1. Vaina LM and Cowey A “Impairment of the perception of
second order motion but not first order motion in a patient with unilateral focal
brain damage”, Proc. R. Soc. Lond. B., 1996; 263:1225-1232.
Unlike first order motion, which is based on spatiotemporal variations in luminance,
second-order motion relies on spatiotemporal variation of attributes derived from
luminance, such as contrast. Here we show that a patient with a small unilateral
cortical lesion adjacent to human cortical area MT (V5) has an apparently permanent
disorder in perceiving several forms of second-order but not first-order motion in
his contralateral visual field. This result indicates that separate pathways for
motion perception exist, either as divergent pathways from area MT or even from primary
visual cortex, or as separate pathways from subcortical areas to extrastriate visual
areas.
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2. Vaina LM, Cowey A, Makris N, Kennedy D “The selective impairment of the perception
of first-order motion by unilateral cortical brain damage”, Visual Neuroscience,
1998; 15:333-348.
First-order (Fourier) motion consists of stable spatiotemporal luminance variations.
Second-order (non-Fourier) motion consists instead of spatiotemporal modulation of
contrast, flicker, or spatial frequency. In spite of extensive psychophysical and
computational analysis of the nature and relationship of these two types of motion,
it remains unclear whether they are detected by the same mechanism or whether separate
mechanisms are involved. Here we report the selective impairment of first-order motion,
on a range of local and global motion tasks, in the contralateral visual hemifield
of a patient with unilateral brain damage centered on putative visual areas V2 and
V3 in the medial part of the occipital lobe. His perception of second-order motion
was unimpaired. As his disorder is the obverse of that reported after damage in the
vicinity of human visual area MT (V5), the results support models of motion processing
in which first- and second-order motion are, at least in part, computed separately
at the extrastriate cortical level.
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3. Clifford CWG and Vaina LM “A computational model of selective deficits in first-
and second-order motion processing”, Vis Res, 1999 (in press).
Recent neurological studies of selective impairments in first- and second-order motion
processing are of considerable relevance in elucidating the mechanisms of motion
perception in normal human observers. We examine the stimuli which have been used
to assess first- and second-order motion processing capabilities in clinical subjects,
and discuss the nature of the computations necessary to extract their motion. We
find that a simple computational model of first- and second-order motion processing
is able to account for the data. The model consists of a first-order channel computing
motion at coarse and fine scales, and a coarse scale second-order channel. The second-order
channel is sensitive to motion information defined by variations in luminance, contrast,
spatial frequency, and flicker. When elements of the model are disabled, its performance
on either first- or second-order motion can be selectively impaired in line with
the neurological data.
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4. Clifford CWG, Freedman J, Vaina LM “First- and second-order motion perception
in Gabor micropattern stimuli: Psychophysical and computational modeling”, Cog.
Br. Res, 1998; 6(4):263-273.
This paper examines the perception of first- and second-order motion in human vision.
In an extension of previous work by Boulton and Baker [2,3], the direction of two-frame
apparent motion is measured for stimuli composed of Gabor or Gaussian micropatterns.
Three conditions are investigated. Condition 1 is that used by Boulton and Baker,
in which motion is defined by the displacement of Gabor micropatterns. In Condition
2, motion is defined by the displacement of Gaussian micropatterns. In Condition
3, the envelopes of Gabor micropatterns are displaced while their carriers remain
static. Using sparsely distributed micropatterns, direction judgements in all three
conditions are determined by the spacing of the micropatterns. With a dense stimulus,
direction judgements vary as a function of displacement in qualitatively different
ways for the three conditions. The psychophysical results are predicted by a two-channel
computational model. In one channel, motion is calculated directly from stimulus
luminance, while in the other it is preceded by a texture-grabbing operation. The
relative activities of the two channels dictates which governs direction judgements
for any given stimulus.
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5. Vaina LM “Complex motion perception and its deficits”, Current Opinion in Neurobiology,
1998; 8:494-502.
