Biomedical subjects

Curtis L Baker

Publications and source records attributed to Curtis L Baker.

7 recordsLinked to original sources

PubMed2005

Contrast response in visual cortex: quantitative assessment with intrinsic optical signal imaging and neural firing.

While previous studies showed that intrinsic optical signals spatially correspond with electrophysiological responses in mammalian visual cortex, the quantitative correspondence of their response strengths is open to question. Measurement of both signals' strength as functions of visual stimulus contrast provides an opportunity for quantitative comparison. Towards that end, the spatial and temporal properties of the optical signal impose important constraints upon quantification of its strength. We used intrinsic optical signal imaging and single unit recording to measure responses to drifting gratings at contrasts ranging from 10-80% in cat area 18. We calculated the average difference images for pairs of oppositely moving, or orthogonally oriented, gratings at each contrast and evaluated three different methods for quantifying optical signal strength. After about 2.5 s, the spatial patterns of optical images and the time course of their strength were contrast-invariant. This "space-time-contrast separability" for optical response implies a spatial uniformity of the optical contrast response functions, provides an objective basis to guide temporal averaging of optical signals, and validates a scalar metric of optical signal strength. Optically measured contrast response functions increase monotonically and saturate at high contrasts, qualitatively resembling those from single units. However, quantitative comparison reveals a nonlinear relationship with neural firing, such that the optical response reaches half of its maximum when the neural response has reached only around 20% of its maximum. This relationship suggests that intrinsic optical signals are relatively more sensitive to weak signals than neural firing.

Animals↗

PubMed2004

First- and second-order information in natural images: a filter-based approach to image statistics.

Previous analyses of natural image statistics have dealt mainly with their Fourier power spectra. Here we explore image statistics by examining responses to biologically motivated filters that are spatially localized and respond to first-order (luminance-defined) and second-order (contrast- or texture-defined) characteristics. We compare the distribution of natural image responses across filter parameters for first- and second-order information. We find that second-order information in natural scenes shows the same self-similarity previously described for first-order information but has substantially less orientational anisotropy. The magnitudes of the two kinds of information, as well as their mutual unsigned correlation, are much stronger for particular combinations of filter parameters in natural images but not in unstructured fractal images having the same power spectra.

Algorithms↗

PubMed2003

Failure of signed chromatic apparent motion with luminance masking.

It has been suggested that there are two types of chromatic motion mechanisms: signed chromatic motion, in which correspondence across successive frames is based on chromatic content of image regions, and unsigned chromatic motion based on movement of chromatically-defined borders. We investigate whether signed and unsigned red-green chromatic motion are mediated by a genuinely chromatic mechanism. Direction discrimination of signed and unsigned red-green chromatic motion were measured in the presence of a dynamic luminance masking noise. Increasing the luminance noise contrast systematically impaired signed motion, regardless of contrast and speed. This result suggests that signed red-green chromatic motion is derived from a luminance-based signal, rather than a genuinely chromatic motion mechanism. In the case of unsigned chromatic motion, there is no effect of luminance masking noise, indicating there exists a genuine chromatic mechanism for second-order motion perception.

Color Perception↗

PubMed2003

Luminance mechanisms mediate the motion of red-green isoluminant gratings: the role of "temporal chromatic aberration".

