Biomedical subjects

M C Morrone

Publications and source records attributed to M C Morrone.

At least 19 recordsLinked to original sources

PubMed2004

Different attentional resources modulate the gain mechanisms for color and luminance contrast.

We used an interference paradigm to investigate whether attention is attribute-specific at early levels of visual processing. We show that the peripheral increment thresholds for luminance contrast deteriorate when the observer is currently performing another luminance (form or contrast) discrimination task in central view, but not when he or she is performing a color discrimination task. Similar results were obtained for color increment thresholds, indicating that the interference is specific to contrast modality. The effects are strong and robust over primary task difficulties and perceptual learning levels. Modeling suggests that attention improves contrast sensitivity by modulating the gain of the neuronal response to contrast. These results suggest that attention is parceled in independent resources for luminance and color contrast.

Attention↗

PubMed2001

Separate visual representations for perception and action revealed by saccadic eye movements.

Some 30 years ago, Trevarthen [1] introduced the idea of two separate visual systems, a focal system for fine motor acts and an ambient system for gross body movements such as ambulation. More recent developments indicating anatomically and physiologically separate pathways in primate vision [2] have led to a different idea of separate visual systems, one for conscious perception and one for action [3]. It has received empirical support from several studies showing that pointing, reaching, and grasping can remain accurate while the perceived position or size of objects is subject to illusory distortion [4-6]. However, much of this evidence has been challenged on the grounds of methodological flaws, particularly failure to match perfectly the conditions for verbal and motor tasks and failure to replicate results [7-10]. Here we take advantage of the strong compression of perceived position that occurs around the time of saccadic eye movements [11, 12]. Under normal lighting conditions, stimuli flashed briefly over a wide range of spatial positions just before saccadic onset are neither seen nor reached for in their veridical positions, but are compressed toward the saccadic target. We validate the idea of separate systems by showing that, in the dark, subjects are able to point accurately to the correct target position, even though their verbal reports are still subject to compression.

Humans↗

PubMed2001

Automatic gain control contrast mechanisms are modulated by attention in humans: evidence from visual evoked potentials.

This study investigated the effect of attention on the contrast response curves of steady-state visual evoked potentials (VEPs) to counter-phased sinusoidal gratings. The 1 cyc/deg gratings were modulated either in luminance or chromaticity (equiluminant red-green). The luminance grating counter-phased at 9 Hz (to favour activation of the magno-cellular system), and the chromatic grating at 2.5 Hz (to favour activation of the parvo-cellular system). Attention was directed towards the gratings (displayed in the left visual field) by requiring subjects to detect and respond to randomly occurring changes in contrast. In the control condition, attention towards the grating was minimised by requiring subjects to detect a target letter amongst distracters briefly flashed in the contra-lateral visual field. Attention increased VEP amplitudes for both luminance and chromatic stimuli, more so at high than at low contrasts, increasing the slope of the contrast amplitude curves (over the non-saturating range of contrasts). The estimates of contrast threshold from extrapolation of amplitudes were unaffected by attention. Attention also changed the VEP phases, but only for luminance gratings, where it acted to reduce the magnitude of phase advance with contrast. Attention had no effect on the average phases for chromatic gratings. The results are consistent with the notion that attention acts on cortical gain control mechanisms, which are known to be different for the magno- and parvo-cellular systems.

Adult↗

PubMed2001

Changes in visual perception at the time of saccades.

We frequently reposition our gaze by making rapid ballistic eye movements that are called saccades. Saccades pose problems for the visual system, because they generate rapid, large-field motion on the retina and change the relationship between the object position in external space and the image position on the retina. The brain must ignore the one and compensate for the other. Much progress has been made in recent years in understanding the effects of saccades on visual function and elucidating the mechanisms responsible for them. Evidence suggests that saccades trigger two distinct neural processes: (1) a suppression of visual sensitivity, specific to the magnocellular pathway, that dampens the sensation of motion and (2) a gross perceptual distortion of visual space in anticipation of the repositioning of gaze. Neurophysiological findings from several laboratories are beginning to identify the neural substrates involved in these effects.