Within the motion hierarchy, the area MSTd is optimally suited for the analysis of
complex motion patterns which are directly useful for tasks of visually guided behaviour
(e.g. computation of heading). I first review the electrophysiological and psychophysical
evidence for the existence of “detectors” in MSTd specialized for complex motion
patterns, and the necessity of combining retinal and extraretinal signals received
by MSTd neurones for the accurate perception of heading. Second, in a small number
of neurological patients I illustrate the devastating effects of lesions involving
the human homologue of MST on their ability to navigate in their surrounding and
discuss these patients’ impaired performance on psychophysical tasks of complex motion
discrimination.
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6. Morrone MC, Burr DC, Vaina LM “Two stages of visual processing for radial and
circular motion”, Nature, 1995; 376(6540):507-509.
As we move through our environment, the flow of the deforming images on our retinae
provides rich information about ego motion and about the three-dimensional structure
of the external world. Flow-fields comprise five independent components, including
radial and circular motion. Here we provide psychophysical evidence for the existence
of neural mechanisms in human vision that integrate motion signals along these complex
trajectories. Signal-to-noise sensitivity for discriminating the direction of radial,
circular, and translational motion increased predictably with the number of exposed
sectors, implying the existence of specialized detectors that integrate motion signals
of different directions from different locations. However, contrast sensitivity for
complex motion did not increase greatly with sector number, implying that the specialized
detectors are preceded by a first stage of local-motion mechanisms that impose a
contrast threshold. These findings fit well with recent electrophysiological evidence
in monkey showing that whereas motion-sensitive neurons in primary visual cortex
respond best to local translation, many neurons in the medial superior temporal cortex
have large receptive fields tuned to radial, circular, or spiral motion.
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7. Burr D, Morrone MC, Vaina LM “Large receptive fields for optic flow detectors
in humans”, Vis Res, 1998; 38(12):1731-1743.
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 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), ad to
this type of stimuli (Morrone MC, Burr DC, Vaina LM, Nature 1995;376:507-509), consistent
with the suggestion that the two techniques examine different levels of motion analysis.
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8. Clifford CWG, Beardsley SA, Vaina LM “The perception and discrimination of speed
in complex motion”, Vis Res (accepted).
Random dot kinematograms were used to simulate radial, rotational, and spiral optic
flow. The stimuli were designed so that, while dot speed increased linearly with
distance from the centre of the display, the density of dots remained uniform throughout
their presentation. In two experiments, subjects were required to perform a temporal
2AFC speed discrimination task. Experiment 1 measured the perceived speed of a range
of optic flow patterns against a rotational comparison stimulus. Radial motions were
found to appear faster than rotations by approximately 10%, with a smaller but significant
effect for spirals. Experiment 2 measured discrimination thresholds for pairs of
similar optic-flow stimuli identical in all respects except mean speed. No consistent
differences were observed between the speed discrimination thresholds of radial,
rotational, and spiral motions and a control stimulus with the same speed profile
in which motion was along fixed random trajectories. The perceived speed results
are interpreted in terms of a model satisfying constraints on motion-in-depth and
object rigidity, while speed discrimination appears to be based upon the pooled responses
of elementary motion detectors.
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9. Beardsley SA and Vaina LM “Computational modelling of optic flow selectivity in
MSTd neurons”, Network: Comput. Neural Sys, 1998; 9:467-493.
In neurophysiological experiments examining the selectivity of MSTd neurons to visual
motion components of optic flow stimuli in monkeys, Duffy and Wurtz (1991) reported
cells with double-component (plano-radial and plano-circular) and triple-component
(plano-radial-circular) selectivities, while Graziano et al (1994) reported neurons
selective to a continuum of optic flow stimuli including spiral motion. Here, we
address these reported findings under simulated experimental conditions by examining
the development of optic flow selectivity in the hidden units of a two-layer back-propagation
network. We also examine network motion sensitivity during simulated psychophysical
tests via the addition of a competitive decision layer. Network analysis with neurophysiological
stimuli identified a majority of hidden units whose position invariance and motion
selectivity were consistent with MSTd responses to the visual motion components of
optic flow stimuli reported by Duffy and Wurtz and Graziano et al. Furthermore, the
hidden units developed a continuum of optic flow selectivities independent of the
biases associated with the specification of the motion selectivity in the output
layer. During psychophysical testing, network responses showed motion sensitivities
which met or exceeded human performance. Within the limitations imposed by the learning
algorithm, the psychophysical results were consistent with a model of global motion
perception via local integration along complex motion trajectories.
