Minimum Stimulus Research Articles

Platelet Activation in Bypass Surgery for Critical Limb Ischemia

Platelet activation contributes to graft occlusion after bypass surgery. This study investigated platelet activation status before, during, and after bypass. Blood was taken preoperatively from patients undergoing femoro-popliteal bypass and at incision, after dissection, after ischemia, after reperfusion, 24 hours after surgery, and almost 2 years after bypass (and given aspirin or warfarin). Platelet aggregation was measured using a turbidimetric method and platelet activation with flow cytometry. Statistical analysis was performed using Mann-Whitney U and Wilcoxon's tests. Resting platelet activation was similar between controls and patients undergoing bypass. Platelet activation decreased at incision but remained highly reactive. Platelet aggregation increased after dissection and the ischemic phase but significantly decreased after reperfusion. Platelet aggregation and activation were increased at 24 hours and subsequently after bypass. Platelets in critical limb ischemia exist in the primed state and become activated by minimum stimuli. Increased platelet activation occurs after bypass grafting for critical limb ischemia despite adjunctive therapy.

El trastorno de la percepción invertida o inclinada

In the context of the increasing number of reported cases of patients presenting tilt or reversal of vision anomaly (here reviewed), we call the attention to the work of Justo Gonzalo (1910-1986), scarcely known in the contemporary literature. His work deals with that anomaly, and with tactile and auditory inversion, in relation to what he called central and paracentral syndromes, interpreted from the functional model he developed.Gonzalo makes reference to 25 patients with chronic tilted vision, some of them with almost inverted perception in visual, tactile and auditory systems, under minimum stimulus. The central syndrome is caused by unilateral lesion in the parieto-occipital cortex, equidistant from the visual, tactile and auditory projection areas, and is characterized by bilateral multisensory involvement, and by dynamic effects following physiological laws of nervous excitability. Thus, as the illumination of an object was diminishing, it was perceived progressively tilted, reduced, and losing form and colors, following a physiological order; however the image was corrected by increasing the illumination, or by facilitation through other sensory stimulus. The central syndrome is compared with the reviewed cases.This syndrome involves a deficit of nervous excitability which would induce an integration deficit, tilted vision emerging as a more common affection than believed. The central syndrome reveals aspects of the cerebral dynamics, suggesting a functional continuity and unity of the cortex. This is reflected in the model that was proposed, based on functional gradations through the cortex and scaling laws of dynamic systems.

P35.20 Influence of stimulus intensity on latency of sympathetic skin response evoked by sound, electric, and magnetic stimulation

Background: The physiological origin of sympathetic skin response (SSR) has not been fully established. The conduction time required for the response traveling through post-ganglionic sudomotor fiber accounts for major part of SSR latency. Because the unmyelinated c-fiber does not show conduction delay, prolongation of SSR latency is still controversial. Aim: The aim of this study was to investigate the influence of stimulus intensity on the SSR latency evoked by three different type of stimulation. Methods: The subjects were 40 healthy subjects with an average age of 28 years. Three different modalities of stimulation (sound, electric, and magnetic) were used for obtaining SSR. A total of 12 stimuli of different intensity in each stimulation modality (70, 90, and 105 dB; 5, 15, and 30 mV; and 10%, 35%, and 70% of the maximum output for sound, electric, and magnetic stimulation, respectively) (36 responses in all). The three stimuli were repeatedly applied in four separate sets. Changes in the latency were analyzed to investigated the changes of these SSR parameters in response to the intensity (intensity-effect) and repetition (repetition-effect, habituation) of stimulation. Results and discussions: The latency of the 12 responses varied considerably during the serial recordings. Two-way repeated-measure-ANOVA showed significant intensity – effect but insignificant repetition – effects on latency for all stimulation modalities. The stronger stimuli reproducibly elicited a shorter response than the minimum stimulus throughout the four sessions. Conclusions: These results suggested that strong responses probably produced a response with short latency. This finding can be explained by a newly presented theory on the neurophysiological origin of SSR, based on a model of equivalent current dipole dependent on the Na concentration gradient.

