Valia Rodríguez

Switching Attention without Shifting the Spotlight: Object-Based Attentional Modulation of Brain Potentials

Although psychophysical evidence for object-based attention has been reported, corresponding studies with event-related potentials (ERPs) are scarce. Here subjects were presented with perceptual fields containing two superimposed objects (transparent surfaces generated by two sets of dots in rigid rotation around fixation, each set of a different color and direction of motion) or only one object (the same dots but either at rest or all rotating in the same direction). Brief (150-msec) rectilinear displacements affected either of the sets at random ISIs of 350 to 550 msec. Attention was directed to one set of dots, guided by color, in order to discriminate the direction of their displacement. Motion-onset ERPs elicited by these displacements were compared for attended and unattended dots. When the perceptual field consisted of two objects, strong suppression of P1 and N1 was obtained in the ERPs associated with the unattended object. No suppression was found with the field containing a single object, although an enhanced selection negativity was found in ERPs associated with attended dots (selected by color). Since the two objects occupied the same region of visual space, the suppression of P1/N1 cannot be explained by the space-based mechanisms but is consistent with object-based attentional selection at early stages of vision. The results highlight the role of perceptual organization in enabling alternative attentional mechanisms.

Testing non-linearity and directedness of interactions between neural groups in the macaque inferotemporal cortex

Information processing in the visual cortex depends on complex and context sensitive patterns of interactions between neuronal groups in many different cortical areas. Methods used to date for disentangling this functional connectivity presuppose either linearity or instantaneous interactions, assumptions that are not necessarily valid. In this paper a general framework that encompasses both linear and non-linear modelling of neurophysiological time series data by means of Local Linear Non-linear Autoregressive models (LLNAR) is described. Within this framework a new test for non-linearity of time series and for non-linearity of directedness of neural interactions based on LLNAR is presented. These tests assess the relative goodness of fit of linear versus non-linear models via the bootstrap technique. Additionally, a generalised definition of Granger causality is presented based on LLNAR that is valid for both linear and non-linear systems. Finally, the use of LLNAR for measuring non-linearity and directional influences is illustrated using artificial data, reference data as well as local field potentials (LFPs) from macaque area TE. LFP data is well described by the linear variant of LLNAR. Models of this sort, including lagged values of the preceding 25 to 60 ms, revealed the existence of both uni- and bi-directional influences between recording sites.

Attention to Surfaces Modulates Motion Processing in Extrastriate Area MT

In the visual system, early atomized representations are grouped into higher-level entities through processes of perceptual organization. Here we present neurophysiological evidence that a representation of a simple object, a surface defined by color and motion, can be the unit of attentional selection at an early stage of visual processing. Monkeys were cued by the color of a fixation spot to attend to one of two transparent random-dot surfaces, one red and one green, which occupied the same region of space. Motion of the attended surface drove neurons in the middle temporal (MT) visual area more strongly than physically identical motion of the non-attended surface, even though both occurred within the spotlight of attention. Surface-based effects of attention persisted even without differential surface coloring, but attentional modulation was stronger with color. These results show that attention can select surface representations to modulate visual processing as early as cortical area MT.

Dividing attention between form and motion during transparent surface perception.

Attending to objects implies the concurrent process of features that are analyzed in different visual subsystems or domains. Previous works have shown that attention cannot be simultaneously directed to the components of motion present in two transparent surfaces [M. Valdés et al., Cognition 66 (1998) B13-B23], even though they occupy overlapping regions of space. In this paper, possible across-domain effects in object-based attention were examined using a conjunction of form and motion in transparent superimposed surfaces. After directing attention to one surface, different combinations of motion and form judgements were performed. If both attributes belonged to the same surface, no interference was found. If the two judgements concerned features from different surfaces, a large performance cost was present for the attribute belonging to the uncued surface. The fact that these effects cut across feature domains supports the integrated competition hypothesis [J. Duncan, Attention and Performance XVI, The MIT Press, 1996, pp. 549-578].

Systemic and Skin Toxicity in Cercopithecus aethiops sabaeus Monkeys Treated During 26 Weeks with a High Intravenous Dose of the Anti- Epidermal Growth Factor Receptor Monoclonal Antibody Nimotuzumab

Nimotuzumab (h-R3) is a humanized anti-epidermal growth factor receptor monoclonal antibody (mAb) registered for treating head and neck tumours. The present study was designed to evaluate the systemic and skin toxicity of chronic intravenous administration of the h-R3 in a relevant species demonstrated by comparing the h-R3 binding affinity constants (Kd) in microsomal placental fractions from Homo sapiens and Cercopithecus aethiops monkeys using an EGF-Receptor radioligand competition assay. The Kd obtained for Nimotuzumab were 9.1x10-8 M for monkeys and 4.5x10-8 M for humans. Monkeys (n=18) were distributed into 3 groups with 3 animals of each sex in each group. Group I received saline; group II received 2.85 mg/kg of h-R3; and group III received 28.57 mg/kg of the h-R3, which represent 1 and 10 times the human dose, and they were weekly intravenously treated during 26 weeks. During the study there were no deaths. Electroneurophysiological, sanguine chemistry and haematological results did not evidence alterations. Areas of haematomas, probably related with the administration procedure, were observed at the administration zones of all animals. The electrocardiography study showed at the end of the study a slight increase in the cardiac frequency of 4 treated animals without others signs. Unexpectedly, skin biopsies and a detailed clinical inspection of the animals did not detect the presence of cutaneous rash or any other skin toxicity sign reported for the majority of the anti-EGF-R monoclonal antibodies. It is concluded that doses up to 28.5 mg/kg of h-R3, intravenously administered during 26 weeks to Cercopithecus aethiops monkeys, do not produce considerable toxic effects.

