Continuity - based and discontinuity - based segmentation in transparent and spatially segregated global motion

The mechanisms underlying the parsing of a spatial distribution of velocity vectors into two adjacent (spatially segregated) or overlapping (transparent) motion surfaces were examined using random dot kinematograms. Parsing might occur using either of two principles. Surfaces might be defined on the basis of similarity of motion vectors and then sharp perceptual boundaries drawn between different surfaces (continuity-based segmentation). Alternatively, detection of a high gradient of direction or speed separating the motion surfaces might drive the process (discontinuity-based segmentation). To establish which method is used, we examined the effect of blurring the motion direction gradient. In the case of a sharp direction gradient, each dot had one of two directions differing by 135°. With a shallow gradient, most dots had one of two directions but the directions of the remainder spanned the range between one motion-defined surface and the other. In the spatial segregation case the gradient defined a central boundary separating two regions. In the transparent version the dots were randomly positioned. In both cases all dots moved with the same speed and existed for only two frames before being randomly replaced. The ability of observers to parse the motion distribution was measured in terms of their ability to discriminate the direction of one of the two surfaces. Performance was hardly affected by spreading the gradient over at least 25% of the dots (corresponding to a 1° strip in the segregation case). We conclude that detection of sharp velocity gradients is not necessary for distinguishing different motion surfaces.

Direction repulsion goes global

When viewing two superimposed, translating sets of dots moving in different directions, one overestimates the direction difference. This phenomenon of direction repulsion is thought to be driven by inhibitory interactions between directionally tuned motion detectors [1, 2]. However, there is disagreement on where this occurs — at early stages of motion processing [1, 3], or at the later, global motion-processing stage following “pooling” of these measures [4–6]. These two stages of motion pro - cessing have been identified as occurring in area V1 and the human homolog of macaque MT/V5, respectively[7, 8]. We designed experiments in which local and global predictions of repulsion are pitted against one another. Our stimuli contained a target set of dots, moving at a uniform speed, superimposed on a “mixed-speed” distractor set. Because the perceived speed of a mixed-speed stimulus is equal to the dots’ average speed [9], a global-processing account of direction repulsion predicts that repulsion magnitude induced by a mixed-speed distractor will be indistinguishable from that induced by a single-speed distractor moving at the same mean speed. This is exactly what we found. These results provide compelling evidence that global-motion interactions play a major role in driving direction repulsion.

Path-Dependency, Stocks, Switching Points, Flows: Reflections on Long-term Global Change and Local Opportunities

This paper examines the possibilities for peripheral localities to achieve upward mobility in the world-system by “hooking on” to larger processes of world-system accumulation. In particular, is it possible for economies that are dependent on foreign investment to receive a flow of investments that is high enough to overcome the negative impacts of a high stock of foreign investment, thus enabling them to cross a threshold and achieve upward mobility in the world-system? An analysis of therecent experience of the southern Irish “Celtic Tiger” economy during 1990-2000 indicates that such an upward movement is possible on the basis of massive foreign investment inflows. On closer examination, however, the Irish-type model appears to be highly deficient, because a high proportion of growth is illusionary and also on grounds of social desirability and lack of generalizability.

The Global Politics of Contemporary Travel Writing

To what extent do bestselling travel books, such as those by Paul
Theroux, Bill Bryson, Bruce Chatwin and Michael Palin, tell us as
much about world politics as newspaper articles, policy documents and
press releases? Debbie Lisle argues that the formulations of genre,
identity, geopolitics and history at work in contemporary travel writing
are increasingly at odds with a cosmopolitan and multicultural world in
which ‘everybody travels’. Despite the forces of globalisation, common
stereotypes about ‘foreignness’ continue to shape the experience of
modern travel. The Global Politics of Contemporary Travel Writing is
concerned with the way contemporary travelogues engage with, and try
to resolve, familiar struggles in global politics such as the protection of
human rights, the promotion of democracy, the management of
equality within multiculturalism and the reduction of inequality. This is
a thoroughly interdisciplinary book that draws from international
relations, literary theory, political theory, geography, anthropology and
history.