Biomedical subjects

M M Conte

Publications and source records attributed to M M Conte.

12 recordsLinked to original sources

PubMed1997

Nonlinear preprocessing in short-range motion.

The phenomenon of non-Fourier motion (visually perceived motion that cannot be explained simply on the basis of the autocorrelation structure of the visual stimulus) is well recognized, and is generally considered to be due to nonlinear preprocessing of the visual stimulus prior to a stage of standard motion analysis. We devised a sequence of novel visual stimuli in which the availability of a motion stimulus depends on the nature of the nonlinear preprocessing: an nth order stimulus Pn will generate a perception of motion if it is preprocessed by a nonlinearity of polynomial order n or greater, but not if preprocessed by a nonlinearity of polynomial order less than n. We found that unambiguous motion direction was perceived for P2, P3, and P4, but not for higher-order stimuli, and we measured the contrast thresholds for direction discrimination with superimposed noise. We found that an asymmetric compressive nonlinearity can, in a unified fashion, account for these results, while a purely quadratic nonlinearity or a rectification of the form T(p) = magnitude of p cannot. We compared velocity discrimination judgements for second-order non-Fourier stimuli (P2) with standard drifting gratings. Although velocity comparisons were veridical, uncertainties were greater for the non-Fourier stimuli. This could be reproduced by substituting a Fourier grating with superimposed noise for the non-Fourier grating. These findings are consistent with a single pathway which processes both Fourier and non-Fourier short-range motion, and are discussed in the context of other investigations which have been interpreted as demonstrating separate pathways.

Adult↗

PubMed1996

The role of high-order phase correlations in texture processing.

Isodipole textures are pairs of texture ensembles whose autocorrelations, and hence power spectra, are equal. Examples of readily discriminable isodipole textures are well known. Such discriminations appear to require feature extraction, since the isodipole condition eliminates ensemble differences in spatial frequency content. We studied the effects of phase decorrelation on VEP indices of discrimination of isodipole texture pairs. Phase decorrelation, which ranged from 0.125 pi radians (slight randomization) to pi radians (complete randomization), was introduced in two ways: by independent jittering of each spatial Fourier component, and by a product method, which preserved correlations among certain quadruples of spatial Fourier components, despite pairwise decorrelation. For the even/random isodipole texture pair, independent phase decorrelation greater than 0.5 pi radians markedly reduced VEP indices of texture discrimination for all check sizes, and eliminated them entirely for check sizes of 8 min or greater. However, the product method preserved texture discrimination signals even with complete pairwise randomization of spatial phases. For the triangle/random isodipole texture pair, both kinds of phase decorrelation eliminated VEP indices of texture discrimination. These results imply that isodipole texture discrimination is based on fundamentally local processing, and not on global Fourier amplitudes-since the phase manipulations which eliminate texture discrimination preserve the Fourier amplitudes. The dependence of the antisymmetric response component (the odd harmonics) on phase decorrelation and texture type is consistent with a previously proposed model for feature extraction, and leads to constraints on how texture processing is modulated by contrast. The limited contribution of global spectral characteristics for small checks is consistent with a previously identified breakdown in scale-invariant processing.

Adult↗

PubMed1994

Investigation of a patient with severely impaired direction discrimination: evidence against the intersection-of-constraints model.

A man with presumed posterior cortical atrophy had a markedly elevated threshold for orientation discrimination (approx. 25 deg) and selective impairment of "pop-out" tasks based on orientation. Direction discrimination for moving plaids was superior to direction discrimination for their component gratings. The superior performance for plaids disappeared when the spatial frequencies of the component gratings were altered to eliminate coherence. This finding implies that extraction of plaid motion is not dependent on pre-processing within narrow spatial frequency bands. It is inconsistent with simulations based on the "intersection of constraints" model, which predict that the error rate for plaids would be larger than the error rate for gratings, particularly for the plaids composed of gratings moving at nearly opposing angles. It is consistent with models such as the Heeger [(1987) Journal of the Optical Society of America A, 4, 1455-1471] model, which extract direction from the pattern of activity across broadly-tuned spatiotemporal filters.

Aged↗

PubMed1992

Evoked potential and psychophysical analysis of Fourier and non-Fourier motion mechanisms.