In this paper we use a dynamic noise-masking paradigm to explore the nature of the mechanisms mediating the motion perception of drifting isoluminant red-green gratings. We compare contrast thresholds for the detection and direction discrimination of drifting gratings (1.5 cpd), over a range of temporal frequencies (0.5-9 Hz) in the presence of variable luminance or chromatic noise. In the first experiment, we used dynamic luminance noise to show that direction thresholds for red-green grating motion are masked by luminance noise over the entire temporal range tested, whereas detection thresholds are unaffected. This result indicates that the motion of nominally isoluminant red-green gratings is mediated by luminance signals. We suggest that stimulus-based luminance artifacts are not responsible for this effect because there is no masking of the detection thresholds. Instead we propose that chromatic motion thresholds for red-green isoluminant gratings are mediated by dynamic luminance artifacts that have an internal, physiological origin. We have termed these "temporal chromatic aberration". In the second experiment, we used dynamic chromatic noise masking to test for a chromatic contribution to red-green grating motion. We were unable to find conclusive evidence for a contribution of chromatic mechanisms to the chromatic grating motion, although a contribution at very high chromatic contrasts cannot be ruled out. Our results add to a growing body of evidence indicating the presence of dynamic, internal luminance artifacts in the motion of chromatic stimuli and we show that these occur even at very low temporal rates. Our results are compatible with our previous work indicating the absence of a chromatic mechanism for first order (quasi-linear) apparent motion [Vision Res. 40 (2000) 1993]. We conclude that previous conclusions based on the motion of chromatic red-green gratings should be reassessed to determine the contribution of dynamic luminance artifacts.

Color Perception↗

PubMed2003

Automatic volumetric segmentation of human visual retinotopic cortex.

Previous identification of early visual cortical areas in humans with phase-encoded retinotopic mapping techniques have relied on an accurate cortical surface reconstruction. Here a 3D phase-encoded retinotopic mapping technique that does not require a reconstruction of the cortical surface is demonstrated. The visual field sign identification is completely automatic and the method directly supplies volumes for a region-of-interest analysis, facilitating the application of cortical mapping to a wider population. A validation of the method is provided by simulations and comparison to cortical surface-based methodology.

Adult↗

PubMed2003

Cortical specialization for processing first- and second-order motion.

Distinct mechanisms underlying the visual perception of luminance- (first-order) and contrast-defined (second-order) motion have been proposed from electrophysiological, human psychophysical and neurological studies; however a cortical specialization for these mechanisms has proven elusive. Here human brain imaging combined with psychophysical methods was used to assess cortical specializations for processing these two kinds of motion. A common stimulus construction was employed, controlling for differences in spatial and temporal properties, psychophysical performance and attention. Distinct cortical regions have been found preferentially processing either first- or second-order motion, both in occipital and parietal lobes, producing the first physiological evidence in humans to support evidence from psychophysical studies, brain lesion sites and computational models. These results provide evidence for the idea that first-order motion is computed in V1 and second-order motion in later occipital visual areas, and additionally suggest a functional dissociation between these two kinds of motion beyond the occipital lobe.

Adult↗

PubMedDate not supplied

The direction-selective contrast response of area 18 neurons is different for first- and second-order motion.

Cortical neurons selective for the direction of motion often exhibit some limited response to motion in their nonpreferred directions. Here we examine the dependence of neuronal direction selectivity on stimulus contrast, both for first-order (luminance-modulated, sine-wave grating) and second-order (contrast-modulated envelope) stimuli. We measured responses from single neurons in area 18 of cat visual cortex to both kinds of moving stimuli over a wide range of contrasts (1.25-80%). Direction-selective contrast response functions (CRFs) were calculated as the preferred-minus-null difference in average firing frequency as a function of contrast. We also applied receiver operating characteristic analysis to our CRF data to obtain neurometric functions characterizing the potential ability of each neuron to discriminate motion direction at each contrast level tested. CRFs for sine-wave gratings were usually monotonic; however, a substantial minority of neurons (35%) exhibited nonmonotonic CRFs (such that the degree of direction selectivity decreased with increasing contrast). The underlying preferred and nonpreferred direction CRFs were diverse, often having different shapes in a given neuron. Neurometric functions for direction discrimination showed a similar degree of heterogeneity, including instances of nonmonotonicity. For contrast-modulated stimuli, however, CRFs for either carrier or envelope contrast were always monotonic. In a given neuron, neurometric thresholds were typically much higher for second- than for first-order stimuli. These results demonstrate that the degree of a cell's direction selectivity depends on the contrast at which it is measured, and therefore is not a characteristic parameter of a neuron. In general, contrast response functions for first-order stimuli were very heterogeneous in shape and sensitivity, while those for second-order stimuli showed less sensitivity and were quite stereotyped in shape.

Animals↗