Proactive control: Endogenous cueing effects in a two-target attentional blink task.

This study examined proactive control in a two-target task using an endogenous cueing method. Participants identified two target words (T1 then T2) presented in rapid succession. T1 was presented alone or interleaved with a distractor word. In Experiment 1, informative pre-cues that signalled T1 selection difficulty were randomly intermixed with uninformative pre-cues. The results revealed a cueing effect for both T1 and T2, with better performance for informative cues than for uninformative cues. In Experiment 2, informative and uninformative cues were mixed for one group, and blocked for another group. In the mixed cue group, we again found a T2 cueing effect. In the blocked cue group, a cueing effect was observed for both T1 and T2, with the T2 cueing effect restricted to the shortest T1-T2 SOA. The results demonstrate that pre-cues of attentional conflictcan modulate performance in a two-target task used to measure the attentional blink.

Attentional influences on memory formation: A tale of a not-so-simple story.

Is there a learning mechanism triggered by mere expectation violation? Is there some form of memory enhancement inherent to an event mismatching our predictions? Across seven experiments, we explore this issue by means of a validity paradigm. Although our manipulation clearly succeeded in generating an expectation and breaking it, the memory consequences of that expectation mismatch are not so obvious. We report here evidence of a null effect of expectation on memory formation. Our results (1) show that enhanced memory for unexpected events is not easily achieved and (2) call for a reevaluation of previous accounts of memory enhancements based on prediction error or difficulty of processing. Limitations of this study and possible implications for the field are discussed in detail.

Gradual proportion congruent effects in the absence of sequential congruent effects.

In perceptual filtering tasks, congruency effects vary as a function of proportion congruent (PC), with smaller congruency effects when congruent trials are rare than when they are frequent. This effect is typically larger with extreme differences between high and low proportion congruent conditions (e.g., 80% congruent-20% incongruent) than with intermediate differences (e.g., 60% congruent-40% incongruent; Logan & Zbofroff, 1979; Blais & Bunge, 2010). Some authors have claimed that both PC effects can be explained in terms of the same reactive cognitive control mechanism that is responsible for sequential congruency (SC) effects (e.g., Botvinick, Braver, Barch, Carter, & Cohen, 2001). In fact, in most previous studies there was a systematic confounding between proportion congruent and the proportion of transitions involving an incongruent trial followed by another incongruent trial. In the present study we eliminated this confound and tested directly whether PC effects can still be measured in the absence of SC effects. Once confirmed, we studied the properties of this pure form of PC effect, in particular whether it is conflict-type specific or general, and whether it decreases gradually as a function of changes in proportion congruency (80% vs. 70% vs. 60%). Our results showed significant PC effects in the absence of SC effects, which replicates our previous findings (Torres-Quesada, Milliken, Lupiáñez, & Funes, 2014), and PC effects that can be conflict-type general or specific, depending on the nature of conflict type where they were produced. Importantly, the congruency effect was modulated by the level of proportion congruent, decreasing systematically as the absolute percentage of incongruent trials decreases.

On the strategic modulation of the time course of facilitation and inhibition of return.

In studies of exogenous attentional orienting, response times for targets at previously cued locations are often longer than those for targets at previously uncued locations. This effect is known widely as inhibition of return (IOR). There has been debate as to whether IOR can be observed in discrimination as well as detection tasks. The experiments reported here confirm that IOR can be observed when target discrimination is required and that the cue-target interval at which IOR is observed is often longer in discrimination than in detection tasks. The results also demonstrate that the later emergence of IOR is related to perceptual discrimination rather than to response selection differences between discrimination and detection tasks. More difficult discrimination tasks lengthen the SOA at which IOR emerges. In contrast, increasing task difficulty by adding a distractor to the location opposite the target shortens the SOA at which IOR emerges. Together, the results reveal an adaptive interaction between exogenous and endogenous attentional systems, in which the action of the orienting (exogenous) system is modulated endogenously in accord with task demands.

Anesthesia for Cesarean section in a patient with paraplegia resulting from tumour metastases to spinal cord.

