methods gave similarly accurate estimates of preferred orientation. The sampling interval, the estimated error in the response measurements and the probable shape of the underlying response function all need to be taken into account in deciding on the best method of estimating referred orientation from physiological measurements of orientation tuning data.

ROC analysis of Heidelberg Retina Tomograph optic disc shape measures in glaucoma.

OBJECTIVE: To determine the capacity of the Heidelberg Retina Tomograph (HRT) optic disc shape measures to detect glaucomatous damage. DESIGN: Prospective study. SETTING: Department of ophthalmology at a university-affiliated hospital in Vancouver. PATIENTS: Ninety-seven consecutive patients from the glaucoma centre and 129 healthy subjects selected from volunteers and employees of the department. One eye of each subject was chosen randomly. OUTCOME MEASURES: Visual fields, as assessed with the Humphrey perimeter, program 30-2, and 12 HRT shape characteristics: disc area, cup area cup/disc area ratio, rim area, cup volume, rim volume, mean cup depth, maximum cup depth, cup shape measure, height variation contour, mean retinal nerve fibre layer thickness and retinal nerve fiber layer cross-section area. ROC (receiver operating characteristic) curves were used to analyse the capacity of each HRT characteristic to detect glaucoma. RESULTS: Statistically significant difference were found between the control and glaucoma groups in age, cup area, cup/disc ratio, rim area, cup volume, rim volume, mean cup depth, cup shape measure, mean retinal nerve fibre layer thickness and retinal nerve fibre layer cross-section area (p < or = 0.003, t-test). The largest (i.e., best) ROC curve area was found for cup shape measure (area = 0.812), rim area (0.809), cup/disc area ratio (0.804) and rim volume (0.768). The mean reference height was 0.31 mm (standard deviation [SD] 0.14 mm) for the control group and 0.29 mm (SD 0.12 mm) for the glaucoma group, a nonsignificant difference. CONCLUSIONS: Cup shape measure was the most predictive HRT shape characteristic.

Visual cortex: a cat's-eye view of the visual system.

Optical imaging data show that the cat's visual cortex contains patches of cells that respond to low spatial and high temporal frequencies; outside the patches, cells respond to high spatial and low temporal frequencies. The results suggest a possible anatomical substrate for psychophysically defined spatial frequency channels.

Sector-based analysis of optic nerve head shape parameters and visual field indices in healthy and glaucomatous eyes.

PURPOSE: To evaluate the correlations between the sector optic nerve head parameters measured by Heidelberg Retina Tomograph (HRT, Heidelberg Engineering, Heidelberg, Germany), version 1.11S, and the visual field. METHODS: One eye was randomly chosen from 55 individuals with glaucoma and 50 healthy individuals. Each participant had at last one Humphrey visual field, program 30-2 (Humphrey Instruments, San Leandro, CA, U.S.A.), and three 10 degrees HRT pictures. From the mean of the three HRT pictures, global measurements, superior (45 degrees-135 degrees), nasal (135 degrees-225 degrees), inferior (225 degrees-315 degrees), and temporal (315 degrees-45 degrees) sector measurements were calculated for the following parameters: disc area, effective area, area below reference, mean height of contour, volume below surface, volume above surface, volume below reference, volume above reference, and third moment. From the visual field results, mean deviation (MD), superior MD, and inferior MD were calculated. For each HRT parameter we calculated the "r" Pearson correlation with the corresponding visual field measures. RESULTS: Within the combined healthy and glaucomatous groups we found highly significant (p < 0.001) correlations between the following HRT parameters and the visual field MD: inferior and mean high of contour (r = -0.53), inferior and third moment (r = -0.52), global and third moment (r = -0.49), inferior and volume above reference (r = 0.47), superior and third moment (r = -0.46), and superior and area below reference (r = -0.44). Correlations between global mean deviation and nasal or temporal sector parameters were generally smaller and less significant. CONCLUSIONS: Inferior and superior HRT sector parameters were correlated with the respective visual field indices. In many cases these correlations were as strong or stronger than with the global equivalent shape measures.

