Perception-action dissociations depend on the luminance contrast of the stimuli.

The observation that near-threshold low-contrast visual distractors can equally influence perceptual state and goal-directed motor responses was recently taken as an argument against a sharp separation between a conscious vision for perception and an unconscious vision for action. However, data supporting the dual visual system theory have principally involved high-contrast stimuli. In the present study, we assessed the effect of varying the contrast of a near-threshold visual distractor while keeping its visibility constant with backward noise masks. Eight participants performed fast manual reaching movements toward a highly visible target while subsequently reporting the presence/absence of a near-threshold distractor appearing at the opposite location with respect to the body midline. For all distractor contrasts, hand trajectory deviations toward the distractor were observed when the distractor was present and detected. When the distractor remained undetected deviations also occurred, but for higher contrasts. The subliminal motor effect traditionally observed in visual masking studies may therefore primarily depend on the luminance contrast of the interfering stimuli. These results suggest that dissociations between perceptual and motor responses can be explained by a single-signal model involving differential thresholds for perception and action that are specifically modulated as a function of both the requirements of the task and the contrast level of the stimuli. Such modulation is compatible with neurophysiological accounts of visual masking in which feedforward activation to--and feedback activation from--higher visual areas are correlated with the actual presence of the stimulation and its conscious perception, respectively.

Influence of near threshold visual distractors on perceptual detection and reaching movements.

Providing evidence against a dissociation between conscious vision for perception and unconscious vision for action, recent studies have suggested that perceptual and motor decisions are based on a unique signal but distinct decisional thresholds. The aim of the present study was to provide a direct test of this assumption in a perceptual-motor dual task involving arm movements. In 300 trials, 10 participants performed speeded pointing movements toward a highly visible target located at 10° from the fixation point and ± 45° from the body midline. The target was preceded by one or two close to threshold distractor(s) (80 ms stimulus onset asynchrony) presented ± 30° according to the target location. After each pointing movement, participants judged whether the distractor was present or not on either side of the target. Results showed a robust reaction time facilitation effect and a deviation toward the distractor when the distractor was both present and consciously perceived (Hit). A small reaction time facilitation was also observed when two distractors were physically present but undetected (double-miss)--this facilitation being highly correlated with the physical contrast of the distractors. These results are compatible with the theory proposing that perceptual and motor decisions are based on a common signal but emerge from a contrast dependent fixed threshold for motor responses and a variable context dependent criterion for perceptual responses. This paper thus extends to arm movement control previous findings related to oculomotor control.

Disentangling signal from noise in visual contrast discrimination.

Human ability to detect stimulus changes (Delta C) decreases with increasing reference level (C). Because detection performance reflects the signal-to-noise ratio within the relevant sensory brain module, this behavior can be accounted for in two extreme ways: first, the internal response change Delta R evoked by a constant Delta C decreases with C (that is, the transducer R = f(C) displays a compressive nonlinearity), whereas the internal noise is independent of R; second, Delta R is constant with C but the noise level increases with R. A newly discovered constraint on human decision-making helps solve this century-old problem: in a detection task where multiple changes occur with equal probabilities, observers use a unique response criterion to decide whether a change has occurred. For contrast discrimination, our results supported the first account above: human performance was limited by the contrast transducer nonlinearity and an almost constant noise.

Interocular interactions reveal the opponent structure of motion mechanisms.

Interactions between motion sensors tuned to the same and to opposite directions were probed by means of measuring summation indexes for sensitivities (d') to contrast increments and/or decrements applied to drifting gratings presented in binocular and in dichoptic vision. The data confirm a phenomenon described by Stromeyer, Kronauer, Madsen & Klein (1984, J. Opt. Soc. Am. A 1, 876-884), whereby opposite polarity contrast changes applied to binocular gratings drifting in opposite directions yield sensitivities significantly higher than same sign changes for which performance complies with probability summation (PS). The effect disappears in dichoptic vision where opposite sign contrast changes yield a performance close to, or below PS, whether they are applied to same or to opposite direction stimuli. Same sign changes in dichoptic drifting stimuli yield a performance higher than PS independently of their relative directions and close to the performances obtained when these same sign changes are applied to dichoptic, static +/- 45 degrees gratings. Opposite sign changes applied to such static gratings yield PS. The data support the view according to which: (i) motion direction is extracted at the monocular site; (ii) motion sensors exhibit mutual inhibition within each eye when tuned to opposite directions; and (iii) binocular summation when tuned to the same direction. The data also suggest that (iv) independently of their directional tuning, all motion sensors converge on a binocular, motion non-specific ('flicker') unit; and that (v) signals carried by ON and OFF pathways are slightly inhibitory to each other.

