Reflexive optokinetic nystagmus in younger and older observers under photopic and mesopic viewing conditions

PURPOSE. To investigate the effect of age on optokinetic nystagmus (OKN) in response to stimuli designed to preferentially stimulate the M-pathway. METHOD. OKN was recorded in 10 younger (32.3 +/- 5.98 years) and 10 older (65.6 +/- 6.53) subjects with normal vision. Vertical gratings of 0.43 or 1.08 cpd drifting at 5 degrees/s or 20 degrees/s and presented at either 8% or 80% contrast were displayed on a large screen as full-field stimulation, central stimulation within a central Gaussian-blurred window of 15 diameter, or peripheral stimulation outside this window. All conditions apart from the high-contrast condition were presented in a random order at two light levels, mesopic (1.8 cdm(-2)) and photopic (71.5 cdm(-2)). RESULTS. Partial-field data indicated that central stimulation, mesopic light levels, and lower temporal frequency each significantly increased slow-phase velocity (SPV). Although there was no overall difference between groups for partial-field stimulation, full-field stimulation, or low-contrast stimulation, a change in illumination revealed a significant interaction with age: there was a larger decrease in SPV going from photopic to mesopic conditions for the older group than the younger group, especially for higher temporal frequency stimulation. CONCLUSIONS. OKN becomes reflexive in conditions conducive to M-pathway stimulation, and this rOKN response is significantly diminished in older healthy adults than in younger healthy adults, indicative of decreased M-pathway sensitivity.

Reflexive OKN with partial and full field stimulation: Effects of age and light level

We recorded reflexive OKN in ten younger (32.3±5.98 years) and older (65.6±6.53) visually normal subjects under viewing conditions designed to differentiate M-pathway functioning from other pathways. Subjects were required to gaze straight ahead while viewing vertical gratings of either 0.43 or 1.08 cpd, drifting at either 5 or 20°/sec and presented at either 8 or 80% contrast. Gratings were presented as full field stimulation, central stimulation or peripheral (>15°) stimulation. The order of presentation of conditions was pseudo-randomised at two blocked light levels: ‘mesopic’ or twilight conditions (1.8 cdm-2) and ‘photopic’ or full light conditions (71.5 cdm-2). For the partial fields, central stimulation, mesopic light level, lower temporal frequencies (i.e. number of stripes passing per second) each contributed to greater OKN strength as measured by slow-phase velocity (SPV). For full field stimulation, and especially for higher temporal frequencies and low contrast, there was a significant interaction between age group × light level (p = 0.017): SPV diminished much more among the older than the younger group for the twilight condition compared to full light. Such a clear diminution in M-pathway sensitivity revealed by OKN response has important implications for everyday situations like crash avoidance under twilight driving conditions.

Impact of scar thickness on the assessment of viability using dobutamine echocardiography and thallium single-photon emission computed tomography - A comparison with contrast-enhanced magnetic resonance imaging

OBJECTIVES We sought to determine whether the transmural extent of scar (TES) explains discordances between dobutamine echocardiography (DbE) and thallium single-photon emission computed tomography (Tl-SPECT) in the detection of viable myocardium (VM). BACKGROUND Discrepancies between DbE and Tl-SPECT are often attributed to differences between contractile reserve and membrane integrity, but may also reflect a disproportionate influence of nontransmural scar on thickening at DbE. METHODS Sixty patients (age 62 +/- 12 years; 10 women and 50 men) with postinfarction left ventricular dysfunction underwent standard rest-late redistribution Tl-SPECT and DbE. Viable myocardium was identified when dysfunctional segments showed Tl activity >60% on the late-redistribution image or by low-dose augmentation at DbE. Contrast-enhanced magnetic resonance imaging (ceMRI) was used to divide TES into five groups: 0%, 75% of the wall thickness replaced by scar. RESULTS As TES increased, both the mean Tl uptake and change in wall motion score decreased significantly (both p < 0.001). However, the presence of subendocardial scar was insufficient to prevent thickening; >50% of segments still showed contractile function with TES of 25% to 75%, although residual function was uncommon with TES >75%. The relationship of both tests to increasing TES was similar, but Tl-SPECT identified VM more frequently than DbE in all groups. Among segments without scar or with small amounts of scar (50% were viable by SPECT. CONCLUSIONS Both contractile reserve and perfusion are sensitive to the extent of scar. However, contractile reserve may be impaired in the face of no or minor scar, and thickening may still occur with extensive scar. (C) 2004 by the American College of Cardiology Foundation.

Incremental benefit of myocardial contrast to combined dipyridamole-exercise stress echocardiography for the assessment of coronary artery disease

Background-Although assessment of myocardial perfusion by myocardial contrast echocardiography (MCE) is feasible, its incremental benefit to stress echocardiography is not well defined. We examined whether the addition of MCE to combined dipyridamole-exercise echocardiography (DExE) provides incremental benefit for evaluation of coronary artery disease (CAD). Methods and Results-MCE was combined with DExE in 85 patients, 70 of whom were undergoing quantitative coronary angiography and 15 patients with a low probability of CAD. MCE was acquired by low-mechanical-index imaging in 3 apical views after acquisition of standard resting and poststress images. Wall motion, left ventricular opacification, and MCE components of the study were interpreted sequentially, blinded to other data. Significant (>50%) stenoses were present in 43 patients and involved 69 coronary territories. The addition of qualitative MCE improved sensitivity for the detection of CAD (91% versus 74%, P=0.02) and accurate recognition of disease extent (87% versus 65% of territories, P=0.003), with a nonsignificant reduction in specificity. Conclusions-The addition of low-mechanical-index MCE to standard imaging during DExE improves detection of CAD and enables a more accurate determination of disease extent.

