Perceptual Cycles Travel Across Retinotopic Space

Visual perception waxes and wanes periodically over time at low frequencies (theta: 4-7 Hz; alpha: 8-13 Hz), creating "perceptual cycles." These perceptual cycles can be induced when stimulating the brain with a flickering visual stimulus at the theta or alpha frequency. Here, we took advantage of the well-known organization of the visual system into retinotopic maps (topographic correspondence between visual and cortical spaces) to assess the spatial organization of induced perceptual cycles. Specifically, we tested the hypothesis that they can propagate across the retinotopic space. A disk oscillating in luminance (inducer) at 4, 6, 8, or 10 Hz was presented in the periphery of the visual field to induce perceptual cycles at specific frequencies. EEG recordings verified that the brain responded at the corresponding inducer frequencies and their first harmonics. Perceptual cycles were assessed with a concurrent detection task-target stimuli were displayed at threshold contrast (50% detection) at random times during the inducer. Behavioral results confirmed that perceptual performance was modulated periodically by the inducer at each frequency. We additionally manipulated the distance between the target and the inducer (three possible positions) and showed that the optimal phase, that is, moment of highest target detection, shifted across target distance to the inducer, specifically when its flicker frequency was in the alpha range (8 and 10 Hz). These results demonstrate that induced alpha perceptual cycles travel across the retinotopic space in humans at a propagation speed of 0.3-0.5 m/sec, consistent with the speed of unmyelinated horizontal connections in the visual cortex.

Rhythmic sampling revisited: Experimental paradigms and neural mechanisms

Sampling of information is thought to be an important aspect of explorative behaviour. Evidence for it has been gained in behavioural assessments of a variety of overt and covert cognitive domains, including sensation, attention, memory, eye movements and dexterity. A common aspect across many findings is that sampling tends to exhibit a rhythmicity at low frequencies (theta, 4-8 Hz; alpha, 9-12 Hz). Neurophysiological investigations in a wide range of species, including rodents, non-human primates and humans have demonstrated the presence of sampling related neural oscillations in a number of brain areas ranging from early sensory cortex, hippocampus to high-level cognitive areas. However, to assess whether rhythmic sampling represents a general aspect of exploratory behaviour one must critically evaluate the task parameters, and their potential link with neural oscillations. Here we focus on sampling during attentive vision to present an overview on the experimental conditions that are used to investigate rhythmic sampling and associated oscillatory brain activity in this domain. This review aims to (1) provide guidelines to efficiently quantify behavioural rhythms, (2) compare results from human and non-human primate studies and (3) argue that the underlying neural mechanisms of sampling can co-occur in both sensory and high-level areas.

Distinct contributions of alpha and theta rhythms to perceptual and attentional sampling

Accumulating evidence suggests that visual perception operates in an oscillatory fashion at an alpha frequency (around 10 Hz). Moreover, visual attention also seems to operate rhythmically, albeit at a theta frequency (around 5 Hz). Both rhythms are often associated to "perceptual snapshots" taken at the favorable phases of these rhythms. However, less is known about the unfavorable phases: do they constitute "blind gaps," requiring the observer to guess, or is information sampled with reduced precision insufficient for the task demands? As simple detection or discrimination tasks cannot distinguish these options, we applied a continuous report task by asking for the exact orientation of a Landolt ring's gap to estimate separate model parameters for precision and the amount of guessing. We embedded this task in a well-established psychophysical protocol by densely sampling such reports across 20 cue-target stimulus onset asynchronies in a Posner-like cueing paradigm manipulating involuntary spatial attention. Testing the resulting time courses of the guessing and precision parameters for rhythmicities using a fast Fourier transform, we found an alpha rhythm (9.6 Hz) in precision for invalidly cued trials and a theta rhythm (4.8 Hz) in the guess rate across validity conditions. These results suggest distinct roles of the perceptual alpha and the attentional theta rhythm. We speculate that both rhythms result in environmental sampling characterized by fluctuating spatial resolution, speaking against a strict succession of blind gaps and perceptual snapshots.

