Absence of placeholders abolishes exogenous attention effect in a peripheral cueing procedure

Most studies of exogenous visuospatial attention use placeholders indicating the regions where the stimuli appear on the screen. Preliminary results from our laboratory provided evidence that the attentional effect is more frequently observed when placeholders are used in these experimental procedures. Four experiments were carried out. Experiment 1 aimed at confirming the finding that the attentional effect of a spatially non-informative cue (S1) observed in the presence of placeholders disappears in their absence. The results confirmed this finding. Experiments 2, 3, and 4 examined several possible processes that could explain this finding. Experiment 2 investigated if the contribution of a faster disengagement of attention from the cued location or a stronger forward masking could explain the absence of attentional effect when no placeholders were used. Experiment 3 investigated if increased difficulty in discrimination of the target (S2) from S1 would favor the appearance of the attentional effect in the absence of placeholders. Experiment 4 investigated if an insufficient focusing of attention towards the cued location could explain the absence of attentional effect when no placeholders were used. The results of the three experiments indicated that placeholders act by reducing the discriminability of the S2. This would presumably lead to the adoption of an attentional set that favors the mobilization of attention by the S1

Premotor and occipital theta asymmetries as discriminators of memory- and stimulus-guided tasks

The saccadic paradigm has been used to investigate specific cortical networks involving visuospatial attention. We examined whether asymmetry in theta and beta band differentiates the role of the hemispheres during the execution of two different prosacadic conditions: a fixed condition, where the stimulus was presented at the same location; and a random condition, where the stimulus was unpredictable. Twelve healthy volunteers (3 male; mean age: 26.25) performed the task while their brain activity pattern was recorded using quantitative electroencephalography. We did not find any significant difference for beta, slow- and fast-alpha frequencies for the pairs of electrodes analyzed. The results for theta band showed a superiority of the left hemisphere in the frontal region when responding to the random condition on the right, which is related to the planning and selection of responses, and also a greater activation of the right hemisphere during the random condition, in the occipital region, related to the identification and recognition of patterns. These results indicate that asymmetries in the premotor area and the occipital cortex differentiate memory- and stimulus-driven tasks. (C) 2011 Elsevier Inc. All rights reserved.

Mercury toxicity in Amazon gold miners: Visual dysfunction assessed by retinal and cortical electrophysiology

Amazonian gold mining activity results in human exposure to mercury vapor. We evaluated the visual system of two Amazonian gold miners (29 and 37 years old) by recording the transient pattern electroretinogram (tPERG) and transient pattern visual evoked potential (tPVEP). We compared these results with those obtained from a regional group of control subjects. For both tPERG and tPVEP, checkerboards with 0.5 or 2 cycles per degree (cpd) of spatial frequency were presented in a 16 degrees squared area, 100% Michelson contrast, 50cd/m(2) mean luminance, and 1 Hz square-wave pattern-reversal presentation. Two averaged waveforms (n = 240 sweeps, Is each) were monocularly obtained for each subject in each condition. Both eyes were monocularly tested only in gold miners. Normative data were calculated using a final pooled waveforin with 480 sweeps. The first gold miner, LCS, had normal tPERG responses. The second one, RNP, showed low tPERG (P50 component) amplitudes at 0.5cpd for both eyes, outside the normative data, and absence of response at 2 cpd for his right eye. Delayed tPVEP responses (P 100 component) were found at 2 cpd for LCS but the implicit times were inside the normative data. Subject RNP also showed delayed tPVEP responses (all components), but only the implicit time obtained with his right eye was outside the normative data at 2cpd. We conclude that mercury exposure levels found in the Amazon gold miners is high enough to damage the visual system and can be assessed by non-invasive electrophysiological techniques. (C) 2007 Elsevier Inc. All rights reserved.

Construção e Evidências Psicométricas de uma Escala de Avaliação da Percepção Visual

Este estudo teve como objetivo construir e conhecer os parâmetros psicométricos de um instrumento para análise da percepção visual de adultos. Para a construção da escala participaram 295 adultos saudáveis, sem déficits cognitivos ou perceptivo-visuais. Nesta etapa foi formulada uma escala tetrafatorial constituída por 20 itens que avaliam quatro dimensões referentes à percepção visual: constância da forma, figurafundo, posição e relação espacial. Para obter evidências de validade foi utilizada uma amostra de 183 voluntários com boa saúde física e mental e acuidade visual normal ou corrigida. Os dados obtidos relatam a existência de concordância interjuízes, adequação semântica e significância no teste-reteste do instrumento. Os coeficientes de fidedignidade variaram de 0,84 a 0,93. Os quatro fatores esperados foram encontrados, cada um contendo 5 itens, e juntos explicaram 57,52% da variância do constructo. O instrumento apresentou parâmetros psicométricos adequados, o que pode justificar sua utilidade em pesquisas básicas e na prática clínica.

