51, t = 2 80; total time: b = 55 08, t = 2 21, go-past time: b = 

51, t = 2.80; total time: b = 55.08, t = 2.21, go-past time: b = 41.51, t = 2.20) with the exception of first fixation duration (b = 3.98, t = 0.60) and single fixation duration (b = 8.11, t = 0.98) whereas predictability was not modulated by task in any reading measure (all ts < 1.37) except for total time (b = 57.60, t = 2.72). These data suggest that, when checking for spelling errors that produce real but inappropriate words, proofreaders

still perform a qualitatively different type EX 527 concentration of word processing, which specifically amplifies effects of word frequency. However, while proofreaders do not appear to change their use of predictability during initial word recognition (i.e., first pass reading), later word processing does show increased effects of how well the word fits into the context of the sentence (i.e., during total time). We return to the issue of why this effect only appears on a late measure in Section 4.2. As with the reading time measures reported in Section 3.2.2.1, fixation probability measures showed a robust effect of task, with a higher probability of fixating the target (frequency items: z = 4.92, p < .001; predictability items: z = 5.41, p < .001), regressing into the target (frequency items: z = 5.60, p < .001; predictability items: z = 6.05, p < .001) and regressing out of the target (frequency items: z = 3.64, p < .001; predictability

items: z = 4.15, p < .001) in the proofreading task than in the reading task. Frequency yielded a main effect on probability of fixating the target (z = 5.77, p < .001) and probability of regressing out click here www.selleck.co.jp/products/Gemcitabine(Gemzar).html of the target (z = 2.56, p < .01) but not probability of regressing into the target (p > .15). Predictability yielded a marginal effect

on the probability of fixating the target (z = 1.77, p = .08) and a significant effect on the probability of regressing into the target (z = 5.35, p < .001) and regressing out of the target (z = 3.71, p < .001). There was a significant interaction between task and frequency on the probability of fixating the target (z = 2.14, p < .05) and a marginal interaction on the probability of regressing out of the target (z = 1.77, p = .08). All other interactions were not significant (all ps > .17). Thus, it seems as if the interactions seen in total time in Experiment 2 were not due to an increased likelihood of making a regression into or out of the target word, but rather to the amount of time spent on the word during rereading. As in Experiment 1, we tested for the three-way interaction between target type (frequency vs. predictability), independent variable value (high vs. low) and task (reading vs. proofreading) to evaluate whether the interactions between independent variable and task were different between the frequency stimuli and the predictability stimuli. As in Section 2.2.2.3, we tested for the three-way interaction in two key measures: gaze duration (Fig.

Numerous conceptual models incorporate some or all of these basic

Numerous conceptual models incorporate some or all of these basic concepts (e.g., Bull, 1991, Simon and Rinaldi, 2006, Wohl, 2010 and Chin et al.,

in press): in this section, I focus on the basic concepts. Connectivity is used to describe multiple aspects of fluxes of matter, energy and organisms (Fig. 1). Hydrologic connectivity refers to the movement of water, such as down a hillslope in the surface and/or subsurface, from hillslopes into channels, or along a river network (Pringle, 2001 and Bracken and Croke, 2007). Sediment connectivity describes the movement or storage of sediment down hillslopes, into channels, along river networks, and selleck so forth (Fryirs et al., 2007). River connectivity refers to water-mediated Cilengitide solubility dmso fluxes within a river network (Ward, 1997). Biological connectivity describes the ability of organisms or plant propagules to disperse between suitable habitats or between isolated populations for breeding (Merriam, 1984). Landscape connectivity refers to the movement of water, sediment, or other materials between individual landforms (Brierley et al., 2006). Structural connectivity characterizes the extent

to which landscape units, which can range in scale from <1 m for bunchgrasses dispersed across exposed soil to the configuration of hillslopes and valley bottoms across thousands of meters, are physically linked to one another (Wainwright et al., 2011). Functional connectivity describes Vildagliptin process-specific interactions between multiple structural characteristics, such as runoff and sediment moving downslope between the bunchgrasses and exposed soil patches (Wainwright et al., 2011). Any of these forms of connectivity can be described in terms of spatial extent, which partly depends on temporal variability. River connectivity, for example, fluctuates through time as discharge fluctuates, just as functional

