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J D Victor

Publications and source records attributed to J D Victor.

At least 55 records · Page 3Linked to original sources
PubMed1994

Population encoding of spatial frequency, orientation, and color in macaque V1.

1. We recorded local field potentials in the parafoveal representation in the primary visual cortex of anesthetized and paralyzed macaque monkeys with a multicontact electrode that provided for sampling of neural activity at 16 sites along a vertical penetration. Differential recordings at adjacent contacts were transformed into an estimate of current source density (CSD), to provide a measure of local neural activity. 2. We used m-sequence stimuli to map the region of visual space that provided input to the recording site. The local field potential recorded in macaque V1 has a population receptive field (PRF) size of approximately 2 deg2. 3. We assessed spatial tuning by the responses to two-dimensional Gaussian noise, spatially filtered to retain power only within one octave. Responses to achromatic band-limited noise stimuli revealed a prominent band-pass spatial tuning in the upper layers, but a more low-pass spatial tuning in lower layers. 4. We assessed orientation tuning by the responses to band-limited noise whose spectrum was further restricted to lie within 45 degrees wedges. The local field potential showed evidence of orientation tuning at most sites. Orientation tuning in upper and lower layers was manifest by systematic variations not only in response size but also in response dynamics. 5. We assessed chromatic tuning by the responses to isotropic band-limited noise modulated in a variety of directions in tristimulus space. Some lower-layer locations showed a nulling of response under near-isoluminant conditions. However, response dynamics in upper and lower layers depended not only on luminance contrast, but also on chromatic inputs. 6. Responses to near-isoluminant stimuli and to low-contrast luminance modulation were shifted to lower spatial frequencies. 7. We determined the extent to which various temporal frequencies in the response conveyed information concerning spatial frequency, orientation, and color under the steady-state conditions used in these studies. In each case, information is distributed in the response dynamics across a broad temporal frequency range, beginning at 4 Hz (the lowest frequency used). For spatial frequency the information rate remains significant up to at least 25 Hz. For orientation tuning and chromatic tuning, the information rate is lower overall and remains significant up to 13 Hz. In contrast, for texture discrimination, information is shifted to lower temporal frequencies.

Animals↗
PubMed1994

A continuum of non-Gaussian self-similar image ensembles with white power spectra.

We introduce a scaling procedure that acts on visual textures to produce new textures having the same resolution, display size, and mean contrast power. We derive the form of the scaling fixed-point textures (image ensembles) resulting from repeated application of this scale transformation to the 'even' texture with an arbitrary amount of sporadic decorrelation. The result is a continuum of scaling fixed-point or 'self-similar' image ensembles, ranging from a strongly non-Gaussian white texture with higher-order spatial correlations at one extreme to Gaussian white noise at the other. The simple construction of a continuum of self-similar ensembles possessing phase correlations provides a tool for investigating human perception of structure in the absence of useful length scales. The fixed-point textures have luminance histogram differences, a direct result of their higher-order spatial properties. This suggests that scaling might afford an extension of our understanding of IID (independent identically distributed) texture discrimination (Chubb et al., J. Opt. Soc. Am. A 11, 2350-2374, 1994) to more general texture discrimination tasks in which spatial correlations play a role.

Form Perception↗
PubMed1992

Intra-arterial cisplatin--associated optic and otic toxicity.

Over a 22-month period, we investigated optic and otic toxicity accompanying intra-arterial cisplatin therapy. Baseline and serial neurologic and ophthalmologic examinations, visual evoked potentials, and brain-stem auditory evoked potentials were performed in six patients, aged 37 to 53 years. Patients received infraophthalmic intra-arterial cisplatin (60 mg/m2) every month for three to 10 treatments (mean, six treatments). Five of the six patients had progressive optic toxicity. In two patients, the visual evoked potential prolongation preceded acuity loss by at least 4 months. Two patients had evidence of otic toxicity by either brain-stem auditory evoked potential or click threshold and brain-stem auditory evoked potential. Intra-arterial cisplatin neurotoxicity may be significant in patients with already limited survival. Visual evoked potential and brain-stem auditory evoked potential should be used to monitor patients receiving potentially neurotoxic therapy.