Long-Term Occupational Exposure to Organic Solvents Affects Color Vision, Contrast Sensitivity and Visual Fields

The purpose of this study was to evaluate the visual outcome of chronic occupational exposure to a mixture of organic solvents by measuring color discrimination, achromatic contrast sensitivity and visual fields in a group of gas station workers. We tested 25 workers (20 males) and 25 controls with no history of chronic exposure to solvents (10 males). All participants had normal ophthalmologic exams. Subjects had worked in gas stations on an average of 9.6 +/- 6.2 years. Color vision was evaluated with the Lanthony D15d and Cambridge Colour Test (CCT). Visual field assessment consisted of white-on-white 24-2 automatic perimetry (Humphrey II-750i). Contrast sensitivity was measured for sinusoidal gratings of 0.2, 0.5, 1.0, 2.0, 5.0, 10.0 and 20.0 cycles per degree (cpd). Results from both groups were compared using the Mann-Whitney U test. The number of errors in the D15d was higher for workers relative to controls (p<0.01). Their CCT color discrimination thresholds were elevated compared to the control group along the protan, deutan and tritan confusion axes (p<0.01), and their ellipse area and ellipticity were higher (p<0.01). Genetic analysis of subjects with very elevated color discrimination thresholds excluded congenital causes for the visual losses. Automated perimetry thresholds showed elevation in the 9 degrees, 15 degrees and 21 degrees of eccentricity (p<0.01) and in MD and PSD indexes (p<0.01). Contrast sensitivity losses were found for all spatial frequencies measured (p<0.01) except for 0.5 cpd. Significant correlation was found between previous working years and deutan axis thresholds (rho = 0.59; p<0.05), indexes of the Lanthony D15d (rho = 0.52; p<0.05), perimetry results in the fovea (rho = -0.51; p<0.05) and at 3, 9 and 15 degrees of eccentricity (rho = -0.46; p<0.05). Extensive and diffuse visual changes were found, suggesting that specific occupational limits should be created.

Examination of the performance of different pointwise linear regression progression criteria to detect glaucomatous visual field change

Background: We aimed to investigate the performance of five different trend analysis criteria for the detection of glaucomatous progression and to determine the most frequently and rapidly progressing locations of the visual field. Design: Retrospective cohort. Participants or Samples: Treated glaucoma patients with =8 Swedish Interactive Thresholding Algorithm (SITA)-standard 24-2 visual field tests. Methods: Progression was determined using trend analysis. Five different criteria were used: (A) =1 significantly progressing point; (B) =2 significantly progressing points; (C) =2 progressing points located in the same hemifield; (D) at least two adjacent progressing points located in the same hemifield; (E) =2 progressing points in the same Garway-Heath map sector. Main Outcome Measures: Number of progressing eyes and false-positive results. Results: We included 587 patients. The number of eyes reaching a progression endpoint using each criterion was: A = 300 (51%); B = 212 (36%); C = 194 (33%); D = 170 (29%); and E = 186 (31%) (P = 0.03). The numbers of eyes with positive slopes were: A = 13 (4.3%); B = 3 (1.4%); C = 3 (1.5%); D = 2 (1.1%); and E = 3 (1.6%) (P = 0.06). The global slopes for progressing eyes were more negative in Groups B, C and D than in Group A (P = 0.004). The visual field locations that progressed more often were those in the nasal field adjacent to the horizontal midline. Conclusions: Pointwise linear regression criteria that take into account the retinal nerve fibre layer anatomy enhances the specificity of trend analysis for the detection glaucomatous visual field progression.