connectivity along a hillslope fluctuates through time in response to precipitation (Wainwright et al., 2011). Connectivity can also be used to describe social components. The terms multidisciplinary, interdisciplinary, holistic, and integrative, as applied to research or management, all refer to disciplinary connectivity, or the ability to convey information originating in different scholarly disciplines, the incorporation of different disciplinary perspectives, and the recognition that critical zone processes transcend any particular scholarly discipline. Beyond the fact that the characteristics of connectivity critically influence process and form in the critical zone, the specifics of connectivity can be used to understand how past human manipulations have altered a particular landscape or ecosystem, and how future manipulations might be used to restore desired system traits. This approach is exemplified by the connectivity diagrams for rivers in Kondolf et al. (2006) (Fig. 2).

Sand released by the erosion of paleo-lobes such as St George I o

Sand released by the erosion of paleo-lobes such as St George I or Sulina (Fig. 1) periodically transferred sand downcoast to construct baymouth barriers and forming the Razelm, Sinoe and Zmeica lagoons (Giosan et al., 2006a and Giosan et al., 2006b). If left to natural forces, such a large scale alongshore sediment transfer may begin as soon as the St. George II lobe is de facto abandoned ( Constantinescu et al., in preparation), once Sacalin Island will attach to the shore with its southern tip or will drown in place. For all periods considered in this study, the shoreline behavior generally

mirrored and was therefore diagnostic for nearshore morphological changes. One exception has been the region downcoast of the St. PD0332991 cell line George mouth where wave sheltering by the updrift delta coast and changes in coastal orientation led to the development of a more complex series of longshore transport cells and an alternation of progradation and retreat sectors. Also several other local mechanisms may be acting to reduce the erosion GDC973 rates locally along the coast. For example, erosion appears to be minimal along the coast of the Chilia lobe where a series of secondary distributaries

still debouche small amounts of sediment. Controlled by the post-damming decrease in fluvial sediment, the sectors of the coast with natural deltaic progradation have shrunk drastically to the two largest secondary mouths of the Chilia distributaries that have become themselves wave dominated. The coast at the St. George mouth has been quite stable probably due to groin-type effects of the river plume and the mouth subaqueous bars and levees (Giosan, 2007). However, the dramatic increase in nearshore erosion

for the anthropogenic mafosfamide period was in large part due to the de facto abandonment of the St. George lobe ( Constantinescu et al., in preparation). Minor depocenters along the coast are not now the result of delta front development per se, but reflect either redirecting of eroded sediments offshore by the Sacalin barrier or trapping near large scale jetties. All in all, the dynamics of the Danube delta coastal fringe clearly shows that the natural pattern of delta coast evolution was a carefully balanced act of deposition and erosion rather than a uniform progradation of the shoreline. And this was aided not only by brute, direct fluvial sediment unloading at the coast but also by more subtle morphodynamic sediment trapping mechanisms. Still the overall budget of the deltaic coastal fringe was in deficit loosing sediment alongshore and offshore. When we take into account the long term history of the Danube delta in addition to insights gained in the current study, we can develop a novel conceptual understanding of its evolution as a function sediment partition between the delta plain and the delta coastal fringe as well as between major and minor distributaries.

Genetic and archeological data suggest that AMH populations moved

Genetic and archeological data suggest that AMH populations moved out of Africa between ∼70,000 and 50,000 years ago, spreading eastward along the southern shores of Asia (Bulbeck, 2007), as well as along inland routes into central and western Eurasia (Fig. 2). From Island Southeast Asia, they crossed oceanic straits

up to 100 km wide to settle Australia, New Guinea, western Melanesia (near Oceania), and the Ryukyu Islands between 50,000 and 35,000 years ago (Erlandson, 2010). These maritime explorers had fishing skills and boats capable of oceanic crossings that enabled them to colonize PD-1/PD-L1 inhibitor clinical trial lands that earlier hominins never reached (O’Connor et al., 2011). Near the end of the Pleistocene, maritime peoples may also have followed the coastlines of Northeast Asia to Beringia, a broad plain connecting Asia and North America that formed as sea levels dropped dramatically during the Last Glacial Maximum. Roughly 16,000 years ago, as the world warmed and the coastlines of Alaska and British Columbia deglaciated, these coastal peoples may have migrated down the Pacific Coast into the Americas, following an ecologically rich ‘kelp highway’ that provided a similar suite of marine resources from northern Japan to Baja California (Erlandson et al., 2007). By 14,000 years ago, these ‘First Americans’ had reached Compound C the coast of central Chile and probably explored much of the

