Biochemical-thermal-hydro-mechanical coupling model for aerobic degradation of landfilled municipal solid waste.

Ventilating solid waste landfills with an oxygen supply can effectively accelerate the degradation of waste, achieve rapid stabilization, and realize the sustainable utilization of landfills. Aiming to understand and verify the aerobic degradation process in landfills, this paper proposed a biochemical-thermal-hydro-mechanical coupling model. The model considers aerobic biochemical reactions, dissolved solute migration, heat transport, two-phase flow, and skeleton deformation. The model was verified by comparison with an in-situ experiment at Jinkou landfill. The results showed the model could accurately represent the observed degradation phenomena during the experiment. The modelling results indicated that the rate of temperature increase and peak temperature of the upper layer, which were lower than those of the middle layer, were affected by heat exchange at the landfill surface. The lowest temperatures occurred near the bottom because of high water content and low oxygen concentrations. The high temperature zone migrated out from the injection well during degradation, reflecting the degradation of degradable organic matter associated with oxygen diffusion rates and aerobic degradation reactions. The initial accumulated settlement value was fast, but slowed and finally stabilized. The surface subsidence also developed from the center around the injection well to the surrounding area, and 70% of the total subsidence occurred within 150 days. This newly developed model provides a theoretical framework for analyzing the multi-field coupling of aerobic degradation of landfilled municipal solid waste (MSW).

Out of sight but not out of mind: the neurophysiology of iconic memory in the superior temporal sulcus.

Iconic memory, the short-lasting visual memory of a briefly flashed stimulus, is an important component of most models of visual perception. Here we investigate what physiological mechanisms underlie this capacity by showing rapid serial visual presentation (RSVP) sequences with and without interstimulus gaps to human observers and macaque monkeys. For gaps of up to 93 ms between consecutive images, human observers and neurones in the temporal cortex of macaque monkeys were found to continue processing a stimulus as if it was still present on the screen. The continued firing of neurones in temporal cortex may therefore underlie iconic memory. Based on these findings, a neurophysiological vision of iconic memory is presented.

A region of right posterior superior temporal sulcus responds to observed intentional actions.

Human adults and infants identify the actions of another agent based not only on its intrinsic perceptual features, but critically on the contingent relationship between its motion path and the environmental context [Trends Cogn. Sci. 7 (1995) 287; Cognition 72 (2003) 237]. Functional neuroimaging studies of the perception of agents and intentional actions, on the other hand, have mostly focussed on the perception of intrinsic cues to agency, like a face or articulated body motion (e.g. [J. Neurosci. 17 (1997) 4302; Neuroimage 8 (1998) 221; Trends Cogn. Sci. 4 (2000) 267; Nat. Neurosci. 3 (2000) 80; Neuroimage 13 (2001) 775; Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 11656; Neuron 35 (2002) 1167; Neuron 34 (2002) 149, Neuroscience 15 (2003) 991; J. Neurosci. 23 (2003) 6819; Philos. Trans. R Soc. Lond. B. Biol. Sci. 358 (2003) 435]. Here we describe a region of the right posterior superior temporal sulcus that is sensitive not to articulated body motion per se, but to the relationship between the observed motion and the structure of the surrounding environment. From this and other aspects of the region's response, we hypothesize that this region is involved in the representation of observed intentional actions.

The speed of sight.

Macaque monkeys were presented with continuous rapid serial visual presentation (RSVP) sequences of unrelated naturalistic images at rates of 14--222 msec/image, while neurons that responded selectively to complex patterns (e.g., faces) were recorded in temporal cortex. Stimulus selectivity was preserved for 65% of these neurons even at surprisingly fast presentation rates (14 msec/image or 72 images/sec). Five human subjects were asked to detect or remember images under equivalent conditions. Their performance in both tasks was above chance at all rates (14--111 msec/image). The performance of single neurons was comparable to that of humans and responded in a similar way to changes in presentation rate. The implications for the role of temporal cortex cells in perception are discussed.

Response latency of macaque area MT/V5 neurons and its relationship to stimulus parameters.