Animals↗

PubMed2000

Extraretinal control of saccadic suppression.

We measured the time course of saccadic suppression and tested whether suppression results entirely from retinal image motion or has an extraretinal source. We measured contrast thresholds for low-frequency gratings modulated either in luminance, at 17 cd/m(2) and 0.17 cd/m(2), or color at 17 cd/m(2). Gratings were flashed on a uniform background before, during, or after voluntary 12 degrees saccades and, additionally in the case of luminance modulated gratings, saccades simulated by mirror motion. A 10-fold decrease in contrast sensitivity was found for luminance-modulated gratings with saccades, but little suppression was found with simulated saccades. Adding high-contrast noise to the display increased the magnitude and the duration of the suppression during simulated saccades but had little effect on suppression produced by real saccades. Suppression anticipates saccades by 50 msec, is maximal at the moment of saccadic onset, and outlasts saccades by approximately 50 msec. At lower luminance, suppression is reduced, and its course is shallower than at higher luminance. Simulated saccades produce shallower suppression over a longer time course at both luminances. No suppression was found for chromatically modulated gratings. Differences between real and simulated saccades in the magnitude and time course of sensitivity loss suggest that saccadic suppression has an extraretinal component. We model the effects of saccades by adding a signal to the visual input, so as to saturate the nonlinear stage of visual processing and make detection of a test stimulus more difficult.

Adult↗

PubMed2000

A cortical area that responds specifically to optic flow, revealed by fMRI.

The continuously changing optic flow on the retina provides information about direction of heading and about the three-dimensional structure of the environment. Here we use functional magnetic resonance imaging (fMRI) to demonstrate that an area in human cortex responds selectively to components of optic flow, such as circular and radial motion. This area is within the region commonly referrred to as V5/MT complex, but is distinct from the part of this region that responds to translation. The functional properties of these two areas of the V5/MT complex are also different; the response to optic flow was obtained only with changing flow stimuli, whereas response to translation occurred during exposure to continuous motion.

Adult↗

PubMed1999

Saccadic suppression precedes visual motion analysis.

There is now good evidence that perception of motion is strongly suppressed during saccades (rapid shifts of gaze), presumably to blunt the disturbing sense of motion that saccades would otherwise elicit. Other aspects of vision, such as contrast detection of high-frequency or equiluminant gratings, are virtually unaffected by saccades [1] [2] [3] [4] [5]. This has led to the suggestion that saccades may suppress selectively the magnocellular pathway (which is strongly implicated in motion perception), leaving the parvocellular pathway unaffected [5] [6]. Here, we investigate the neural level at which perception of motion is suppressed. We used a simple technique in which an impression of motion is generated from only two frames, allowing precise control over the stimulus [7] [8]. One frame has a certain fixed contrast, whereas the contrast of the other (the test frame) is varied to determine the threshold for motion discrimination (that is, the lowest test-frame contrast level at which the direction of motion can be correctly guessed). Contrast thresholds of the test depended strongly and non-monotonically on the contrast of the fixed-contrast frame, with a minimum at medium contrast. To study the effect of saccadic suppression, we triggered the two-frame sequence by a voluntary saccade. Thresholds during saccades increased in a way that suggested that saccadic suppression precedes motion analysis: when the test frame was first in the motion sequence there was a general depression of sensitivity, whereas when it was second, the contrast response curve was shifted to a higher contrast range, sometimes even resulting in higher sensitivity than without a saccade. The dependence on presentation order suggests that saccadic suppression occurs at an early stage of visual processing, on the single frames themselves rather than on the combined motion signal. As motion detection itself is thought to occur at an early stage, saccadic suppression must take place at a very early phenomenon.

Humans↗

PubMed1999

Developmental changes in optokinetic mechanisms in the absence of unilateral cortical control.