New World. Another significant maritime migration occurred between about 4000 and 1000 years ago, when agricultural peoples with sophisticated sailing vessels loaded with domesticated plants and animals spread out of Asia to populate thousands of islands throughout the Pacific and Indian oceans (Kirch, 2000 and Rick et al., 2014). Often referred to as the Austronesian Radiation after the family of languages these maritime peoples spoke, the result was the introduction of humans and domesticated animals (pigs, dogs, Sulfite dehydrogenase rats, chickens, etc.) and plants to fragile island ecosystems throughout

the vast Indo-Pacific region. A similar process occurred in the North Atlantic, as the Vikings settled several islands or archipelagos—including the Faroes, Iceland, and Greenland—between about AD 700 and 1100, carrying a ‘transported landscape’ of domesticated plants and animals with them (Erlandson, 2010). Within this broad overview of human evolution, geographic expansion, and technological innovation, we can also see a general acceleration of behavioral and technological change through the past 2.5 million years (Fig. 3). Beginning with the Oldowan Complex, technological change was initially very slow, with limited evidence of innovation from the initial Oldowan, through the Developed Oldowan, to the appearance of the Acheulean Complex about 1.7 million years ago. The Acheulean, marked by a widespread (but not universal) reliance on large handaxes and cleavers, shows a similar conservatism, with only limited evidence of technological change through almost a million years of prehistory.

Because these costs and benefits are assumed to be correlated int

Because these costs and benefits are assumed to be correlated intrinsically Androgen Receptor Antagonist with one another, being influenced by a common underlying inhibition

process, the overall relationship between inhibitory ability and retrieval-induced forgetting should be muddied. Consequently, the correlation between inhibitory control ability and retrieval-induced forgetting should be stronger when retrieval-induced forgetting is measured using category-plus-stem cues at final test than when measured using category cues alone. These dynamics are illustrated in Fig. 1, which depicts a hypothetical function relating inhibitory control ability to the two hypothesized components of retrieval-induced forgetting, separately for the two types of test (adapted from Anderson & Levy, 2007). In both the top and bottom

panels the amount of retrieval-induced forgetting attributable to the persisting aftereffects of inhibition increases monotonically with increasing inhibitory control ability. Thus, for simplicity, we assume that regardless of the nature of the final test, the amount of retrieval-induced forgetting caused by the aftereffects of inhibition from the earlier retrieval practice phase remains the same. However, the two panels differ in the amount of retrieval-induced forgetting attributable to blocking at final test, with greater blocking arising on a category-cued final test than on a category-plus-stem final test, with this difference growing Saracatinib as inhibitory control ability weakens. This reflects our assumption that searching memory with a distinctive compound cue should greatly reduce competition,

and focus search. Crucially, because we assume both components may contribute to the observed retrieval-induced forgetting effect to varying degrees, the Adenosine relationship between inhibitory control ability and overall forgetting should vary substantially by test type. Because persisting inhibition and blocking are oppositely related to inhibitory control ability, the contribution of blocking at test, when combined with the aftereffects of inhibition, should dilute the relationship between inhibition ability and forgetting. Specifically, the stronger the blocking component at test, the weaker the observed relationship between retrieval-induced forgetting and inhibition ability should become. For example, the correlation should be more strongly positive in the category-plus-stem condition than in the category-cued condition. Indeed, if the contribution of blocking to category-cued recall is great enough—as in the hypothetical example—then retrieval-induced forgetting may be unrelated or even negatively related to inhibitory control ability.