The dynamic nature of antipredator behavior: prey fish integrate threat-sensitive antipredator responses within background levels of predation risk

Prey animals often have to face a dynamic tradeoff between the costs of antipredator behavior and the benefits of other fitness-related activities such as foraging and reproduction. According to the threat-sensitive predator avoidance hypothesis, prey animals should match the intensity of their antipredator behavior to the degree of immediate threat posed by the predator. Moreover, longer-term temporal variability in predation risk (over days to weeks) can shape the intensity of antipredator behavior. According to the risk allocation hypothesis, changing the background level of risk for several days is often enough to change the response intensity of the prey to a given stimulus. As the background level of risk increases, the response intensity of the prey decreases. In this study, we tested for possible interactions between immediate threat-sensitive responses to varying levels of current perceived risk and temporal variability in background risk experienced over the past 3 days. Juvenile convict cichlids were preexposed to either low or high frequencies of predation risk (using conspecific chemical alarm cues) for 3 days and were then tested for a response to one of five concentrations (100, 50, 25, 12.5%, or a distilled water control). According to the threat-sensitive predator avoidance hypothesis, we found greater intensity responses to greater concentrations of alarm cues. Moreover, in accordance with the risk allocation hypothesis, we found that cichlids previously exposed to the high background level of risk exhibited a lower overall intensity response to each alarm cue concentration than those exposed to the low background level of risk. It is interesting to note that we found that the background level of risk over the past 3 days influenced the threshold level of response to varying concentrations of alarm cues. Indeed, the minimum stimulus concentration that evoked a behavioral response was lower for fish exposed to high background levels of predation than those exposed to low background levels of predation. These results illustrate a remarkable interplay between immediate (current) risk and background risk in shaping the intensity of antipredator responses.

Hastening orientation sensitivity

Previous perceptual learning studies have shown that sensitivity to subtle orientation differences improves with practice at oblique axes but not with practice at cardinal axes. The cause of this anisotropy in angular resolution is uncertain, and it is not known whether the same anisotropy pertains to temporal resolution-the minimum stimulus duration needed to achieve a specified angular resolution. Here, we investigated the hypothesis that cardinal improvements were previously absent because long stimulus durations yielded maximal precision, even at the start of training. Accordingly, we exploited the relatively imprecise responses that occur naturally when masked stimuli are presented for extremely brief durations. After 110,000 trials were completed over seven daily sessions, temporal resolution improved by 51% at cardinal axes and by 86% at oblique axes. This hastening of the visual response was accompanied by significant improvements in angular resolution, which were specific to the trained axis. The data demonstrate plasticity in the response to cardinal orientations and indicate that sufficient initial levels of neural imprecision may be necessary for perceptual learning.

Objective Measurement of Visual Acuity by Optokinetic Response Determination in Patients With Ocular Diseases

To investigate the efficacy of optokinetic nystagmus (OKN) suppression and induction methods for the objective estimation of visual acuities in patients with various ocular diseases. Prospective, descriptive study. One hundred seventy-three eyes of 89 patients aged between 27 and 75 years who registered at our institution from January to December 2004 were prospectively enrolled onto this study. Ocular diseases included generalized retinal diseases (47 eyes), media opacity (32 eyes), refractive errors (31 eyes), glaucoma (27 eyes), maculopathies (26 eyes), and optic neuropathies (10 eyes). Horizontal optokinetic stimuli were presented on a 17-inch monitor screen at a distance of 40 cm from the subject in a dark room. Horizontal eye movements were recorded in each eye separately by infrared oculography. Objective visual acuities measured by using OKN suppression or induction methods were compared with subjective visual acuity assessments. The logarithm of minimal angle of resolution visual acuity was 1.03, and ranged from -0.08 to hand movement. Linear regression identified minimum stripe stimuli required to induce OKN by using the OKN induction method, and the minimum dot size required to suppress OKN was found to be correlated with subjective visual acuity (P<.01). The induction method was useful in patients with visual acuities of 20/60 or worse, and the suppression method was useful in patients with visual acuities of 20/200 or better. Combined use of the OKN induction and suppression methods provides a satisfactory means of determining objective visual acuity.

Experimental designs and processing strategies for fMRI studies involving overt verbal responses

Event-related paradigms have been used increasingly in the past few years for the localization of function in tasks involving overt speech. These designs exploit the differences in the temporal characteristics between the rapid motion-induced and the slower hemodynamic signal changes. The optimization of these designs and the best way to analyze the acquired data has not yet been fully explored. The purpose of this study is to investigate various design and analysis strategies for maximizing the detection of function while minimizing task-induced motion artifacts. Both event-related and blocked paradigms can be specifically designed to meet these goals. Various event-related and blocked designs were compared both in simulation and in experiments involving overt word reading in their ability to detect function and to avoid speech-induced motion artifact. A blocked design with task and control durations of 10 s and an event-related design with a minimum stimulus duration (SD) of 5 s and an average interstimulus interval (ISI) of 10 s were found to optimally detect blood oxygenation level-dependent signal changes without significant motion artifact. Ignoring images acquired during the speech can help recover function in areas particularly affected by motion but substantially reduces the detection power in other regions. Using the stimulus timing as an additional regressor to model the motion offers little benefit in practice due to the variability of the motion-induced signal change.