Specific Visual Subregions of TPJ Mediate Reorienting of Spatial Attention

The temporo-parietal junction (TPJ) has been associated with various cognitive and social functions, and is critical for attentional reorienting. Attention affects early visual processing. Neuroimaging studies dealing with such processes have thus far concentrated on striate and extrastriate areas. Here, we investigated whether attention orienting or reorienting modulate activity in visually driven TPJ subregions. For each observer we identified 3 visually responsive subregions within TPJ: 2 bilateral (vTPJant and vTPJpost) and 1 right lateralized (vTPJcent). Cortical activity in these subregions was measured using fMRI while observers performed a 2-alternative forced-choice orientation discrimination task. Covert spatial endogenous (voluntary) or exogenous (involuntary) attention was manipulated using either a central or a peripheral cue with task, stimuli and observers constant. Both endogenous and exogenous attention increased activity for invalidly cued trials in right vTPJpost; only endogenous attention increased activity for invalidly cued trials in left vTPJpost and in right vTPJcent; and neither type of attention modulated either right or left vTPJant. These results demonstrate that vTPJpost and vTPJcent mediate the reorientation of covert attention to task relevant stimuli, thus playing a critical role in visual attention. These findings reveal a differential reorienting cortical response after observers' attention has been oriented to a given location voluntarily or involuntarily.

Distinct perceptual rhythms for feature and conjunction searches

Feature and conjunction searches are widely used to study attentional deployment. However, the spatiotemporal behavior of attention integration in these tasks remains under debate. Are multiple search stimuli processed in parallel or sequentially? Does sampling of visual information and attentional deployment differ between these two types of search? If so, how? We used an innovative methodology to estimate the distribution of attention on a single-trial basis for feature and conjunction searches. Observers performed feature- and conjunction-search tasks. They had to detect and discriminate a tilted low-spatial-frequency grating among three low-spatial-frequency vertical gratings (feature search) or low-spatial-frequency vertical gratings and high-spatial-frequency tilted gratings (conjunction search). After a variable delay, two probes were flashed at random locations. Performance in reporting the probes was used to infer attentional deployment to those locations. By solving a second-degree equation, we determined the probability of probe report at the most (P1) and least (P2) attended locations on a given trial. Were P1 and P2 equal, we would conclude that attention had been uniformly distributed across all four locations. Otherwise, we would conclude that visual information sampling and attentional deployment had been nonuniformly distributed. Our results show that processing was nonuniformly distributed across the four locations in both searches, and was modulated periodically over time at ∼5 Hz for the conjunction search and ∼12 Hz for the feature search. We argue that the former corresponds to the periodicity of attentional deployment during the search, whereas the latter corresponds to ongoing sampling of visual information. Because different locations were not simultaneously processed, this study rules out a strict parallel model for both search types.

Transcranial Magnetic Stimulation Reveals Intrinsic Perceptual and Attentional Rhythms

Oscillatory brain activity has functional relevance for perceptual and cognitive processes, as proven by numerous electrophysiology studies accumulating over the years. However, only within the past two decades have researchers been able to study the causal role of such oscillations using transcranial magnetic stimulation (TMS) technology. Two complementary approaches exist. A majority of research employs rhythmic TMS (rTMS) to entrain oscillatory activity and investigate its effect on targeted brain functions. On the other hand, single pulses of TMS (spTMS) that can be delivered with a high spatio-temporal resolution, can be used to precisely probe the state of the system. In this mini-review, we concentrate on this second approach. We argue that, with no a priori hypothesis on the oscillatory frequency of the targeted cortical regions, spTMS can help establish causal links between spontaneous oscillatory activity and perceptual and cognitive functions. Notably, this approach helped to demonstrate that the occipital cortex is periodically involved during specific attentional tasks at the theta (~5 Hz) frequency. We propose that this frequency reflects periodic inter-areal communication for attentional exploration and selection. In the future, clever combination of non-invasive recording and stimulation with well-controlled psychophysics protocols will allow us to further our understanding of the role of brain oscillations for human brain functions.

An expedient synthesis of flexible nucleosides via a regiocontrolled enzymatic glycosylation of functionalized imidazoles

A versatile two-step synthesis of C4- and C5-arylated 2′-deoxyribosylimidazoles was elaborated by enzymaticN-transglycosylation followed by microwave-assisted Pd-catalysed arylation reactions.