Absence of face-specific cortical activity in the complete absence of awareness: Converging evidence from functional magnetic resonance imaging and event-related potentials

In this study, we explored the neural correlates of perceptual awareness during a masked face detection task. To assess awareness more precisely than in previous studies, participants employed a 4-point scale to rate subjective visibility. An event-related fMRI and a high-density ERP study were carried out. Imaging data showed that conscious face detection was linked to activation of fusiform and occipital face areas. Frontal and parietal regions, including the pre-SMA, inferior frontal sulcus, anterior insula/frontal operculum, and intraparietal sulcus, also responded strongly when faces were consciously perceived. In contrast, no brain area showed face-selective activity when participants reported no impression of a face. ERP results showed that conscious face detection was associated with enhanced N170 and also with the presence of a second negativity around 300 msec and a slow positivity around 415 msec. Again, face-related activity was absent when faces were not consciously perceived. We suggest that, under conditions of backward masking, ventral stream and fronto-parietal regions show similar, strong links of face-related activity to conscious perception and stress the importance of a detailed assessment of awareness to examine activity related to unseen stimulus events.

Sensory suppression during shifts of attention between surfaces in transparent motion

During transparent motion, attention to changes in the direction of one illusory surface will impede recognition of a similar event affecting the other surface if both are close together in time. This is a form of object-based attentional blink (AB). Here, we show that this AB is related to a smaller N200 response to the change in direction and that the response is even smaller for trials on which the subject makes mistakes compared to those with correct responses consistent with signal detection theory models. The variation of N200 associated with the AB can be modeled by an attenuation of current sources estimated in visual extrastriate cortex. These results suggest that the AB in the transparent motion paradigm is due to the suppression of sensory signals in early visual areas.

Therapeutic Effect of the Combined Use of Growth Hormone Releasing Peptide-6 and Epidermal Growth Factor in an Axonopathy Model

Amyotrophic lateral sclerosis (ALS) is a disease of the central nervous system characterized by loss of spinal motor neurons, for which no effective treatment exists. Epidermal growth factor (EGF) and growth hormone releasing peptide-6 (GHRP-6) have been considered as good candidates for the treatment of this disease, due to their well documented effects in eliciting pleiotrophic and cell survival mechanisms. The aim of the present work was to evaluate the separate and combined effects of both peptides in an experimental animal model of ALS, the proximal axonopathy induced by 1,2 diacetylbenzene (1,2 DAB) in mice. The evaluations were conducted by means of behavioral tests (trapeze, tail suspension, gait pattern, and open field) and by recording the complex muscle action potential (CMAP) in three different hind limb segments: proximal S1, medial S2, and distal S3. Intraperitoneal daily administration of 1,2 DAB produced significant reduction in body weight, muscle strength, extensor reflex, spontaneous activity, and changes in gait pattern parameters. In parallel 1,2 DAB produced significant prolongation of onset latency and decrease in amplitude of CMAP and in the integrated complex action potential index. Daily administration of the separate compounds did not accelerate the recovery of the affected parameters, except for the gait pattern. The combined treatment produced significant improvement in behavioral parameters, as well as in electrophysiological recovery, particularly in the proximal segment of CMAP. The latter results confirm the proximal character of 1,2 DAB neuropathy, and suggest that combined therapy with EGF and GHRP-6 might be a good therapeutic strategy for the treatment of ALS.

Ictal–interictal continuum: A proposed treatment algorithm

The ictal-interictal continuum (IIC) is characterized by periodic and/or rhythmic EEG patterns that occur with relative high frequency in critically ill patients. Several studies have reported that some patterns seen within the continuum are independently associated with poor outcome. However there is no consensus regarding when to treat them or how aggressive treatment should be. In this review we examine peer-reviewed original scientific articles, guidelines and reviews indexed in PubMed and summarize current knowledge related to the ictal-interictal continuum. A treatment algorithm to guide management of critically ill patients with EEG patterns that fall along the IIC is proposed. The algorithm-based on best current practice in adults-takes into account associated clinical events, risk factors for developing seizures, response to medication trials and biomarkers of neuronal injury.

Hematological, biochemical, respiratory, cardiovascular and electroneurophysiological parameters in African green monkeys (Cercopithecus aethiops sabaeus). Its use in non-clinical toxicological studies.

Background  The purpose of this study is to better characterize the hematological, biochemical, respiratory, cardiovascular and electroneurophysiological parameters in young adult Cercopithecus aethiops sabaeus of both sexes. The rhesus and cynomolgus monkeys are widely used as experimental primate models. However, only few articles have been published testing toxicological effects of pharmaceuticals on African green monkey.