Adult↗
PubMed1992

A relation between the Akaike criterion and reliability of parameter estimates, with application to nonlinear autoregressive modelling of ictal EEG.

The Akaike minimum information criterion provides a means to determine the appropriate number of lags in a linear autoregressive model of a time series. We show that the Akaike criterion is closely related to the reliability estimates of successively determined parameters of a linear autoregressive (LAR) model. A similar criterion may be applied to determine whether the addition of a nonlinear term to an LAR model provides a statistically significant improvement in the description of the time series. As an example, we use this method to identify quadratic contributions to a nonlinear autoregressive characterization of a typical 3/s spike and wave seizure discharge.

Data Interpretation, Statistical↗
PubMed1992

Broadband temporal stimuli decrease the integration time of neurons in cat striate cortex.

We have studied the responses of striate cortical neurons to stimuli whose contrast is modulated in time by either a single sinusoid or by the sum of eight sinusoids. The sum-of-sinusoids stimulus resembles white noise and has been used to study the linear and nonlinear dynamics of retinal ganglion cells (Victor et al., 1977). In cortical neurons, we have found different linear and second-order responses to single-sinusoid and sum-of-sinusoids inputs. Specifically, while the responsivity near the optimal temporal frequency is lower for the sum-of-sinusoids stimulus, the responsivity at higher temporal frequencies is relatively greater. Along with this change in the response amplitudes, there is a systematic change in the time course of responses. For complex cells, the integration time, the effective delay due to a combination of actual delays and low-pass filter stages, changes from a median of 85 ms with single sinusoids to 57 ms with a sum of sinusoids. For simple cells, the integration times for single sinusoids range from 44-100 ms, but cluster tightly around 40 ms for the sum-of-sinusoids stimulus. The change in time constant would argue that the increased sensitivity to high frequencies cannot be explained by a static threshold, but must be caused by a fundamental alteration in the response dynamics. These effects are not seen in the retina (Shapley & Victor, 1981) and are most likely cortical in origin.

Animals↗
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

Asymptotic approach of generalized orthogonal functional expansions to Wiener kernels.

Wiener-like orthogonal functional expansions may be constructed with respect to test ensembles that are non-Gaussian, nonwhite, or both. Although the original Wiener expansion has particularly advantageous analytical properties, orthogonal expansions constructed with respect to other ensembles have practical advantages for laboratory implementation. We show how functional expansions based on two classes of input ensembles--white but non-Gaussian discrete noises and the sum of sinusoids--converge to the standard Wiener kernels. For discrete noises, the disparity between the standard and nonstandard kernels of a linear-static nonlinear transducer is proportional to the kurtosis of the input signal and inversely proportional to the ratio of the integration time of the linear filter to the time discretization. For the sum of sinusoids, the disparity is inversely proportional to the effective number of sinusoids passed by the initial linear stage.

Models, Statistical↗
PubMed1991

A new statistic for steady-state evoked potentials.

Steady-state evoked potentials are often characterized by the amplitude and phase of the Fourier component at one or more frequencies of interest. We introduce a new statistic for the evaluation of these Fourier components. This statistic, denoted T2circ, is based on the same physiologic assumptions concerning the sources of variability of a Fourier component that are made in the use of the Rayleigh phase-coherence statistic as well as the standard T2 statistic (Hotelling 1931) for multivariate data. However, the T2circ statistic also exploits the relationship between the real and imaginary components of Fourier estimates, which is not exploited by T2, and utilizes amplitude information, which is ignored by the Rayleigh criterion. For these reasons, the T2circ statistic is more efficient than previously used criteria for detection and quantitation of steady-state responses, both in principle and in practice.

Adult↗
PubMed1991

Fluctuations of steady-state VEPs: interaction of driven evoked potentials and the EEG.

