-Conotoxin PVIIA (-PVIIA), a 27-amino acid peptide with three disulfide cross-links, isolated from the venom of Conus purpurascens, is the first conopeptide shown to inhibit the Shaker K ؉ channel (Terlau, H., Shon, K., Grilley, M., Stocker, M., Stü hmer, W., and Olivera, B. M. (1996) Nature 381, 148 -151). Recently, two groups independently determined the solution structure for -PVIIA using NMR; although the structures reported were similar, two mutually exclusive models for the interaction of the peptide with the Shaker channel were proposed. We carried out a structure/function analysis of -PVIIA, with alanine substitutions for all amino acids postulated to be key residues by both groups. Our data are consistent with the critical dyad model developed by Mé nez and co-workers (Dauplais, M.,

SUMMARY1. The visual resolving ability of different types of macaque retinal ganglion cells was estimated at different retinal eccentricities, by measuring the amplitude of modulated responses to black-white gratings of spatial frequencies near the resolution limit for each cell.2. The resolving ability of tonic, spectrally opponent ganglion cells was usually similar to that of phasic, non-opponent ganglion cells at similar eccentricities, except that at eccentricities greater than 10 deg some tonic ganglion cells with remarkably high resolution (up to ca. 15 cycles/deg) were found. Our cell sample was limited within the central 2 deg of the visual field, however.3. Only a small proportion of phasic ganglion cells showed an increase of mean firing level to gratings near the resolution limit. The maintained firing of tonic ganglion cells was higher than that of phasic ganglion cells.4. With red-black or green-black gratings, the resolution of phasic ganglion cells was unaffected. For red or green on-centre ganglion cells. a marked deterioration of resolving ability occurred when the grating was of a colour to which a cell responded poorly (green-black gratings for red on-centre cells. and red-black gratings for green on-centre cells). A slight improvement in resolving ability occurred when the grating was of an excitatory colour.5. For a sub-sample of cells. we compared resolution limit with centre size as determined from area-threshold curves. For both phasic and tonic ganglion cells, resolution limit (the period length just resolved) was about half the centre diameter, as is the case for cat ganglion cells. This implies that the centre sizes of phasic and tonic monkey ganglion cells are similar at most eccentricities.6. We attempt to relate these results to primate retinal anatomy and visual resolution, determined behaviourally.

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Visual motion perception is essential for appropriate behavior in a dynamic visual world. It is influenced by voluntary attention towards or away from moving objects as well as by the capture of automatic attention by salient stimuli. Both kinds of attention play a major role in the Eriksen Flanker Task (EFT),where a central stimulus has to be identified in the presence of flanking distractors. For static visual stimuli incongruent peripheral flankers are known to reduce accuracy rates and prolong reaction times. However,it is not known if a similar flanker effect also affects speeded responses to moving stimuli. We therefore examined whether a flanker effect exists for moving random dot patterns (RDPs) and compared it to the effect elicited by static visual triangles in human subjects. We observed a motion flanker effect,both for response times and accuracy rates. Incongruently moving peripheral flankers caused a slowing of response time and a reduction of accuracy rates compared to congruently moving RDPs. These motion flanker effects were not significantly different from those in the static flanker task. The presence of a motion flanker effect and its similarity to the flanker effect for static stimuli suggests that visual motion engages competitive attention and control mechanisms for perception and decision-making similar to those engaged by non-moving features.

We have investigated the darkness induction of surround fields of various composition on a centrally located test field. Darkness induction can be described as a linear subtraction of the luminance of the induction region from the test field luminance, weighted for the size, the length of immediate contact and the distance of the induction field from the test field. Furthermore, closer induction fields exert a shunting effect on the induction effect of fields which are more distally located on the same radius, and neighbouring fields mutually interact. A model is discussed which takes into account these variables. It is compared with older models as well as with the Retinex-model as formulated by Land (1983). Our data and model are closer to the model of Jameson and Hurvich (1964). Neurophysiological correlates and mechanisms are discussed.