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Christopher W Tyler

Publications and source records attributed to Christopher W Tyler.

At least 19 recordsLinked to original sources
PubMed2006

Spatial summation of face information.

Do all parts of the face contribute equally to face detection or are some parts more detectable than others? To evaluate this issue, we studied detection of the presence of normalized frontal-face images within aperture windows of varying extent. We performed a face summation study using two-alternative forced-choice psychophysics. The face stimuli were scaled to equal eye-to-chin distance, centered on the bridge of the nose, and windowed by fourth-power Gaussian envelopes of various sizes. The faces were intermixed with control stimuli consisting inverted faces to test for configuration effects, split-half inverted faces to perturb the symmetry, and phase-scrambled versions of the faces with equal Fourier energy. Face detectability improved rapidly at first, then at a progressively shallower rate for larger window sizes, in a similar fashion for the three face-based stimulus types. The spectrally equated noise stimuli were less detectable than the face stimuli for all except the smallest apertures. The results were fit with a model incorporating global face-specific and local nonspecific spatial integration mechanisms. Detection of the noise images was consistent with local detection mechanisms accessed through a wide-field attention mechanism. The data for face detection implied detection mechanisms that integrated linearly up to some small size, integrated more slowly up to an intermediate size, and failed to gain any improvement for information beyond some larger size. This performance supports the concept of a specialized face configuration mechanism operating at detection threshold, similar in extent among the observers.

Artifacts↗
PubMed2006

Face configuration processing in the human brain: the role of symmetry.

Symmetry is an important cue in face perception. We manipulated symmetry and other configurational variables to study their role in face processing in the human brain. We employed 2 types of symmetry: image symmetry (where one part of the image is defined as the mirrored transform of the other part about an axis) and object symmetry (where the spatial relationships among the image components are interpreted as parts of a symmetric 3-dimensional object). We compared blood oxygenation level dependent responses in healthy human observers for upright front-view faces with responses to different symmetry-controlled images. The cortical areas activated by the face images, relative to Fourier-matched scrambled images, were the fusiform (FFA) and occipital (OFA) face areas, the middle occipital gyri (MOG), and areas around the superior temporal and intraoccipital sulci (IOS). Contrasting faces and their image-symmetric scrambled versions showed a similar activation pattern except in the right OFA, suggesting an involvement in facial symmetry processing. The upright versus inverted faces (with the same image symmetry but unfamiliar object identity) showed robust differential activation in the FFA, OFA, MOG, IOS, and precuneus. The response to frontal-view versus 3/4-view faces (having the same object symmetry but disrupted image symmetry) showed little differential activation in the FFA or the OFA but strong responses in the MOG and IOS, suggesting that face processing in the FFA and the OFA is holistic and viewpoint invariant.

Brain Mapping↗
PubMed2006

Evidence for elongated receptive field structure for mechanisms subserving stereopsis.

To study the spatial extent and shape of the binocular disparity mechanisms subserving depth perception, we employ the spatial summation paradigm of contrast threshold for front/back depth discrimination at a fixed binocular disparity. The stimuli were Gabor patches with disparity set at either 4 or 8 arcmin and spatial frequency set at an optimal value of 4 cy/deg. Contrast threshold was measured as a function of length and width of the Gabor patches to determine the aspect ratio of greatest efficiency. The space constant of the Gaussian envelope varied between 0.0375 degrees and 0.9 degrees in either vertical or horizontal directions, or both simultaneously. For vertical elongation of the Gabor patches, discrimination sensitivity improved by 4-6 dB for a doubling of the length of the Gabor patches, then reduced more slowly as the length further increased. However, extending the Gabor patches horizontally across cycles produced little or no sensitivity improvement. Instead, discrimination performance collapsed in a fashion that is incompatible with many models of disparity processing. The results imply that the main mechanisms subserving stereoscopic depth discrimination are vertically elongated for vertical-bar Gabors and encounter special difficulties integrating horizontal disparity information. Disparity discrimination sensitivity for very small targets was also much greater than predicted by the single-mechanism fit, implying the presence of a second, independent mechanism with a very small summation field, which may underlie the fine stereoscopic processing system.