1), draining an area of ∼742,400 km2 which covers semi-arid and s

1), draining an area of ∼742,400 km2 which covers semi-arid and semi-humid climatic zones. Its upper reaches (from the headwater to Toudaoguai) drain the northern Qinghai-Tibetan mountains and provide approximately 60% of the river’s water discharge. The middle reaches of the Huanghe (from Toudaoguai to Huayuankou) cross the soil-rich Loess Plateau, where the soils are highly

erodible during rain-storm events. The river gains ∼90% of its sediment load during this journey. As the Huanghe enters its flatter lower basin, however, it loses considerable energy for sediment transport and deposits large amounts of sediment (primarily coarser-grained) on the riverbed. Moreover, the lower reaches have few tributaries, further diminishing water flux and transportation capacity. The heavy sedimentation results in an elevated riverbed several meters (locally > 10 m) MDV3100 cell line above the surrounding floodplain. River discharge of the Huanghe is highly dependent on the monsoon flood season (July–October), which brings about 60% of the annual precipitation for the drainage basin. But water discharge is also affected by short-term climatic oscillations. The lower reaches of the Huanghe experienced

no flow Selleck SCR7 or low flow conditions during the 1970s–1990s, which was mainly due to low basin precipitation associated with drought. The sediment load is also sensitive to human-controlled Thiamet G land use in its source region, the Loess plateau. Since the 1960s, more than 20 large reservoirs have been constructed in the Huanghe and its tributaries to meet demands for water. In particular, four large dams (Longyangxia, Liujiaxia, Sanmenxia, Xiaolangdi) on the Huanghe (Fig. 1) each exceeds 100 m in height (Table 1). The four reservoirs have a total impoundment capacity of 55.7 × 109 m3, roughly equaling the river’s annual water discharge. This capacity enables modulation of the river’s runoff by storing flood water within reservoirs

in wet seasons and releasing it in dry seasons (Wang et al., 2007). Given the different source regions for Huanghe’s water and sediment, the Sanmenxia and Xiaolangdi reservoirs in the lower middle reaches have major impacts on sediment entrapment. The upstream reservoirs (Longyangxia and Liujiaxia) play a more significant role in modulating runoff. The Xiaolangdi dam (location shown in Fig. 1) situates at the end of the middle reaches and thus controls the runoff entering the lower Huanghe (Table 1). Long-term (1950–2012) datasets of water and sediment recorded at gauging stations on the Huanghe (see Fig. 1) allow an assessment of how dams affect the delivery of material to the sea.

Castellnou and Miralles (2009) further

Castellnou and Miralles (2009) further Tanespimycin cell line detailed the industrial fire epoch by differentiating among five “generations of large wildfires” (Fig. 1), where a wildfire is defined

as an uncontrolled fire in an area of combustible vegetation that occurs in the countryside or a wilderness area. Both typological systems can be applied in most regions of the world. In this review paper we integrate these definitions for the first time in the long-term and recent forest fire history of the Alpine region. In fact, despite the considerable literature produced for specific areas, e.g., Conedera et al. (2004a), Carcaillet et al. (2009), Favilli et al. (2010), Colombaroli et al. (2013), no synthesis on historical, present and future fire regimes so far exists for the European Alpine region. The proposed approach additionally allows to insert the analyzed fire history in a more global context of ongoing changes as experienced also by other regions

of the world. To this purpose, the impact of the evolution of human fire uses, and fire suppression policies, on the fire regime and on the value of ecosystem services is presented; the potential influence of present and future fire management strategies on the cultural landscape maintenance, post-management forest ecosystems evolution, and the general landscape and habitat diversity is discussed. Looking at common traits in the worldwide fire regime trajectories, Pyne Selleckchem AC220 (2001) identified three main fire epochs consisting of a pre-human phase driven by natural fire regimes, a successive phase dominated by land-use related anthropogenic fires, and a third phase resulting from the rise of industrial technology and the progressive banning of the use of fire in land management (Fig. Orotidine 5′-phosphate decarboxylase 1): – First fire epoch: when the human population was too scarce and scattered to have a significant impact

on the fire regime and ignition sources were mostly natural (lightning and volcanoes). In this first fire epoch, fire became an important ecological factor along with climate fluctuations, influencing the selection of species life-history traits related to fire, e.g., Johnson (1996), Keeley and Zedler (2000), Pausas and Keeley (2009), and the evolution of fire-adapted and fire dependent ecosystems, e.g., Bond et al. (2005), Keeley and Rundel (2005), Beerling and Osborne (2006). Charcoal fragments stratified in alpine lakes and soils sediments have been used as proxy of fire activity in the European Alpine region (Ravazzi et al., 2005, Tinner et al., 2006 and Favilli et al., 2010). Early evidence of relevant fires in the Alps date back to interglacial periods during the Early Pleistocene (Ravazzi et al., 2005). However, due to multiple glaciations most of the Alpine stratigraphic record was eroded. Consequently, most fire regime reconstruction date-back to the Lateglacial-Holocene transition at around 15,000 cal. yrs BC (Favilli et al., 2010 and Kaltenrieder et al., 2010).