Some visual stimuli produce a strong percept of motion, even though they fail to excite motion detectors based on Fourier energy or cross correlation. Models which suffice to explain the motion percept in these non-Fourier motion (NFM) stimuli include linear spatiotemporal filtering, followed by rectification, followed by standard motion analysis (Chubb & Sperling 1988). We used the human "motion-onset" evoked potential, which has been assigned to area 17 on the basis of work in the macaque (van Dijk et al., 1986; van Dijk & Spekreijse, 1989), to investigate the neural substrate of the processing stages postulated in the above models. Motion-onset VEPs elicited by FM and NFM matched for spatial and temporal characteristics were indistinguishable in temporal characteristics and scalp topography at a transverse chain of electrodes. Addition of textural cues (granularity and higher-order form) did not influence the response dynamics or scalp topography of NFM responses. However, comparison of responses to NFM stimuli and related stimuli without coherent motion but similar spatial and temporal properties showed that the motion-onset responses were distinct from responses to the onset of fixed flicker-defined contours not undergoing coherent motion. We discuss the implications of these results for computational models of motion analysis.

Adult↗

PubMed1992

Coherence and transparency of moving plaids composed of Fourier and non-Fourier gratings.

We examined the perceptual coherence of two-component moving plaids. The gratings that constituted the plaids were either standard Fourier gratings (F), in which luminance was determined by a drifting sinusoid, or non-Fourier gratings (NF), in which the contrast of a random background was modulated by a drifting sinusoid. These NF gratings are examples of stimuli that generate a compelling percept of motion, even though they fail to elicit a motion signal from motion analyzers based on standard cross-correlation (Chubb & Sperling, 1988). Naive observers viewed three types of stimuli consisting of superpositions of these two components: (1) two standard drifting gratings (F/F), (2) two non-Fourier drifting gratings (NF/NF), and (3) one standard and one non-Fourier drifting grating (F/NF). As expected, the F/F stimulus yielded a compelling percept of coherent motion. The dominant percept of all the observers for the NF/NF stimulus was one of coherent motion, provided that both gratings were visible and of approximately equal contrast. None of the observers reported a dominant percept of coherent motion for the F/NF condition, over a wide range of contrasts for the two grating components and across two varieties of NF gratings. In view of the results of Albright (1992) and Albright and Chaudhuri (1989), that show that single cells in macaque V1 and MT respond to both F and NF motion, one cannot interpret our findings as evidence that F and NF motion are processed independently. Alternative, "higher level" interpretations based on the intrinsically ambiguous nature of the stimuli and physical laws governing the appearance of transparent objects are discussed.

Adult↗

PubMed1991

Spatial organization of nonlinear interactions in form perception.

We examined the perception of structure in a family of visual textures whose second-order correlation structure is flat. These textures were generated by two-dimensional recursion rules, in a manner which extends the construction of Julesz, Gilbert and Victor (1978; Biological Cybernetics, 31, 137-140). Textures generated by some recursion rules elicited a visually salient percept of structure, while textures generated by other recursion rules did not. Textures whose statistical structure was visually salient produced evoked responses which differed from the response evoked by completely random textures. The size of this VEP difference correlated well with psychophysical measures. Since the textures were constructed to have identical global spatial frequency spectra, models for the extraction of visual structure must be essentially nonlinear. Models based on symmetry, information content, or simple spatial extent (but not pattern) of correlation fail to explain the observed results. Models based on the cooperative interaction of pairs of nonlinear subunits provide a reasonable qualitative account of the findings. The critical model features are (i) the presence of multiple nonlinear subunits, and (ii) a second nonlinearity, such as a threshold, at the stage of combination of subunit signals.

Adult↗

PubMed1990

Motion mechanisms have only limited access to form information.

We investigate the roles of spatial frequency content, flicker and higher-order elements of form ("features") in the generation of motion percepts. These cues are separated through the use of dynamic visual stimuli based on stochastic textures. Flicker alone and spatial frequency content alone suffice to generate a strong motion percept, but higher-order elements of form alone generate a much weaker motion percept. Thus, even for achromatic stimuli, all pattern information is not equally available for motion processing. Furthermore, higher-order form information, which by itself does not provide a strong cue to motion, is shown to interact with other visual information to facilitate determination of direction of motion.

Adult↗

PubMed1989

Cortical interactions in texture processing: scale and dynamics.