Attention Reorients Periodically

Reorienting of voluntary attention enables the processing of stimuli at previously unattended locations. Although studies have identified a ventral fronto-parietal network underlying attention [1, 2], little is known about whether and how early visual areas are involved in involuntary [3, 4] and even less in voluntary [5] reorienting, and their temporal dynamics are unknown. We used transcranial magnetic stimulation (TMS) over the occipital cortex to interfere with attentional reorienting and study its role and temporal dynamics in this process. Human observers performed an orientation discrimination task, with either valid or invalid attention cueing, across a range of stimulus contrasts. Valid cueing induced a behavioral response gain increase, higher asymptotic performance for attended than unattended locations. During subsequent TMS sessions, observers performed the same task, with high stimulus contrast. Based on phosphene mapping, TMS double pulses were applied at one of various delays to a consistent brain location in retinotopic areas (V1/V2), corresponding to the evoked signal of the target or distractor, in a valid or invalid trial. Thus, the stimulation was identical for the four experimental conditions (valid/invalid cue condition × target/distractor-stimulated). TMS modulation of the target and distractor were both periodic (5 Hz, theta) and out of phase with respect to each other in invalid trials only, when attention had to be disengaged from the distractor and reoriented to the target location. Reorientation of voluntary attention periodically involves V1/V2 at the theta frequency. These results suggest that TMS probes theta phase-reset by attentional reorienting and help link periodic sampling in time and attention reorienting in space.

Expedient and generic synthesis of imidazole nucleosides by enzymatic transglycosylation

A straightforward route to original imidazole-based nucleosides that makes use of an enzymatic N-transglycosylation step is reported in both the ribo- and deoxyribo-series. To illustrate the scope of this approach, a diverse set of 4-aryl and 4-heteroaryl-1H-imidazoles featuring variable sizes and hydrogen-bonding patterns was prepared using a microwave-assisted Suzuki-Miyaura cross-coupling reaction. These imidazole derivatives were examined as possible substrates for the nucleoside 2'-deoxyribosyltransferase from L. leichmannii and the purine nucleoside phosphorylase from E. coli. The optimum transglycosylation conditions, including the use of co-adjuvants to address solubility issues, were defined. Enzymatic conversion of 4-(hetero)arylimidazoles to 2'-deoxyribo- or ribo-nucleosides proceeded in good to high conversion yields, except bulky hydrophobic imidazole derivatives. Nucleoside deoxyribosyltransferase of class II was found to convert the widest range of functionalized imidazoles into 2'-deoxyribonucleosides and was even capable of bis-glycosylating certain heterocyclic substrates. Our findings should enable chemoenzymatic access to a large diversity of flexible nucleoside analogues as molecular probes, drug candidates and original building blocks for synthetic biology.

Appendicitis after right colectomy? How can this be possible?

This case report concerns a man with appendicitis following right colectomy.

Theta oscillations modulate attentional search performance periodically

Visual search--finding a target element among similar-looking distractors--is one of the prevailing experimental methods to study attention. Current theories of visual search postulate an early stage of feature extraction interacting with an attentional process that selects candidate targets for further analysis; in difficult search situations, this selection is iterated until the target is found. Although such theories predict an intrinsic periodicity in the neuronal substrates of attentional search, this prediction has not been extensively tested in human electrophysiology. Here, using EEG and TMS, we study attentional periodicities in visual search. EEG measurements indicated that successful and unsuccessful search trials were associated with different amounts of poststimulus oscillatory amplitude and phase-locking at ∼6 Hz and opposite prestimulus oscillatory phase at ∼6 Hz. A trial-by-trial comparison of pre- and poststimulus ∼6 Hz EEG phases revealed that the functional interplay between prestimulus brain states, poststimulus oscillations, and successful search performance was mediated by a partial phase reset of ongoing oscillations. Independently, TMS applied over occipital cortex at various intervals after search onset demonstrated a periodic pattern of interference at ∼6 Hz. The converging evidence from independent TMS and EEG measurements demonstrates that attentional search is modulated periodically by brain oscillations. This periodicity is naturally compatible with a sequential exploration by attention, although a parallel but rhythmically modulated attention spotlight cannot be entirely ruled out.

Anisomycin injection in area CA3 of the hippocampus impairs both short-term and long-term memories of contextual fear

Protein synthesis is involved in the consolidation of short-term memory into long-term memory. Previous electrophysiological data concerning LTP in CA3 suggest that protein synthesis in that region might also be necessary for short-term memory. We tested this hypothesis by locally injecting the protein synthesis inhibitor anisomycin in hippocampal area CA1 or CA3 immediately after contextual fear conditioning. As previously shown, injections in CA1 impaired long-term memory but spared short-term memory. Conversely, injections in CA3 impaired both long-term and short-term memories. We conclude that early steps of experience-induced plasticity occurring in CA3 and underlying short-term memory require protein synthesis.