To test this hypothesis, we measured the association of CaMKIIα m

To test this hypothesis, we measured the association of CaMKIIα mRNA with PABP in the hippocampus of WT and Paip2a−/− mice using a ribonucleoprotein

immunoprecipitation (RIP) assay with PABP antibody. The association of PABP with CaMKIIα mRNAs was increased after contextual training in both groups. However, the AT13387 price increase was greater in Paip2a−/− mice as compared to WT mice ( Figure 6F). Taken together, our data demonstrate that, while translation of CaMKIIα mRNA is not altered in Paip2a−/− mice under basal conditions, contextual training of Paip2a−/− mice leads to enhanced CaMKIIα mRNA translation. This is consistent with previous studies showing that the CaMKIIα mRNA contains two cytoplasmic polyadenylation elements (CPEs), binds the CPE binding protein, and undergoes NMDA- and experience-dependent elongation of poly(A) tail at synapses ( Huang et al., 2002; Wu et al., 1998). Translational activation by newly formed poly(A) tail depends on PABP binding, which, in turn, is regulated by PAIP2A.

We next examined the enhancement of CaMKIIα mRNA translation in Paip2a−/− mice by using immunostaining. Previous studies reported Stem Cell Compound Library that tetanic stimulation increases CaMKIIα levels in CA1 pyramidal cell dendrites of acute hippocampal slices as early as 5 min after the stimulation in a protein synthesis-dependent manner ( Gong et al., 2006; Ouyang et al., 1999). Tetanus-induced dendritic translation of CaMKIIα mRNA in CA1 pyramidal cells in acute hippocampal slices from WT and Paip2a−/− mice was examined. A surgical cut was made across the CA1 area perpendicularly to the pyramidal cell layer to separate tetanized and untetanized slice

regions ( Gong et al., 2006). Thirty minutes after tetanic stimulation, slices Loperamide were fixed and processed for CaMKIIα fluorescent immunostaining, and the ratio of the CaMKIIα fluorescent signal from the dendritic area of the stimulated and the control sides was calculated. 1HFS induced no change in CaMKIIα amounts in WT slices ( Figures 7A and 7D), but in Paip2a−/− slices, 1HFS led to a significant increase in CaMKIIα expression (WT: 3.8% ± 1.9%; Paip2a−/−: 34.5% ± 9.7%, p < 0.01; Figures 7B and 7D). The increase in dendritic expression of CaMKIIα in Paip2a−/− slices was abolished when anisomycin was present during tetanization ( Figures 7C and 7D), demonstrating that increased levels of CaMKIIα protein is due to upregulation of CaMKIIα mRNA translation. These results indicate that, as with L-LTP, the threshold for induction of dendritic CaMKIIα mRNA translation is lowered in Paip2a−/− slices. It is striking that TBS increased CaMKIIα levels to a greater degree in Paip2a−/− slices than in WT slices (WT: 14.5% ± 2.3%; Paip2a−/−: 45.8% ± 15.4%, p < 0.05; Figure 7E), which supports in vivo results that demonstrate increased CaMKIIα mRNA translation following behavioral training.

, 2003) Stressful experiences exert biphasic, time-dependent eff

, 2003). Stressful experiences exert biphasic, time-dependent effects upon the prefrontal cortex, as shown in animal models. In

3- to 4-week-old rats, diverse acute stressors (forced swim, restraint, elevated platform) facilitate both PFC-dependent behavior, as well as long-term potentiation (LTP), tested 4 hr after stress exposure. Adrenal steroids mediate these effects and facilitate LTP, as well as behaviors known to depend on mPFC via mechanisms dependent not only on glucocorticoid receptors (GRs), but also on signaling pathways involving serum- and glucocorticoid-inducible kinase (SGK) and Rab4-mediated recycling of NMDA and AMPA receptors (NMDARs and AMPARs, respectively) (Yuen et al., 2009 and Yuen et al., 2011a). Yet, at this same age, chronic unpredictable stress

