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D J McKeefry

Publications and source records attributed to D J McKeefry.

15 recordsLinked to original sources
PubMed2006

The segregation and integration of colour in motion processing revealed by motion after-effects.

Analysis of the colour and motion of objects is widely believed to take place within segregated processing pathways in the primate visual system. However, it is apparent that this segregation cannot remain absolute and that there must be some capacity for integration across these sub-modalities. In this study, we have assessed the extent to which colour constitutes a separable entity in human motion processing by measuring the chromatic selectivity of two kinds of after-effect resulting from motion adaptation. First, the traditional motion after-effect, where prolonged inspection of a unidirectional moving stimulus results in illusory motion in the opposite direction, was found to exhibit a high degree of chromatic selectivity. The second type of after-effect, in which motion adaptation induces misperceptions in the spatial position of stationary objects, was completely insensitive to chromatic composition. This dissociation between the chromatic selectivities of these after-effects shows that chromatic inputs remain segregated at early stages of motion analysis, while at higher levels of cortical processing there is integration across chromatic, as well as achromatic inputs, to produce a unified perceptual output.

Adaptation, Physiological↗
PubMed2002

The influence of stimulus chromaticity on the isoluminant motion-onset VEP.

Motion-onset visual evoked potentials (VEPs) were elicited by low spatial frequency chromatic isoluminant gratings presented in a central 7 degrees circular field. The chromatic composition of the stimuli was varied so as to modulate along different axes in colour space. For slow speeds (<5 degrees/s) changing the chromatic axis induced large response differences between the S- and L/M-cone VEPs. At faster speeds (5-12 degrees/s) the effects were not as marked. A dichotomy between the slow and fast responses was also shown to exist in terms of their contrast dependencies, the former exhibiting a stronger dependency on contrast than the latter. These findings suggest that neural substrates with chromatic sensitivity are involved in the generation of S- and L/M-cone mediated motion-onset VEPs at low velocities. At higher velocities, responses are generated by different mechanisms that possess little or no chromatic sensitivity.

Adult↗
PubMed2001

Red--green and blue--yellow mechanisms are matched in sensitivity for temporal and spatial modulation.

The spatial and temporal properties of human colour vision are examined using isoluminant, red--green and blue--yellow tritanopic gratings. Chromatic sensitivity is found to be low-pass as a function of both spatial and temporal frequency along all the chromatic axes investigated, including the tritanopic confusion lines employed to examine the properties of the S-cone driven mechanism. Comparison of sensitivity to on-off and contrast reversing stimuli indicates that transient mechanisms contribute to the detection of red--green patterns but that the detection of S-cone specific patterns is governed by sustained mechanisms. By compensating for transient contributions to red--green sensitivity, it is shown that sensitivity of chromatic mechanisms dominated by L- and M-cone input are closely matched to those with S-cone input.

Color Perception↗
PubMed2001

Visual evoked potentials elicited by chromatic motion onset.

Visually Evoked Potentials (VEPs) were recorded in response to the onset of chromatic and luminance motion gratings of 1 cpd and luminance 40 cd m(-2) subtending a 7 degrees field. At slow speeds (< or =2 cycles s(-1)) the motion onset response exhibits a clear amplitude minimum at isoluminance. Over the Michelson contrast range tested (0.05-0.75) the chromatic response at 2 cycles s(-1) possesses a linear response function compared to the saturating function of the luminance response and the contrast dependency of the former is a factor of 5-6 times greater than for the latter. These differences are suggestive of different neural substrates for the chromatic and luminance motion VEPs at slow speeds. At 10 cycles s(-1) the chromatic motion onset VEP exhibits no amplitude minimum at isoluminance and becomes more like its luminance counterpart in terms of its saturating contrast response function. Furthermore, the contrast dependency of the chromatic and luminance responses differs by only a factor of 1.6 at this faster rate. These findings are consistent with the idea of separate motion mechanisms that operate at fast and slow speeds, the latter having separate channels for colour and luminance motion.

Adult↗
PubMed2001

Motion adaptation in chromatic motion-onset visual evoked potentials.

The aim of this study was to investigate the influence of motion adaptation on visual evoked potentials (VEPs) elicited by the onset of isoluminant chromatic motion. VEPs were recorded from the occipital cortex of human subjects using a sinusoidal grating stimulus of one cycle per degree which moved at either a velocity of 2 or 10 degrees/s and subtended a field of 7 degrees with a mean luminance of 30 cdm(-2). In the first experiment the effects of adaptation were investigated via the manipulation of the stimulus duty cycle which was varied between 11-90%. The results showed a significant (p < 0.001) reduction in the N2-P2 amplitude of the chromatic response. In contrast, P1-N2 amplitude was not significantly affected by motion adaptation. Subsequent experiments demonstrated that the chromatic motion onset VEP was attenuated not only following adaptation to isoluminant chromatic motion, but also to luminance motion as well. These results indicate that the chromatic motion onset VEP is just as susceptible as its luminance counterpart to motion after effects (MAEs) and as a result it is highly likely that it is a motion specific response. Furthermore, the fact that the VEP shows that there are cross-adaptation effects between motion defined by change in colour and by change in luminance, suggests that the two types of motion stimuli have inputs into a common motion mechanism.