Psilocybin impairs high-level but not low-level motion perception

The hallucinogenic serotonin(IA&2A) agonist psilocybin is known for its ability to induce illusions of motion in otherwise stationary objects or textured surfaces. This study investigated the effect of psilocybin on local and global motion processing in nine human volunteers. Using a forced choice direction of motion discrimination task we show that psilocybin selectively impairs coherence sensitivity for random dot patterns, likely mediated by high-level global motion detectors, but not contrast sensitivity for drifting gratings, believed to be mediated by low-level detectors. These results are in line with those observed within schizophrenic populations and are discussed in respect to the proposition that psilocybin may provide a model to investigate clinical psychosis and the pharmacological underpinnings of visual perception in normal populations.

A parametric study of droplet deformation through a microfluidic contraction: Low viscosity Newtonian droplets

Numerical simulations are conducted to investigate how a droplet of Newtonian liquid. entrained in a higher viscosity Newtonian liquid, behaves when passing through an axisymmetric microfluidic contraction. Simulations are performed using a transient Volume of Fluid finite volume algorithm, and cover ranges of Reynolds and Weber numbers relevant to microfluidic flows. Results are presented for a droplet to surrounding fluid viscosity ratio of 0.001. In contrast to behaviour at higher viscosity ratios obtained previously by the authors, shear and interfacial tension driven instabilities often develop along the droplet Surface. leading to complex shape development, and in some instances, droplet breakup. (c) 2006 Elsevier Ltd. All rights reserved.

Local edge detectors: A substrate for fine spatial vision at low temporal frequencies in rabbit retina

Visual acuity is limited by the size and density of the smallest retinal ganglion cells, which correspond to the midget ganglion cells in primate retina and the beta- ganglion cells in cat retina, both of which have concentric receptive fields that respond at either light- On or light- Off. In contrast, the smallest ganglion cells in the rabbit retina are the local edge detectors ( LEDs), which respond to spot illumination at both light- On and light- Off. However, the LEDs do not predominate in the rabbit retina and the question arises, what role do they play in fine spatial vision? We studied the morphology and physiology of LEDs in the isolated rabbit retina and examined how their response properties are shaped by the excitatory and inhibitory inputs. Although the LEDs comprise only similar to 15% of the ganglion cells, neighboring LEDs are separated by 30 - 40 mu m on the visual streak, which is sufficient to account for the grating acuity of the rabbit. The spatial and temporal receptive- field properties of LEDs are generated by distinct inhibitory mechanisms. The strong inhibitory surround acts presynaptically to suppress both the excitation and the inhibition elicited by center stimulation. The temporal properties, characterized by sluggish onset, sustained firing, and low bandwidth, are mediated by the temporal properties of the bipolar cells and by postsynaptic interactions between the excitatory and inhibitory inputs. We propose that the LEDs signal fine spatial detail during visual fixation, when high temporal frequencies are minimal.

The effect of reduced contrast on drivers' speed perception

It has been demonstrated, using abstract psychophysical stimuli, that speeds appear slower when contrast is reduced under certain conditions. Does this effect have any real life consequences? One previous study has found, using a low fidelity driving simulator, that participants perceived vehicle speeds to be slower in foggy conditions. We replicated this finding with a more realistic video-based simulator using the Method of Constant Stimuli. We also found that lowering contrast reduced participants’ ability to discriminate speeds. We argue that these reduced contrast effects could partly explain the higher crash rate of drivers with cataracts (this is a substantial societal problem and the crash relationship variance can be accounted for by reduced contrast). Note that even if people with cataracts can calibrate for the shift in their perception of speed using their speedometers (given that cataracts are experienced over long periods), they may still have an increased chance of making errors in speed estimation due to poor speed discrimination. This could result in individuals misjudging vehicle trajectories and thereby inflating their crash risk. We propose interventions that may help address this problem.

Can myocardial contrast echo provide incremental benefit to exercise echo? Implications of the intensity and duration of hyperaemia

Background. Stress myocardial contrast echo (MCE) is technically challenging with exercise (Ex) because of cardiacmovementandshort duration ofhyperemia.Vasodilators solve these limitations, but are less potent for inducing abnormal wall motion (WM). We sought whether a combined dipyridamole (DI; 0.56 mg/kg i.v. 4 min) and Ex stress protocol would enable MCE to provide incremental benefit toWManalysis for detection of CAD. Methods. Standard echo images were followed by real time MCE at rest and following stress in 85 pts, 70 undergoing quantitative coronary angiography and 15 low risk pts.WMAfrom standard and LVopacification images, and then myocardial perfusion were assessed sequentially in a blinded fashion. A subgroup of 13 pts also underwent Ex alone, to assess the contribution of DI to quantitative myocardial flow reserve (MFR). Results. Significant (>50%) stenoses were present in 43 pts, involving 69 territories. Addition of MCE improved SE sensitivity for detection of CAD (91% versus 74%, P = 0.02) and better appreciation of disease extent (87% versus 65%territories, P=0.003), with a non-significant reduction in specificity. In 55 territories subtended by a significant stenosis, but with no resting WM abnormality, ability to identify ischemia was also significantly increased by MCE (82% versus 60%, P = 0.002). MFR was less with Ex alone than with DIEx stress (2.4 ± 1.6 versus 4.0 ± 1.9, P = 0.05), suggesting prolongation of hyperaemia with DI may be essential to the results. Conclusions. Dipyridamole-exercise MCE adds significant incremental benefit to standard SE, with improved diagnostic sensitivity and more accurate estimation of extent of CAD.