First-
and Second-order Motion
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 First-
and second-order motion perception in Gabor micropattern stimuli: psychophysics
and computational modelling. 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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Studies
of neurological patients. We developed novel stimuli for investigating
the relationship between local and global first- and second-order (1
and 2). We provided further evidence for
separate processing pathways for first- and second-order motion from two
neurological patients with unilateral brain damage (case RA and FD). Both
patients were submitted to a detailed battery of tests spanning various
levels and types of moving and static displays. Both patients presented
a double dissociation of deficits between first- and second-order motion.
a) FD was selectively impaired on all second-order motion tasks but had
normal performance on their first-order motion counterparts and other first-order
motion tasks. b) RA was selectively impaired on first-order motion tasks
but performed normally on all second-order motion tasks.
Both, RA and FD?s deficits were long lasting
(more than 2 years after the lesion occurred) and were confined to stimuli
presented in the visual field contralateral to the lesion. Using the Cardview
parcellation method we delineated the locus and extent of the patients?
brain lesions and compared them anatomically. The parcellation of the lesions
can be seen in [2]. Briefly, F.D.'s lesion
involved mostly the dorsolateral caudal aspect of the temporal lobe while
RA's lesion involved parts of the caudal and superficial cuneus, the superior
and caudal part of the lingual gyrus and the rostro-medial occipital cortex.
Together, the locus of the lesions suggests that these two types of motion
are mediated by different neural circuitry. These results suggest second-order
motion may be mediated by a region in the visual cortex lying adjacent
and dorsal to area MT which does not directly require cortical areas such
as V1 and V3.
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Anomalous
perception of coherence and transparency. Moving plaid patterns
generated by additive superposition of two sine wave gratings oriented
at ±30i were viewed through a circular aperture subtending 8°
of visual angle. The spatial frequency of one grating remained fixed at
1 cyc/deg while the other was varied between 0.25 and 2.5 cyc/deg in steps
of 0.25 cyc/deg. Observers were required to indicate whether the moving
pattern appeared "coherent" or "transparent". Data from five neurological
patients and six normal subjects has been interpreted in terms of the operation
and interaction of parallel first- and second-order motion processing channels.
While some patients perceive coherent motion over a much smaller range
of spatial frequencies than normal controls, others reported coherence
over almost the entire range tested. Our data suggest a motion processing
architecture in which linear and non-linear channels are present at multiple
scales. At each scale the pairs of channels interact competitively with
the outputs from each scale combined in additional stages (Clifford CWG,
Vaina LM ?Anomalous Perception of Coherence and transparency in moving
plaid Patterns: Evidence for Competing Processes?, Cognitive Brain Research
, submitted).
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Modeling
first- and second -order motion. The clear cut double dissociation
of deficits on first- and second-order motion discussed above motivated
us to develop a model of first- and second-order motion processing in the
normal human visual system. When elements of the model are disabled, performance
on either first- or second-order motion can be selectively impaired in
line with the neurological data. The model should be able to generate testable
predictions on a range of psychophysical tasks involving normal and clinical
subjects. We conducted two studies with this aim:
a) To establish the nature of the
computations necessary to extract first- or second-order motion we examined
stimuli which have been used by us and other laboratories to assess first-
and second-order motion processing capabilities in neurological patients.
We proposed and implemented a computational model consisting of two parallel
channels geared toward the perception of first- and second-order motion
respectively [3]. The image signal in the
second-order channel was processed by a low-pass texture grabbing mechanism
prior to motion analysis. In both channels motion was extracted by identical
motion energy computations, although the second-order channel operated
only at a coarse spatial scale. We investigated the response of the first-
and second-order channels to stimuli where motion was defined by a range
of attributes and compared the performance of the model with patient data
from three recent clinical studies.
The model was sensitive to motion defined
by first-order variations in luminance, and to second-order motion defined
by contrast, spatial frequency and flicker. The ability of the model to
account for data from recent neurological studies supports the theory that,
at the lower levels of processing, two parallel channels underlie the detection
and direction discrimination of first- and second-order motion.
b) We designed a novel psychophysical stimulus
(a variant of that used by Boulton & Baker) to investigate the interaction
of first- and second-order motion information in judgements of motion direction.
The performance of normal subjects on the test was compared with the model
predictions. The direction of two-frame apparent motion was measured for
stimuli composed of Gabor or Gaussian micro-patterns. Three conditions
were investigated. In condition 1, used by Boulton and Baker, motion was
defined by the displacement of Gabor micro-patterns. In condition 2, motion
was defined by the displacement of Gaussian micro-patterns and in condition
3, the envelopes of Gabor micro-patterns were displaced while their carriers
remain static. The performance of normal subjects was compared with the
predictions of the model for each condition. Using sparsely distributed
micro-patterns, direction judgements in all three conditions were determined
by the spacing of the micro-patterns. With a dense stimulus, direction
judgements varied as a function of displacement in qualitatively different
ways for the three conditions which were predicted by the two-channel computational
model. In one channel, motion was calculated directly from stimulus luminance,
while in the other it was preceded by a texture-grabbing operation. The
relative activities of the two channels dictated which governed direction
judgements for a given stimulus [4].
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Publications
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Vaina, L.M., Royden, C., Bienfang, D.C., Makris,
N., and Kennedy, D. Normal perception of heading in a patient with
impaired structure from motion, Invest. Opth. Vis. Sci., 1996; 37:
4890.
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Vaina, L.M., Burin des Roziers, E., and Belliveau,
J.W. Cerebellar activation during direction discrimination of learning
in visual motion stimuli: an fMRI study. Society of Neuroscience,
1997; 23: 1401.
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Beardsley, S.A. and Vaina, L.M. Computational
modeling of optic flow selectivity in MSTd neurons, Investigative Ophthalmology
& Visual Science , 1997; 38(4): S80.
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Vaina, L.M., Belliveau, J.W., Burin des Roziers,
E., Zeffiro, T. The relationship between cortical activation and psychophysical
performance in humans during fast learning of motion discrimination. NeuroImage,
1997; 5(4): S137.
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Clifford, C.W.G. and Vaina, L.M. A computational
model of selective deficits in first- and second-order motion processing.
Vision Research, 1999 (in press).
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Clifford, C.W.G. and Vaina, L.M. Anomalous Perception
of Coherence and transparency in moving plaid Patterns: Evidence for Competing
Processes. Cognitive Brain Research (accepted).
-
Vaina, L.M. and Cowey, A. Selective deficits
to first or second order order motion in stroke patients provides further
evidence for separate mechanisms. Soc. Neurosci. Abstr., 1996; 22(1):
1718.
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Vaina, L.M, Makris, N., and Cowey, A. The neuroanatomical
damage producing selective deficits of first or second order motion in
stroke patients provides further evidence for separate mechanisms in Functional
Mapping of the Human Brain, NeuroImage, 1996, 3(3): 360.
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Clifford, C.W.G. and Vaina, L.M. A computational
model of selective deficits in first- and second- order motion perception,
Investigat. Opthalmology & Visual Science 1998; 39(4):
S1077.
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