Occlusion and the interpretation of visual motion: perceptual and neuronal effects of context.

Visual motion can be represented in terms of the dynamic visual features in the retinal image or in terms of the moving surfaces in the environment that give rise to these features. For natural images, the two types of representation are necessarily quite different because many moving features are only spuriously related to the motion of surfaces in the visual scene. Such "extrinsic" features arise at occlusion boundaries and may be detected by virtue of the depth-ordering cues that exist at those boundaries. Although a number of studies have provided evidence of the impact of depth ordering on the perception of visual motion, few attempts have been made to identify the neuronal substrate of this interaction. To address this issue, we devised a simple contextual manipulation that decouples surface motion from the motions of visual image features. By altering the depth ordering between a moving pattern and abutting static regions, the perceived direction of motion changes dramatically while image motion remains constant. When stimulated with these displays, many neurons in the primate middle temporal visual area (area MT) represent the implied surface motion rather than the motion of retinal image features. These neurons thus use contextual depth-ordering information to achieve a representation of the visual scene consistent with perceptual experience.

Luminance contrast affects motion coherency in plaid patterns by acting as a depth-from-occlusion cue.

Moving plaid patterns composed of component gratings that differ in luminance contrast tend not to cohere perceptually. Plaid patterns configured to mimic one occlusive grating overlying another also fail to cohere. We hypothesized that plaids constructed of components with different luminance contrasts fail to cohere because these components are interpreted as occlusive surfaces lying in different depth planes. It is known that when depth-from-occlusion and depth-from-binocular disparity cues support the same depth-ordering, both segregation in depth and motion non-coherency are more likely to be perceived than when these two cues conflict. We exploited this interaction and tested our hypothesis by introducing horizontal binocular disparity between two superimposed component gratings of different luminance contrasts. We found that both depth segregation and motion non-coherency were much more likely when the high-contrast grating was stereoscopically in front of the low-contrast grating. From these results we infer that luminance contrast acts as a depth-cue in plaid patterns, with higher contrast gratings appearing to lie in front of lower contrast gratings. Perceptual motion coherency parallels these depth-ordering judgments. We conclude that luminance contrast affects motion coherency by acting as a depth-from-occlusion cue.

Perceptual, oculomotor, and neural responses to moving color plaids.

Moving plaids constructed from two achromatic gratings of identical luminance contrast are known to yield a percept of coherent pattern motion, as are plaids constructed from two identical chromatic (e.g. isoluminant red/green) gratings. To examine the interactive influences of chromatic and luminance contrast on the integration of visual motion signals, we constructed plaids with gratings that possessed both forms of contrast. We used plaids of two basic types, which differed with respect to the phase relationship between chromatic and luminance modulations (after Kooi et al, 1992 Perception 21 583-598). One plaid type ('symmetric') was made from component gratings that had identical chromatic/luminance phase relationships (e.g. both components were red-bright/green-dark modulation). The second plaid type ('asymmetric') was made from components that had complimentary phase relationships (i.e. one red-bright/green-dark grating and one green-bright/red-dark grating). Human subjects reported that the motion of symmetric plaids was perceptually coherent, while the components of asymmetric plaids failed to cohere. We also recorded eye movements elicited by both types of plaids to determine if they are similarly affected by these image cues for motion coherence. Results demonstrate that, under many conditions, eye movements elicited by perceptually coherent vs noncoherent plaids are distinguishable from one another. To reveal the neural bases of these perceptual and oculomotor phenomena, we also recorded the responses of neurons in the middle temporal visual area (area MT) of macaque visual cortex. Here we found that individual neurons exhibited differential tuning to symmetric vs asymmetric plaids. These neurophysiological results demonstrate that the neural mechanism for motion coherence is sensitive to the phase relationship between chromatic and luminance contrast, a finding which has implications for interactions between 'color' and 'motion' processing streams in the primate visual system.

The interpretation of visual motion: evidence for surface segmentation mechanisms.