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10. Vaina LM, Grzywacz NM, LeMay M, Bienfang D, Wolpow E “Perception of motion discontinuities
in patients with selective motion deficits”, in High-level motion processing: computational,
neurobiological, and psychophysical perspectives. T. Watanabe (ed), MIT press, 1998;
pp. 213-247.
Humans can find image discontinuities, which usually correspond to object borders,
based solely on motion measurements (Anstis, 1970). This computation is an important
component of figure-ground segregation (Gibson et al., 1959; Braddick, 1973, 1980),
a fundamental function of the visual system. However, despite the importance of discontinuity
extraction, two points are still unclear: What are the visual measurements underlying
it? And what is its relationship to the spatial integration of motion signals?
Several theoretical investigators have studied the problem of finding motion discontinuities
and its relationship to how the visual system measures velocity (Nakayama and Loomis,
1974; Grzywacz and Yuille, 1990) and integrates motion signals over space and time
(Hildreth, 1984; Bulthoff, Little, and Poggio, 1989; Yuille and Grzywacz, 1988, 1989).
The proposed theories may be divided into three classes in terms of their hierarchical
organization: (1) Discontinuities are computed from direction of optic-flow vectors
or spatiotemporal signal measurements (Hildreth 1984; Grzywacz and Yuille, 1990),
but prior to the computation of full optic-flow velocity in relatively localized
regions of the image; (2) Discontinuities are computed from the outputs of full local
velocity measurement, but prior to a stage that integrates the local vectors to obtain
a coherent, global percept of motion (Nakayama and Loomis, 1974; Clocksin, 1980);
(3) Discontinuities are computed simultaneously with the motion integration stage
(Koch et al., 1989). Common to all these theoretical frameworks is the question of
whether the processing of information related to the extraction of discontinuity
is hierarchically organized (see also Grossberg, chapter 1; Hildreth and Royden,
chapter 9; Yuille and Grzywacz, chapter 6).
Physiological and anatomical studies indicate a degree of hierarchical processing
of motion in the cortex (for a review see Maunsell and Newsome, 1987). For example,
layer 4b of area V1 (Dow, 1974; Blasdel and Fitzpatrick, 1984) is one of the earliest
stages providing an explicit representation of direction of motion at the level of
individual neurons, which is then further processed in the middle temporal area (MT)
(Zeki, 1969; Maunsell and Van Essen, 1983). It is unclear what cortical area computes
visual speed, that is, what area has a speed representation which is independent
of spatial and temporal frequencies. While it might be MT (Newsome, Gizzi, and Movshon,
1983), it is almost certainly not V1 (Tolhurst and Movshon, 1975; Holub and Morton-Gibson,
1981). The spatial integration of motion signals might be performed in MT or in the
(later) medial superior temporal area (MST), since these areas tend to have large
receptive fields (Gattass and Gross, 1981; Tanaka et al., 1986; Desimoneand Ungerleider,
1986; for a review see Tanaka, chapter 10). These functions might also be assigned
to other extrastriate areas that process motion.
Psychophysics provides valuable information pertaining to hierarchical processing
of motion information in humans. Precise measurements of velocity seem to require
spatiotemporal integration (McKee, 1981; McKee and Welch, 1985). Motion discontinuities
appear to be computed after some absolute motion measurements are made (Baker and
Braddick, 1982; vanDoorn and Koenderink, 1982, 1983). Computation of discontinuities
has been suggested to involve motion antagonistic mechanisms (Hildreth, 1984) or
the exploitation of the population code of directionally selective cells (Grzywacz
and Yuille, 1990).
In this paper we take a different approach to test the theoretical proposals on the
organization of motion processing underlying discontinuity extraction. We investigate
performance on seven psychophysical motion tasks of patients with focal brain lesions
involving the neural circuits mediating specific aspects of motion perception. This
investigation is complemented by magnetic resonance imaging (MRI) studies, and neurological,
neuro-ophthalmological, and neuropsychological evaluations. Such patient studies
offer a valuable opportunity to infer the neuroanatomical substrate of specific motion
computations in humans and to learn how these computations relate to each other.
The results of this study were previously presented in a short form (Vaina et al.,
1990a; Vaina and Grzywacz, 1992).
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