Electrophysiological correlates of texture segregation in the human visual evoked potential.

We investigated whether the visual evoked potential (VEP) reflects cortical processing associated with preattentive texture segregation. On a visual display unit we presented stimuli with various arrangements of oriented line segments that either led to the appearance of a "preattentive" checkerboard or did not. Two presentation modes were used (pattern onset at 1 Hz and rapid pattern change at 4.3 Hz), while luminance (57 cd/m2) and contrast (92%) of the line segments remained constant. VEPs were recorded in 7 human subjects. The VEP was analyzed as a linear combination of putative components, which are evoked by either local pattern, quasi-local orientation contrast or global preattentive structure. In the transient VEP, we found a negativity over the posterior pole at a latency between 161 and 225 msec (FWHM) in the linear combination designed to extract segregation-specific components. Peak amplitude reached 3.1 +/- 0.8 microV (mean +/- SEM) at 199 msec. This negative peak appeared only for textures containing orientation contrast. Steady-state analysis of the rapid presentation also revealed a significant component (P = 0.002) associated with texture segregation. These potentials either represent processing of orientation contrast or global processing of texture segregation. The results suggest that specific surface potentials, differing from cognitive potentials, can be derived which are associated with preattentive processing.     

A visual evoked potential correlate of global figure-ground segmentation.

Human observers discriminated the global orientation of a texture-defined figure which segregated from a texture surround. Global figure discriminability was manipulated through within-figure collinearity, figure-surround interaction, and figure connectedness, while the local orientation contrast at edges between figure and surround was kept constant throughout all the experiments. Visual evoked potentials (VEPs) were recorded during onset-offset stimulation in which the figure cyclically appeared and disappeared from a uniform texture background. A difference component was obtained by subtraction of offset-from onset-VEP. Two negative peaks of the difference component are found with latencies around 140-160 and 200-260 ms, respectively. Enhanced discriminability of the global figure reduced (11-25 ms) the latency of the second peak, hence indicating that the 200-260 ms component was produced by global figure-ground segmentation.     

Movement aftereffects contingent on the colour or pattern of a stationary surround

Other studies have shown that movement aftereffects (MAEs) contingent on several visual dimensions, such as colour, texture, luminance or depth, can be obtained from suitable stationary test fields if the appropriate characteristics of the moving stimulus are varied during adaptation. We have obtained CMAEs from an adaptation paradigm in which only the direction of movement of a moving stimulus is changed while the colour or pattern of a surrounding stationary area is varied. Stationary test fields then elicit MAEs contingent on the colour or pattern of their surround.     

Sensitivity for structure gradient in texture discrimination tasks

Recent experiments indicate that the segregation of visual structures ("texture discrimination") depends not only on the form of texture elements but also on their spacing. Structures with discriminable elements in close proximity can be segregated more easily than patterns in which the same texture elements are more widely spaced. In dot arrays with areas of different dot luminance, segregation was found to depend on both the luminance difference and dot spacing; discrimination of texture areas in coarse dot rasters required greater differences in luminance than in fine rasters. Also, in regular arrays of iso-luminant line patterns, the maximal spacing between neighbouring lines for which different texture areas could still be discriminated was found to be influenced by the degree of dissimilarity between elements. For lines of a given length, texture areas with small differences in orientation became indiscriminable at smaller spacings than texture areas with orthogonal line orientations. Line length additionally had a strong effect on texture discrimination; increasing the line length for a given spacing provided easier segregation of texture areas. However, over a range of raster widths, discrimination of texture areas with a given difference in line orientation varied not with absolute values of line length but with the ratio of line length to interline spacing. Overall, the data suggest that texture discrimination in man is based on the evaluation of variation in structure over space (defined as the "texture gradient"). If local variation of structure is too small, texture areas cannot be discriminated, though differences between texture elements themselves may be apparent. As far as the dependence on variation over space is concerned, discrimination of iso-luminant textures resembles the limited sensitivity of the visual system for differences in texture luminance.     

Attention and visual texture segregation

Visual texture segregation is believed to be performed preattentively. Recent evidence, however, suggests that attention does play an important role. Using visual evoked potentials (VEPs), we investigated the effect of different tasks on texture segregation. Stimuli consisted of Gabor-filtered binary noise patterns. In segregated stimuli, local texture orientation contrasts defined global checkerboard patterns. VEP responses specific to texture segregation were obtained by computing the difference between VEPs to homogeneous and segregated stimuli. Four conditions were examined that required attending either the global pattern, the local structure, random numbers displayed on the screen, or a series of tones. Responses specific to texture segregation were dominated by two occipital negativities peaking around 110 and 230 ms. The earlier one was not affected by the task, whereas the later one was completely abolished when the subjects attended to either numbers or tones (p = .0005 and p = .006, respectively). The results suggest that early stages of texture segregation are not affected by attention, whereas task relevance is crucial for later processes. The timing is compatible with a recurrent processing pattern with initial bottom-up processing of basic stimulus characteristics and a subsequent top-down flow of higher level modulatory information. As attention effects occur across modalities, they cannot be simply explained by competition within the visual cortex.     