PURPOSE: Spinal cord injured patients present multiple unique challenges to the anesthesiologist. These include choice of muscle relaxant and management of autonomic hyperreflexia. We report the anesthetic management for Cesarean delivery in a patient who was paraplegic due to spinal canal metastases. Preeclampsia and fever complicated this case. CLINICAL FEATURES: The patient presented at 29 wk gestation with progressive paraplegia at the T10 level due to metastatic osteosarcoma. She had a decompressive laminectomy without improvement in her paralysis. She subsequently developed preeclampsia at 31 wk gestation, and underwent Cesarean delivery for breech presentation under general anesthesia. Anatomical concerns left us unsure of the efficacy or safety of neuraxial anesthesia. CONCLUSIONS: Preeclampsia and autonomic hyperreflexia are generally indications for regional anesthesia for Cesarean section. Tumour in her spinal canal and laboratory abnormalities including thrombocytopenia and a potential urosepsis dissuaded us from this option. Additionally, rapid sequence induction and intubation were not preferred due to paraplegia, leading us to secure the airway fibreoptically.

Attending, ignoring, and repetition: on the relation between negative priming and inhibition of return.

A series of spatial localization experiments is reported that addresses the relation between negative priming and inhibition of return. The results of Experiment 1 demonstrate that slowed responses to repeated location stimuli can be obscured by repetition priming effects involving stimulus dimensions other than spatial location. The results of Experiments 2, 3A, and 3B demonstrate that these repetition priming effects may occur only when participants are required to respond to the prime display. Together, these results suggest that differences between attended and ignored repetition effects in selective attention studies of spatial localization do not provide a basis for distinguishing between spatial negative priming and inhibition of return.

A comparison of the microvascular response in the healing wound in the spontaneously hypertensive and non-hypertensive rat.

BACKGROUND: In the Spontaneously Hypertensive Rat (SHR), there is a significantly greater blood flow at the paw but not at the back than in the non-hypertensive Wistar Kyoto (WKY) rat. We wanted to assess the effect of this higher blood flow on wound healing at the paw. MATERIALS AND METHODS: We characterized the microvascular composition of wounds at the back and paw of 9 SHR rats and 10 WKY rats using a quantitative imaging program. Blood flow was compared using laser Doppler technology. RESULTS: The blood flow response to wounding at the back was identical in the SHR and WKY rats. There was an immediate sharp increase in flow at the center of the wound. Blood flow reached a peak at 3 days and then decreased somewhat by day 7, but still remained five-fold higher than the prewound baseline values. There was also a two-fold increase at the back wound perimeter. There were no differences in microvascular composition at the back between the SHR and WKY rats. In contrast, there was an immediate enormous increase in blood flow at paw wound center in the SHR rats. Flow increased to 75 ml/min/100 gm by 24 h then fell back sharply. Blood flow at the paw in the WKY rats changed very little over the 7 days post wounding. At 3 days, the flow was about twice as high in the SHR than in the WKY wound, but, by day 7, flow was similar in the two rat strains. At the SHR wound perimeter, there was a small increase in flow which was sustained through day 7. Although the microvascular composition at the paw wound center was similar in the SHR and WKY rats, there was a notable difference at the paw perimeter. At baseline, there was a slightly greater capillary density in the SHR paw (32 +/- 1 per mm3) than the WKY paw (25 +/- 8 per mm3). At 7 days after wounding, there was a substantial increase in capillary number in the SHR rats (48 +/- 8 per mm3) as compared to baseline (p = 0.05). In contrast, there was no significant difference in capillary number in the WKY paw wound perimeter (20 +/- 3 per mm3) as compared to baseline. CONCLUSIONS: There is a substantial difference in wound blood flow response between the hypertensive and the non-hypertensive rat. At the back, the blood flow effects of wounding are similar, but, at the paw, the SHR rat shows a dramatic transient increase in flow in the early phases of wound healing. There is apparently no capability to upmodulate microvascular resistance in response to increased pressure at this early stage of wound healing. However, within several days, the granulation tissue microvasculature becomes capable of controlling the effects of raised pressure in the SHR rat. In the SHR paw wound perimeter, there are significantly more capillaries than in the WKY rat. It is possible that greater capillary proliferation in the SHR rat results from higher blood flow in the early phase of wounding. The contrast between the WKY rat and the SHR rat serves to further illustrate the complexity of blood flow regulation which occurs during wound healing.

Inhibition of return and the attentional set for integrating versus differentiating information.