Adaptation, Ocular↗
PubMed1997

The activity in human areas V1/V2, V3, and V5 during the perception of coherent and incoherent motion.

We have used the technique of positron emission tomography to study and compare the cortical activity produced when humans view a pattern of small squares moving incoherently with respect to one another and when the same pattern moves coherently and unidirectionally. A stationary version of the stimulus acted as a control. Our choice of paradigm was inspired by psychophysical models and physiological studies in the macaque monkey which show that directionally selective cells in V5 respond optimally to unidirectional coherent motion, whereas those of V1 respond to motion within their receptive fields, regardless of the motion in surrounding parts. Our results show that human V1/V2, V3, and V5 are all activated by both types of motion stimuli. Incoherent motion, however, proved to be more effective than coherent motion in activating V1/V2 and V5. Thus the higher perceptual salience of unidirectional coherent motion in comparison to incoherent motion is not reflected by any increased activation of human area V5.

Adult↗
PubMed1997

The position and topography of the human colour centre as revealed by functional magnetic resonance imaging.

We used a colour Mondrian--an abstract scene with no recognizable objects--and its achromatic version to image the change in blood oxygenation in the brains of 12 human subjects, with the aim of learning more about the position and variability of the colour centre in the human brain. The results showed a consistent association of colour stimulation with activation of an area that is distinct from the primary visual areas, and lies in the ventral occipitotemporal cortex; we refer to it as human V4. The position of human V4, as defined on functional grounds, varies between individuals in absolute terms but is invariably found on the lateral aspect of the collateral sulcus on the fusiform gyrus. There was no indication of lingual gyral activation. In further studies designed to reveal the topographic map within V4, we stimulated the superior and inferior visual fields separately, using the same stimuli. We found that human V4 contains a representation of both the superior and inferior visual fields. In addition, there appears to be retinotopic organization of V4 with the superior visual field being represented more medially on the fusiform gyrus and the inferior field more laterally, the two areas abutting on one another. We find no evidence that suggests the existence of a separate representation of the inferior hemifield for colour in more dorsolateral regions of the occipital lobe.

Adult↗
PubMed1997

Selective stimulation of colour mechanisms: an empirical perspective.

Anatomically distinct parvo and magno visual pathways show considerable functional overlap. However, specific stimulation of the most sensitive colour-opponent parvo-neurones is still possible, provided that colour stimuli are verified for selectivity. The authors have shown that gratings of low contrast, low spatial frequency and of restricted spatial content (6 or less spatial cycles) are optimal stimuli for distinguishing between colour-related (tritan and red/green) from achromatic or partly chromatic responses. This is particularly important when recording global responses, such as visual evoked potentials, VEPs. The crucial point is that at low presentation rates (< 2 Hz), colour-related onset VEPs are maximally different from contrast reversal VEPs, thereby reflecting the activity of sustained-type parvo mechanisms. Achromatic onset, offset and reversal VEPs are similar, reflecting mediation by transient-type magno mechanisms. A stringent test of colour-response specificity is to check whether the chromatic reversal VEP has a low-pass temporal tuning curve, since it becomes band-pass when substantial achromatic intrusions are present. Specification of chromatic isoluminant stimuli, e.g. along cardinal axes, does not guarantee their colour-selectivity, if chromatic aberration and variable macular pigmentation changes the chromatic content of the retinal image. It is shown here how chromatic stimuli, namely (1) red/green and (2) purple/green (tritanopic) gratings, can be optimized for selective stimulation of the colour system.

Color Perception↗
PubMed1997

Pattern ERGs from isoluminant gratings; poor selectivity compared with VEPS.

The PERGs elicited by low contrast achromatic (luminance-modulated) and chromatic (isoluminant) gratings were studied as a function of the presentation mode (rapid onset, offset, reversal), departure from isoluminance, contrast and spatial frequency; predominantly phasic-type responses were found in all cases. We propose that the PERGs for low and moderate contrast, achromatic and chromatic grating stimuli, are generated by non-linear, transient retinal cells of the magno-system. Parvo retinal cells seem to contribute to the response only at higher chromatic contrasts. The transient nature of the PERG obtained from isoluminant gratings makes it doubtful that these signals represent the activity of colour-related processes.

Adult↗
PubMed1996

Specificity and selectivity of chromatic visual evoked potentials.

A paper by Rabin et al. (1994) Vision Research, 34, 2657-2671, claimed that spatially extensive grating stimuli could be used to generate chromatic-specific visual evoked potentials from subjects assumed to have standard spectral sensitivity and tritanopic confusion lines. Here we demonstrate that such spatially extensive stimuli may generate responses which are contaminated by luminance-contrast intrusions. Such intrusions are mainly due to chromatic aberrations and are compounded by the abovementioned assumptions. Claims regarding the chromatic selectivity of VEPs must, therefore, be substantiated by establishing correlations with the known properties of the chromatic system.