The independent motions of objects in a visual scene are commonly manifest as overlapping retinal motions. A consequence of this overlap is the creation of spurious retinal image features--such as corners and terminated contours--that bear no direct relation to the motions of the objects that give rise to them. To reconstruct object motions, these emergent features must be distinguished from the retinal motions of real object features. This process can be studied using visual stimuli known as plaid patterns, which provide a laboratory archetype for the ubiquitous real-world circumstance of two surfaces with overlapping retinal projections. By adjusting luminance relationships in a plaid pattern it is possible to influence the perceptual interpretation of image features, such that they are seen as either an emergent consequence of occlusion or as real variations in surface reflectance. In the former case, the plaid is most likely to be to perceived as two independently moving surfaces, whereas the latter generally elicits a percept of a single moving surface. This dependence of motion perception on luminance configuration can be viewed as evidence for the involvement of surface segmentation mechanisms, which distinguish between real and emergent image features by promoting a depth-ordered neural representation of surfaces. An alternative interpretation, which does not demand such depth-ordering and feature classification, asserts that the effect of luminance configuration can be accounted for by attendant variations in the distribution of moving Fourier components. To evaluate these two proposed mechanisms, we designed novel plaid stimuli in which surface segmentation cues could be varied independently of changes in the distribution of Fourier components. Perceived motion was found to be highly correlated with the presence of appropriate segmentation cues and uncorrelated with the distribution of Fourier components. These results refute the Fourier components hypothesis, and they support our proposal that surface segmentation plays a critical role in the interpretation of visual motion signals.

Visual motion perception.

The primate visual motion system performs numerous functions essential for survival in a dynamic visual world. Prominent among these functions is the ability to recover and represent the trajectories of objects in a form that facilitates behavioral responses to those movements. The first step toward this goal, which consists of detecting the displacement of retinal image features, has been studied for many years in both psychophysical and neurobiological experiments. Evidence indicates that achievement of this step is computationally straightforward and occurs at the earliest cortical stage. The second step involves the selective integration of retinal motion signals according to the object of origin. Realization of this step is computationally demanding, as the solution is formally underconstrained. It must rely--by definition--upon utilization of retinal cues that are indicative of the spatial relationships within and between objects in the visual scene. Psychophysical experiments have documented this dependence and suggested mechanisms by which it may be achieved. Neurophysiological experiments have provided evidence for a neural substrate that may underlie this selective motion signal integration. Together they paint a coherent portrait of the means by which retinal image motion gives rise to our perceptual experience of moving objects.

Interrelationships between porcine somatotropin and dietary lysine on growth performance and carcass characteristics of finishing swine.

Seventy-two barrows (initial weight = 57.1 kg) were used to determine the interrelationship between porcine somatotropin (pST) and dietary lysine and their effects on growth performance and carcass characteristics. Pigs were injected daily in the extensor muscle of the neck with either 4 or 8 mg of pST and fed a pelleted corn-soybean meal-sesame meal diet (.8% lysine; 17.8% CP) or diets containing 1.0, 1.2, or 1.4% lysine provided by additions of L-lysine.HCl (2 x 4 factorial arrangement). Control pigs (placebo injection) received the .8% lysine diet. All diets were formulated to contain > or = 200% of current recommendations for other amino acids, vitamins, and minerals. A tendency for a pST x lysine interaction was observed for cumulative ADG (P < .15) and feed conversion (G/F; P < .05). Average daily gain and G/F were improved by increasing dietary lysine level in pigs injected with 4 mg/d of pST; however, pigs injected with 8 mg/d of pST had greater improvements in cumulative ADG and G/F with added lysine. Feed intake was reduced (quadratic, P < .10) as dietary lysine level and pST dosage increased. Increasing pST dosage and dietary lysine increased (linear, P < .05) longissimus muscle area and decreased backfat thickness. Trimmed ham and loin weights were increased (linear, P < .10) by pST dosage. Chemical composition of samples taken from the loin, ham, and belly indicated increased moisture and CP and decreased lipid content as pST dosage and dietary lysine level increased (quadratic, P < .05). Shear force values from loin and semimembranosus increased with increasing lysine level (quadratic, P < .01) and pST dosage (linear, P < .05); however, these differences were not detected by sensory analysis (P > .20). Plasma urea concentrations on d 28 decreased with increasing lysine level (quadratic, P < .05), and plasma lysine concentrations increased (linear, P < .01). Based on the pST x lysine interaction for ADG and G/F, these data suggest that the lysine level needed to maximize growth performance and carcass characteristics may be proportional to the pST dosage provided. Growth and carcass characteristics were maximized by dietary lysine intakes of 27 to 32 and > or = 36 g/d for pigs injected with 4 and 8 of mg/d of pST, respectively.