Cuing a location with an uninformative cue leads to a facilitatory effect at that location shortly afterward and later (about 300 ms) to a negative effect called inhibition of return (IOR). Until recently, it was argued that IOR occurs in detection and localization tasks, but not in discrimination tasks. However, the authors of several recent studies have demonstrated IOR effects in discrimination tasks, although at a later cue-to-target stimulus onset asynchrony (SOA). In this study, the authors examined why IOR occurs at a later SOA in discrimination tasks. In Experiments 1 and 2, different time courses of exogenous cuing effects in detection and discrimination tasks were established. In Experiment 3, the authors examined the role of an attentional set on the time course of exogenous cuing effects by manipulating the proportion of trials in which a distractor is presented in the location opposite the target. A new framework for understanding exogenous cuing effects and their dependence on endogenous attention is proposed.

Initial clinical experience with a video-based patient positioning system.

PURPOSE: To report initial clinical experience with an interactive, video-based patient positioning system that is inexpensive, quick, accurate, and easy to use. METHODS AND MATERIALS: System hardware includes two black-and-white CCD cameras, zoom lenses, and a PC equipped with a frame grabber. Custom software is used to acquire and archive video images, as well as to display real-time subtraction images revealing patient misalignment in multiple views. Two studies are described. In the first study, video is used to document the daily setup histories of 5 head and neck patients. Time-lapse cine loops are generated for each patient and used to diagnose and correct common setup errors. In the second study, 6 twice-daily (BID) head and neck patients are positioned according to the following protocol: at AM setups conventional treatment room lasers are used; at PM setups lasers are used initially and then video is used for 1-2 minutes to fine-tune the patient position. Lateral video images and lateral verification films are registered off-line to compare the distribution of setup errors per patient, with and without video assistance. RESULTS: In the first study, video images were used to determine the accuracy of our conventional head and neck setup technique, i.e., alignment of lightcast marks and surface anatomy to treatment room lasers and the light field. For this initial cohort of patients, errors ranged from sigma = 5 to 7 mm and were patient-specific. Time-lapse cine loops of the images revealed sources of the error, and as a result, our localization techniques and immobilization device were modified to improve setup accuracy. After the improvements, conventional setup errors were reduced to sigma = 3 to 5 mm. In the second study, when a stereo pair of live subtraction images were introduced to perform daily "on-line" setup correction, errors were reduced to sigma = 1 to 3 mm. Results depended on patient health and cooperation and the length of time spent fine-tuning the position. CONCLUSION: An interactive, video-based patient positioning system was shown to reduce setup errors to within 1 to 3 mm in head and neck patients, without a significant increase in overall treatment time or labor-intensive procedures. Unlike retrospective portal image analysis, use of two live-video images provides the therapists with immediate feedback and allows for true 3-D positioning and correction of out-of-plane rotation before radiation is delivered. With significant improvement in head and neck alignment and the elimination of setup errors greater than 3 to 5 mm, margins associated with treatment volumes potentially can be reduced, thereby decreasing normal tissue irradiation.

A comparison of the cutaneous microvascular properties of the spontaneously hypertensive rat and the Wistar-Kyoto rat.

The Spontaneously Hypertensive rat (SHR) and its non-hypertensive companion strain, the Wistar-Kyoto (WKY) rat, provide an excellent comparative model to permit study of the differential properties of cutaneous microvascular beds. We explored the possibility that chronically elevated vascular pressures in the SHR rat might affect the microvascular constitution of the skin. We measured skin blood flow at the back and at the paw of a group of 20-week-old WKY rats and a contrast group of SHR rats. We then performed skin biopsies at these two locations and used the NIH Image program to count and measure the size of capillaries, arterioles, and venules. We also determined microvascular density as percentage of total tissue area. At basal temperature, skin blood flow was similar in the two rat strains at both the back and paw. Heat induced vasodilatation resulted in a 50% increase in blood flow at the back, reaching the same level in the two rat groups. However, at the paw site, thermal stimulation resulted in significantly greater flow (39.3 +/- 3.1 ml/100 gm tissue per min) in the SHR rats than the WKY rats (28.6 +/- 1.9 ml/100 gm tissue per min, P < 0.05). The ratio of systemic arterial pressure to skin blood flow was computed as an index of vascular resistance to flow. At basal temperature, this index was 50% greater for the SHR rats at both skin sites. At 44 degrees C, the resistance index decreased at both sites in both rat groups but was still approximately 50% higher at the back of the SHR than the WKY rats. In contrast, the resistance index at 44 degrees C at the paw site fell to the same level in both the SHR and WKY rats. There were twice as many capillaries at the back of the WKY rats than at the back of the SHR rats (9.2 +/- 2.0 per mm2 vs. 4.7 +/- 1.2 per mm2, P < 0.05). Expressed as a percentage of total tissue area, the capillary density at the back in the WKY rats was 0.064 +/- 0.010% as compared to 0.034 +/- 0.008% in the SHR rats (P < 0.05). There were five times more arterioles at the paw compared to the back in both rat groups with no significant difference between the groups. We measured the diameter of the lumen and the thickness of the wall of each arteriole and computed their ratio as an index of possible media hypertrophy. There were minimal differences seen in these parameters between the two rat groups at the back and paw sites. The venular density was significantly higher at the paw than at the back in both rat groups with no significant difference between them. Reduced capillary density at the back of the SHR rats may be a developmental adaptation to high blood pressure. Such a reduction in the pathways of blood flow may help account for increased flow resistance at that site, independent of arteriolar vasoconstriction.