Pain reactivity in Alzheimer patients with different degrees of cognitive impairment and brain electrical activity deterioration

Pain perception and autonomic responses to pain are known to be altered in dementia, although the mechanisms are poorly understood. We studied patients with Alzheimer's disease (AD) whose cognitive status was assessed through the Mini Mental State Examination test and whose brain electrical activity was measured by means of quantitative electroencephalography. After assessment of both cognitive impairment and brain electrical activity deterioration, these patients underwent sensory measurements in which the minimum stimulus intensity for both stimulus detection and pain sensation was determined. In addition, heart rate responses to pain threshold×1.5 were recorded. We found that neither stimulus detection nor pain threshold was correlated to cognitive status and brain electrical activity decline. By contrast, we found a correlation between heart rate responses and deterioration of both cognitive functions and brain electrical activity. In particular, the heart rate increase after pain stimulation was correlated to the presence of slow brain electrical activity (delta and theta frequencies). This correlation was also found for the anticipatory heart rate increase just before pain stimulation. These results indicate that pain anticipation and reactivity depend on both the cognitive status and the frequency bands of the electroencephalogram, whereas both stimulus detection and pain threshold are not affected by the progression of AD. These findings indicate that, whereas the sensory-discriminative components of pain are preserved even in advanced stages of AD, the cognitive and affective functions, which are related to both anticipation and autonomic reactivity, are severely affected. This sensory-affective dissociation is well correlated with the neuropathological findings in AD.

Spike generator limits efficiency of information transfer in a retinal ganglion cell.

The quality of the signal a retinal ganglion cell transmits to the brain is important for preception because it sets the minimum detectable stimulus. The ganglion cell converts graded potentials into a spike train with a selective filter but in the process adds noise. To explore how efficiently information is transferred to spikes, we measured contrast detection threshold and increment threshold from graded potential and spike responses of brisk-transient ganglion cells. Intracellular responses to a spot flashed over the receptive field center of the cell were recorded in an intact mammalian retina maintained in vitro at 37 degrees C. Thresholds were measured in a single-interval forced-choice procedure with an ideal observer. The graded potential gave a detection threshold of 1.5% contrast, whereas spikes gave 3.8%. The graded potential also gave increment thresholds approximately twofold lower and carried approximately 60% more gray levels. Increment threshold "dipped" below the detection threshold at a low contrast (<5%) but increased rapidly at higher contrasts. The magnitude of the "dipper" for both graded potential and spikes could be predicted from a threshold nonlinearity in the responses. Depolarization of the cell by current injection reduced the detection threshold for spikes but also reduced the range of contrasts they can transmit. This suggests that contrast sensitivity and dynamic range are related in an essential trade-off.

Pattern ERG correlates of abnormal fundus autofluorescence in patients with retinitis pigmentosa and normal visual acuity.

To examine the functional significance of central abnormalities present in fundus autofluorescence (AF) images in patients with rod-cone dystrophy and good visual acuity. Thirty patients were selected according to three criteria: a clinical diagnosis of retinitis pigmentosa (RP) confirmed with International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERGs, a parafoveal ring of increased high density on fundus AF imaging, and a visual acuity of 20/30 or better. Macular function was assessed with pattern electroretinography (PERG) to checkerboard stimuli of different field sizes. Fundus AF imaging was performed with a confocal scanning laser ophthalmoscope. The radius of the parafoveal ring of high density varied between 1.5 degrees and 9 degrees. The PERG P50 amplitude correlated highly with the radius of the ring of increased autofluorescence (r = 0.80, P < 0.0005, n = 30). PERGs to smaller circular field sizes were present, but increasing field size to beyond that of the high-density autofluorescence ring did not produce further increases in P50 amplitude. There was a high correlation between the minimum stimulus size required to elicit a maximum-amplitude PERG and the radius of the ring (r = 0.87). The high correlation between AF imaging and PERG, an established technique in the assessment of central retinal function, demonstrates the likelihood that autofluorescence abnormalities have functional significance and may therefore be a valuable additional parameter in the monitoring of these patients.