Image segmentation cues in motion processing: implications for modularity in vision.

Abstract The problem of processing visual motion is underconstrained-many possible real world motions are compatible with any given dynamic retinal image. Recent psychophysical and neurophysiological experiments have shown that the primate visual system's normally veridical interpretation of moving patterns is attained through utilization of image segmentation cues unrelated to motion per se. These findings challenge notions of modularity in which it is assumed that the processing of specific scene properties, such as motion, can be studied in isolation from other visual processes. We discuss the implications of these findings with regard to both experimental and computational approaches to the study of visual motion.

Neural correlates of perceptual motion coherence.

The motions of overlapping contours in a visual scene may arise from the physical motion(s) of either a single or multiple surface(s). A central problem facing the visual motion system is that of assigning the most likely interpretation. The rules underlying this perceptual decision can be explored using a visual stimulus formed by superimposing two moving gratings. The resultant percept is either that of a single coherently moving 'plaid pattern' (coherent motion) or of the two component gratings sliding noncoherently across one another (noncoherent motion). When plaid patterns are configured to mimic one transparent grating overlying another, the percept of noncoherent motion dominates. We now report that neurons in the visual cortex of rhesus monkeys exhibit changes in direction tuning that parallel this perceptual phenomenon: sensitivity to the motions of the component gratings is enhanced under conditions that favour the perception of noncoherent motion. These results challenge models of cortical visual processing that fail to take into account the contribution of figural image segmentation cues to the analysis of visual motion.

Motion coherency rules are form-cue invariant.

Object features can be made manifest by differences in a variety of physical attributes or figural cues. In this study we provide evidence that motion signals arising from different figural cues can be combined to produce a percept of a coherently moving (heterogeneous-cue) pattern. Moreover, as is true for homogeneous-cue patterns, similarity along the dimensions of contrast (we introduce the idea of contrast equivalence) and spatial frequency determines whether coherent motion is perceived. These results are in accordance with recent physiological evidence demonstrating that directional selectivity of many neurons in cortical visual area MT is invariant over changes in the figural cue that defines the moving stimulus ("form-cue invariance").

Visual receptive field organization and cortico-cortical connections of the lateral intraparietal area (area LIP) in the macaque.

The visual receptive field physiology and anatomical connections of the lateral intraparietal area (area LIP), a visuomotor area in the lateral bank of the inferior parietal lobule, were investigated in the cynomolgus monkey (Macaca fascicularis). Afferent input and physiological properties of area 5 neurons in the medial bank of the intraparietal sulcus (i.e., area PEa) were also determined. Area LIP is composed of two myeloarchitectonic zones: a ventral zone (LIPv), which is densely myelinated, and a lightly myelinated dorsal zone (LIPd) adjacent to visual area 7a. Previous single-unit recording studies in our laboratory have characterized visuomotor properties of area LIP neurons, including many neurons with powerful saccade-related activity. In the first part of the present study, single-unit recordings were used to map visual receptive fields from neurons in the two myeloarchitectonic zones of LIP. Receptive field size and eccentricity were compared to those in adjacent area 7a. The second part of the study investigated the cortico-cortical connections of area LIP neurons using tritiated amino acid injections and fluorescent retrograde tracers placed directly into different rostrocaudal and dorsoventral parts of area LIP. The approach to area LIP was through somatosensory area 5, which eliminated the possibility of diffusion of tracers into area 7a. Unlike many area 7a receptive fields, which are large and bilateral, area LIP receptive fields were much smaller and exclusively confined to the contralateral visual field. In area LIP, an orderly progression in visual receptive fields was evident as the recording electrode moved tangentially to the cortical surface and through the depths of area LIP. The overall visual receptive field organization, however, yielded only a rough topography with some duplications in receptive field representation within a given rostrocaudal or dorsoventral part of LIP. The central visual field representation was generally located more dorsally and the peripheral visual field more ventrally within the sulcus. The lower visual field was represented more anteriorly and the upper visual field more posteriorly. In LIP, receptive field size increased with eccentricity but with much variability with in the sample. Area LIPv was found to have reciprocal cortico-cortical connections with many extrastriate visual areas, including the parieto-occipital visual area PO; areas V3, V3A, and V4: the middle temporal area (MT); the middle superior temporal area (MST); dorsal prelunate area (DP); and area TEO (the occipital division of the intratemporal cortex). Area LIPv is also connected to area TF in the lateral posterior parahippocampal gyrus.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of select menhaden fish meal in starter diets for pigs.