Using vascular structure for CT-SPECT registration in the pelvis.

UNLABELLED: The authors outline a method for three-dimensional registration of pelvic CT and 111In-labeled monoclonal antibody capromab pendetide (111In MoAb 7E11.C5) images using 99mTc-labeled red blood cell SPECT data. METHODS: This method of CT-SPECT registration relies on the identification of major blood vessels in the CT and 99mTc SPECT images. The vessels are segmented from the image datasets by outlining them on transverse planar slices using a mouse-based drawing tool. Stacking the transverse outlines provides a three-dimensional representation of the vascular structures. Registration is performed by matching the surfaces of the segmented volumes. Dual isotope acquisition of 111In and 99mTc activities provides precise SPECT-SPECT registration so that registration in three dimensions of the 111In MoAb and CT images is achieved by applying the same transformation obtained from the 99mTc SPECT-CT registration. RESULTS: This method provided accurate registration of pelvic structures and significantly improved interpretation of 111In MoAb 7E11.C5 exams. Furthermore, sites of involvement by prostate cancer suggested by the 111In MoAb examination could be interpreted with the bony and soft tissue (nodal) anatomy seen on CT. CONCLUSION: This method is a general clinical tool for the registration of pelvic CT and SPECT imaging data. There are immediate applications in conformal radiation therapy treatment planning for certain prostate cancer patients.

Verification of the omni wedge technique.

The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.

Diazepam-treated female rats: flumazenil- and PK 11195-induced withdrawal in the hippocampus CA1.

Six female rats had a loading dose of 180 mg of diazepam (DZ) contained in two Silastic capsules implanted in their backs. Thereafter, a single 90-mg capsule was implanted weekly for 4 weeks prior to weekly microinjections of 1 microl of flumazenil (6.25, 12.5, or 25 microg) and PK 11195 (3.125, 6.25, or 12.5 microg) or vehicle into the CA1. Three control rats had empty capsules implanted but received only the high dose of flumazenil after 5 weeks. The time of DZ exposure spanned 8 weeks. Mean steady-state plasma levels of DZ were 1.06 +/- 0.11, and the mean total (DZ + metabolites) was 2.46 microg/ml +/- 0.37. Flumazenil elicited a dose-related precipitated withdrawal score (PAS) in DZ-treated rats (but not in controls) characterized by dose-related increases in convulsive (twitches and jerks), motor and autonomic signs, dose-related increases in the percent of total power in the low frequency (1-4 Hz), and decreases in the high-frequency (18-26 Hz) bands of the EEG recorded from the dentate and the amygdala. PK 11195 produced a dose-related increase in the 4-12 Hz band of the EEG recorded from the CA1, whereas the PAS was mild and not dose-related. However, the 6.25 and 12.5-microg doses elicited a significant PAS that tended to increase with dose. These data indicate that chronic DZ produces dependence, and that in the CA1 it involves the participation of central and possibly peripheral benzodiazepine (BZ) receptors located within this structure.

Selective attention: a reevaluation of the implications of negative priming.

The notion that inhibitory processes play a critical role in selective attention has gained wide support. Much of this support derives from studies of negative priming. The authors note that the attribution of negative priming to an inhibitory mechanism of attention draws its support from a common assumption underlying priming procedures, together with the procedure that has been used to measure negative priming. The results from a series of experiments demonstrate that selection between 2 competing prime items is not required to observe negative priming. This result is demonstrated across several experiments in which participants named 1 of 2 items in a second display following presentation of a single-item prime. The implications of these results for existing theories of negative priming are discussed, and a theoretical framework for interpreting negative priming and several related phenomena is forwarded.

