Visual contrast gain control in migraine: measures of visual cortical excitability and inhibition

The present study examined the extent to which migraineurs demonstrate interictal visual cortical hyperexcitability as a result of poor inhibitory control in the visual system. We employed a well-established psychophysical measure of inhibition, visual contrast gain control. The task involved detecting a briefly presented target that was superimposed on a highly excitable high contrast masking pattern. The strength of inhibition was assessed by comparing target detection thresholds with and without the operation of gain controls. Migraineurs with and without aura (n=25, n=22, respectively) were compared with those with no history of migraine (n=25). Our results do not indicate a loss of inhibition in migraine; the strength of inhibitory feedback contrast gain controls was similar between migraineurs and controls. We did however, find a statistically greater masking effect in migraineurs compared with controls in the zero delay condition, suggesting cortical hyperexcitability in migraine. Possible mechanisms of cortical hyperexcitability are discussed in light of the results.

Orientation discrimination thresholds in migraine: a measure of visual cortical inhibition

Orientation discrimination is a visual task dependent on inhibitory mechanisms in the visual cortex. In this study, orientation discrimination thresholds for bar and grating patterns were measured at two visual field locations in subjects with migraine with (n = 20) and without aura (n = 20) and in migraine-free control subjects (n = 20). No statistically significant differences were found between migraine groups and the control group on either task at foveal or peripheral visual field locations. No significant correlations were found between psychophysical thresholds and age, total lifetime auras or total lifetime migraine episodes. However, a trend was seen toward slightly impaired performance on the two foveal tasks in a subgroup of subjects with the highest total lifetime aura count. Thus we have found no convincing evidence that impaired cortical inhibitory mechanisms are a predisposing characteristic in migraine, but cannot rule out the possibility that cortical inhibitory mechanisms may be adversely affected by repeated visual auras.

Dynamics of perceptual oscillations in form vision

Certain periodic dot patterns (Marroquin patterns) generate a percept of dynamically oscillating circles, and analogous effects were explored by op artists in the 1960s. Here we show psychophysically that circles are perceived in these patterns only around specific points that are quantitatively predicted by a neural model of configural units hypothesized to reside in cortical area V4. Circles superimposed on the pattern mask perception of illusory circles. A neural model of lateral inhibitory interactions among V4 configural units showing spike-frequency adaptation quantitatively accounts for the human data. The model is consistent with ideas on the neural basis of attention in V4, and it suggests that attention may be biased via neuromodulation of slow hyperpolarizing potentials in cortical neurons.

Measurement of the texture-coherence limit for bandpass arrays

Casual observation suggests that when the elements of a visual array are packed at a sufficiently high density they cohere to generate the percept of a texture. This 'texture-coherence limit' has been quantified by using arrays composed of Gabor functions, sixth Gaussian derivatives, or differences of Gaussians. In all cases the texture-coherence limit was a power-law function of the size of the elements as quantified by their space constants with an exponent averaging 0.7. Furthermore, the texture-coherence limit was independent of both element spatial frequency and contrast over a considerable range. A quantitative fit to the data is provided by a model in which the texture-coherence limit is determined by activation of complex cells, which pool a spatial range of subunit inputs, throughout the stimulus region. Possible extensions to two dimensions are considered.

Studies of the photophysics and spectroscopy of alloxazine and related compounds in solution and in the solid state

The absorption and emission spectra and the fluorescence lifetimes and quantum yields of alloxazine and several of its N- and C- methyl-substituted derivatives, including two isoalloxazines, have been measured in a nonpolar solvent (1,2-dichloroethane), a polar aprotic solvent (acetonitrile), and a polar protic solvent (ethanol). The excited state decays are all single exponential, suggesting that only one emitting species is present in all cases. The spectroscopic data show that the emitting species is the π,π* electronic excited state corresponding to the ground state of the absorbing molecule; there is no evidence of excited state proton transfer for any of these solutes in any of the three solvents. The longer lifetimes and larger fluorescence quantum yields of the isoalloxazines compared with the alloxazines can be attributed exclusively to significantly slower rates of radiationless relaxation in the excited isoalloxazines. The remission and emission spectra of these same compounds adsorbed on cellulose are similar to the corresponding absorption and emission spectra in homogeneous solution, suggesting that the emitting species are the same in the adsorbed state as they are in homogeneous solution. However, the adsorbed excited species exhibit non-exponential temporal decay, attributed to inhomogeneities in the binding of the chromophore to cellulose. The emission spectra of polycrystalline samples of alloxazine and lumichrome suggest the possible occurrence of double intermolecular excited state proton transfer, but this interpretation is not supported by the further photophysical data obtained here.Key words: photophysics, spectroscopy, alloxazine, lumiflavin, cellulose.

