Psychophysical evidence for fast region-based segmentation processes in motion and color.

Theories of image segmentation suggest that the human visual system may use two distinct processes to segregate figure from background: a local process that uses local feature contrasts to mark borders of coherent regions and a global process that groups similar features over a larger spatial scale. We performed psychophysical experiments to determine whether and to what extent the global similarity process contributes to image segmentation by motion and color. Our results show that for color, as well as for motion, segmentation occurs first by an integrative process on a coarse spatial scale, demonstrating that for both modalities the global process is faster than one based on local feature contrasts. Segmentation by motion builds up over time, whereas segmentation by color does not, indicating a fundamental difference between the modalities. Our data suggest that segmentation by motion proceeds first via a cooperative linking over space of local motion signals, generating almost immediate perceptual coherence even of physically incoherent signals. This global segmentation process occurs faster than the detection of absolute motion, providing further evidence for the existence of two motion processes with distinct dynamic properties.

Order and disorder in fluid motion.

The development of complex states of fluid motion is illustrated by reviewing a series of experiments, emphasizing film flows, surface waves, and thermal convection. In one dimension, cellular patterns bifurcate to states of spatiotemporal chaos. In two dimensions, even ordered patterns can be surprisingly intricate when quasiperiodic patterns are included. Spatiotemporal chaos is best characterized statistically, and methods for doing so are evolving. Transport and mixing phenomena can also lead to spatial complexity, but the degree depends on the significance of molecular or thermal diffusion.

Perceptual learning in motion discrimination that generalizes across motion directions

When human subjects discriminate motion directions of two visual stimuli, their discrimination improves with practice. This improved performance has been found to be specific to the practiced directions and does not transfer to new motion directions. Indeed, such stimulus-specific learning has become a trademark finding in almost all perceptual learning studies and has been used to infer the loci of learning in the brain. For example, learning in motion discrimination has been inferred to occur in the visual area MT (medial temporal cortex) of primates, where neurons are selectively tuned to motion directions. However, such motion discrimination task is extremely difficult, as is typical of most perceptual learning tasks. When the difficulty is moderately reduced, learning transfers to new motion directions. This result challenges the idea of using simple visual stimuli to infer the locus of learning in low-level visual processes and suggests that higher-level processing is essential even in “simple” perceptual learning tasks.

Structure and function in rhodopsin: Rhodopsin mutants with a neutral amino acid at E134 have a partially activated conformation in the dark state*

The Glu-134–Arg-135 residues in rhodopsin, located near the cytoplasmic end of the C helix, are involved in G protein binding, or activation, or both. Furthermore, the charge-neutralizing mutation Glu-134 to Gln-134 produces hyperactivity in the activated state and produces constitutive activity in opsin. The Glu/Asp-Arg charge pair is highly conserved in equivalent positions in other G protein-coupled receptors. To investigate the structural consequences of charge-neutralizing mutations at Glu-134 and Arg-135 in rhodopsin, single spin-labeled side chains were introduced at sites in the cytoplasmic domains of helices C (140), E (227), F (250), or G (316) to serve as “molecular sensors” of the local helix bundle conformation. In each of the spin-labeled rhodopsins, a Gln substitution was introduced at either Glu-134 or Arg-135, and the electron paramagnetic resonance spectrum of the spin label was used to monitor the structural response of the helix bundle. The results indicate that a Gln substitution at Glu-134 induces a photoactivated conformation around helices C and G even in the dark state, an observation of potential relevance to the hyperactivity and constitutive activity of the mutant. In contrast, little change is induced in helix F, which has been shown to undergo a dominant motion upon photoactivation. This result implies that the multiple helix motions accompanying photoactivation are not strongly coupled and can be induced to take place independently. Gln substitution at Arg-135 produces only minor structural changes in the dark- or light-activated conformation, suggesting that this residue is not a determinant of structure in the regions investigated, although it may be functionally important.