Color Perception↗
PubMedDate not supplied

Amplitude and phase variations of harmonic components in human achromatic and chromatic visual evoked potentials.

Occipital visual evoked potentials (VEPs) were recorded in response to low-contrast, low spatial-frequency chromatic, and achromatic gratings. Fast Fourier Transform (FFT) and time-domain analysis were used to reveal differences in harmonic content and amplitude of chromatic and achromatic response components over a wide range of temporal frequencies. The chromatic ON/OFF VEP is dominated by the fundamental component indicating that onset and offset responses are different. This type of response is typical of neurons with sustained type response characteristics. Conversely, the achromatic onset VEP contains a predominant second harmonic component in addition to the fundamental. This similarity between onset and offset responses suggests that transient mechanisms are responsible for the generation of achromatic components. Frequency analysis of VEPs elicited by phase-reversing stimuli reveals that all of the response energy is concentrated at the second harmonic of the stimulating frequency. The magnitude of the second harmonic component is maximal for achromatic stimuli and undergoes a distinct minimum for isoluminant, chromatic stimuli. This behavior indicates that under the stimulus conditions used, magnocellular neurons with transient characteristics dominate the reversal VEP.

Adult↗
PubMedDate not supplied

Effect of foveal tritanopia on reaction times to chromatic stimuli.

To investigate the effect of foveal inhomogeneities on sensitivity to chromatic stimuli, we measured simple reaction times (RTs) and detection thresholds to temporally and spatially blurred isoluminant stimuli at retinal eccentricities from 0 deg to 8 deg. Three color-normal subjects participated. Contrast gain was derived from the slope of the RT versus contrast function. With a Gaussian spatial distribution (S.D. = 0.5 deg) and modulation between white (CIE x, y, L = 0.31, 0.316, 12.5 cd x m(-2)) and blue (MBDKL 90 deg), gain was maximal at about 2-deg eccentricity and declined by approximately 1 log unit towards the center and the periphery. The red (0 deg) and green (180 deg) cardinal axes showed maximum gain in the center, whilst the yellow (270 deg) data were intermediate. Although the spatial extent of the Gaussian spot was much larger than the S-cone free zone, we wished to determine whether foveal tritanopia was responsible for the marked drop in sensitivity to the 90-deg stimulus. To align the color vector along a tritan line, we used a smaller disk (0.3 deg) with a blurred edge and measured detection threshold, rotating the vector until minimum central sensitivity was obtained. Other workers have used transient tritanopia or minimally distinct border to similar effect. By repeating this at different locations in color space, a group of vectors were obtained. These converged near to the S-cone co-punctal point, evidence that they lay along tritan confusion lines. These threshold findings were then confirmed using the RT-derived contrast gain function. The tritan vectors were less pronounced as stimulus size increased. With the vector optimized to produce foveal tritanopia, the RT gain versus eccentricity functions for the 90-deg and 270-deg stimuli both fell markedly in the center and periphery, and sensitivity peaked at about 3-deg eccentricity. There are some similarities between these findings and the underlying photoreceptor distributions. As a result, there is a greater difference in gain between red-green and blue-yellow systems in the center than in the near periphery. We conclude that the RT versus contrast function is a sensitive index of foveal opponency.

Color Perception↗
PubMedDate not supplied

Chromatic adaptation, perceived location, and color tuning properties.

We have studied the influence of chromatic adaptation upon the perceived visual position of a test stimulus using a Vernier alignment task. Maximum and minimum offsets in spatial position are generated when the adapting and test stimuli lie on the same and orthogonal axes in MBDKL color space, respectively. When the test stimuli lie on intermediate color axes, the measured positional shifts decrease as a function of the angular separation in color space (phi) from the adapting stimulus. At low stimulus contrasts, these shifts follow a sinusoidal function of phi and exhibit broad chromatic tuning and can be accounted for by a model in which the centroid is extracted from the linear combination of after-image, formed by the adapting stimulus, and the test stimulus. Such linear, broadband behavior is consistent with the response properties of chromatic neurons in the precortical visual pathway. At high contrast, and when adaptation gets closer to the S/(L+M) axis, the tuning functions become narrower and require sinusoids raised to increasingly higher exponents in order to describe the data. This narrowing of chromatic tuning is consistent with the tuning properties of chromatic neurons in the striate cortex, and implies the operation of a nonlinear mechanism in the combination of cone outputs.

Adaptation, Ocular↗
PubMedDate not supplied

Cone opponency in the near peripheral retina.

Changes of color perception in the peripheral field are measured using an asymmetric simultaneous matching paradigm. The data confirm previous observations in that saturation changes can be neutralized if the test target is increased in size. However, this compensation does not apply to hue shifts. We show that some hues remain unchanged with eccentricity whereas others exhibit substantial changes. Here the color shifts are plotted in terms of a second-stage cone opponent model. The data suggest that the S-L+M channel is more robust to increasing eccentricity than the L-M channel. Observations are interpreted in terms of the known underlying morphological and physiological differences in these channels.

Adaptation, Ocular↗