Different effects from spatial frequency masking in texture segregation and texton detection tasks

The paper reports psychophysical experiments set up to study the visual cues used in texture discrimination. In particular, the special role of "textons", i.e. distinct visual features such as blobs of different size, lines at different orientation, line intersections ("crossings") and line ends ("terminators") which have been proposed to provide the basis of perceptual segregation of texture areas, has been investigated. Texture pairs were briefly presented and simultaneously masked with two-dimensional visual noise at various spatial frequency bands. In different tasks on similar patterns, observers had to estimate the orientation of globally dissecting texture areas ("texture segregation") and to identify and distinguish the texture elements themselves ("texton detection"). Differential masking effects between these tasks indicate that texture segregation is often based on visual cues different from the supposed texton features. The segregation of crossing or terminator differences is also achieved from associated differences in the spatial frequency composition, that of differences in blob size from associated differences in mean luminance. Only differences in line orientation revealed similar masking curves in texture segregation and texton detection tasks.     

Texture filling-in and texture segregation revealed by transient masking.

When a texture pattern was briefly presented followed by a small annular mask, it was found that the central area of the texture was strongly suppressed within the mask. Analogous to filling-in of brightness in a uniform luminance area (Paradiso, M. A. & Nakayama, K. (1991) Vision Research, 31, 1221-1236), this phenomenon demonstrates filling-in of texture; the texture area was unperceived because filling-in of the texture area was interrupted by the contour in the mask. However, odd local features within the texture, which were assumed to pop out, were selectively perceived while other features were suppressed within the mask. These results suggest that: (1) rapid pattern segregation occurs before and/or separately from texture filling-in, and that (2) filling-in is initiated at boundaries between surfaces rather than at luminance gradients.     

Texture segregation is processed by primary visual cortex in man and monkey. Evidence from VEP experiments.

We investigated whether the process of texture segregation can be allocated to a specific visual cortical area. We designed a stimulus to reveal the presence of a mechanism, which is specifically sensitive to a checkerboard, that is solely defined by textures segregating due to orientation differences of the constituting line segments. We recorded evoked potentials to this stimulus in man and awake monkey. A difference component, signalling texture segregation sensitivity, could be recorded from both types of subjects. Its presence depended on the spatial extent of the textures, in a manner correlating with the perceptibility of the checkerboard. This difference response could be localized in primary visual cortex by means of equivalent dipole estimations.     

Flicker and movement constituents of the pattern reversal response

A counterphase-modulated (i.e. contrast reversing) pattern will affect hypothetical neural elements sensitive to motion as well as hypothetical elements sensitive to changes in contrast and other elements sensitive to local luminance. Attempts to describe visual evoked potentials (VEPs) to contrast reversal entirely in terms of luminance change or entirely in terms of contrast change have been unsuccessful; strong interactions between responses to contrast decrease and increase had to be postulated ad hoc in order to account for the contrast reversal VEP, and no explanation for these interactions has been proposed. Rather than attempting to separate the reversal VEP into responses to contrast decrease and increase, we attempt here to analyze the reversal VEP into responses to motion onset and offset. We find that VEPs to an abrupt displacement of less than one square's width of a checkerboard pattern are qualitatively similar to the contrast reversal VEP, and that the displacement VEP can be seen as the limiting case of a motion onset response shortly followed by a motion offset response. When displacement occurs during steady motion, the displacement and motion responses interact. We conclude that the contrast reversal VEP is better described in terms of motion onset and offset response than in terms of contrast onset and offset responses.     

Simultaneously recorded retinal and cerebral potentials to windmill stimulation

Visual evoked retinal and cerebral potentials were recorded to onset rotation of an isoluminant sectored disc. While the retinal potentials recorded to onset rotation closely resembled the electroretinogram to a checkerboard or stripe pattern of fixed element size, the visual evoked potential changed interindividually and intraindividually from a fast positive wave at high contrasts, velocities and number of windmill segments to a later negative component at low contrasts, velocities and windmill segments. With change in luminance, contrast, speed and extent of rotation field size and number of disc segments, the visual evoked potential was generally less affected than the electroretinogram.     