Two trials were conducted to evaluate a select menhaden fish meal (SMFM) as a protein source in starter diets for 390, 3-wk-old weaned pigs. Initial weights averaged 4.8 and 5.5 kg in Trials 1 and 2, and trials were conducted for 5 and 4 wk, respectively. Diets in Trial 1 were formulated by substituting levels of 0, 4, 8, 12, 16 or 20% SMFM for soybean meal plus corn on a protein basis. The 20% fish meal diet contained no soybean meal; all diets contained between 19.8 and 20.2% CP, between 1.34 and 1.40% lysine and 25% dried whey. Replacement of soy protein with fish meal elicited a quadratic improvement (P = .01) in cumulative ADG and average daily feed intake (ADFI) by the end of wk 5. The diet containing 8% SMFM resulted in the maximum observed ADG; however, the maximum ADFI occurred in pigs fed the diet containing 12% SMFM. Breakpoint analysis indicated that 4.5 and 9.3% SMFM maximized ADG and ADFI, respectively. Addition of SMFM did not affect efficiency of feed utilization (F/G). In Trial 2, a 2 X 3 factorial with two levels of dried whey (10 or 20%) and three levels of SMFM (0, 4 or 8%), a SMFM X dried whey interaction (P less than .05) was observed for cumulative ADG and F/G by the of wk 4 with greater benefit from SMFM with 10% than with 20% dried whey.(ABSTRACT TRUNCATED AT 250 WORDS)

Transparency and coherence in human motion perception.

When confronted with moving images, the visual system often must decide whether the motion signals arise from a single object or from multiple objects. A special case of this problem arises when two independently moving gratings are superimposed. The gratings tend to cohere and move unambiguously in a single direction (pattern motion) instead of moving independently (component motion). Here we report that the tendency to see pattern motion depends very strongly on the luminance of the intersections (that is, to regions where the gratings overlap) relative to that of the gratings in a way that closely parallels the physics of transparency. When the luminance of these regions is chosen appropriately, pattern motion is destroyed and replaced by the appearance of two transparent gratings moving independently. The observations imply that motion detecting mechanisms in the visual system must have access to tacit 'knowledge' of the physics of transparency and that this knowledge can be used to segment the scene into different objects. The same knowledge could, in principle, be used to avoid confusing shadows with real object boundaries.

Preferential effects of caffeine on limbic and cortical dopamine systems.

In this study, we investigated the effects of acute caffeine administration on the activity of midbrain dopamine neurons. Caffeine significantly depressed the firing rates of dopamine neurons in the ventral tegmental area (A10 group), but had no significant effect on the firing rates of dopamine neurons in the substantia nigra zona compacta (A9 group). The action of caffeine in A10 was completely blocked by pretreatment with the adenosine agonist L-phenyl-isopropyl-adenosine (L-PIA), confirming numerous lines of evidence that caffeine and other xanthines act as competitive antagonists at adenosine receptors. The dopamine antagonist haloperidol also antagonized the effects of caffeine. This finding is consistent with a mechanism of caffeine-induced depression of dopamine neuron activity involving dopamine release, similar to that observed during amphetamine administration. Finally, the benzodiazepine diazepam also antagonized the dopaminergic effects of caffeine. It appears that, in the rat, caffeine administration inhibits mesolimbic and mesocortical projecting dopamine neurons, but has no effect on dopamine neurons that project to the striatum.

Differential effects of an anxiogenic beta-carboline on single unit activity in the locus coeruleus and substantia nigra of the rat brain.

The firing rates of single units in the substantia nigra pars reticulata, substantia nigra pars compacta and the locus coeruleus were recorded during the intravenous administration of beta-carboline-3-carboxylic acid ethyl ester (beta CCE). beta-Carboline-3-carboxylic acid produced a dose-dependent excitation in all units tested in the substantia nigra pars reticulata and a small inhibitory effect on some units in the substantia nigra pars compacta. It had no effect on the firing rates of single units in the locus coeruleus, but did prove effective in reversing inhibition induced by diazepam in this nucleus. The relevance of the findings to the anxiogenic effects of beta-carboline-carboxylic acid ethyl ester are discussed.