Biomedical subjects

Mark W Pettet

Publications and source records attributed to Mark W Pettet.

12 recordsLinked to original sources

PubMed2006

Sensitivity and configuration-specificity of orientation-defined texture processing in infants and adults.

Here we use textures made up of widely spaced Gabor patches to compare infant and adult sensitivity to the global organization of the elements comprising the textures. Visual Evoked Potentials (VEPs) were recorded to alterations between random images and images containing varying proportions of patches that were of the same orientation. The patches were placed on rectangular, hexagonal or random lattices. Texture-specific responses were robust in adults and their VEP threshold was reached when 1-17% of the patches had the same orientation in the structured image. Infant thresholds were approximately 20-60%. While infants are capable of detecting the global structure of our textures, their sensitivity is low. In adults we found, unexpectedly, that sensitivity and response gain were higher for horizontal compared to vertical global orientations. Infant sensitivity was the same for the two orientations. Comparable orientation anisotropies have not been previously reported for gratings, suggesting that the Gabor-defined textures are tapping different mechanisms. There were small, but measurable effects of the lattice type in adults, with the rectangular lattice producing the largest responses.

Adult↗

PubMed2006

Cue-invariant networks for figure and background processing in human visual cortex.

Lateral occipital cortical areas are involved in the perception of objects, but it is not clear how these areas interact with first tier visual areas. Using synthetic images portraying a simple texture-defined figure and an electrophysiological paradigm that allows us to monitor cortical responses to figure and background regions separately, we found distinct neuronal networks responsible for the processing of each region. The figure region of our displays was tagged with one temporal frequency (3.0 Hz) and the background region with another (3.6 Hz). Spectral analysis was used to separate the responses to the two regions during their simultaneous presentation. Distributed source reconstructions were made by using the minimum norm method, and cortical current density was measured in a set of visual areas defined on retinotopic and functional criteria with the use of functional magnetic resonance imaging. The results of the main experiments, combined with a set of control experiments, indicate that the figure region, but not the background, was routed preferentially to lateral cortex. A separate network extending from first tier through more dorsal areas responded preferentially to the background region. The figure-related responses were mostly invariant with respect to the texture types used to define the figure, did not depend on its spatial location or size, and mostly were unaffected by attentional instructions. Because of the emergent nature of a segmented figure in our displays, feedback from higher cortical areas is a likely candidate for the selection mechanism by which the figure region is routed to lateral occipital cortex.

Adult↗

PubMed2006

Neural correlates of shape-from-shading.

Visual evoked potentials were recorded during presentation of a single stimulus that generated bi-stable perceptual alternation between two different three-dimensional percepts. One interpretation (asymmetric) changed depth structure from flat to corrugated in depth and the other (symmetric) had the appearance of a flat surface translating laterally behind a set of apertures. Responses during perception of the asymmetric three-dimensional structure contained larger negative components than did responses during perception of the symmetric three-dimensional structure. Control experiments suggest that the interpretation of depth structure is selected after junction information caused by the interplay between shading and object shape is extracted.

Adult↗

PubMed2005

Event-related potentials show configural specificity of global form processing.

Glass patterns are a type of moiré created when a random-dot field is overlaid with a rotated, translated or dilated copy. The overall form of the moiré cannot be detected using local processing mechanisms, and because of this, Glass patterns are useful probes of global form processing. Here, we use event-related potentials to show that certain global organizations (concentric structure created by rotation and radial structure produced by dilation) produce much larger brain responses than others (linear structure created by translation). The results are consistent with the existence of specialized form processing mechanisms in the extrastriate cortex.

Evoked Potentials, Visual↗

PubMed2005

Experience-expectant development of contour integration mechanisms in human visual cortex.