Recognition thresholds for plane-rotated pictures of familiar objects

We investigated picture plane rotation effects on the minimum stimulus duration required to recognise pictures of familiar objects in a picture–word verification task. Participants made unspeeded responses, selecting from 126 written alternatives. Longer stimulus durations were needed to identify plane-misoriented views. These orientation effects were non-linear, arguing against a simple mental rotation account of compensation for plane misorientation in identification tasks. Orientation effects were found for almost all items, in particular including those labelled at the basic level (cf. Hamm, McMullen, 1998, Journal of Experimental Psychology: Human Perception and Performance 24, 413–426). We suggest that plane misorientation increases the difficulty of basic level as well as subordinate level identification unless only a small, visually dissimilar set of stimuli are presented. Errors in the task were analysed to provide an alternative, objective measure of perceived visual similarity, by assessing the number and nature of mistaken identifications made to a given target object. We propose that misorientation effects are best understood in terms of the effects of the perceived visual similarity of a target to its set of response alternatives rather than in terms of the level (basic or subordinate) at which the target is to be identified.

Neonatal Transient Evoked Otoacoustic Emissions Screening: How Many Stimuli Are Enough?

The present study examined the effect of stimulus number on two pass criteria in neonatal transient evoked otoacoustic emission (TEOAE) screening. Participants were 25 normal, full-term neonates (mean age = 2.7 days, SD = 1.3), with normal peripheral auditory function in both ears. The Otodynamics ILO88 (Quickscreen) program was used and the TEOAE spectrum was analysed after the presentation of 10, 50, 100 and 260 stimuli. Two pass criteria were used: Criterion 1 - whole wave reproducibility (WWR) ≥ 60%, and Criterion 2 - SNR ≥ 3 dB for all three frequency bands centered at 2.4, 3.2 and 4.0 kHz. The results revealed that the number of stimuli used to achieve a Pass status depended on the pass criterion and the activity state of neonates. Although the use of an initial minimum stimulus number of 50 in neonatal TEOAE screening is justified, clinicians should continue testing if necessary until a set of predetermined stopping criteria are satisfied.

Large sample test of defibrillation waveform sensitivity.

An unknown mechanism causes defibrillation efficacy to be sensitive to the temporal pattern (waveform) of the delivered energy. Using a guinea pig model, we tested hypotheses in 140 defibrillation waveforms. Two hundred seven male guinea pigs (950 +/- 100 g) were instrumented to continuously monitor the ECG and an optical plethysmographic signal from a forepaw. Two amplifiers served as a voltage-based defibrillator with a maximum output of 400 V at 2 A. Defibrillation electrodes (12-mm diameter) were placed 40 mm apart on the thorax. Thirty ventricular fibrillation episodes were induced where the first 10 episodes were used to estimate ED50 for a biphasic pulse (7/2 msec) and the remaining episodes were defibrillated with 18 test waveforms and two control waveforms all at the ED50 energy. Seven groups of 20 waveforms were tested. We directly tested hypotheses based on charge banking/burping, frequency concentration, and stimulus strength/duration. Of the hypotheses tested, nine are able to predict at least a 10% change in efficacy (P < 0.05): parabolic fit to duration; maximum, minimum, and remaining delivered charge; power at peak frequency; stimulus charge; and maximum, minimum, and maximum of the absolute value of stimulus strength. However, of these, only three are independent predictors of waveform efficacy (P < 0.05, near-minimum residual variance): power at peak frequency; parabolic fit to the stimulus duration; and minimum stimulus strength. Stimulus strength and duration are the main determinants of the efficacy of a defibrillation waveform.

Perceptual latencies to discriminate surface orientation in stereopsis.

The difference in sensitivity to stereoscopic surfaces oriented horizontally or vertically (the stereoscopic orientation anisotropy) can be redescribed as a difference in sensitivity to shear or compression transformations that relate the binocular images. The present experiment was designed to test this by dissociating the image transformation from the orientation of the surface. Surfaces were presented in isolation or in the presence of a surrounding frame that formed step and gradient discontinuities in the disparity field. Without discontinuities, observers required considerably more time to discriminate between surfaces differing in compression than between those differing in shear, irrespective of surface orientation. Disparity discontinuities facilitated the perception of the disparity gradients; minimum stimulus durations were reduced by over an order of magnitude when the reference frame was present. These results support the hypothesis that the disparity field is decomposed into different primitives during the recovery of depth and surface structure.