The Acute Effect of Cigarette Smoking on Pattern Visual Evoked Potentials

Reports of tobacco-induced electrocortical activation and decrements in ocular blood flow in the acute faze indicated that this effect is mediated via nicotin’s action or neuronal systems. In this study, pattern visual evoked potentials were investigated in a group of male smokers (22 right eyes of 22 subjects) in separate real smoking and sham smoking sessions. On each session, pattern visual evoked potentials were recorded before smoking, immediately after smoking, and five minutes after smoking. Latency and amplitude values for P100 peaks were assessed and analyzed in each smoking condition for both real smoking and sham smoking sessions. Real smoking significantly decreased P100 latency values (p value related to difference between pre-smoking and immediately after smoking conditions is 0.009) and increased P100 amplitude values (p value related to difference between pre-smoking and fifth minute after smoking is 0.039). Statistically no significant difference was observed in sham smoking sessions. Our results are consistent with smoking-induced stimulant effects on pattern visual evoked potentials.     

Correlates of figure-ground segregation in fMRI

We investigated which correlates of figure-ground-segregation can be detected by means of functional magnetic resonance imaging (fMRI). Five subjects were scanned with a Siemens Vision 1.5 T system. Motion, colour, and luminance-defined checkerboards were presented with alternating control conditions containing one of the two features of the checkerboard. We find a segregation-specific activation in V1 for all subjects and all stimuli and conclude that neural mechanisms exist as early as in the primary visual cortex that are sensitive to figure-ground segregation.     

Assessment of central visual function after successful retinal detachment surgery by pattern visual evoked cortical potentials.

The pattern of visual recovery after successful surgery by pattern visual evoked cortical potentials (VECP), visual acuity, colour vision, and critical fusion frequency was investigated in 14 eyes with retinal detachment involving the macula. The temporal tuning characteristics in the evoked potentials were measured as based on the P100 amplitude and the frequency necessary for evoking 0 mu V amplitude, which was defined as an objective critical fusion frequency by extrapolating the curve. Significant improvement in visual acuity and colour vision was observed within 2 months postoperatively. A significantly increased P100 peak latency became shorter as the postoperative days increased. In general, a good correlation was noted between the P100 peak latency and subjectively measured visual acuity, colour vision, and critical fusion frequency. The objective critical fusion frequency measured by VECP recovered gradually during the 6 months after surgery. Functional recovery was not related to the length of time the macula was detached before surgery.     

Luminance processing in apparent motion, Vernier offset and stereoscopic depth

We obtained (apparently) linear responses to luminance from three special displays of apparent motion, Vernier offset and stereoscopic depth. In our motion stimulus a dark and a light bar exchanged luminances repetitively on a grey surround. Motion was attributed to the bar that differed more from the surround, that is, on a dark surround the light bar appeared to jump, and on a light surround the dark bar appeared to jump. The apparent motion disappeared when the luminance of the surround lay halfway between that of the bars--on a linear, not a logarithmic scale. Similar results were obtained for special Vernier offset and stereo stimuli. These results cannot be explained if all luminances are processed within the same luminance pathway and that pathway transforms input luminance using non-linear compression. However, the apparent linearity of our results could arise from opposite and equal non-linearities cancelling out within separate ON- and OFF-spatial luminance pathways. A second set of experiments presented one bar separately into each eye on different surrounds (dichoptic presentation of competing apparent motion signals) or manipulated the display spatially so that different surrounds were associated with different bars (binocular presentation of competing Vernier targets). Results showed that apparent motion and Vernier signals of equal Weber contrast (normalisation of linear difference to surround luminance) evoked equal-motion and equal Vernier offset strengths. Given that motion and Vernier strength followed Weber's law, we infer that the ON- and OFF-pathways transform luminance non-linearly. Our third experiment presents an example of a brightness bisection task in which we were able to influence the bisection steps, to follow either a linear or non-linear series. The benefits of parsing the visual scene so that visual information is processed within two opposite luminance pathways is discussed.     

"Colour constancy" in Mondrian patterns: a partial cancellation of physical chromaticity shifts by simultaneous contrast

Edwin Land's Mondrian demonstrations (Land 1977, 1983, 1986a) are striking examples that the perceived colours of objects are largely independent of the chromaticity of the light incident upon them. Attempts to implement this independence in artificial vision systems have renewed interest in colour constancy and contrast, and the explanation of these phenomena in the Retinex theory. We use colour matches to demonstrate that departures from "colour constancy" are large and that it is possible to obtain the same colour shifts when the complex Mondrian pattern is replaced by a homogeneous grey field surrounding a test patch. A given patch has the same colour when surrounded by the Mondrian as when set in a grey background, provided that the grey represents the spatially weighted average of the Mondrian. Neither the colour shifts nor the equivalence of this neutral surround are correctly predicted by the Retinex theory. The phenomenon of partial cancellation of physical chromaticity shifts with changes of illuminant thus reduces to one of simultaneous contrast and adaptation where a spatio-chromatic and luminance average over a Mondrian pattern is the same as for a grey surround. Experiments with simultaneous contrast demonstrate that spatial weighting factors need to be applied in computations of the effect of the separate areas of a complex Mondrian pattern.     