Comparison of psychophysical and electrophysiological testing in early glaucoma.

PURPOSE: To compare the sensitivity and specificity of a wide range of psychophysical and electrophysiological tests in the detection of early glaucomatous damage. METHODS: Forty-three normals and 43 patients with early glaucoma, some still without field defects, were tested with differential light threshold perimetry, short-wavelength automated perimetry, high-pass resolution perimetry, motion detection, flicker contrast sensitivity, flickering and isoluminantly matched letter tests, and pattern and flash electroretinography, including photopic, scotopic, oscillatory potentials, and 30 Hz flicker. Receiver operating characteristic analysis was applied to continuous variables derived from each of the tests. RESULTS: Most parameters reflected glaucomatous loss to some degree, even though only single variables were analyzed separately in the receiver operating characteristic analysis. The pattern electroretinogram and some of the letter acuity tests had the best sensitivity and specificity, followed by short-wavelength automated perimetry and high-pass resolution-perimetry. Motion detection, flicker contrast, and flash electroretinogram parameters scored poorly. Six patients with normal results on the Humphrey field test had abnormal results on many of the other tests. CONCLUSIONS: Applying different psychophysical and electrophysiological tests may add to our ability to detect early glaucomatous damage.

Contrast sensitivity for flickering and static letters and visual acuity at isoluminance in glaucoma.

PURPOSE: Psychophysical testing was used to assess the effects of early glaucomatous damage on visual functions mediated by the magnocellular and parvocellular pathways. METHODS: We studied 57 eyes with early glaucoma, 92 eyes suspect for glaucoma, and 88 normal eyes. Tests were designed to target the functions of the magnocellular (M) or the parvocellular (P) pathways, and included measurements of contrast sensitivity for letter recognition, using both static presentation, and counterphase modulation at 25 Hz. We assumed that ability to perform the latter task would depend on the integrity of the M-cell pathway. To evaluate P-cell function we measured spatial acuity for a green letter presented against an isoluminant white background. Tests were carried out in the fovea, and at 3 degrees nasal, superior, temporal and inferior to the fovea. RESULTS: Glaucomatous eyes showed, on average, defects in almost all of the functions tested. Threshold elevations for static and isoluminant presentations were generally as large as those for flickering stimuli. The inferior paracentral part of the retina was generally more severely affected than the other locations tested. The defects observed in the suspect population were similar to, although smaller in magnitude than, those found in eyes with glaucoma. A discriminant analysis identified a subset of five tests, including P as well as M pathway tests, which was able to correctly classify 89% of normal and 79% of glaucomatous eyes. CONCLUSIONS: Putative M- and P-cell functions are both impaired at a relatively early stage of glaucoma. Deficits for both types of test are most severe in the inferior paracentral area of the retina (superior visual fields). Psychophysically based detection of the condition can benefit if results from different tests, done at different retinal locations, are combined.

The development of topography in the visual cortex: a review of models.

The repetitive stochastic patterns of eye dominance and orientation preference found in the mammalian visual cortex have attracted much attention from theoretical neurobiologists during the last two decades. Reasons for this include the visually intriguing nature of the patterns and the fact that many aspects of their development seem likely to be dependent upon both spontaneous and visually driven patterns of neural activity. Understanding these processes holds out the promise that general theories of learning and memory may be derived from those found to be applicable to the visual cortex. It has turned out, in fact, that remarkably simple models, based on Hebbian synaptic plasticity, intracortical interactions and competitive interactions between cells and growing axons, have been able to explain much of the phenomenology. This article reviews the models of topographic organization in the visual cortex in a roughly historical sequence, beginning with von der Malsburg's paper 1973 paper in Kybernetik on self-organization of orientation selectivity. The principles on which each of the models is based are explained, and the plausibility of each model and the extent to which it is able to account for the relevant experimental data are evaluated. Attention is drawn to the underlying similarities and differences between the models and suggestions are made for future directions in research.