Inhibitory processes in auditory selective attention: evidence of location-based and frequency-based inhibition of return.

The possibility that there is an inhibitory component to auditory covert orienting was addressed. Each trial consisted of a cue followed by a target, and listeners were required to detect, localize, or identify the frequency of the target. At 150-msec stimulus onset asynchrony (SOA), performance was best when stimuli sounded from the same location or were of the same frequency. However, at 750-msec SOA, performance was best when stimuli differed in location or were of different frequencies. These results document the existence of both location-based and frequency-based auditory inhibition of return.

The relationship of laser-Doppler skin blood flow measurements to the cutaneous microvascular anatomy.

The hairless plantar paw surface of the rat shows high skin blood flow with a substantial response to thermal stimulation. This contrasts with hair-covered areas such as the back, where there is much lower basal flow and thermal response. These properties are similar to the differences seen in humans between skin sites which have a high density of arterioles and venules (AV areas) and sites with predominantly nutritive (NUTR) capillary perfusion. However, there has been no previous study of the microvascular anatomy of rodent skin. We used NIH Image, a quantitative imaging program, to count the capillaries, arterioles, and venules in the skin of the plantar paw surface and the back of 14 Wistar-Kyoto rats. We also used laser-Doppler techniques to determine skin blood flow at these sites. We found significantly more vessels per unit area at the paw. There were twice as many capillaries in the paw (19.6 +/- 2.4 per mm2) compared to the back (9 +/- 1.5 per mm2) (P < 0.001). Similarly, there were three times as many venules (11.8 +/- 1.2 per mm2 vs 3. 48 +/- 0.45 per mm2; P < 0.001). The largest difference was in the number of arterioles (7.76 +/- 0.74 per mm2 vs 0.79 +/- 0.13 per mm2 at the back; P < 0.001). The greater microvascular density at the paw was reflected in a threefold higher basal blood flow (6.6 +/- 0. 44 ml/min/100 g) compared to that in the back (1.99 +/- 0.07 ml/min/100 g) (P < 0.001). Microvascular volume at the back was 0.14 +/- 0.01 x 10(6) RBC/ml in the basal state compared to 0.31 +/- 0.01 x 10(6) RBC/ml at the paw. Thus, the increased number of vessels at the paw resulted in a twofold increase in microvascular volume. The plantar paw surface has considerably more vessels than the back. As might be expected, there is a higher proportion of arterioles and venules compared to capillaries at the paw than at the back. Thus, the plantar paw surface is an AV site compared to the back, which is a NUTR site. Although our prior studies have largely assumed that we could use the paw and back as contrast sites comparable to AV and NUTR sites in humans, we have now for the first time conclusively established this fact. The increased microvascular density at the paw results in higher skin blood flow at this site.

The microvascular composition of the healing wound compared at skin sites with nutritive versus arteriovenous perfusion.

Background. In the rat, there is a significantly greater blood flow response to wounding at the back, a site perfused mainly by small capillaries, than at the paw, which has a much higher density of arterioles and venules. Materials and methods. We characterized the microvascular composition of wounds at the two skin sites in 11 Wistar Kyoto rats using a quantitative imaging program. Blood flow was compared using laser Doppler technology. Results. Prior to wounding, skin blood flow was much greater at the paw (7.1 +/- 0.5 ml 100 g tissue-1 min-1) than at the back (2.1 +/- 0.1 ml 100 g tissue-1 min-1, P < 0.01) at baseline. Seven days after wounding, blood flow both at the center (8.3 +/- 1.4 ml 100 g tissue-1 min-1) and at the perimeter of the back wound (4.1 +/- 0.5 ml 100 g tissue-1 min-1) had increased substantially. In contrast, skin blood flow at the perimeter of the paw wound had increased moderately (12. 7 +/- 2.0 ml 100 g tissue-1 min-1), but there was no change at the center of the wound (6.9 +/- 0.9 ml 100 g tissue-1 min-1). There were three times more microvessels per mm2 at the paw site (39.3 +/- 3.6) than at the back (13.1 +/- 1.5) prior to wounding. The wound granulation tissue was very vascular; the numerical density of vessels was identical at back (166 +/- 9) and at paw (154 +/- 6). Despite the marked increase in blood flow at the perimeter of the back wound, there was no difference in the microvascular density (15. 2 +/- 1.4) compared to baseline, nor was there a difference at the paw perimeter (39.4 +/- 3.6) compared to baseline. Conclusions. This study demonstrates that the microvascular constitutions of granulation tissues at the paw and back are identical. Thus, the rise in flow at the back wound and reduction in flow at the paw wound are entirely consistent with similar microvascular compositions of these two sites. Yet, there is increased flow at the back wound perimeter where there is no significant change in microvascular constitution compared to unwounded skin. Therefore, a microvascular structure no different from that prior to wounding functions very differently after wounding. Clearly vasoregulatory factors impact on the wound to modify flow through the microvascular network.