Contrast Sensitivity↗
PubMed2005

The specificity of cortical region KO to depth structure.

Functional MRI studies have identified a cortical region designated as KO between retinotopic areas V3A/B and motion area V5 in human cortex as particularly responsive to motion-defined or kinetic borders. To determine the response of the KO region to more general aspects of structure, we used stereoscopic depth borders and disparate planes with no borders, together with three stimulus types that evoked no depth percept: luminance borders, line contours and illusory phase borders. Responses to these stimuli in the KO region were compared with the responses in retinotopically defined areas that have been variously associated with disparity processing in neurophysiological and fMRI studies. The strongest responses in the KO region were to stimuli evoking perceived depth structure from either disparity or motion cues, but it showed negligible responses either to luminance-based contour stimuli or to edgeless disparity stimuli. We conclude that the region designated as KO is best regarded as a primary center for the generic representation of depth structure rather than any kind of contour specificity.

Adult↗
PubMed2005

The structure of stereoscopic masking: position, disparity, and size tuning.

The masking effect of a Gaussian blob on detection of a Gaussian target was measured as a function of the position, disparity, width and polarity of the mask. The data reveal a large degree of disparity-specific masking that cannot be explained by the masking of its monocular constituents. At 5 degrees eccentricity, the masking range extends about +/-1 degrees around the lines of sight of the two eyes and 1-3 degrees in disparity, depending on the size of the test stimuli. The masking effects can be modeled as having three additive components, one that has a fixed disparity range and is polarity independent, one with a center/surround form keyed to both the disparity and the polarity of the mask, and one that derives from the monocular masking in each eye. Thus, the profound disparity interaction behavior is not limited to the simple monocular masking properties of the stimuli but reveals extensive connectivity across the disparity domain. Future models of disparity encoding will need to take these properties into account.

Adult↗
PubMed2005

Lateral modulation of BOLD activation in unstimulated regions of the human visual cortex.

After staring at a blank region surrounded by a dynamic background for a few seconds, observers report a twinkle aftereffect in the unstimulated blank region. The significance of this twinkle aftereffect is that it occurs at a location that received no stimulation, and therefore reflects a rebound from lateral inhibition within the dynamic processing system. To study this inhibitory rebound effect, the blood oxygenation level dependent (BOLD) activation in the visual cortex was measured while the observers were viewing a flickering pin-wheel pattern alternating with a blank test. Retinotopic regions corresponding to the inter-wedge regions in the pin-wheel pattern showed activation negatively correlated with the test sequence. While the BOLD activation in the visual cortex is generally considered to be retinotopically driven by the visual stimuli, we were able to show a sustained negative activation in the unstimulated regions, with properties that correspond to those of the inhibitory rebound of the perceived aftereffect.

Adult↗
PubMed2004

Predominantly extra-retinotopic cortical response to pattern symmetry.

Symmetry along one or more axes is a key property of objects and biological organisms. We report on a bilateral visual region of occipital cortex that responds strongly to the presence of multiple symmetries in the viewed image. The stimuli consisted of random dots organized in fourfold and onefold mirror-symmetric patterns, against random control stimuli. The contrast between symmetric and random patterns produced negligible or inconsistent activation of the primary visual projection area V1 or of other medial occipital projection areas. However, there was strong symmetry-specific activation in extra-retinotopic lateral occipital cortex. The high level of activation in this region of cortex may represent part of a general class of computations that require integration of information across a large span of the visual field.

Brain Mapping↗
PubMed2004

Representation of stereoscopic structure in human and monkey cortex.

Tsao et al. have recently used functional magnetic resonance imaging to compare processing for moving stereoscopic forms in macaque and human brains. Most humans exhibited activation in a swath of lateral occipital areas, extending into the intraparietal sulcus, with a limited version of the same pattern in monkeys. However, neither species showed strong activation of the motion area known as MT in monkey or its human homolog.