We performed a detailed analysis of the variability of a steady-state human evoked potential (EP) and the spectral properties of the simultaneously recorded electroencephalogram (EEG). This allowed us to determine whether the background EEG was influenced by the evoked potential stimulus, and to what extent variability of evoked potential estimates is simply due to the addition of the background EEG. Steady-state visual evoked potentials (VEPs) were elicited by a checkerboard undergoing contrast-reversal modulation at 1 of 3 fundamental frequencies f: 5.0 Hz, 7.5 Hz, and 10.0 Hz. To a first approximation, the evoked potential (at frequency 2 f) and the undriven components of the EEG combined linearly. However, two kinds of interactions were present: (i) patterned visual stimulation decreased the power in the undriven EEG in the 5-17 Hz range by as much as a factor of 2; (ii) superimposed on this overall effect of pattern stimulation, there were changes in the EEG power at specific harmonics of the input frequency. Power increased by as much as 6-fold at the stimulus reversal rate (2 f) and its second harmonic (4 f). These findings imply a complex non-linear interaction between the visual input and the EEG.

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↗
PubMed1989

Temporal impulse responses from flicker sensitivities: causality, linearity, and amplitude data do not determine phase.

A recent paper [J. Opt. Soc. Am. A 4, 1130 (1987)] advances a method for the determination of an impulse response from amplitudes of a measured amplitude spectrum. The crucial step in this derivation is the assumption that if a transfer function is analytic and causal, then so is its logarithm. This assumption is tantamount to that of minimum phase and is not justified, both in principle and in practice. In the absence of additional information or assumptions, every amplitude spectrum is consistent with a multiplicity of phase spectra.

Algorithms↗
PubMed1988

The dynamics of the cat retinal Y cell subunit.

1. The dynamics of the subunit mechanism of individual cat Y retinal ganglion cells are investigated. In order to isolate the response of the non-linear subunit mechanism, the visual stimuli were sine gratings of a spatial frequency sufficiently high so that contrast reversal of the grating elicited no fundamental response in any spatial phase. For study of the non-linear subunit mechanism, the contrast of the spatial spine grating was varied in time by a temporal modulation signal, consisting of either a square wave or a sum of sinusoids. 2. The responses of twenty-three Y ganglion cells (sixteen on-centre, seven off-centre) to these two stimulus types were measured at a range of contrasts. Responses to the sum-of-sinusoids signal were characterized by the second-order frequency kernel. The overall size of the second-order frequency kernel was approximately proportional to contrast. The deviation from proportionality suggested a power-law scaling, with a power in the range 0.8-0.9. 3. Square-wave responses, as characterized by the post-stimulus histogram, demonstrated identical responses at both reversals of the grating. A similar contrast dependence was observed in the overall size of the square-wave responses. 4. In order to attempt to predict the square-wave responses from the sum-of-sinusoids responses, the second-order frequency kernel measured at each contrast level was fitted with a lumped linear-static non-linear-linear model. In eighteen of twenty-three cells (eleven on-centre, seven off-centre), this model provided an adequate description of the response to the sum-of-sinusoids stimulus. In these cells, the linear-static non-linear-linear model accurately reproduced the square-wave response. 5. In the remaining five ganglion cells (all on-centre), the second-order frequency kernel could not be fitted by a linear-static non-linear-linear model. This diversity of dynamical properties among Y cells was not apparent from the responses of these Y cells to the square-wave temporal stimulus. 6. In the eighteen Y ganglion cells that were fitted well with the linear-static non-linear-linear model, substantial variation of the dynamical parameters was found. However, there were systematic differences between the dynamics of the typical on-centre and off-centre ganglion cells. These differences relate to both linear stages of the model, and are not merely consequences of the lower firing rate of the off-centre cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1988

Models for preattentive texture discrimination: Fourier analysis and local feature processing in a unified framework.

Spatial frequency analysis and local feature analysis may be considered to be examples of a class of models for texture discrimination. In this theoretical framework, texture discrimination relies on differences in the distribution of responses generated in linear receptive fields placed randomly on the texture. If the set of receptive fields is taken to be a collection of gratings, spatial-frequency analysis is recovered. If the set of receptive-fields is taken to be a collection of local feature templates, a corresponding local-feature model is recovered. In order to test such models, it is necessary to construct distinct texture pairs that elicit similar distributions of responses for all of the postulated receptive field profiles: the model prediction is that such textures are not discriminable. A method is provided for construction of such textures which test generic models within this framework. This framework includes not only strict Fourier analysis, but also models which postulate a collection of arbitrarily-shaped local feature detectors, and models which postulate both Fourier analysis and local feature detection.

Form Perception↗