Animal models suggest that the asymmetry of monocular optokinetic nystagmus (OKN) in young infants can be explained by a direct pathway from retina to the midbrain nucleus of the optic tract. However, earlier studies with hemispherectomized infants showed no evidence for OKN responses towards the damaged cortex that could be ascribed to this subcortical pathway. In longitudinal testing of two infants with very extensive unilateral cortical damage, we have now shown that OKN responses in both directions do occur before 10 months of age. OKN towards the damaged cortex, indicating functioning of the direct pathways in the absence of cortical control, drops out in the later development. The neural circuitry responsible for OKN in humans appears to undergo a plastic reorganization.

Brain↗

PubMed1999

Cardinal directions for visual optic flow.

As we move through our environment, the flow of deforming images on the retinae provides a rich source of information about the three-dimensional structure of the external world and how to navigate through it. Recent evidence from psychophysical [1] [2] [3] [4], electrophysiological [5] [6] [7] [8] [9] and imaging [10] [11] studies suggests that there are neurons in the primate visual system - in the medial superior temporal cortex - that are specialised to respond to this type of complex 'optic flow' motion. In principle, optic flow could be encoded by a small number of neural mechanisms tuned to 'cardinal directions', including radial and circular motion [12] [13]. There is little support for this idea at present, however, from either physiological [6] [7] or psychophysical [14] research. We have measured the sensitivity of human subjects for detection of motion and for discrimination of motion direction over a wide and densely sampled range of complex motions. Average sensitivity was higher for inward and outward radial movement and for both directions of rotation, consistent with the existence of detectors tuned to these four types of motion. Principle component analysis revealed two clear components, one for radial stimuli (outward and inward) and the other for circular stimuli (clockwise and counter-clock-wise). The results imply that the mechanisms that analyse optic flow in humans tend to be tuned to the cardinal axes of radial and rotational motion.

Adult↗

PubMed1999

The effects of ageing on reaction times to motion onset.

We have measured reaction time (RT) to motion onset in two groups of subjects (average ages: 70 and 29 years), for horizontal gratings of 1 c deg-1, modulated in either luminance or colour (equiluminant red-green), for various contrasts and speeds. For both old and young subjects, RTs depended on both speed and contrast, being faster at high speeds and high contrasts, and showed a stronger contrast dependency for chromatic gratings. The older subjects were systematically slower than the younger subjects. The difference between old and young RTs varied with condition, being 30-40 ms more at the slow than at the fast speed. The relative difference in RTs in different stimulus conditions shows that at least some of the increase in response time with age has a sensory origin. The results relate well to previous work on visual evoked potentials.

Adult↗

PubMed1998

Seeing biological motion.

One of the more stunning examples of the resourcefulness of human vision is the ability to see 'biological motion', which was first shown with an adaptation of earlier cinematic work: illumination of only the joints of a walking person is enough to convey a vivid, compelling impression of human animation, although the percept collapses to a jumble of meaningless lights when the walker stands still. The information is sufficient to discriminate the sex and other details of the walker, and can be interpreted by young infants. Here we measure the ability of the visual system to integrate this type of motion information over space and time, and compare this capacity with that for viewing simple translational motion. Sensitivity to biological motion increases rapidly with the number of illuminated joints, far more rapidly than for simple motion. Furthermore, this information is summed over extended temporal intervals of up to 3 seconds (eight times longer than for simple motion). The steepness of the summation curves indicates that the mechanisms that analyse biological motion do not integrate linearly over space and time with constant efficiency, as may occur for other forms of complex motion, but instead adapt to the nature of the stimulus.

Humans↗

PubMed1998

Large receptive fields for optic flow detection in humans.