Animals↗
PubMed2004

What makes Mona Lisa smile?

To study the ability of humans to read subtle changes in facial expression, we applied reverse correlation technique to reveal visual features that mediate understanding of emotion expressed by the face. Surprising findings were that (1) the noise added to a test face image had profound effect on the facial expression and (2) in almost every instance the new expression was meaningful. To quantify the effect, we asked naïve observers to rank the face of Mona Lisa superimposed with noise, based on their perception of her emotional state along the sad/happy dimension. Typically, a hundred trials (with 10 or more samples for each rank category) were sufficient to reveal areas altering the facial expression, which is about two orders of magnitude less than in the other reverse correlation studies. Moreover, the perception of smiling in the eyes was solely attributable to a configurational effect projecting from the mouth region.

Emotions↗
PubMed2004

Theory of texture discrimination of based on higher-order perturbations in individual texture samples.

This analysis addresses the issue that texture properties are defined on ensembles of possible textures, while psychophysical judgments of texture properties must be made on individual texture samples, or regions of uniform texture within a larger texture field. Since the basic discrimination task requires comparison of two sample images (or regions) specified by different ensemble rules, the viewer is thus required to compare the estimates of their ensemble statistics of single textures. This paper develops a theory of texture discrimination incorporating a roving local sampling window that allows the visual system to derive an estimate of the ensemble statistics over the window from any particular texture image, without the need to present multiple samples for evaluation. This approach to texture explains how we can have a clear sense that two patterns derive from different statistical generation rules even though we see only one example of each type. In providing the theoretical basis for texture discrimination of individual samples, this analysis goes beyond previous work to account for our intuitive sense that we can estimate the generation rule underlying particular textures. It also analyzes the decision process for discriminating texture boundaries in extended images, defining a novel "Gregorian attractor" that replaces and extends standard Bayesian decision rules.

Discrimination, Psychological↗
PubMed2004

Beyond fourth-order texture discrimination: generation of extreme-order and statistically-balanced textures.

Julesz introduced the concept of statistically defined textures and their perceptual discrimination. Julesz showed that discrimination was possible with statistics equated to third-order, specifying fourth-order textures. Klein and Tyler offered a variety of paradigms suggesting that fourth order might be the limit on human texture processing. To go beyond this limit, new texture paradigms are now introduced to avoid contamination by luminance extrema, to control local and long-range texture properties, and to provide textures without global statistical structure. Local luminance contamination is avoided by novel orientation plaids, in which higher-order rules govern the orientation of local elements rather than their coloring. These textures allow evaluation of texture discrimination up to thirty-second order by cortical pattern elements. Long-range processing is studied by random strip rotation and by interlacing of independent textures. Each substantially degrades the visibility of the fourth-order textures, revealing that the fourth-order information is conveyed largely by local rather than long-range perturbations from random statistics. Finally, textures equated at all orders can be defined in terms of their global statistics, but may nevertheless readily be discriminated in human vision. The discrimination on the basis of local perturbations implies that human vision assesses textures through a local sampling window, and is largely insensitive to longer-range statistical properties.

Discrimination, Psychological↗
PubMed2003

Peak localization of sparsely sampled luminance patterns is based on interpolated 3D surface representation.

Objects in the world are typically defined by contours and local features separated by extended featureless regions. Sparsely sampled profiles were therefore used to evaluate the cues involved in localizing objects defined by such separated features (as opposed to typical Vernier acuity or other line-based localization tasks). Objects, in the form of Gaussian blobs, were defined at the sample positions by luminance cues, binocular disparity cues or both together. Remarkably, the luminance information in the sampled profiles was unable to support localization for objects requiring interpolation when the perceived depth from the luminance cue was cancelled by a disparity cue. Disparity cues, on the other hand, improved localization substantially over that for luminance cues alone. These data indicate that it is only through the interpolated depth representation that the position of the sampled object can be recognized. The dominance of a depth representation in the performance of such tasks shows that the depth information is not just an overlay to the 2D sketch of the positional information, but a core process that must be completed before the position of the object can be recognized.