Visual sensitivity to temporal modulations of temporal noise.

The present endeavor is meant (a) to provide a direct comparison between first- and second-order temporal modulation and, by so doing, (b) to eliminate all spatial clues that might have contaminated previous assessments of the second-order temporal modulation transfer function (TMTF). The second aim was achieved by means of the temporal modulation of a purely temporal white noise, a stimulus used frequently in psychoacoustics but not used as yet in visual stimulation. Luminance and contrast temporal modulation thresholds were measured with a 2AFC staircase procedure. In the first case, the mean luminance of a spatially homogeneous, 30 degrees field was modulated sinusoidally over time (first-order modulation). In the second case, the luminance of the same or of a 60 degrees field was randomized over time at a rate of 150 Hz and this temporal white noise (the carrier) was modulated sinusoidally over time (second-order modulation). First-order thresholds reproduce the classical (large field) flicker sensitivity. Second-order thresholds (measured for the first time with purely temporal stimuli) are at least 100 times higher than first-order ones, display a low-pass characteristic (at least up to 0.5 Hz) and yield a critical fusion frequency (measured at 100% modulation) of approximately 10 Hz. The data are in accord with other estimates of the TMTF of the second-order system and thus confirm the effective neutralization of the spatial cues present in these previous studies.

Failure to handle more than one internal representation in visual detection tasks.

Perceptual studies make a clear distinction between sensitivity and decision criterion. The former is taken to characterize the processing efficiency of the underlying sensory system and it increases with stimulus strength. The latter is regarded as the manifestation of a subjective operation whereby individuals decide on (as opposed to react reflexively to) the occurrence of an event based on factors such as expectation and payoff, in addition to its strength. To do so, individuals need to have some knowledge of the internal response distributions evoked by this event or its absence. In a natural, behaviorally relevant multistimulus environment, observers must handle many such independent distributions to optimize their decision criteria. Here we show that they cannot do so. Instead, while leaving sensitivity unchanged, lower and higher visibility events tend to be reported respectively less and more frequently than when they are presented in isolation. This behavior is in quantitative agreement with predictions based on the notion that observers represent a multistimulus environment as a unitary internal distribution to which each stimulus contributes proportionally to its probability of occurrence. Perceptual phenomena such as blindsight, hemineglect, and extinction may be, at least in part, accounted for in such a way.

Attention-based texture segregation.

Luminance- or color-defined +/- 45 degrees-oriented bars were arranged to yield single-feature or double-conjunction texture pairs. In the former, the global edge between two regions is formed by differences in one attribute (orientation, or color, or luminance). In the color/orientation double-conjunction pair, one region has +45 degrees red and -45 degrees green textels, the other -45 degrees red and +45 degrees green textels (the luminance/orientation double-conjunction pair is similar); such a pair contains a single-feature orientation edge in the subset of red (or green) textels, and a color edge in the subset of +45 degrees (or -45 degrees) textels. We studied whether edge detection improved when observers were instructed to attend to such subsets. Two groups of observers participated: in the test group, the stimulus construction was explained to observers, and they were cued to attend to one subset. The control group ran through the same total number of sessions without explanations/cues. The effect of cuing was week but statistically significant. Feature cuing was more effective for color/orientation than for luminance/orientation conjunctions. Within each stimulus category, performance was nearly the same no matter which subset was attended to. On average, a global performance improvement occurred over time even without cuing, but some observers did not improve with either cuing or practice. We discuss these results in the context of one-versus two-stage segregation theories, as well as by reference to signal enhancement versus noise suppression. We conclude that texture segregation can be improved by attentional strategies aimed to isolate specific stimulus features.

Local versus global contrasts in texture segregation.

In a texture pair (TP) yielding a vertical or horizontal edge, the local (luminance or color) contrast or the local orientation of the individual textels is traded off with the global strength of the luminance-, color-, or orientation-defined TP edge so as to keep the latter at the detection threshold. Local and global contrasts are defined along the same (within-domain conditions) or along distinct physical dimensions (transdomain conditions). In the latter case local luminance or color contrast is traded off against global orientation. In all cases TP's are presented for 66.7 or 333.3 ms. Textels differ from the background in either luminance or color so that the TP's are respectively equichromatic or equiluminant. TP edge strength is modulated by means of swapping variable proportions of textels between the two textures in the TP. The observed local--global relationships are fitted with a version of the equivalent noise model for contrast coding modified to include the presentation time factor. The extension of the standard model in the time domain is meant to allow comparison between equivalent noise estimates for variable duration stimuli. Model fits of the within-domain data yield equivalent noise energy values significantly different for color- and luminance-defined TP's but are not applicable for the transdomain experiments, which indicates that global orientation processing is independent of both local luminance and local color contrast insofar as the latter are above the detection threshold. Finally, this study points to the equivalence among the local--global, the equivalent noise, and the statistical approaches to texture segregation.

