digitized by an 8-bit analog-to-digital converter (Matrox) installed on a 486-66 PC. Initially, a reference map of the blood vessel pattern at the surface of the cortex was obtained by means of light filtered at 550 Ϯ 40 nm (Ealing). The camera was then focused 300 m below the surface of the cortex. Light from a 100-W tungsten-halogen light source driven by a dc power supply (Kepco) was passed through a 610-nm filter and used to illuminate the cortex during data collection. Frames were summed between 0.9 to 3.6 s after stimulus onset, corresponding to the time of maximum signal as determined by our previous experiments (26). Data were analyzed with the use of in-house programs written in Cϩϩ (Borland) and IDL Research Systems. 8. Square-wave luminance gratings (typical parameters for V2: spatial frequency, 0.15 cycle per degree; drift velocity, 12°per second; 64% contrast) and subjective gratings (typical spatial frequency of subjective orientation, 0.15 cycle per degree; spatial frequency of inducing lines, 0.45 cycle per degree; drift velocity, 12°per second for V2) were shown to the animal on a 17-inch monitor positioned 28.5 cm in front of it. In some experiments, the spatial frequency of subjective edges was systematically varied. Neutral gray intensity was 6.0 cd/ m 2 . All stimuli were shown binocularly. Subjective grating stimuli of four different orientations (0°, 45°, 90°, and 135°) were randomly interleaved with square-wave luminance grating stimuli and presented 80 to 100 times. Luminance-defined inducing lines, 1 to 2 pixels wide, were orthogonal to the subjective orientation for all subjective grating stimuli. Gratings were drifted normal to the subjective edge orientation and parallel to the orientation of the inducing lines in both directions separately. Eye position and area centrali of both eyes were checked at the start of imaging by use of a reverse opthalmoscope to project an image of the retinal vasculature onto the screen. 9. Orientation maps obtained with luminance gratings composed of thin lines (of the same width as the subjective grating inducing lines) were identical to orientation maps obtained with thicker bars (Fig. 2C) and relatively independent of grating spatial frequency. 10. We compared orientation strengths (magnitude of the orientation vector) for those pixels in V2 (Fig. 2, C and D) that had similar orientation preferences for luminance and subjective gratings (within Ϯ22.5°o rientation difference). The mean orientation strength for luminance gratings (4.56 ϫ 10 Ϫ3 units) was threefold higher than the mean orientation strength for subjective gratings (1.31 ϫ 10 Ϫ3 units) for these pixels. 11. We vectorially summed the responses of each pixel to all orientations of luminance and subjective gratings separately, obtained the pixel's resultant orientation preferences for both, and derived the difference between the two values. 12. Single-unit experiments were carried out in 12 cats.Single units were recorded with parylene-insulated tungsten microelectrodes. Responses were conve...

SummaryExposure of macrophages to lipopolysaccharide (LPS) leads to production of the pro-inflammatory cytokine, tumour necrosis factor alpha (TNF-␣). Previous studies have suggested that pathogenic Yersinia spp. inhibit LPS-mediated production of TNF-␣ in macrophages, and that one of the Yop proteins secreted by the plasmid-encoded type III pathway is required for this activity. We found that TNF-␣ production was inhibited when J774A.1 murine macrophages were infected with wild-type Y. pseudotuberculosis but not with an isogenic ysc mutant defective for Yop secretion. We inactivated multiple yop genes to identify which of these factors are required for the inhibition of TNF-␣ production. A mutant unable to express yopJ was defective for the inhibition of TNF-␣ production. Production of TNF-␣ is regulated at the transcriptional and translational levels by several mitogen-activated protein (MAP) kinases. The MAP kinases p38 and JNK underwent sustained activation in macrophages infected with the yopJ mutant. Conversely, p38 and JNK were downregulated in macrophages infected with the wild-type strain. The ability of the yopJ mutant to downregulate p38 and JNK and to inhibit production of TNF-␣ was restored by the expression of yopJ þ in trans. Therefore, YopJ is required for Y. pseudotuberculosis to downregulate MAP kinases and inhibit the production of TNF-␣ in macrophages.

SummaryThe ShlB protein in the outer membrane of Serratia marcescens is the only protein known to be involved in secretion of the ShlA protein across the outer membrane. At the same time, ShlB converts ShlA into a haemolytic and a cytolytic toxin. Surface-exposed residues of ShlB were determined by reaction of an M2 monoclonal antibody with the M2 epitope DYKDDDDK inserted at 25 sites along the entire ShlB polypeptide. The antibody bound to the M2 epitope at 17 sites in intact cells, which indicated surface exposure of the epitope, and to 23 sites in isolated outer membranes. Two insertion mutants contained no ShlB(M2) protein in the outer membrane. The ShlB derivatives activated and/or secreted ShlA. To gain insights into the secretion mechanism, we studied whether highly purified