Cues↗
PubMed2003

Statistical properties of BOLD magnetic resonance activity in the human brain.

We investigated the random variability of BOLD (blood oxygen level dependent) activation during rest, or null-hypothesis, conditions in which the observers were neither receiving controlled sensory stimuli nor performing cognitive tasks. The data indicate that the distributions for the BOLD variation across space are skewed, with non-Gaussian tails, while the distributions for the temporal variation within individual voxels are predominantly Gaussian. The proportion of voxels that show non-Gaussian properties is highly correlated with the magnitude of head movement of the observers. In all observers, the white matter showed less variability than the gray matter. The distributions for the spatial and the temporal variations are robust across observers despite differences in the data acquisition methods (EPI vs. spiral) and magnetic field strength (1.5 vs. 3 T). In most cases, the non-Gaussian tails of the spatial distribution can be eliminated by normalizing the amplitude in each voxel to its standard deviation before cumulating across voxels. We therefore recommend such a normalization procedure before any data manipulations are performed on fMRI data.

Adult↗
PubMed2003

Spatiotemporal relationships in a dynamic scene: stereomotion induction and suppression.

We establish the existence of purely stereoscopic motion induction, i.e., perceived depth motion induced into a fixed-disparity target by disparity changes in a surround region. The stimuli were dynamic autostereograms consisting of a target and a surround, both consisting of horizontal lines of discs. We explored the stereomotion induction process by (i) direct estimation of the perceived distance moved, (ii) a cancellation technique with compensatory target motion, and (iii) extension of the compensatory motion into the zone beyond the null point. Adding compensatory stereomotion to the target reduced the induced motion experience to a null point. Beyond the cancellation point, two surprising results were obtained; perceived motion in the target increased, while the surround stereomotion perception was almost suppressed over a wide range of disparity changes (reciprocal stereomotion suppression). A model of the target/surround interactions was developed in the context of dynamic organization principles operating in stereomotion perception and misperception.

Depth Perception↗
PubMed2002

Separating the effects of response nonlinearity and internal noise psychophysically.

A psychophysical method is proposed to separate the contrast dependence of internal response and its noise. The resulting contrast relationships represent a signature of the visual processing stage that limits the human observer's performance. The method was applied to contrast discrimination for sustained and transient Gabor patches with a 3 cycle/deg spatial carrier. For both stimulus types the predominant noise was found to be multiplicative with a power exponent of 0.76-0.85 and the source of this noise preceded by an accelerating signal transducer with a power of 2-2.7. These exponents combine to account for the classic compressive power of about 0.4 for the signal-to-noise ratio in contrast discrimination. The estimated transducer acceleration suggests that there is a direct computation of contrast energy in the visual cortex.

Adolescent↗
PubMed2002

Temporal dynamics of the human response to symmetry.

Symmetry is a highly salient feature of animals, plants, and the constructed environment. Although the perceptual phenomenology of symmetry processing is well understood, little is known about the underlying neural mechanisms. Here we use visual evoked potentials to measure the time course of neural events associated with the extraction of symmetry in random dot fields. We presented sparse random dot patterns that were symmetric about both the vertical and horizontal axes. Symmetric patterns were alternated with random patterns of the same density every 500 msec, using new exemplars of symmetric and random patterns on each image update. Random/random exchanges were used as a control. The response to updates of random patterns was multiphasic, consisting of P65, N90, P110, N140 and P220 peaks. The response to symmetric/random sequences was indistinguishable from that for random/random sequences up to about 220 msec, after which the response to symmetric patterns became relatively more negative. Symmetry in random dot patterns thus appears to be extracted after an initial response phase that is indifferent to configuration. These results are consistent with the hypothesis (Lee, Mumford, Romero, & Lamme, 1998; Tyler & Baseler, 1998) that the symmetry property is extracted by processing in extrastriate cortex.

Adult↗