We investigate the neural computations underlying pattern processing with stimuli based on textures balanced for spatial frequency content (and second-order correlations) but not for higher-order correlations (Julesz et al. 1978). Interchange between two such isodipole textures produces a robust human visual evoked potential (VEP). The difference in population activity driven by two isodipole textures is quantified by the antisymmetric component of the VEP. Statistical properties of the textures eliminate contributions from linear mechanisms to the antisymmetric VEP. The dependence of the antisymmetric VEP on check size and fourth-order correlation statistics is used to test nonlinear models for the underlying neural computations. Linear summation, followed by a simple nonlinearity (such as rectification, saturation, or threshold), is inconsistent with the data. More elaborate models, in which a second nonlinear stage combines the output of local nonlinear mechanisms, are consistent with the data, provided that an appropriate spatial scale is chosen for the second stage of processing. For checks 4 min or smaller, the deduced interaction length is 10-15 min. For checks larger than 4 min, the interaction length is proportional to check size.

Adult↗

PubMedDate not supplied

Visual evoked potentials in dyslexics and normals: failure to find a difference in transient or steady-state responses.

We measured transient and steady-state checkerboard contrast-reversal visual evoked potentials (VEPs) in ten dyslexics, five patient controls, and 11 normals over a range of contrasts and luminances. Latency, amplitude, and phase measurements failed to distinguish the responses of dyslexics from those of normals or patient controls. Decreases in luminance or contrast resulted in an increased latency of the transient VEP in all groups, but these changes also did not distinguish the responses of dyslexics from those of the controls. Response variability was similar in dyslexics and normals, but was increased in subjects with attention deficit-hyperactivity disorder (ADHD). Performance on standardized psychometric testing did differentiate the dyslexics from controls, but did not correlate with VEP responses.

Adolescent↗

PubMedDate not supplied

Dynamic shifts of the contrast-response function.

We recorded visual evoked potentials in response to square-wave contrast-reversal checkerboards undergoing a transition in the mean contrast level. Checkerboards were modulated at 4.22 Hz (8.45-Hz reversal rate). After each set of 16 cycles of reversals, stimulus contrast abruptly switched between a "high" contrast level (0.06 to 1.0) to a "low" contrast level (0.03 to 0.5). Higher contrasts attenuated responses to lower contrasts by up to a factor of 2 during the period immediately following the contrast change. Contrast-response functions derived from the initial second following a conditioning contrast shifted by a factor of 2-4 along the contrast axis. For low-contrast stimuli, response phase was an advancing function of the contrast level in the immediately preceding second. For high-contrast stimuli, response phase was independent of the prior contrast history. Steady stimulation for periods as long as 1 min produced only minor effects on response amplitude, and no detectable effects on response phase. These observations delineate the dynamics of a contrast gain control in human vision.

Adaptation, Physiological↗

PubMedDate not supplied

Chromatic and luminance interactions in spatial contrast signals.

We report VEP studies which delineate interactions between chromatic and luminance contrast signals. We examined responses to sinusoidal luminance gratings undergoing 4-Hz square-wave contrast reversal, upon which standing gratings with various admixtures of luminance and chromatic contrast were alternately superimposed and withdrawn. The presence of the standing grating induced a VEP component at the fundamental frequency of the contrast-reversal grating. This VEP component appeared without any appreciable lag, and did not vary in amplitude over the 4 s during which the standing grating was present. The observed fundamental response differed from the fundamental component that would be expected from the known interaction between the luminance component of the standing grating with the modulated grating (Bodis-Wollner et al., 1972; Bobak et al., 1988), in three ways: (1) The fundamental response was not nulled for standing gratings that were isoluminant or near-isoluminant. (2) The chromatic dependence of the fundamental response implied an S-cone input to the interaction. (3) No single mechanism (driven by a linear combination of cone signals) could account quantitatively for the size of this response, particularly when the standing grating strongly modulated two cones in phase.

Adult↗

PubMedDate not supplied

Stimulus orientation and contrast constancy.

The human visual system requires less contrast to detect patterns oriented vertically or horizontally than those oriented obliquely. We investigated whether this orientational anisotropy persists at suprathreshold contrasts. Using a contrast-matching technique, we found that it disappears at contrasts just above threshold, given the stimulus conditions employed. These results suggest that a neural mechanism, at suprathreshold, adjusts the gain of orientational subsystems to compensate for the lower sensitivity of the visual system to oblique patterns. A similar suprathreshold effect, referred to as contrast constancy, has been observed in studies employing grating patterns varying in spatial and temporal frequency. We argue, based on previous electrophysiological findings, that this neural compensation results from antagonism between excitatory and inhibitory processes in the visual cortex.

Anisotropy↗