Performance of a video-image-subtraction-based patient positioning system.

PURPOSE: We have developed and tested an interactive video system that utilizes image subtraction techniques to enable high precision patient repositioning using surface features. We report quantitative measurements of system performance characteristics. METHODS AND MATERIALS: Video images can provide a high precision, low cost measure of patient position. Image subtraction techniques enable one to incorporate detailed information contained in the image of a carefully verified reference position into real-time images. We have developed a system using video cameras providing orthogonal images of the treatment setup. The images are acquired, processed and viewed using an inexpensive frame grabber and a PC. The subtraction images provide the interactive guidance needed to quickly and accurately place a patient in the same position for each treatment session. We describe the design and implementation of our system, and its quantitative performance, using images both to measure changes in position, and to achieve accurate setup reproducibility. RESULTS: Under clinical conditions (60 cm field of view, 3.6 m object distance), the position of static, high contrast objects could be measured with a resolution of 0.04 mm (rms) in each of two dimensions. The two-dimensional position could be reproduced using the real-time image display with a resolution of 0.15 mm (rms). Two-dimensional measurement resolution of the head of a patient undergoing treatment for head and neck cancer was 0.1 mm (rms), using a lateral view, measuring the variation in position of the nose and the ear over the course of a single radiation treatment. Three-dimensional repositioning accuracy of the head of a healthy volunteer using orthogonal camera views was less than 0.7 mm (systematic error) with an rms variation of 1.2 mm. Setup adjustments based on the video images were typically performed within a few minutes. The higher precision achieved using the system to measure objects than to reposition them suggests that the variability in repositioning is dominated by the ability of the therapist to make small, controlled changes in the position of the patient. CONCLUSION: Using affordable, off-the-shelf technology, we have developed a patient positioning system that achieves repositioning accuracy normally associated with fractionated stereotactic systems. The technique provides real-time guidance and can be used to easily and quickly correct patient setup before every treatment, thus significantly reducing overall random positioning error. This improved positioning capability provides the precision required to realize the potential gains of conformal radiotherapy.

Negative Priming, Attention, and Discriminating the Present from the Past

Priming effects have been used widely as a tool to study attentional processes. However, inferences regarding attention depend on how priming effects are interpreted. In the case of negative priming, an activation-based framework for interpreting priming suggests that attention inhibits the representation of prime distractors and that this inhibition is measured in performance to subsequent probes. Data summarized in this article point out that negative priming does not depend on selection of one of two primes and that attentional influences during retrieval play an important role in determining negative priming. Also, two experiments are described that demonstrate a correlation between priming effects and knowledge of the relation between primes and probes. We suggest that negative priming is not determined directly by a process of ignoring, but instead occurs because a repeated probe is less temporally distinct when ignored as a prime than when attended.

Negative priming, attention, and discriminating the present from the past.

Priming effects have been used widely as a tool to study attentional processes. However, inferences regarding attention depend on how priming effects are interpreted. In the case of negative priming, an activation-based framework for interpreting priming suggests that attention inhibits the representation of prime distractors and that this inhibition is measured in performance to subsequent probes. Data summarized in this article point out that negative priming does not depend on selection of one of two primes and that attentional influences during retrieval play an important role in determining negative priming. Also, two experiments are described that demonstrate a correlation between priming effects and knowledge of the relation between primes and probes. We suggest that negative priming is not determined directly by a process of ignoring, but instead occurs because a repeated probe is less temporally distinct when ignored as a prime than when attended.