The influence of connectivity on the firing rate in a neuronal network with electrical and chemical synapses

We study the firing rate properties of a cellular automaton model for a neuronal network with chemical synapses. We propose a simple mechanism in which the nonlocal connections are included, through electrical and chemical synapses. In the latter case, we introduce a time delay which produces self-sustained activity. Nonlocal connections, or shortcuts, are randomly introduced according to a specified connection probability. There is a range of connection probabilities for which neuron firing occurs, as well as a critical probability for which the firing ceases in the absence of time delay. The critical probability for nonlocal shortcuts depends on the network size according to a power-law. We also compute the firing rate amplification factor by varying both the connection probability and the time delay for different network sizes. (C) 2011 Elsevier B.V. All rights reserved.

Single intravitreal bevacizumab injection effects on contrast sensitivity in macular edema from branch retinal vein occlusion

Purpose: To evaluate the effect of a single intravitreal bevacizumab injection on visual acuity, contrast sensitivity and optical coherence tomography-measured central macular thickness in eyes with macular edema from branch retinal vein occlusion. Methods: Seventeen eyes of 17 patients with macular edema from unilateral branch retinal vein occlusion were treated with a single bevacizumab injection. Patients were submitted to a complete evaluation including best corrected visual acuity, contrast sensitivity and optical coherence tomography measurements before treatment and one and three months after injection. Visual acuity, contrast sensitivity and optical coherence tomography measurements were compared to baseline values. Results: Mean visual acuity measurement improved from 0.77 logMAR at baseline to 0.613 logMAR one month after injection (P=0.0001) but worsened to 0.75 logMAR after three months. Contrast sensitivity test demonstrated significant improvement at spatial frequencies of 3, 6, 12 and 18 cycles/degree one month after injection and at the spatial frequency of 12 cycles/degree three months after treatment. Mean +/- standard deviation baseline central macular thickness (552 +/- 150 mu m) reduced significantly one month (322 +/- 127 mu m, P=0.0001) and three months (439 perpendicular to 179 mu m, P=0.01) after treatment. Conclusions: Bevacizumab injection improves visual acuity and contrast sensitivity and reduces central macular thickness one month after treatment. Visual acuity returns to baseline levels at the 3-month follow-up, but some beneficial effect of the treatment is still present at that time, as evidenced by optical coherence tomography-measured central macular thickness and contrast sensitivity measurements.

Thyroid hormone action is required for normal cone opsin expression during mouse retinal development

PURPOSE. The expression of S- and M-opsins in the murine retina is altered in different transgenic mouse models with mutations in the thyroid hormone receptor (TR)-beta gene, demonstrating an important role of thyroid hormone (TH) in retinal development. METHODS. The spatial expression of S- and M-opsin was compared in congenital hypothyroidism and in two different TR mutant mouse models. One mouse model contains a ligand-binding mutation that abolishes TH binding and results in constitutive binding to nuclear corepressors. The second model contains a mutation that blocks binding of coactivators to the AF-2 domain without affecting TH binding. RESULTS. Hypothyroid newborn mice showed an increase in S- opsin expression that was completely independent of the genotype. Concerning M-opsin expression, hypothyroidism caused a significant decrease (P < 0.01) only in wild-type animals. When TR beta 1 and -beta 2 were T3-binding defective, the pattern of opsin expression was similar to TR beta ablation, showing increased S- opsin expression in the dorsal retina and no expression of M-opsin in the entire retina. In an unexpected finding, immunostaining for both opsins was detected when both subtypes of TR beta were mutated in the helix 12 AF-2 domain. CONCLUSIONS. The results show, for the first time, that the expression of S- and M-opsin is dependent on normal thyroid hormone levels during development.

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.

Contrast Sensitivity Mediated by Inferred Magno- and Parvocellular Pathways in Type 2 Diabetics with and without Nonproliferative Retinopathy

PURPOSE. To evaluate achromatic contrast sensitivity (CS) with magnocellular-(M) and parvocellular-(P) probing stimuli in type 2 diabetics, with (DR) or without (NDR) nonproliferative retinopathy. METHODS. Inferred M-and P-dominated responses were assessed with a modified version of the steady-/pulsed-pedestal paradigm (SP/PP) applied in 26 NDR (11 male; mean age, 55 +/- 9 years; disease duration, 5 +/- 4 years); 19 DR (6 male; mean age, 58 +/- 7 years; disease duration = 9 +/- 6 years); and 18 controls (CTRL; 12 male; mean age, 55 +/- 10 years). Thresholds were measured with pedestals at 7, 12, and 19 cd/m(2), and increment durations of 17 and 133 ms. The thresholds from the two stimulus durations were used to estimate critical durations (Tc) for each data set. RESULTS. Both DR and NDR patients had significant reduction in CS in both SP and PP paradigms in relation to CTRL (Kruskal-Wallis, P < 0.01). Patients` critical duration estimates for either paradigm were not significantly different from CTRL. CONCLUSIONS. The significant reduction of CS in both paradigms is consistent with losses of CS in both M and P pathways. The CS losses were not accompanied by losses in temporal processing speed in either diabetic group. Significant CS loss in the group without retinopathy reinforces the notion that neural changes associated with the cellular and functional visual loss may play an important role in the etiology of diabetic visual impairment. In addition, the results show that the SP/PP paradigm provides an additional tool for detection and characterization of the early functional damage due to diabetes. (Invest Ophthalmol Vis Sci. 2011; 52:1151-1155) DOI:10.1167/iovs.09-3705