Extended contours are a common feature of natural images. Most previous studies have considered contour integration as a two-dimensional process of linking like-oriented elements along their common orientation axis. Yet contours exist in a three-dimensional world, and one might therefore ask about the relationship between contour integration and binocular vision. Using an event-related potential assay of contour integration, we demonstrate that patients with strabismic amblyopia show a relative insensitivity to Gabor-defined contours in their dominant eyes, all of which had normal acuity. These deficits were not seen in the dominant eyes of patients with anisometropic amblyopia without strabismus, but were present in the amblyopic eyes of patients with either strabismus or anisometropia. Deficits were also found in both eyes of strabismus patients who had normal visual acuity in each eye, but who had strongly reduced or absent stereopsis. These results suggest that the maturation of contour detection mechanisms depends at least in part on the presence of normal binocular interaction during a developmental critical period.

Amblyopia↗

PubMed2005

Suppression of monocular visual direction under fused binocular stimulation: evoked potential measurements.

Visual evoked potentials (VEPs) were recorded in response to a vernier onset/offset target presented to one eye that was combined with matching static targets in the other eye. The monocular response was dominated by a negative peak at 160 ms that occurred after a set of offsets was introduced into a one-dimensional random bar pattern. The static targets produced no discernible VEP response by themselves, but when fused binocularly with the oscillating vernier target, they produced shifts in perceived visual direction that influenced the VEP response. A vernier target fused with static vertical bars was perceived to alternate in depth between a flat surface and one broken into two interleaved surfaces. The response to this "surface-breaking" was as large or larger than the response to the monocular vernier offset. This response was much reduced when the oscillating vernier was fused with a static offset vernier (5' offset) that produced a percept of segregated regions moving in depth. Apparently, the VEP is strongly driven by shifts in visual direction that alter surface, texture, or contour contiguity.

Depth Perception↗

PubMed2005

Development of sensitivity to texture and contour information in the human infant.

Texture discrimination and bounding contour extraction are essential parts of the object segmentation and shape discrimination process. As such, successful texture and contour processing are key components underlying the development of the perception of both objects and surfaces. By recording visual-evoked potentials, we investigate whether young infants can detect orientation-defined textures and contours. We measured responses to an organized texture comprised of many Gabor patches of the same orientation, alternated with images containing the same number of patches, but all of random orientation. These responses were compared with a control condition consisting of the alternation between two independently random configurations. Significant difference potentials were found as early as 2-5 months, as were significant odd harmonics in the test conditions. Responses were also measured to Gabor patches organized either as circles (all patches tangent to an imaginary circular path) alternated with pinwheels (all patches having a fixed orientation offset from the path). Infants between 6 and 13 months also showed sensitivity to the global organization of the elements along contours. Differential responses to our texture and contour stimuli and their controls could only have been generated by mechanisms that are capable of comparing the relative orientation of 2 or more patches, as no local information at a single patch distinguished the random and organized textures or the circle and pinwheel configurations.

Adult↗

PubMed2005

A VEP measure of the binocular fusion of horizontal and vertical disparities.

PURPOSE: Because of the lateral separation of the orbits, the retinal images differ in the two eyes. These differences are reconciled into a single image through sensory and motor fusional mechanisms. This study demonstrates electrophysiologically the effects that normal horizontal and vertical fusional processes have on the processing of monocular position signals. METHODS: VEPs were recorded in 16 healthy adults in response to a vernier onset-offset target presented to one eye. The vernier offsets appeared and disappeared at 2 Hz and were introduced into bar targets that were oriented either vertically (horizontal offsets) or horizontally (vertical offsets). The magnitude of the offsets was varied over the range of 0.5 to 10 arc min. VEP amplitude was measured as a function of the size of the dynamic offset under monocular viewing conditions and in the presence of two different static targets presented to the other eye. One of the static targets matched the dynamic test, except that it had no vernier offsets. The other static target, the static pedestal, matched the dynamic test, but contained a set of static vernier offsets in locations corresponding to the locations of the dynamic offsets presented to the other eye. RESULTS: VEP amplitude was a monotonically increasing function of vernier offset size under monocular viewing conditions. The addition of the static target without offsets in the other eye resulted in an increased amplitude VEP response. The addition of the static target with vernier offsets resulted in a decrease in VEP amplitude for both horizontal and vertical disparities. CONCLUSIONS: The normal process of fusion results in a single visual direction. To obtain a single visual direction, the visual system must synthesize a binocular visual direction that differs from the monocular components. One of the conditions (the static pedestal with offsets) produces binocular visual direction shifts that degrade the appearance of vernier onset-offset, and reduce VEP amplitude for both horizontal and vertical disparities. This characteristic evoked response marker is a promising tool for measuring binocular fusion objectively in patients with strabismus.