A deficit in strabismic amblyopia for global shape detection

Using a task which relied upon the detection of sinusoidal deformations from circularity, we show that strabismic amblyopes exhibit deficits which are not critically dependent on either the scale of deformation or the spatial frequency characteristics of the stimulus (circular D4) itself. We show that this loss is not due to the restricted passband of the amblyopic eye. Furthermore, in a pedestal distortion experiment, we show that the suprathreshold form of this loss is consistent with an elevated level of 'intrinsic noise' rather than a loss in 'sampling efficiency'.

Modulation frequency and orientation tuning of second-order texture mechanisms

Modulation frequency and orientation tuning of second-order mechanisms underlying the detection of modulation in local spatial-frequency information are assessed by using an oblique-masking paradigm. Stimuli were Gabor-filtered noise patterns in which the local carrier spatial frequency was modulated about an average value of 4.7 cycles per degree (cpd) according to sinusoidal function. Thresholds were determined for spatial-frequency modulated test patterns (0.2 and 0.8 cpd) with fixed vertical carrier and modulation orientations presented alone and in the presence of spatiotemporally superimposed masks. Mask modulation frequency (0.1, 0.2, 0.4, 0.8, or 1.6 cpd), modulation orientation (0 degree, 45 degrees, or 90 degrees relative to vertical), and carrier orientation (18.5 degrees or 90 degrees relative to vertical) were manipulated independently while the mask modulation amplitude remained fixed at 0.25. Manipulating the modulation frequency of the mask revealed some modulation frequency specificity, particularly at lower test modulation frequencies. Spatial-frequency modulated masks produced threshold elevations regardless of the local carrier orientation. However, there was no evidence of threshold elevation when the mask modulation orientation was orthogonal to that of the test pattern. These results suggest a second-order texture mechanism that is tuned to both modulation frequency and modulation orientation but is not selective in terms of the orientation of first-order inputs.

Detection and recognition of radial frequency patterns

Detection thresholds for radial deformations of circular contours were measured using a range of radii and contour peak spatial frequencies. For radial frequencies above two cycles, thresholds were found to be a constant fraction of the mean radius across a four-octave range of pattern radii and peak spatial frequencies (mean Weber fraction: 0.003-0.004). At low radial frequencies, thresholds were unaffected by contrast reduction. In 167 ms presentations, subjects were able to identify radial frequencies of six cycles and below with an accuracy of over 90% correct even when phase was randomized. The extreme sensitivity of subjects to these radial deformations (as low as 2-4 s of arc) cannot be explained by local orientation or curvature analysis, and points instead to the global pooling of contour information at intermediate levels of form vision.

Detection of global structure in Glass patterns: implications for form vision

Glass (Nature 1969;223:578-580) patterns are random dot stimuli that generate a percept of global structure. To study the mechanisms underlying this global form perception, concentric, radial, hyperbolic, and parallel Glass patterns were constructed. Thresholds for detecting each type of pattern were measured by degrading the patterns through the addition of noise. Concentric patterns yielded the lowest thresholds for all subjects, while radial and hyperbolic patterns produced somewhat higher thresholds. For all subjects the parallel patterns produced the highest thresholds. Threshold measurements as a function of the area containing pattern structure provided evidence for global pooling of orientation information in the detection of radial and concentric Glass patterns but only local pooling in the detection of parallel patterns. Monte-Carlo simulations demonstrate that plausible neural models can accurately predict the data. These models indicate that the visual system contains networks that pool orientation information within regions 3.5-4.5 degrees in diameter in central vision. This pooling is organized to extract cross-shaped, X-shaped, and quasi-circular forms from the retinal image. The results are in good agreement with recent single unit physiology of primate area V4, an intermediate level of the form vision pathway.

Apparent contrast and spatial frequency of local texture elements

We measured the apparent contrast and spatial frequency of a parafoveal Gabor signal located at the center of an array of Gabor signals as a function of both element density and the direction of contrast and spatial frequency of the surrounding elements. The target Gabor appeared lower in contrast and higher in spatial frequency when the elements were in close proximity, regardless of the direction of contrast and spatial frequency of the surrounding elements. Overall, the evidence suggests that the appearance of a parafoveal target is strongly affected by its visual context. These findings provide additional support for the existence of spatial interactions among neurons implicated in textural processing.

Evolving concepts of spatial channels in vision: from independence to nonlinear interactions

By the 1960s it was evident from neuroanatomy that there were extensive recurrent interactions, both excitatory and inhibitory, among visual cortical neurons. Nevertheless, the psychophysical discovery of 'spatial-frequency channels' gave rise to a decade in which parallel, independent channels were thought to subserve early spatial vision. Recent work, however, has clearly demonstrated that early visual channels do not perform a Fourier or wavelet decomposition of the image. Instead, they interact through a variety of nonlinear pooling mechanisms. Such nonlinear interactions perform important computations in texture perception, stereopsis, and motion and form vision.