Light adaptation and the saturation of colours

The effect of the luminance L of an achromatic surround on the saturation of central colour stimuli has been measured by means of the haploscopic colour matching method. It was found that the opponent purity p_m of the matching stimuli varied according to a power function of the luminance L of the achromatic surround field. The exponent is the same for all five stimuli tested. A saturation index ΔS is defined that is a measure of the variation of saturation with adaptation luminance.     

Texture segregation by motion under low luminance levels

We examined the effect of average luminance level on texture segregation by motion. We determined the minimum presentation duration required for subjects to detect a target defined by motion direction against a moving background. The average luminance level and retinal position of the target were systematically varied. We found that the minimum presentation duration needed for texture segregation depends significantly on the average luminance level and on retinal position. The minimum presentation duration increased as the mean luminance decreased. At a very low (presumably scotopic) luminance level, the motion-defined target was never detected rapidly. Under scotopic conditions, the minimum presentation duration was shorter in the periphery than in a near foveal region when the task was simple detection of the target. When the task included identifying the shape of the target patch, however, the target presented near the fovea was identified faster at all luminance levels. These results suggest that the performance of texture segregation is constrained by the spatiotemporal characteristics of the early visual system.     

Texture discrimination: representation of orientation and luminance differences in cells of the cat striate cortex

Neuronal texture discrimination in the cat striate cortex was investigated by measuring the responses of single cells to different pattern structures. The representation of two independent features, texture orientation and texture luminance, was analysed in detail and the sensitivity of neurones to either feature was studied at different levels of structure density. Texture patterns were systematically moved across the receptive field. From the cell response to various parts of the pattern, "response patterns" were generated which displayed the cell transform of the textured stimulus pattern. Only when texture structures were coarse, were cells able to encode the texture orientation of an area. Differences in texture luminance, on the other hand, were detected only in fine texture structures. Further, these textural features were processed in a different manner: Cells responded to differences in texture luminance but continuously to areas of similar texture orientation. Thus, responses of striate cells reveal an ambiguous representation of texture features and a failure to uniquely encode texture borders.     

Spatial frequency of the human short-wavelength-sensitive (blue) cone mechanism

The interactions of spatial and chromatic processing of the short-wavelength-sensitive cone mechanism were studied in humans with patterned (checkerboard) stimuli of various spatial frequency (10, 22, 44, and 85 min of arc respectively), under steady exposure to yellow light (575 nm, 390 cd/m²). Psychophysical studies and pattern-reversal visual evoked potentials were employed. Parameters of the transient pattern-reversal visual evoked potentials (pattern reversal rate of 2.4 s^–1) especially observed were the latencies of P2 (P100) and N3 and the amplitude of P2-N3. It was only with the largest applicable check size (85 min of arc) that both the psychophysical studies and visual evoked potentials could succeed in satisfactorily isolating the short-wavelength-sensitive cone mechanism. Pattern-reversal visual evoked potential latencies are recommended in the evaluation of this cone mechanism because of their smaller variance and higher selectivity in isolating the short-wavelength-sensitive cone mechanism than the amplitude. The peak sensitivity of this cone mechanism was shown to be at about 449 nm at the corneal level. The short-wavelength sensitive cone mechanism represented the characteristics of low spatial resolution and long latencies of the pattern-reversal visual evoked potentials.Abbreviations LERG luminance electroretinograms - LVEP luminance visual evoked potentials - LWS long-wavelength-sensitive - MWS medium-wavelength-sensitive - P-VEP pattern-reversal visual evoked potentials - SWS short-wavelength-sensitive     

Luminance-unbalanced pattern onset-offset electroretinogram and visual evoked cortical potential

The effects of a luminance-unbalanced pattern onset-offset mode of stimulation on electroretinograms and visual evoked cortical potentials were investigated. With the use of originally devised software, only offset luminance was varied from 14.7 to 62.3 cd/m². A vertical grating pattern (1.5c/deg, 38.8 cd/m² mean luminance, 0.95 contrast) was presented for 260 ms and was absent for 260 ms to normal subjects. With an increase in the luminance level of the offset pattern from the lowest level, the amplitude of the onset electroretinogram increased by degrees, while that of the offset electroretinogram gradually decreased. Conversely, onset visual evoked cortical potential responses decreased gradually, and offset visual evoked cortical potentials increased correspondingly. Furthermore, the spatial tuning of the onset pattern electroretinogram was already ambiguous, even when there was only a 4-cd/m² difference between onset and offset pattern luminances. Thus, luminance control is indispensable for pattern onset-offset stimulation.