We used a psychophysical summation technique to study the properties of detectors tuned to radial, circular and translational motion, and to determine the spatial extent of their receptive fields. Signal-to-noise motion thresholds were measured for patterns curtailed spatially in various ways. Sensitivity for radial, circular and translational motion increased with stimulus area at a rate predicted by an ideal integrator. When sectors of noise were added to the stimulus, sensitivity decreased at a rate consistent with an ideal integrator. Summation was tested for large annular stimuli, and shown to hold up to 70 degrees in some cases, suggesting very large receptive fields for this type of motion (consistent with the physiology of neurones in the dorsal region of the medial superior temporal area (MSTd)). This is a far greater area than observed for summation of contrast sensitivity to gratings (Anderson SJ and Burr DC, Vis Res 1987;29:621-635, and to this type of stimuli (Morrone MC, Burr DC and Vaina LM, Nature 1995;376:507-509, consistent with the suggestion that the two techniques examine different levels of motion analysis.

Humans↗

PubMed1998

Motion analysis by feature tracking.

We have developed a two-stage model of motion perception that identifies moving spatial features and computes their velocity, achieving both high spatial localisation and reliable estimates of velocity. Features are detected in each frame by locating the peaks of the spatial local energy functions, as for stationary images (Morrone MC and Burr DC. Proc R Soc Lond 1988;B235:221-245.). The energy functions are calculated for different scales and orientations, and integrated within a temporal Gaussian window. The velocity of features is determined by the direction of maximal elongation of the energy in space-time, evaluated by calculating the three characteristic curvatures of the energy at each feature point. To circumvent the aperture problem, the energy maps are blurred in space by various amounts, and velocity is computed separately for each spatial blur. The Weber fraction of the local curvatures (curvature contrast) describes the spatio-temporal energy elongation at each feature point, giving a reliability index for each velocity estimate. For each point, the velocity of the spatial blur that yielded the highest curvature contrast was selected, with no further constraints, such as rigidity of motion. Dynamic recruitment of operators of different size allows maximum flexibility of the analysis, allowing it to simulate human visual performance in the detection of noise images, transparent motion, some motion illusions, and second-order motion.

Algorithms↗

PubMed1997

Apparent position of visual targets during real and simulated saccadic eye movements.

It is now well established that briefly flashed single targets are mislocalized in space, not only during saccades but also before them. We show here by several techniques (including a vernier judgment that did not require absolute location in space) that errors appear up to 100 msec before saccades are made and are maximal just before they start. The size and even the sign of errors depend strongly on position in the visual field, the complete pattern of errors suggesting a compression of visual space around the initial fixation point and the target of the impending saccade. The compression was confirmed by displaying multiple rather than single targets and was found to be powerful enough to reduce or even to remove vernier offset for pairs of bars shown simultaneously and to create offsets for colinear bars separated in time by 75 msec. It also reduced the apparent number of parallel bars. When saccades were simulated by moving the display at saccadic speed, there were sometimes errors of location, but only for tasks requiring absolute judgment of position. The pattern of errors differed greatly from that during saccades and, in particular, showed no signs of compression. We can model our saccade results by assuming a shift in the point in space associated with eye position compression of eccentricity along the axis of saccades.

Adult↗

PubMed1997

Compression of visual space before saccades.

Saccadic eye movements, in which the eye moves rapidly between two resting positions, shift the position of our retinal images. If our perception of the world is to remain stable, the visual directions associated with retinal sites, and others they report to, must be updated to compensate for changes in the point of gaze. It has long been suspected that this compensation is achieved by a uniform shift of coordinates driven by an extra-retinal position signal, although some consider this to be unnecessary. Considerable effort has been devoted to a search for such a signal and to measuring its time course and accuracy. Here, by using multiple as well as single targets under normal viewing conditions, we show that changes in apparent visual direction anticipate saccades and are not of the same size, or even in the same direction, for all parts of the visual field. We also show that there is a compression of visual space sufficient to reduce the spacing and even the apparent number of pattern elements. The results are in part consistent with electrophysiological findings of anticipatory shifts in the receptive fields of neurons in parietal cortex and superior colliculi.

