Visual sensitivity, probed through perceptual detectability of very brief visual stimuli, is strongly impaired around the time of rapid eye movements. This robust perceptual phenomenon, called ...saccadic suppression, is frequently attributed to active suppressive signals that are directly derived from eye movement commands. Here we show instead that visual-only mechanisms, activated by saccade-induced image shifts, can account for all perceptual properties of saccadic suppression that we have investigated. Such mechanisms start at, but are not necessarily exclusive to, the very first stage of visual processing in the brain, the retina. Critically, neural suppression originating in the retina outlasts perceptual suppression around the time of saccades, suggesting that extra-retinal movement-related signals, rather than causing suppression, may instead act to shorten it. Our results demonstrate a far-reaching contribution of visual processing mechanisms to perceptual saccadic suppression, starting in the retina, without the need to invoke explicit motor-based suppression commands.
Global agricultural production will likely need to increase in the future due to population growth, changing diets, and the rising importance of bioenergy. Intensifying already existing cropland is ...often considered more sustainable than converting more natural areas. Unfortunately, our understanding of cropping patterns and intensity is weak, especially at broad geographic scales. We characterized and mapped cropping systems in Europe, a region containing diverse cropping systems, using four indicators: (a) cropping frequency (number of cropped years), (b) multi-cropping (number of harvests per year), (c) fallow cycles, and (d) crop duration ratio (actual time under crops) based on the MODIS Normalized Difference Vegetation Index (NDVI) time series from 2000 to 2012. Second, we used these cropping indicators and self-organizing maps to identify typical cropping systems. The resulting six clusters correspond well with other indicators of agricultural intensity (e.g., nitrogen input, yields) and reveal substantial differences in cropping intensity across Europe. Cropping intensity was highest in Germany, Poland, and the eastern European Black Earth regions, characterized by high cropping frequency, multi-cropping and a high crop duration ratio. Contrarily, we found lowest cropping intensity in eastern Europe outside the Black Earth region, characterized by longer fallow cycles. Our approach highlights how satellite image time series can help to characterize spatial patterns in cropping intensity-information that is rarely surveyed on the ground and commonly not included in agricultural statistics: our clustering approach also shows a way forward to reduce complexity when measuring multiple indicators. The four cropping indicators we used could become part of continental-scale agricultural monitoring in order to identify target regions for sustainable intensification, where trade-offs between intensification and the environmental should be explored.
Ocular position drifts during gaze fixation are significantly less well understood than microsaccades. We recently identified a short-latency ocular position drift response, of ∼1 min arc amplitude, ...that is triggered within <100 ms by visual onsets. This systematic eye movement response is feature-tuned and seems to be coordinated with a simultaneous resetting of the saccadic system by visual stimuli. However, much remains to be learned about the drift response, especially for designing better-informed neurophysiological experiments unraveling its mechanistic substrates. Here we systematically tested multiple new feature tuning properties of drift responses. Using highly precise eye tracking in three male rhesus macaque monkeys, we found that drift responses still occur for tiny foveal visual stimuli. Moreover, the responses exhibit size tuning, scaling their amplitude (both up and down) as a function of stimulus size, and they also possess a monotonically increasing contrast sensitivity curve. Importantly, short-latency drift responses still occur for small peripheral visual targets, which additionally introduce spatially directed modulations in drift trajectories toward the appearing peripheral stimuli. Drift responses also remain predominantly upward even for stimuli exclusively located in the lower visual field and even when starting gaze position is upward. When we checked the timing of drift responses, we found it was better synchronized to stimulus-induced saccadic inhibition than to stimulus onset. These results, along with a suppression of drift response amplitudes by peristimulus saccades, suggest that drift responses reflect the rapid impacts of short-latency and feature-tuned visual neural activity on final oculomotor control circuitry in the brain.