A total of 310 MT/V5 single cells were tested in anesthetized, paralyzed macaque monkeys with moving random-dot stimuli. At optimum stimulus parameters, latencies ranged from 35 to 325 ms with a mean of 87+/-45 (SD) ms. By examining the relationship between latency and response levels, stimulus parameters, and stimulus selectivities, we attempted to isolate the contributions of these factors to latency and to identify delays representing intervening synapses (circuitry) and signal processing (flow of information through that circuitry). First, the relationship between stimulus parameters and latency was investigated by varying stimulus speed and direction for individual cells. Resulting changes in latencies were explainable in terms of response levels corresponding to how closely the actual stimulus matched the preferred stimulus of the cell. Second, the relationship between stimulus selectivity and latency across the population of cells was examined using the optimum speed and direction of each neuron. A weak tendency for cells tuned for slow speeds to have longer latencies was explainable by lower response rates among slower-tuned neurons. In contrast, sharper direction tuning was significantly associated with short latencies even after taking response rate into account, (P = 0.002, ANCOVA). Accordingly, even the first 10 ms of the population response fully demonstrates direction tuning. A third study, which examined the relationship between antagonistic surrounds and latency, revealed a significant association between the strength of the surround and the latency that was independent of response levels (P < 0.002, ANCOVA). Neurons having strong surrounds exhibited latencies averaging 20 ms longer than those with little or no surround influence, suggesting that neurons with surrounds represent a later stage in processing with one or more intervening synapses. The laminar distribution of latencies closely followed the average surround antagonism in each layer, increasing with distance from input layer IV but precisely mirroring response levels, which were highest near the input layer and gradually decreased with distance from input layer IV. Layer II proved the exception with unexpectedly shorter latencies (P< 0.02, ANOVA) yet showing only modest response levels. The short latency and lack of strong direction tuning in layer II is consistent with input from the superior colliculus. Finally, experiments with static stimuli showed that latency does not vary with response rate for such stimuli, suggesting a fundamentally different mode of processing than that for a moving stimulus.

Influence of stimulus speed upon the antagonistic surrounds of area MT/V5 neurons.

We investigated the effect of stimulus speed upon surround antagonism in macaque MT/V5 neurons, using probe stimuli placed at different positions in the surround. Their speed was varied, while the stimulation of the excitatory receptive field (RF) was held at optimal speed. Most Surrounds proved asymmetric, arising from a single region on one side of RF, although bilaterally and circularly symmetric surrounds were occasionally observed. Surround organization was generally retained at faster or slower surround speeds. Speed-dependent changes usually entailed diminished position dependence of surround influence, consequent to reduced surround effect at the position producing maximum inhibition. The effect of a stimulus covering the entire surround was much less dependent upon motion speed. Results show that surround non-uniformity is a robust finding in MT/V5 and endows neurons with multiple mechanisms for extracting surface orientation in depth.

The spatial distribution of the antagonistic surround of MT/V5 neurons.

The majority (217/325, 66%) of the neurons in the middle temporal (MT) area/V5 show strong antagonistic surrounds, defined here by a decrease of at least 50% in the summation curve. We mapped the antagonistic surround in 145 such cells, using eight circularly distributed surround stimulus patches (Surround Asymmetry Test, SAT) and also mapped the surround in 51 of these 145 cells using a grid consisting of 25 square patches (Surround Mapping Test, SMT). Both tests showed that the angular surround distribution was non-uniform in the majority of these neurons. In half the neurons, the antagonistic surround was asymmetric, and arose from a single region on one side of the excitatory receptive field (ERF). In another quarter of the sample the surround was bilaterally symmetric, and arose from a pair of regions on opposite sides of the ERF. Only the remaining 20% showed a circularly symmetric surround distribution. These three groups differed in their laminar distribution. The SMT showed that, radially, the surround antagonism reached a maximum, on average, at 1.5 times the ERF radius. Detailed comparisons of the spatial relationships of excitatory and inhibitory regions of the RF components shows that non-homogeneity of the surround influence appears to be an intrinsic property of the surround. Such a property may underly the extraction of the surface orientation and curvature from speed patterns.

Size and shape of receptive fields in the medial superior temporal area (MST) of the macaque.

