Encoding of Naturalistic Stimuli by Local Field Potential Spectra in Networks of Excitatory and Inhibitory Neurons

Recordings of local field potentials (LFPs) reveal that the sensory cortex displays rhythmic activity and fluctuations over a wide range of frequencies and amplitudes. Yet, the role of this kind of activity in encoding sensory information remains largely unknown. To understand the rules of translation between the structure of sensory stimuli and the fluctuations of cortical responses, we simulated a sparsely connected network of excitatory and inhibitory neurons modeling a local cortical population, and we determined how the LFPs generated by the network encode information about input stimuli. We first considered simple static and periodic stimuli and then naturalistic input stimuli based on electrophysiological recordings from the thalamus of anesthetized monkeys watching natural movie scenes. We found that the simulated network produced stimulus-related LFP changes that were in striking agreement with the LFPs obtained from the primary visual cortex. Moreover, our results demonstrate that the network encoded static input spike rates into gamma-range oscillations generated by inhibitory–excitatory neural interactions and encoded slow dynamic features of the input into slow LFP fluctuations mediated by stimulus–neural interactions. The model cortical network processed dynamic stimuli with naturalistic temporal structure by using low and high response frequencies as independent communication channels, again in agreement with recent reports from visual cortex responses to naturalistic movies. One potential function of this frequency decomposition into independent information channels operated by the cortical network may be that of enhancing the capacity of the cortical column to encode our complex sensory environment

Myocardial hypertrophy and left ventricular diastolic function in hypertensive patients: An echo Doppler evaluation

The presence and the characteristics of left ventricular diastolic dysfunction in mild to moderate systemic hypertension were evaluated in 13 normotensive subjects (Group I), in 12 hypertensive subjects without (Group II) and 28 with (Group III) LV hypertrophy who underwent two-dimensional Doppler echocardiagraphic study. Among Group III patients, a subset (n = 12) with a dilated left ventricle was identified. Diastolic filling parameters were impaired in Group III patients while, in Group II, they were intermediate between Groups I and III. In all Group III patients normalized peak filling rate (nPFR) correlated directly with mean velocity of circumferential fibre shortening (mVCF) (r = 0.55; P < 0.001) and inversely with left ventricular mass index (LVM) (r = -0.60; P < 0.001), left ventricular end-diastolic diameter (LVIDd) (r = -0.63; P < 0.001), LV peak systolic stress (LVWST) (r = 0.64; P < 0.01). A separate analysis showed that these correlations were also present in patients without left ventricular dilation; in the subset with left ventricular dilation nPFR correlated only with LVWST (r = -0.73; P < 0.01), but not with LVM, mVCF, LVIDd. Thus, left ventricular hypertrophy is one of the major determinants of diastolic dysfunction in hypertensives; other factors influence nPFR values in hypertensive patients when the left ventricle dilates

Effects of late administration of tissue-type plasminogen activator on left ventricular remodeling and function after myocardial infarction.