Motion blur and motion sharpening: temporal smear and local contrast non-linearity.

Blurred images may appear sharper when drifting than when stationary. But, paradoxically, moving sharp edges may appear more blurred. To resolve this paradox, the perceived sharpness of drifting, blurred, square wave gratings was compared with that of their static analogues over a range of speeds, blurs and spatial frequencies. Both motion blur and motion sharpening occurred, depending upon the physical blur of the patterns. For large extents of blur (> 10 min arc) moving patterns always appeared sharper than their static analogues, but for small blurs (< 10 min arc) moving edges appeared more blurred than stationary ones. We present a quantitative model for the distortion of waveforms in motion based on two factors: (i) visual temporal integration that smears moving images, and (ii) a local contrast non-linearity that increasingly sharpens the effective profile of edges as speed and contrast increase. We suggest that a plausible account of the speed-dependent non-linearity is the differential recruitment of M and P cells at different speeds.

Spatio-temporal vernier acuity.

The study of space-time vernier (STV) provides information on the spatio-temporal structure of the visual system in the same way that the classical spatio-spatial vernier (SSV) provides information on its spatial structure. The transposition of a SSV task into a STV one yields the following experimental format: an object (in the present case a Gaussian Blob) drifts with a constant velocity, V, disappears at x0, t0 and reappears after a variable duration delta t at a position x1 +/- delta x with x1 the correct position (given a constant V) and delta x the minimum (positive and negative) spatial offset discriminable from x0, i.e. the STV threshold. Observer's task is to specify whether the reappearance position is ahead of, or behind x1. The STV functions of delta t measured for 1, 5 and 10 deg/s reference velocities are linear with non-zero spatial and temporal intercepts at the origin. We refer to these x and t intercepts as dynamic dmin and tmin. Dynamic dmin is the smallest instantaneous displacement (infinite velocity) discriminable from a continuous drift, V. Dynamic tmin is the shortest 'motion stop' discriminable from the same continuous drift, V. To our knowledge these quantities have not yet been assessed. Estimated dynamic dmin increases with V. whereas tmin is more or less V independent suggesting that the motion sensors presumably involved in the STV task have peak spatial frequencies inversely proportional with V and a temporal frequency characteristic independent of V (at least within the studied range). The observed STV linearity with the spatio-temporal separation implies that the STV task is equivalent to a velocity discrimination. Two additional observations yield support to this conclusion. (i) The slopes of these functions yield velocities very similar to those discriminable from the reference V in a standard V-discrimination experiment. (ii) The predicted STV performances based on a decomposition of the task into two velocity discrimination tasks run as independent experiments are reasonably accurate.

2D motion aliasing yielding 3D ambiguity. A study with variants of a Necker cube.

The 2D projection of a rotating Necker cube yields an ambiguous 3D interpretation based on both 2D shape and kinetic depth information. The present study shows that the alternation rate of the two 3D interpretations is constant with the rotation speed up to some critical value (around 25 turns/min for a cube whose sides subtend 2.5 deg) and increases monotonically thereafter. It is proposed that the additional perceptual reversals (PRs) observed at high rotation speeds are due to the increased frequency of the crossovers of the cube's edges. These crossovers yield 2D motion "aliasing" (or discontinuity) and "veridical" (or continuity) motion components. The motion aliasing (or crossover) hypothesis states that, in addition to the inherent ambiguity of the dynamic 2D projection of 3D objects, perceptual motion/perspective reversals will occur any time the discontinuity speed takes over the continuity speed. It is proposed that the relative strengths of the two components depend on the linear speed of the projected edges and that the discontinuity components take over the continuity one in the speed range where contrast sensitivity (or, above threshold, efficiency) is a decreasing function of speed. The motion aliasing hypothesis was tested and supported in a series of independent experiments showing that, for rotation speeds higher than 25 turns/min the PR rate increases with the crossover frequency at a constant speed, with linear speed at a constant crossover frequency and with the similarity of the crossing bars in terms of their orientation, polarity and spatial overlap. In addition, some of these experiments suggest that 2D shape and kinetic depth 3D-cues combine in such a way that the average PR rate they yield together is the same as the PR rate yielded by each of them independently. In the Discussion section we elaborate on issues related to the perceptual combination of ambiguous shape and kinetic depth, 3D cues.