Software for subjective visual vertical assessment: an observational cross-sectional study

Spatial orientation in relation to the gravitational axis is significantly important for the maintenance of the posture, gait and for most of the human's motor activities. The subjective visual vertical exam evaluates the individual's perception of vertical orientation. Objectives: The aims of this study were (1) to develop a virtual system to evaluate the subjective visual vertical exam, (2) to provide a simple tool to clinical practice and (3) to assess the subjective visual vertical values of h ealthy subjects using the new software. Study Design: observational cross-sectional study. Methods: Thirty healthy volunteers performed the subjective visual vertical exam in both static and dynamic conditions. The exam consisted in adjusting a virtual line in the vertical position using the computer mouse. For the static condition, the virtual line was projected in a white background. For the dynamic condition, black circles rotated in clockwise or counterclockwise directions. Six measurements were taken and the mean deviations in relation to the real vertical calculated. Results: The mean values of subjective visual vertical measurements were: static -0.372 degrees; +/- 1.21; dynamic clockwise 1.53 degrees +/- 1.80 and dynamic counterclockwise -1.11 degrees +/- 2.46. Conclusion: This software showed to be practical and accurate to be used in clinical routines.

Visual impairment in children with spastic cerebral palsy measured by psychophysical and electrophysiological grating acuity tests

Background: This study measured grating visual acuity in 173 children between 6-48 months of age who had different types of spastic cerebral palsy (CP). Method: Behavioural acuity was measured with the Teller Acuity Cards (TAC) using a staircase psychophysical procedure. Electrophysiological visual acuity was estimated using the sweep VEP (sVEP). Results: The percentage of children outside the superior tolerance limits was 44 of 63 (69%) and 50 of 55 (91%) of tetraplegic, 36 of 56 (64%) and 42 of 53 (79%) of diplegic, 10 of 48 (21%) and 12 of 40 (30%) of hemiplegic for sVEP and TAC, respectively. For the sVEP, the greater visual acuity deficit found in the tetraplegic group was significantly different from that of the hemiplegic group (p < 0.001). In the TAC procedure the mean visual acuity deficits of the tetraplegic and diplegic groups were significantly different from that of hemiplegic group (p < 0.001). The differences between sVEP and TAC means of visual acuity difference were statistically significant for the tetraplegic (p < 0.001), diplegic (p < 0.001), and hemiplegic group (p = 0.004). Discussion: Better visual acuities were obtained in both procedures for hemiplegic children compared to diplegic or tetraplegic. Tetraplegic and diplegic children showed greater discrepancies between the TAC and sVEP results. Inter-ocular acuity difference was more frequent in sVEP measurements. Conclusions: Electrophysiologically measured visual acuity is better than behavioural visual acuity in children with CP.

Comparison of the reliability of multifocal visual evoked cortical potentials generated by pattern reversal and pattern pulse stimulation

This study compared the effectiveness of the multifocal visual evoked cortical potentials (mfVEP) elicited by pattern pulse stimulation with that of pattern reversal in producing reliable responses (signal-to-noise ratio >1.359). Participants were 14 healthy subjects. Visual stimulation was obtained using a 60-sector dartboard display consisting of 6 concentric rings presented in either pulse or reversal mode. Each sector, consisting of 16 checks at 99% Michelson contrast and 80 cd/m² mean luminance, was controlled by a binary m-sequence in the time domain. The signal-to-noise ratio was generally larger in the pattern reversal than in the pattern pulse mode. The number of reliable responses was similar in the central sectors for the two stimulation modes. At the periphery, pattern reversal showed a larger number of reliable responses. Pattern pulse stimuli performed similarly to pattern reversal stimuli to generate reliable waveforms in R1 and R2. The advantage of using both protocols to study mfVEP responses is their complementarity: in some patients, reliable waveforms in specific sectors may be obtained with only one of the two methods. The joint analysis of pattern reversal and pattern pulse stimuli increased the rate of reliability for central sectors by 7.14% in R1, 5.35% in R2, 4.76% in R3, 3.57% in R4, 2.97% in R5, and 1.78% in R6. From R1 to R4 the reliability to generate mfVEPs was above 70% when using both protocols. Thus, for a very high reliability and thorough examination of visual performance, it is recommended to use both stimulation protocols.

O envolvimento da atenção na codificação da informação visuoespacial integrada

O objetivo deste estudo foi investigar o envolvimento de recursos da atenção na codificação e manutenção da informação visual e espacial na memória de trabalho. Utilizou-se um paradigma de tarefas duplas em que uma tarefa primária de localização espacial foi realizada simultaneamente a uma tarefa atentiva secundária de discriminação de tons. O desempenho dos participantes (n = 20) na tarefa primária foi afetado pela presença e pela similaridade entre os tons da tarefa secundária, e também, pela instrução de priorizar uma ou outra tarefa. Os resultados indicam que recursos atentivos (do executivo central) estão envolvidos na codificação e na manutenção ativa da informação integrada na memória visuoespacial, assim como na manutenção dos objetivos das tarefas a serem realizadas simultaneamente.