Ninety-one single units were recorded in area MSTd of anesthetized and paralyzed macaques. Receptive fields (RFs) were mapped quantitatively using small patches of moving random dots in 25 different positions (the two-dimensional position test, or P2D). The dimensions of the receptive fields (RFs) were estimated by fitting P2D data with a generalized Gaussian function. The half-height areas of the RFs in MSTd were found to average 1085 deg2 and were not dependent upon eccentricity, in contrast to those in MT/V5 (n = 295) which averaged 31 deg2 at the fovea but at the periphery approached the RFs of MSTd in size. The RFs of some MSTd neurons extended 30-40 degrees into the ipsilateral hemifield. In comparison, the overlap was only 10-15 degrees in area MT/V5.

Selectivity of macaque MT/V5 neurons for surface orientation in depth specified by motion.

Area MTN5 in the macaque brain is one of the major cortical regions involved in the analysis of retinal image motion. The majority of the neurons in this cortical area have non-uniform antagonistic surrounds as components of their receptive field complexes. Theoretical studies indicate that such asymmetrical surrounds should enable neurons to extract orientation in depth from motion. Here we show that nearly half of the MTN5 neurons encode the tilt component of the orientation in depth of a plane specified by motion. Furthermore, we show that such selectivity for depth from motion depends on the presence of an asymmetrical surround and on the speed tuning of those asymmetrical surround influences.

Orientation discrimination in the cat: its cortical locus II. Extrastriate cortical areas.

Luminance-defined edges or bars are among the basic units of visual analysis: a "primitive" component of perception. We have utilized this stimulus in a psychophysical study of bar orientation discrimination in the cat before and after selective lesions in visual cortical areas. The cortices have been divided on the basis of their connectivity into three tiers. Tier I refers to areas 17 and 18, tier II includes areas that receive directly from tier I, and tier III includes those areas that receive directly from tier II. Previous studies (Vandenbussche et al. [1991] J. Comp. Neurol. 305:632-658) have shown that the discrimination of bar orientation depends heavily upon the integrity of areas 17 and 18 (tier I). The present study indicates that several extrastriate areas in tiers II and III contribute to this discrimination task. Our data suggest that the anterior medial lateral suprasylvian, the posterior lateral lateral suprasylvian (tier II), and the anterior lateral lateral suprasylvian (tier III) areas are most likely to contribute to bar orientation discrimination.

Spatial heterogeneity of inhibitory surrounds in the middle temporal visual area.

A recurrent theme in the organization of vertebrate visual cortex is that of receptive fields with an associated "silent" opponency component. In the middle temporal area (area MT), a cortical visual area involved in the analysis of retinal motion in primates, this opponency appears in the form of a region outside the classical receptive field (CRF) that in itself gives no response but suppresses responses to motion evoked within the CRF. This antagonistic motion surround has been described as very large and symmetrically arrayed around the CRF. On the basis of this view, the primary function of the surround has long been thought to consist of simple figure-ground segregation based on movement. We have made use of small stimulus patches to map the form and extent of the surround and find evidence that the surround inhibition of many MT cells is in fact confined to restricted regions on one side or on opposite sides of the CRF. Such regions endow MT cells with the ability to make local-to-local motion comparisons, capable of extracting more complex features from the visual environment, and as such, may be better viewed as intrinsic parts of the receptive field, rather than as separate entities responsible for local-to-global comparisons.

Shape and spatial distribution of receptive fields and antagonistic motion surrounds in the middle temporal area (V5) of the macaque.

The spatial organization of receptive fields in the middle temporal (MT) area of anaesthetized and paralysed macaque monkeys was studied. In all, 288 neurons were successfully recorded. The size and shape of the receptive field (RF) was mapped with small patches of translating random dots and the resulting data were fitted with a generalized Gaussian. Results show that the RF area increases with eccentricity, and is larger in lamina 5 than in other layers. Most of these RFs are elongated, and the axis of elongation tends to be orthogonal to the preferred direction of motion. The direction selectivity is maintained in all positions in the RF, but layer 5 cells are less direction-selective than cells in other layers. In a second series of experiments, radial dimensions of the classical RF and the antagonistic surround were estimated from area summation tests. These data were fitted with the difference of the integrals of two Gaussians. Surrounds were weakest in layer 4 and strongest in layer 2. Optimal stimulus diameters, also estimated from the area summation curve, were larger in the infragranular layers than in the other layers. The maximum sensitivity of the surround was clearly displaced from the classical RF (CRF) centre, indicating that the surround is not concentric with the CRF. This radial offset and the extent of the surround were largest in layers 2 and 5 and smallest in 3a. The extent of the surround half-height equalled, on average, 3-4 times that of the CRF. These results suggest that antagonistic surrounds are constructed in MT, probably through horizontal connections, and that a strong vertical organization exists in area MT, as has been shown for V1.