Adult↗

PubMed2005

Measurement of position acuity in strabismus and amblyopia: specificity of the vernier VEP paradigm.

PURPOSE: An objective measure of positional acuity is desirable in the nonverbal clinical population. This study was conducted to investigate the specificity of the vernier VEP as a measure of positional acuity, evaluating the potential confound of asymmetric motion responses that may be present in some groups of patients. These motion responses could masquerade as position-specific responses, since they occur at the same response frequency as the vernier-related response. METHODS: Twelve observers with early-onset esotropia (EOE), 30 children with untreated amblyopia, and 15 control children underwent swept vernier VEP acuity testing accompanied by a swept motion control stimulus. The control condition was used to detect the presence of artifactual responses not related to position sensitivity. The patients with EOE were selected for high levels of motion asymmetry as documented with oscillating gratings presented monocularly. As a measure of motion confound (penetration), the proportion of first-harmonic responses recorded in the control condition was determined. RESULTS: The penetration rate in the vernier condition in each study group (EOE: 0.93%; amblyopes: 4.26%; normal subjects: 2.40%) and the entire group (2.85%) was acceptably low. The level of penetration was not significantly influenced by the presence of amblyopia. CONCLUSIONS: The vernier VEP paradigm, when applied in the manner described, can be interpreted as a measure of position sensitivity. The presence of motion asymmetry or untreated amblyopia does not affect the validity of vernier measurements made.

Adolescent↗

PubMed2003

Development of the spatial organization and dynamics of lateral interactions in the human visual system.

Psychophysical thresholds and neuronal responses for isolated stimuli are strongly modified by nearby stimuli in the visual field. We studied the orientation and position specificity of these contextual interactions using a dual-frequency visual-evoked potential technique in developing human infants and adults. One set of small, oriented stimulus elements (targets) was tagged with a temporal frequency f1 of 4.52 Hz. The addition of an abutting second set of similar patches (flankers) tagged at f2 = 2.58 Hz had three effects: (1) The flankers reduced the second and fourth harmonic responses to the targets. This reduction was independent of flanker orientation or position and age. (2) The response to the combination of targets and flankers also contained nonlinear interaction terms (1f1 +/- 1f2) that were tuned for flanker orientation and position in adults, but only for flanker orientation in infants 8-31 weeks of age. (3) Nonlinear interaction terms recorded at 2f1 +/- 2f2 were large and untuned for flanker orientation and position in adults but were nearly absent in the youngest infants. The three forms of nonlinear interaction, thus, have differences in sensitivity to flanker orientation and position and differential growth trends, indicating that they are generated by different mechanisms. These three forms of interaction could serve different functional roles. The first process provides a nonselective gain control that is fully functional in early infancy. The second process, which develops slowly, is selective for the specific form of the stimuli. The third process, which is also immature, pools across orientation.

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↗

PubMed2002

Neural correlates of object-based attention.

Much research has been directed toward disentangling the "units" of attention: Is attention directed to locations in space, visual objects, or to individual features of an object? Moreover, there is considerable interest in whether attention increases the gain of neural mechanisms (signal enhancement) or acts by other means, such as reducing noise or narrowing channel tuning. To address these questions, we used a direct measure of signal strength: the amplitude of visual evoked potentials and a task in which selection could be based on a depth order cue but not on location. Attended and nonattended stimuli were presented at different temporal frequencies, and, thus, responses to the two stimuli could be analyzed separately even though they were presented simultaneously. Attention increased the amplitude of the second harmonic component of the response, but not the fourth harmonic. In addition, responses measured at the second harmonic, but not at the fourth harmonic, were larger for stimuli seen as behind. The results are consistent with the fourth harmonic being generated at a stage of processing that is not accessible to attention and where depth order has not been extracted. The second harmonic, on the other hand, is modifiable by attention and shows evidence for differential encoding of depth order.

Adult↗