or restraint I-BET-762 molecular weight stress for 7 days impaired temporal order recognition memory in rats, a cognitive process controlled by the mPFC and caused reduced AMPAR- and NMDAR-mediated synaptic transmission and glutamate receptor expression in mPFC (Yuen et al., 2012). All these effects relied on activation of glucocorticoid receptors and the subsequent enhancement of ubiquitin/proteasome-mediated degradation SCH772984 molecular weight of GluR1 and NR1 subunits, which was controlled by the E3 ubiquitin ligase Nedd4-1 and Fbx2, respectively. Inhibition of proteasomes or knockdown of Nedd4-1 and Fbx2 in PFC prevented the loss of glutamatergic responses and recognition memory in stressed animals. Thus, repeated stress dampens PFC glutamatergic transmission by facilitating glutamate receptor turnover. Indeed, the effects of chronic stress carry over to older ages Forskolin research buy since, in adult

rats, 21 days of chronic restraint stress impaired working memory and caused spine loss and debranching of dendrites on mPFC neurons (Hains et al., 2009), as will be discussed further below. However, in adult rats, acute mild stress impairs working memory during and immediately after stress exposures and does so via excessive stimulation of dopaminergic and noradrenergic receptors (Arnsten, 2009b). This acute stress effect on working memory and working memory-related activity in dlPFC monitored by fMRI is reported in volunteer subjects viewing movie clips with extremely aversive material (Qin et al., 2009). Intracellular signaling pathways activated by stress exposure have feedforward interactions that rapidly impair PFC-dependent cognitive function. High levels of dopamine (DA) D1-receptor stimulation and noradrenaline (NA) β1-receptor stimulation activate adenylyl cyclases (ACs) to produce cyclic AMP (cAMP); cAMP opens hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN channels) on dendritic spines to produce the h current (Ih), which weakens network inputs and decreases delay-related firing. High levels of NA also stimulate α1-receptors, which activate phosphatidylinositol biphosphate (PIP2)-protein kinase C (PKC) signaling (Arnsten, 2009b).

As described above, the correlation between LGN inputs is necessa

As described above, the correlation between LGN inputs is necessary for this variability to appear in simple cells despite the pooling of multiple inputs at the simple cell membrane. Unlike the variability in Vm of both the model and data (Figures 5B and 5C), the variability in the modeled synaptic input from the LGN (conductance, g) is strongly orientation dependent ( Figures 7B and 7F). This dependence is a function of the elongation of the subfields,

and that larger numbers of LGN afferents are activated simultaneously by the preferred stimulus compared to the null stimulus. As discussed above, the orientation dependent variability in g is transformed into the orientation independent variability in Vm by the saturating nonlinear relationship between g and Vm; removing the nonlinearity increases the orientation dependence of Vm variability ( Figures 6G–6I). selleck products selleck screening library The mechanism

underlying this transformation is illustrated in Figure 7C. The variability in g at the preferred orientation (gray) is higher than at the null orientation (cyan). Because that variability is occurring around a high mean g ( Figure 7C, gray)—where the slope of the g-Vm curve is flatter—it gives rise to a comparable level of variability in Vm as does the variability in g at the null orientation, which varies around the much lower resting g ( Figure 7C, cyan). The same compressive effect occurs, to a lesser degree, at low contrast ( Figures 7F and 7G, magenta and green). As a result, the variability in Vm is less dependent on orientation ( Figures 7D and 7G) than Monoiodotyrosine the variability in visually evoked conductance. Note that a more-rapidly saturating relationship between LGN activity

and Vm could potentially make the variability more equal across orientations. Historically, the feedforward model of visual cortex has been rightfully questioned for its failure to account for a large number of the response properties of simple cells: the sharpness of orientation tuning and its mismatch with receptive field maps, contrast invariance of orientation tuning and contrast-set gain control, cross-orientation suppression, contrast dependence of response phase, contrast dependence of preferred temporal frequency, and direction selectivity. All of these properties can be accounted for in models that incorporate cross-orientation inhibition or orientation-independent inhibition (Heeger, 1992, Troyer et al., 1998, Kayser et al., 2001, Lauritzen et al., 2001, Martinez et al., 2002, Lauritzen and Miller, 2003 and Hirsch et al., 2003). In gain-control models, almost all of these properties emerge from a single underlying mechanism: a large shunting inhibition that is contrast dependent and orientation independent (Heeger, 1992, Carandini and Heeger, 1994 and Carandini et al., 1997).