A human vision based computational model for chromatic texture segregation.

We have developed a computational model for texture perception which has physiological relevance and correlates well with human performance. The model attempts to simulate the visual processing characteristics by incorporating mechanisms tuned to detect luminance-polarity, orientation, spatial frequency and color, which are characteristic features of any textural image. We obtained a very good correlation between the model's simulation results and data from psychophysical experiments with a systematically selected set of visual stimuli with texture patterns defined by spatial variations in color, luminance, and orientation. In addition, the model predicts correctly texture segregation performance with key benchmarks and natural textures. This represents a first effort to incorporate chromatic signals in texture segregation models of psychophysical relevance, most of which have treated grey-level images so far. Another novel feature of the model is the extension or the concept of spatial double opponency to domains beyond color, such as orientation and spatial frequency. The model has potential applications in the areas of image processing, machine vision and pattern recognition, and scientific visualization.

Precise assessment of the mean effective luminance of texture patches--an approach based on reverse-phi motion.

In studying the response of mechanisms to contrast-defined texture stimuli, it is critical that the average effective luminance of these textures be equal to that of the background, to minimize net luminance-based signals. We present an efficient and accurate technique for constructing such equiluminant textures to isolate contrast-sensitive mechanisms for investigating their properties. The technique is based on the reverse-phi motion phenomenon, and the resulting settings agree closely with those obtained by photometric means for the class of textures studied. The method also allows one to explore the properties of contrast- and luminance-driven motion mechanisms and, in particular, to evaluate the contribution of putative second-order mechanisms to the motion percept. Results of applying the method are presented, and its advantages over the minimum-flicker and minimum-motion techniques are discussed.

Penetration of clindamycin and metronidazole into inflamed appendiceal tissue.

OBJECTIVE: To calculate the penetration of clindamycin and metronidazole into inflamed appendiceal tissue. DESIGN: Prospective study. SETTING: Teaching hospital, Israel. SUBJECTS: 20 Consecutive men and women operated on for acute appendicitis. INTERVENTIONS: Appendicectomy. Each patient was given three intravenous injections of gentamicin 80 mg combined with either clindamycin 600 mg or metronidazole 500 mg immediately before operation over a period of 15 minutes. MAIN OUTCOME MEASURES: Serum and tissue concentrations of the antibiotics. RESULTS: There was no significant difference between the mean serum concentrations of the drugs (clindamycin 17.86 micrograms/ml and metronidazole 9.75 micrograms/ml) but the mean tissue concentrations of clindamycin (10.41 micrograms/g in the base and 9.86 micrograms/g in the tip of the appendix) were significantly higher than those of metronidazole (5.65 micrograms/g in the base and 5.89 micrograms/g in the tip; p = 0.02 and p = 0.05, respectively). Tissue concentrations of clindamycin and serum concentration of both drugs were more than twice their MIC90. The tissue concentrations of metronidazole were close to its MIC90. CONCLUSIONS: Clinical trials are necessary before any conclusion about therapeutic superiority of one or other agent can be drawn.

Spatiotemporal characterization of a Fourier and non-Fourier motion system.

Direct-phi perception elicited by a reverse-phi (i.e. reversed-polarity) stimulus may well be accounted for, if the front-end filters of a classical Reichardt unit are full-wave rectifiers. It is shown that reverse-phi perception is progressively replaced by direct-phi perception when either the spatial or the temporal modulation of the reversed-polarity stimuli are decreased. Reverse-phi perception is very weak or absent for spatial and temporal frequencies < or = 1 c/deg and < or = 3.75 Hz, respectively, indicating that the sensitivity of a linear mechanism is weak or null within this frequency range. By pitting against each other reverse- and direct-phi stimuli, the relative sensitivities of the putative motion mechanisms with linear (Fourier) and full-wave rectified (non-Fourier) front-end filters were assessed for a large range of spatial and temporal frequencies. Absolute sensitivities of the two mechanisms were estimated on the assumption that they contribute through probability summation to the overall spatiotemporal sensitivity surface described by Kelly [(1979) Journal of the Optical Society of America, 69, 1340-1349]. In conjunction with related evidence it is suggested that the Fourier/non-Fourier distinction may be generalized in terms of a specific/unspecific dichotomy in motion processing.

Morphological transformation of Helicobacter pylori during prolonged incubation: association with decreased acid resistance.