Appendiceal actinomycosis complicated by multiple hepatic abscesses

We report the case of a patient with appendicitis due to actinomycosis, complicated by multiple liver abscesses. Definitive diagnosis was based on histopathologic examination of the resected appendix. Accurate identification of the pathogen led to curative antibiotic therapy of the liver abscesses.

Transcranial magnetic stimulation reveals attentional feedback to area V1 during serial visual search

Visual search tasks have been used to understand how, where and when attention influences visual processing. Current theories suggest the involvement of a high-level “saliency map” that selects a candidate location to focus attentional resources. For a parallel (or “pop-out”) task, the first chosen location is systematically the target, but for a serial (or “difficult”) task, the system may cycle on a few distractors before finally focusing on the target. This implies that attentional effects upon early visual areas, involving feedback from higher areas, should be visible at longer latencies during serial search. A previous study from Juan & Walsh (2003) had used Transcranial Magnetic Stimulation (TMS) to support this conclusion; however, only a few post-stimulus delays were compared, and no control TMS location was used. Here we applied TMS double-pulses (sub-threshold) to induce a transient inhibition of area V1 at every post-stimulus delay between 100 ms and 500 ms (50 ms steps). The search array was presented either at the location affected by the TMS pulses (previously identified by applying several pulses at supra-threshold intensity to induce phosphene perception), or in the opposite hemifield, which served as a retinotopically-defined control location. Two search tasks were used: a parallel (+ among Ls) and a serial one (T among Ls). TMS specifically impaired the serial, but not the parallel search. We highlight an involvement of V1 in serial search 300 ms after the onset; conversely, V1 did not contribute to parallel search at delays beyond 100 ms. This study supports the idea that serial search differs from parallel search by the presence of additional cycles of a select-and-focus iterative loop between V1 and higher-level areas.

Synthesis and recognition by DNA polymerases of a reactive nucleoside for DNA diversification

The synthesis of 1-(2-deoxy-beta-D-erythro-pentofuranosyl)imidazole-4-hydrazide having the features of an ambigous base is reported. The recognition of the analogue by DNA polymerases as an incoming triphosphate as well as a template base was investigated. The mutagenic properties was evaluated by PCR. The potential of this new monomer for DNA diversification is illustrated by the reactivity of the nucleobase towards various aldehydes.

Preoperative systemic 5-fluorouracil does not increase the risk of liver resection

BACKGROUND/AIMS

The majority of patients who underwent surgery for colorectal liver metastases have been previously treated with 5-FU either as adjuvant chemotherapy or as a primary treatment. We have performed a retrospective study to assess whether this chemotherapy increases the risk of liver resection.

METHODOLOGY

Mortality, morbidity and histology of the resected liver of two groups of patients having colorectal liver metastases who underwent major resection were studied. The first group included 17 patients who had received at least 2 courses of 5-FU chemotherapy within 3 months prior to liver resection. The second group included 18 patients who had received no chemotherapy and who were used as controls.

RESULTS

Perioperative mortality was nil. Intraoperative blood loss during liver resection (1 +/- 2.5 vs. 1.2 +/- 2 units) was similar in the two groups. Changes of liver function tests on days 2 and 5 were similar in the two groups. Morbidity rate was similar in the two groups (29 vs. 22%) with a mean duration of postoperative hospital stay of 19 +/- 9 days in the 5-FU group and 16 +/- 6 days in the control group. Although 7 (41%) patients in the 5-FU group had an abnormal parenchyma consistency as compared to only 3 (17%) in the control group, the pathological findings within the resected specimen were not different.

CONCLUSIONS

5-FU based systemic chemotherapy does not increase the risk of liver resections.

[Treatment of adenocarcinoma of the lower esophagus and cardia: resection with or without thoracotomy?]

STUDY AIM

In the treatment of adenocarcinoma of the cardia and lower oesophagus, the choice of the approach (with or without thoracotomy) to perform a proximal oesogastrectomy (POG) is still debated. The aim of this retrospective study was to compare mortality, morbidity and long-term survival in a series of patients operated on with or without thoracotomy.