Humans↗

PubMed1997

Capture and transparency in coarse quantized images.

This study examines the effect of coarse quantization (blocking) on image recognition, and explores possible mechanisms. Thresholds for noise corruption showed that coarse quantization reduces drastically the recognizability of both faces and letters, well beyond the levels expected by equivalent blurring. Phase-shifting the spurious high frequencies introduced by the blocking (with an operation designed to leave both overall and local contrast unaffected, and feature localization) greatly improved recognizability of both faces and letters. For large phase shifts, the low spatial frequencies appear in transparency behind a grid structure of checks or lines. We also studied a more simple example of blocking, the checkerboard, that can be considered as a coarse quantized diagonal sinusoidal plaid. When one component of the plaid was contrast-inverted, it was seen in transparency against the checkerboard, while the other remained "captured" within the block structure. If the higher harmonics are then phase-shifted by pi, the contrast-reversed fundamental becomes captured and the other seen in transparency. Intermediate phase shifts of the higher harmonics cause intermediate effects, which we measured by adjusting the relative contrast of the fundamentals until neither orientation dominated. The contrast match varied considerably with the phase of the higher harmonics, over a range of about 1.5 log units. Simulations with the local energy model predicted qualitatively the results of the recognizability of both faces and letters, and quantitatively the apparent orientation of the modified checkerboard pattern. More generally, the model predicts the conditions under which an image will be "captured" by coarse quantization, or seen in transparency.

Contrast Sensitivity↗

PubMed1996

Sensitivity to spatial phase at equiluminance.

We have measured sensitivity for discriminating the spatial phase of multi-harmonic and two-harmonic patterns modulated either in luminance or in chromaticity (red-green). The multi-harmonic patterns were either highpass squarewaves, lines or ramps. For all patterns, contrast thresholds for discriminating 0 from 180 deg phase were similar to those for discriminating -90 from 90 deg, for luminance or chromatic modulation (or both). For all types of multi-harmonic patterns, the ratio of contrast thresholds for the phase discrimination to that for pattern detection was the same for luminance and chromatic modulation, and for combinations of both. Similarly, phase thresholds, the minimum detectable differences in phase (about a mean 0 deg), were the same for chromatic and luminance patterns, provided that contrast was scaled to equate detection thresholds of the patterns. Similar results were observed for simple three-harmonic patterns (f + 2f + 3f), and for (f + 2f) two-harmonic patterns. Strangely, however, two-harmonic patterns of f + 3f (first two terms of square-wave) of moderate to high spatial frequency did show a two-fold advantage for luminance over colour, as Troscianko and Harris (1988) have previously reported (Vision Research, 28, 1041-1049), possibly because the two harmonics have a greater separation in frequency. However, for most classes of patterns, sensitivity for spatial phase is as good for chromatic as for luminance modulation, suggesting that similar sorts of mechanisms operate under these two conditions.

Color Perception↗

PubMed1996

Temporal impulse response functions for luminance and colour during saccades.

Previous work has shown that during saccadic eye movements, contrast sensitivity for low spatial frequency patterns modulated in luminance is selectively reduced by up to one logarithmic unit, while high spatial frequency patterns, and equiluminant patterns of all spatial frequencies are not suppressed at all [Burr et al. (1994). Nature, 371, 511-513]. Here we study the temporal characteristics for sensitivity to luminance and chromatic patterns during saccades, using the two-pulse summation technique. Sensitivity was measured for detecting two successive pulses as a function of stimulus-onset asynchrony, during normal viewing and during saccades. Impulse response functions were estimated from the summation data, for all conditions. For equiluminance, the functions were monophasic during normal viewing and saccades. For luminance modulation, the impulse response functions were di-phasic in both normal viewing and saccades. However, during saccades the impulse responses were faster in normal viewing. This result is consistent with the suggestion that saccadic suppression is mediated by contrast gain control mechanisms, known to occur in M-cells but not P-cells.

Color Perception↗