Visual processing is segregated into ON and OFF channels as early as in the retina, and the superficial (output) layers of the primary visual cortex (V1) are dominated by neurons preferring dark ...stimuli. However, it is not clear how the timing of neural processing differs between "darks" and "brights" in general, especially in light of psychophysical evidence; it is also equally not clear how subcortical visual pathways that are critical for active orienting represent stimuli of positive (luminance increments) and negative (luminance decrements) contrast polarity. Here, we recorded from all visually-responsive neuron types in the superior colliculus (SC) of two male rhesus macaque monkeys. We presented a disk (0.51° radius) within the response fields (RFs) of neurons, and we varied, across trials, stimulus Weber contrast relative to a gray background. We also varied contrast polarity. There was a large diversity of preferences for darks and brights across the population. However, regardless of individual neural sensitivity, most neurons responded significantly earlier to dark than bright stimuli. This resulted in a dissociation between neural preference and visual response onset latency: a neuron could exhibit a weaker response to a dark stimulus than to a bright stimulus of the same contrast, but it would still have an earlier response to the dark stimulus. Our results highlight an additional candidate visual neural pathway for explaining behavioral differences between the processing of darks and brights, and they demonstrate the importance of temporal aspects in the visual neural code for orienting eye movements.
Objects in our environment, such as birds flying across a bright sky, often project shadows (or images darker than the surround) on our retina. We studied how primate superior colliculus (SC) neurons visually process such dark stimuli. We found that the overall population of SC neurons represented both dark and bright stimuli equally well, as evidenced by a relatively equal distribution of neurons that were either more or less sensitive to darks. However, independent of sensitivity, the great majority of neurons detected dark stimuli earlier than bright stimuli, evidenced by a smaller response latency for the dark stimuli. Thus, SC neural response latency can be dissociated from response sensitivity, and it favors the faster detection of dark image contrasts.
The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute ...to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.
The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.
•Ensemble trajectory approaches robustly map disturbances in tropical dry forest.•About 8% (~24,900 km2) of forest in the Argentine Dry Chaco disturbed since 1990.•Disturbances particularly ...widespread during drought years.•Diverse spatial patterns point to multiple disturbance agents.•Degradation of remaining forest overlooked in this global deforestation hotspot.
Forest loss in the tropics affects large areas, but whereas full forest conversions are routinely assessed, forest degradation patters remain often unclear. This is particularly so for the world’s tropical dry forests, where remote sensing of forest disturbances is challenging due to high canopy complexity, strong phenology and climate variability, and diverse degradation drivers. Here, we used the full depth of the Landsat archive and devised an approach to detect disturbances related to forest degradation across the entire Argentine Dry Chaco (about 489,000 km2) over a 30-year timespan. We used annual time series of different spectral indices, summarized for three seasonal windows, and applied LandTrendr to temporally segment each time series. The resulting pixel-level forest disturbance metrics then served as input for a Random Forests classification which we used to produce an area-wide disturbance map, and associated yearly area estimates of disturbed forest. Finally, we evaluated disturbance trends in relation to climate and soil conditions. Our best model produced a disturbance map with an overall accuracy of 79%, with a balanced error distribution. A total of 8% (24,877 ± 860 km2) of the remaining forest in the Argentine Dry Chaco have been affected by forest disturbances between 1990 and 2017. Diverse spatial patterns of forest disturbances indicate a variety of agents driving disturbances. We also found the disturbed area to vary strongly between years, with larger areas being disturbed during drought years. Our approach shows that it is possible to robustly map forest disturbances in tropical dry forests using Landsat time series, and demonstrates the value of ensemble approaches to capture spectrally-complex and heterogeneous land-change processes. For the Chaco, a global deforestation hotspot, our analyses provide the first Landsat-based assessment of forest disturbance in remaining forests, highlighting the need to better consider such disturbances in assessments of carbon budgets and biodiversity change.
Display omitted
► Land-use decisions in post-Soviet Russia shifted toward market-based principles. ► Agricultural land use followed micro-economic theories of von Thünen and Ricardo. ► Areas with low ...agricultural productivity were abandoned first. ► Abandonment was driven by macro-economic factors, e.g. the withdrawal of subsidies. ► We expect further abandonment of lands with low productivity and population density.