Concentric orientation summation in human form vision

Psychophysical data demonstrate that orientation information in concentric, random-dot Glass patterns is summed linearly to extract a global form percept. Surprisingly, no such global pooling was found for Glass patterns with parallel structure. A simple neural model explains these results and agrees with recent V4 single unit physiology. As V4 provides the major input to IT, global concentric units may play an important role in analyzing complex images such as faces. In support of this possibility, deficits in the perception of concentric Glass patterns have recently been linked to prosopagnosia.

Lateral interactions in peripherally viewed texture arrays

A horizontal array of vertically oriented Gabor elements was used to examine lateral masking in the near periphery (1.9 degrees-5.7 degrees eccentricity). Thresholds were assessed for detecting changes in the contrast, the spatial frequency, and the orientation of the central element within the array. The presence of surround elements induced marked threshold elevations that increased in strength as interelement spacing decreased and as retinal eccentricity increased. A model incorporating spatial summation by complex cells and reciprocal inhibition between simple and complex cells is shown to provide a quantitative fit to the data. This model suggests that complex cells analyze highly redundant textures, whereas simple cells function predominantly in the presence of isolated contours.

Isolation and identification of a protein with capsaicin-inhibited NADH oxidase activity from culture media conditioned by growth of HeLa cells

A ca. 33.5-kDa protein has been identified as a soluble NADH oxidase activity of culture media conditioned by growth of HeLa cells. The protein appears to be derived from a 34-kDa protein of the HeLa plasma membrane. Both proteins are characterized by an ability to oxidize NADH in the absence of exogenous electron acceptors. The activity is inhibited by 8-methyl-N-vanillyl-6-noneamide (capsaicin). The soluble and the plasma membrane forms of the activity exhibit a similar EC50 of about 5 nM for inhibition of the activity by capsaicin. The activity was purified from culture media conditioned by growth of HeLa cells using DEAE ion exchange chromatography, G-200 size exclusion chromatography, and preparative SDS-PAGE. Purification was monitored on the basis of the capsaicin-inhibited oxidation of NADH, including the final electrophoretic purification. Activity was restored following SDS-PAGE by reduction with dithiothreitol or reduced glutathione in the presence of NADH followed by the addition of 0.03% hydrogen peroxide and preincubation in the presence of NADH for 5-15 min. For affinity purification, the vanillylamine portion of capsaicin was linked to agarose. The agarose-linked vanillylamine bound a ca. 33.5-kDa protein band with capsaicin-inhibited NADH activity from total defined culture media conditioned by growth of HeLa cells. The NADH oxidase activity of both the soluble and the plasma membrane-associated form of the activity was inhibited by antisera corresponding to the 33.5-kDa protein. The antisera also immunoprecipitated and reacted on Western blots with both the soluble (33.5 kDa) and plasma membrane (34 kDa)-associated forms of the capsaicin-inhibited activity. The results identify the capsaicin-inhibited NADH oxidase of the conditioned media of HeLa cells as being a ca. 33.5-kDa shed form of the previously reported capsaicin-inhibited NADH oxidase of the HeLa cell plasma membrane.

Orientation, density and size as cues to texture segmentation in kittens

The ability of kittens (45-135 days of age) to segment images based on textural differences was examined using a two-alternative forced-choice procedure on the jumping stand. Tasks based on 3 textural cues--element size, element density and element orientation--were presented concurrently in a within-subject design. Texture segmentation based on element size appeared as early as 47 days of age, and segmentation based on element density as early as 57 days. In both cases, onset age varied with the specific stimulus parameters. Segmentation based on a 90 deg difference in element orientation did not appear until after 90 days and its time of appearance was independent of element size over a 2 octave range. For all segmentation cues, age was a more powerful determinant of when a task would be solved than was amount of training. The late onset of segmentation based orientation, relative to other cues, closely parallels recent findings in human infants. This evidence of differences in developmental time course provides strong support for the idea that texture segmentation based on orientation differences does not share a common neural substrate with texture segmentation based on other visual cues.

Acuities for textures and gratings in kittens assessed by preferential looking

A forced-choice preferential looking paradigm, similar to that developed for human infants, was used to assess visual thresholds in kittens between 21 and 45 days of age. In agreement with the earlier work of Sireteanu [19,30], the technique was found to be successful for grating acuity measurement. Acuity for square wave gratings (vs. matched greys) increased from 0.12 cycles/degrees at 21 days to 1.6 cycles/degrees at 40 days of age. Comparable results were obtained at three viewing distances: 20, 40, and 60 cm, indicating that accommodation is not a limiting factor in spatial resolution over this range of distances. Kittens also showed preferences for texture arrays with strong oriented components and 'texture acuities' were found to be comparable to acuity for square wave gratings. A consistent orientation anisotropy favouring horizontal line elements was seen in very young kittens but disappeared by 5 weeks of age. Preferences for texture arrays composed of dots were much weaker and thresholds could not be determined in all cases. The results are discussed in the context of the Banks and Ginsberg [2] model of infant preferences.