To evaluate the effects of late thrombolysis on left ventricular volume and function in acute myocardial infarction, two-dimensional echocardiography and radionuclide angiography were performed before discharge and after 1 year of follow-up study in 34 patients with acute anterior myocardial infarction. Of these, 10 admitted to the coronary care unit within 4 h from the onset of symptoms were treated with recombinant tissue-type plasminogen activator (rt-PA) (Group A) and 24 admitted between 4 and 8 h after onset were randomly assigned to receive either rt-PA (Group B, n = 12) or conventional therapy (Group C, n = 12). Seven to 10 days after admission, all patients underwent cardiac catheterization and coronary angiography. Patency of the infarct-related vessel was 70% in Group A, 66% in Group B and 33% in Group C and the average Thrombolysis in Myocardial Infarction (TIMI) coronary perfusion grade was 1.9 +/- 0.8 for Group A, 1.6 +/- 1.0 for Group B and 0.84 +/- 0.95 for Group C (Group A versus Group C p less than 0.01; Group B versus Group C p less than 0.05). At predischarge evaluation, mean left ventricular end-systolic and end-diastolic volumes were higher in Group C than in Group B (p less than 0.001 and 0.05, respectively) and Group A (p less than 0.005 for both); mean left ventricular ejection fraction at rest was lower in Group C than in Group B and Group A (p less than 0.05 for both). At 1 year follow-up study, end-systolic and end-diastolic volumes remained higher in Group C than in Group B (p less than 0.05 for both) and Group A (p less than 0.005 for end-systolic volume and p less than 0.001 for end-diastolic volume); ejection fraction at rest was lower in Group C than in Groups A and B (p less than 0.05 for both); during exercise, it increased more in Group A than in Group C (p less than 0.01). Comparison of data obtained before discharge and at the 1 year follow-up study revealed a significant differences in end-systolic volume (p less than 0.05) in Group C patients and in end-diastolic volume in patients in Groups B (p less than 0.05) and C (p less than 0.001). The beneficial effect of late thrombolysis with rt-PA may be related to a reduction in myocardial expansion and thus to a favorable influence on postinfarction left ventricular remodeling

Doppler echocardiographic evaluation of three models of bioprosthetic mitral valves

The aim of this study was to compare the Doppler echocardiographic characterists of three models of normally functioning bioprosthetic valves in the mitral position. Echocardiographic and Doppler studies were performed in 87 patients in whom Hancock (17), Carpentier-Edwards (32), and Liotta (38) models had been implanted in the mitral position. Two parameters of bioprosthetic transvalvular flow were assessed: maximum velocity of diastolic left ventrcular inflow (V(mas)), and pressure half-time (P 1/2 T). Hancock valves showed higher V(max) and P 1/2 T values than Carpentier-Edwards and Liotta valves. Moreover, only Hancock valves presented a significant difference in P 1/2 T values between larger and smaller valves (p < 0.001). Neither V(max) nor P 1/2 T were correlated to valve size or patient age; a significant inverse correlation was found between P 1/2 T and postoperative interval in Hancock (p < 0.05) and Carpentier-Edwards (p < 0.001), but not in Liotta valves. This study indicates that Hancock valve implantation in the mitral position is not preferable if a small bioprosthesis is need; Liotta and Carpentier-Edwards valves appear to perform better than Hancock in terms of pressure gradients, and Liotta valves seem to offer better long-term hemodynamic performance

Effects of captopril treatment on left ventricular remodeling and function after anterior myocardial infarction: Comparison with digitalis

The effects of captopril and digoxin treatment on left ventricular remodeling and function after anterior myocardial infarction were evaluated in a randomized unblinded trial. Forty-two patients with a first transmural anterior myocardial infarction and a radionuclide left ventricular ejection fraction <40% were randomly assigned to treatment with captopril (Group A) or digoxin (Group B). The two groups had similar baseline hemodynamic, coronary angiographic, echocardiographic and radionuclide angiographic variables. Among the 40 patients (20 in each group) who were followed up for 1 year, echocardiographic end-diastolic and end-systolic volumes were unmodified in Group A and global wall motion index was improved (p < 0.01); in Group B, end-diastolic and end-systolic volumes increased (p < 0.001 for both) and global wall motion index was unchanged. Rest radionuclide ejection fraction increased significantly in both groups (p < 0.001, Group A; p < 0.005, Group B). A comparison of the changes in the considered variables bewteen the two groups after 1 year of treatment showed a difference in end-diastolic (p < 0.005) end-systolic volumes (p < 0.001) and global wall motion index (p < 0.005) without differences in radionuclide ejection fraction, which improved to a similar degree in both groups. The results of this study suggest that captopril therapy, started 7 to 10 days after symptom onset in patients with anterior myocardial infarction and an ejection fraction <40%, improves both left ventricular remodeling and function and prevents left ventricular enlargement and in these patients performs better than digitalis