The breakdown of socialism caused massive socio-economic and institutional changes that led to substantial agricultural land abandonment. The goal of our study was to identify the determinants of agricultural land abandonment in post-Soviet Russia during the first decade of transition from a state-controlled economy to a market-driven economy (1990–2000). We analyzed the determinants of agricultural land abandonment for approximately 150,550km2 of land area in the provinces (oblasts) of Kaluga, Rjazan, Smolensk, Tula and Vladimir in European Russia. Based on the economic assumptions of profit maximization, we integrated maps of abandoned agricultural land from five ∼185km×185km Landsat TM/ETM+ footprints with socio-economic, environmental and geographic variables, and we estimated logistic regressions at the pixel level to identify the determinants of agricultural land abandonment. Our results showed that a higher likelihood of agricultural land abandonment was significantly associated with lower average grain yields in the late 1980s and with higher distances from the nearest settlements, municipality centers, and settlements with more than 500 citizens. Hierarchical partitioning showed that the average grain yields in the late 1980s had the greatest power to explain agricultural land abandonment in our models, followed by the locational attributes of the agricultural land. We hypothesize that the termination of 90% of state subsidies for agriculture from 1990 to 2000 was an important underlying cause for the decrease of cultivation in economically and environmentally marginal agriculture areas. Thus, whereas the spatial patterns corresponded to the land rent theory of von Thünen, it was primarily the macro-scale driving forces that fostered agricultural abandonment. Our study highlighted the value of spatially explicit statistical models for studying the determinants of land-use and land-cover change in large areas.
Detailed knowledge of forest cover dynamics is crucial for many applications from resource management to ecosystem service assessments. Landsat data provides the necessary spatial, temporal and ...spectral detail to map and analyze forest cover and forest change processes. With the opening of the Landsat archive, new opportunities arise to monitor forest dynamics on regional to continental scales. In this study we analyzed changes in forest types, forest disturbances, and forest recovery for the Carpathian ecoregion in Eastern Europe. We generated a series of image composites at five year intervals between 1985 and 2010 and utilized a hybrid analysis strategy consisting of radiometric change classification, post-classification comparison and continuous index- and segment-based post-disturbance recovery assessment. For validation of the disturbance map we used a point-based accuracy assessment, and assessed the accuracy of our forest type maps using forest inventory data and statistically sampled ground truth data for 2010. Our Carpathian-wide disturbance map achieved an overall accuracy of 86% and the forest type maps up to 73% accuracy. While our results suggested a small net forest increase in the Carpathians, almost 20% of the forests experienced stand-replacing disturbances over the past 25years. Forest recovery seemed to only partly counterbalance the widespread natural disturbances and clear-cutting activities. Disturbances were most widespread during the late 1980s and early 1990s, but some areas also exhibited extensive forest disturbances after 2000, especially in the Polish, Czech and Romanian Carpathians. Considerable shifts in forest composition occurred in the Carpathians, with disturbances increasingly affecting coniferous forests, and a relative decrease in coniferous and mixed forests. Both aspects are likely connected to an increased vulnerability of spruce plantations to pests and pathogens in the Carpathians. Overall, our results exemplify the highly dynamic nature of forest cover during times of socio-economic and institutional change, and highlight the value of the Landsat archive for monitoring these dynamics.
•Regional monitoring of forest dynamics with Landsat has recently become feasible.•We analyzed six large-area composites employing a hybrid change detection approach.•We addressed forest types, disturbances and recovery in the Carpathians since 1985.•Forests increased slightly while 1/5 of Carpathian forests experienced disturbances.•Disturbances increasingly occurred in conifers, forest composition shifted markedly.
The wealth of complementary data available from remote sensing missions can hugely aid efforts towards accurately determining land use and quantifying subtle changes in land use management or ...intensity. This study reviewed 112 studies on fusing optical and radar data, which offer unique spectral and structural information, for land cover and use assessments. Contrary to our expectations, only 50 studies specifically addressed land use, and five assessed land use changes, while the majority addressed land cover. The advantages of fusion for land use analysis were assessed in 32 studies, and a large majority (28 studies) concluded that fusion improved results compared to using single data sources. Study sites were small, frequently 300–3000 km 2 or individual plots, with a lack of comparison of results and accuracies across sites. Although a variety of fusion techniques were used, pre-classification fusion followed by pixel-level inputs in traditional classification algorithms (e.g., Gaussian maximum likelihood classification) was common, but often without a concrete rationale on the applicability of the method to the land use theme being studied. Progress in this field of research requires the development of robust techniques of fusion to map the intricacies of land uses and changes therein and systematic procedures to assess the benefits of fusion over larger spatial scales.
PDF