PATIENTS AND METHOD

From January 1991 to June 1997, 59 patients (mean ages: 65 +/- 10 years, range: 30-83) underwent POG through a transthoracic (n = 31) or a transhiatal approach (n = 28). All patients underwent both coeliac and left gastric lymphadenectomy. A mediastinal subaortic lymphadenectomy was only performed in patients who had a transthoracic approach. Both groups were comparable concerning age, weight and height, and tumoral staging according to preoperative imaging and pathologic examination. The transhiatal group included more high-risk patients (respiratory insufficiency, ASA score = 3) (NS).

RESULTS

Resection was palliative in four patients in the transthoracic group and two patients in the transhiatal group. Operative mortality was 9% in the transthoracic group and 0% in the transhiatal group (NS). Pulmonary complications were as frequent with and without thoracotomy (35% versus 32% respectively). Global (curative and palliative resections) 3-year actuarial survival was similar in both groups (transthoracic: 39% versus transhiatal: 46%, NS), as well as survival after curative resection (44% versus 49% respectively, NS). The operative approach did not influence survival in patients N+ (22% versus 17% respectively, NS) and in patients N- (86% versus 77% respectively, NS).

CONCLUSION

These results suggest that, for adenocarcinoma of the cardia and lower oesophagus, the theoretical carcinologic benefit of mediastinal lymphadenectomy can be balanced with an higher operative risk related to the transthoracic approach.

Is there a role of preservation of the spleen in distal pancreatectomy?

BACKGROUND

The spleen may be preserved during distal pancreatectomy (DP) for benign disease. The aim of this retrospective study was to compare the postoperative course of DP with or without splenectomy.

STUDY DESIGN

From June 1992 to June 1997, 40 adult patients without chronic pancreatitis underwent elective DP for benign lesions. Fifteen underwent spleen-preserving DP (Conservative Group) and 25 DP with splenectomy (Splenectomy Group). In spleen-preserving DP, we attempted to preserve the splenic artery and vein.

RESULTS

Spleen-preserving DP was successfully performed in all 15 cases. Patient groups were comparable for clinical features, indication for DP, and surgical procedure. There were no postoperative deaths. The overall incidence of pancreatic fistula was 23%, but was significantly higher in the Conservative Group (40%) than in the Splenectomy Group (12%; p < 0.05). Subphrenic abscesses were more frequently observed in the Conservative Group than in the Splenectomy Group (p < 0.05). The mean duration of postoperative hospital stay was 19 days (range 6 to 46 days) in the Conservative Group and 12.5 days (range 7 to 45 days) in the Splenectomy Group (p < 0.05). At the end of mean followup of 30 months (range 8 to 40 months), no severe postsplenectomy sepsis was observed in the Splenectomy Group.

CONCLUSIONS

In our experience, DP with splenectomy has a lower morbidity rate and we consider it to be the best procedure for benign pancreatic disease.

[Palliative treatment of adenocarcinoma of the cardia: is there a role for surgery?]

OBJECTIVES

The value of palliative surgery for adenocarcinoma of the cardia (AC) is controversial, and specific studies are lacking. The aim of this study was to report the results of a palliative resection for AC in 69 patients.

METHODS

From 1980 to 1993, 69 patients (mean age 59 +/- 10 years) underwent a palliative resection for AC. Palliative resection was defined by macroscopically incomplete resection, tumoral involvement of resection margins, visceral or serosal metastasis, or N3 metastatic nodes. Patients were classified according to the diagnosis of palliation established preoperatively (group A, n = 26), peroperatively (group B, n = 35), or postoperatively (group C, n = 8) respectively.

RESULTS

Six patients (8.7%) died postoperatively. Mortality rates were 3.8%, 8.6% and 25% in groups A, B and C, respectively. Twenty one patients (30%) had postoperative non-fatal complications. Median global survival was 9 months (mean 11 +/- 7 months) without significant difference between groups A, B and C. Forty-four out of 51 patients (86%) followed until death did not have dysphagia. The other patients were free of dysphagia during an average of 70% of the follow-up duration. Among the 14 patients surviving postoperatively with a tumoral esophageal margin, none experienced dysphagia from anastomotic recurrence during follow-up.

CONCLUSIONS

In selected patients with AC, a palliative resection can be achieved with an acceptable mortality and a very good functional result. This result can justify a prospective comparison between palliative surgery and alternative treatments.