Processing of kinetically defined boundaries in the cortical motion area MT of the macaque monkey.

1. Electrophysiological recordings of 68 cells in the middle temporal area MT were made in paralyzed and anesthetized macaque monkeys. 2. Testing with our kinetic boundary stimuli always occurred under optimized conditions. To this end, the preferred direction, speed, stimulus position, and stimulus size of each cell were determined by quantitative tests. 3. The orientation selectivity to stationary luminance contrast edges served as a reference by which a response to kinetic boundaries could be compared. We found cells in area MT to be less selective to the orientation of luminance contrast stimuli than to the direction of motion. We confirmed the presence of neurons with preferred orientation aligned with their preferred direction. 4. The responses to kinetic edges defined by motion vectors moving in opposite directions, kinetic gratings with motion vectors in opposite directions, kinetic edges containing coherent motion and a stationary complementary field or coherent motion and a complementary field containing visual dynamic noise were compared. Kinetic boundaries were generated so that the motion vectors moved either parallel or orthogonal to the orientation of the discontinuity. For a cell to be considered as responding to the orientation of a kinetic boundary, it had to exhibit the same preferred orientation when the local motion vectors changed from parallel to orthogonal to the orientation of the kinetic boundary. 5. All cells in area MT changed their preferred orientation by 90 degrees when the coherent motion vectors changed from moving parallel to moving orthogonal to the boundary. This was the case independent of the types of kinetic boundary tested. We concluded that cells in area MT appear to respond to the motion vector over their classical receptive field (CRF) only and were unable to code the orientation of the kinetic boundary. 6. In those cells exhibiting an antagonistic surround, we examined the ability of the cell to code the position of a kinetic boundary. None of the cells tested signaled the position of a kinetic boundary. The side preference of the stimulus of the cells changed from left to right as the motion vectors in the stimulus reversed. This indicates that the cells were only selective for the motion vectors present over their CRF. 7. We found that the directional sensitivity of cells in area MT remained unaltered by the presence of additional motion vectors within the CRF. This suggests that cells in area MT extract a specific motion vector from a spatial configuration of vectors.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of macaque STS neurons to optic flow components: a comparison of areas MT and MST.

1. We recorded and tested quantitatively 65 middle temporal (MT) and 82 middle superior temporal (MST) cells in paralyzed and anesthetized monkeys. 2. Responses to the three elementary optic flow components (EFCs)--rotation, deformation, and expansion/contraction--and to translation (in the display) were compared after optimization of stimulus direction, speed, size, and position. As a control responses to flicker were measured. 3. Response windows were adapted in correspondence with our finding that latencies of MT and MST cells decrease with increasing speed for all types of motion. 4. There was a response continuum in MT as well as in MST cells. Compared with translation, MST cells responded significantly more to rotation but less to flicker than MT cells. MST cells were significantly more direction selective for expansion/contraction than MT cells. 5. MST cells generally responded to fewer motion types than MT cells. 6. Position invariance of EFC direction selectivity was tested over a region of the visual field centered on the translation receptive field (RF). Direction selectivity for an EFC was not position invariant in MT cells but it was invariant in 40% of the MST cells tested. These cells were considered EFC selective. 7. Most EFC-selective MST cells were selective for a single EFC, possibly combined with translation. Few of them were selective for deformation. 8. EFC selectivity was also speed invariant and EFC-selective MST cells usually had RFs summating inputs over wide portions of the visual field. 9. EFC-selective MST cells with similar selectivities were clustered.