The survival of clinical isolates of H pylori at two cultural ages (two and four days) at pH 2, in the presence of different buffers, with and without urea, was investigated. It was found that the morphological changes which occur with longer incubation of H pylori have an inverse correlation with its resistance to an acidic environment. The finding that the addition of urea almost reversed this phenomenon and prolonged survival of the cultures emphasises the role of urea in the survival of H pylori in acidic environments.

[Antimicrobial sensitivity and treatment of Helicobacter pylori infections].

Helicobacter pylori (HP) is considered the etiological agent of chronic active gastritis and suspicion is strong that it plays an important role in duodenal ulcer. Recently, several clinical studies reported that eradication of HP markedly reduces the frequency of ulcer relapse. Triple-drug treatment, including a bismuth salt and 2 antibiotics (usually metronidazole with either amoxycillin or tetracycline) is considered the treatment of choice. It has been shown that the most important factor for predicting success of treatment is the sensitivity of HP to metronidazole, which varies considerably. In the present study we evaluated antimicrobial susceptibility of 18 HP clinical isolates, as well as effectiveness of triple therapy for eradicating HP infections in 65 patients. In vitro, HP was highly sensitive to amoxycillin, erythromycin and tetracycline (100%), and also to metronidazole and tinidazole (94%). Sensitivity to chloramphenicol was low (50%). In our clinical study, the overall eradication rate was 66%; it was higher among women (80%) than men (54%), probably due to better compliance. It is concluded that HP strains in Israel are highly sensitive to metronidazole and that triple therapy is effective, providing compliance is good.

Motion perception with spatiotemporally matched chromatic and achromatic information reveals a "slow" and a "fast" motion system.

Recent reports dealing with apparent motion challenged the standard view according to which motion processing should be impossible if the visual attributes matched across space and time are processed in independent channels (the similarity principle). The present work examines this possibility insofar as it relates to the spatiotemporal combination of pure chromatic and pure luminance information. The data indicate that the "similarity principle" is indeed infringed at low (< or = 2.5 Hz, i.e. velocities of 2.5 deg/sec for spatial modulations of 1 c/deg, in this study) but not at high (> or = 7.5 Hz) temporal frequencies. The fact that colour and luminance may or may not combine to yield motion perception depending on their temporal modulation reconciliates contradictory results in the literature and supports the idea of two motion systems, a "fast"/specific one, integrating information only from similar subunits, and a "slow"/unspecific one, integrating information across dissimilar subunits (in the present case, across the chromatic and achromatic "domains"). This dichotomy is also supported by the finding that chromatic reverse-phi (i.e. with equiluminant, red and green stimuli) can be observed at medium temporal frequencies but is replaced by direct motion at low temporal frequencies, presumably within the range of the "slow"/unspecific system. Using a modified "minimum motion" technique (referred to as the Reverse-Phi equiluminance method) we present data allowing to assess the relative weights of the two systems as a function of temporal frequency.

Two motion systems with common and separate pathways for color and luminance.

We present psychological experiments that reveal two motion systems, a specific and an unspecific one. The specific system prevails at medium to high temporal frequencies. It comprises at least two separate motion pathways that are selective for color and for luminance and that do not interact until after the motion signal is extracted separately in each. By contrast, the unspecific system prevails at low temporal frequencies and it combines color and luminance signals at an earlier stage, before motion extraction. The successful implementation of an efficient and accurate technique for assessing equiluminance corroborates further the main findings. These results offer a general framework for understanding the nature of interactions between color and luminance signals in motion perception and suggest that previously proposed dichotomies in motion processing may be encompassed by the specific/unspecific dichotomy proposed here.

Nosocomial acinetobacter meningitis secondary to invasive procedures: report of 25 cases and review.

The medical records of 25 patients with nosocomial meningitis due to Acinetobacter baumannii were retrospectively reviewed. Most cases occurred in the neurosurgical intensive care unit over a 5-year period, with an increased rate during summer. The majority of infections were associated with indwelling ventriculostomy tubes or CSF fistulae in patients receiving antimicrobial therapy. Repeated environmental cultures failed to reveal a source of the microorganism, and control measures had no apparent effect on the outbreak. However, no further cases appeared following a sharply reduced total intake of antibiotics in the neurosurgical department. Forty-one cases of acinetobacter meningitis, secondary to invasive procedures, were found in the English-language literature and were compared with the cases presented. To our knowledge, our series is the largest of acinetobacter meningitis reported hitherto. Although not one of the most common pathogens in hospitals, Acinetobacter constitutes an increasing threat for patients, especially those receiving antimicrobial therapy in intensive care units who are being maintained by various life-support systems.