電流刺激による皮膚受容感覚の安定化のための絶対しきい値と皮膚インピーダンスの関連性の検討

This study focused on stabilizing the sensations elicited by electrical stimulation applied to the skin through surface electrodes, a technique widely used in sensory feedback research. The relationship between absolute threshold (the minimum stimulus amplitude that causes sensation) and the parameters of an electrical equivalent circuit for skin impedance was examined with neurologically intact subjects. Electric current pulse stimulation at 100 Hz frequency and pulse widths between 0.08 and 0.5 ms was used to elicit a cutaneous sensation. The electrical equivalent circuit for skin impedance consisted of resistance Rp and capacitance Cp connected in parallel, plus resistance Rs connected in series to both.First, we estimated the parameters of the equivalent circuit by the skin impedance measured with sinusoidal signals (Rps and Cps), and we examined their relevance to the absolute threshold. For Ag-AgCl electrodes used with electroconductive paste, variations over time of the frequency characteristics of Rps and Cps were found to be similar to the changes caused by repeatedly stripping the skin. Values of Cps and Rps at 10 Hz were found to relate to the absolute threshold with high correlation coefficients. For a solid-gel Ag-AgCl electrode, changes of absolute threshold, Rps and Cps caused by increasing size of the solid-gel electrode were similar to variations over time obtained using electrodes with electroconductive paste. Correlations between the threshold and values of Rps and Cps at 10 Hz were also observed.Next, we estimated the parameters of the equivalent circuit by measuring stimulus pulse current and voltage waves of square-wave stimulus pulses using solid-gel electrodes (Rps and Cpr). We then examined their relevance to the absolute threshold. The absolute threshold increased with repeated electrical stimulation to the skin and decreased after adequate rest with no electrical stimulation. The value of Rpr decreased with repeated electrical stimulation and increased after rest, while the value of Cpr did not change. These variations became minor after preparation of the skin for removing the stratum corneum. A significant correlation between Rpr and the absolute threshold was observed. These results show that electrical stimulation to the stratum corneum affects both absolute threshold and skin impedance; they also indicate that real-time evaluation is important for stabilizing cutaneous sensation elicited by electrical stimulation.The relevance of Rp to the threshold was shown in both methods that were used to estimate the parameters. Possible reasons for decreasing Rp were an increase of ionic conductivity due to osmosis of electroconductive paste into the stratum corneum or an increase of electric charge in the stratum corneum due to charge transfer by electrical stimulation through the skin. The increase of the threshold amplitude was considered to be the result of broadened current path due to increases of ionic or electric conductivities in the stratum corneum. It is necessary to clarify the mechanism of the relationship between the threshold and impedance parameters in order to develop a technique for stabilizing electrocutaneous sensation.

The minimum stimulus energy required to produce a cooling sensation in the human cornea.

To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus. The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan-Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea. The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2-32.9 degrees C (i.e. 0.3 degrees C), over a 1 mm diameter circular area (0.785 mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0 ergs, or 7.0 x 10(-7) joules. The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.

The minimum stimulus energy required to produce a cooling sensation in the human cornea

Purpose: To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus.Methods: The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan–Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea.Results: The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2–32.9°C (i.e. 0.3°C), over a 1mm diameter circular area (0.785mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0ergs, or 7.0×10−7 joules.Conclusion: The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.

Passive current redistribution in the heart.

Electrical stimuli produce a spatial distribution of voltage across the heart. We hypothesized that the potential difference between tissue near a stimulating electrode and a remote site could cause sufficient current to flow through a highly conductive wire connecting the two sites to directly stimulate the remote tissue. In six open chest pigs, we inserted a catheter with right ventricular (RV) and superior vena cava (SVC) coil electrodes. A wire was placed with one end contacting the epicardium 10 +/- 2 mm away from the RV electrode and the other end on left ventricular (LV) epicardium 56 +/-14 mm from the RV end of the wire. Stimuli of 10 to 100 V were delivered to the RV and SVC electrodes. Potentials were recorded from two 252-electrode arrays placed over the RV and LV ends of the passive wire. The current induced in the wire was measured. A minimum stimulus of 15 +/- 6 V was needed between RV --> SVC electrodes to not only pace the RV but also to pace the LV through the passive wire as detected by the electrode array recordings. Current in the wire varied linearly with RV-SVC stimulus strength. Coupling RV and LV epicardium with a passive wire while stimulating the RV endocardium induced sufficient current in the wire to synchronously pace the LV.

Theoretical study of intracellular stress fiber orientation under cyclic deformation

We studied stress fiber orientation under a wide range of uniaxial cyclic deformations. We devised and validated a hypothesis consisting of two parts, as follows: (1) a stress fiber aligns to avoid a mechanical stimulus in the fiber direction under cyclic deformation. This means that, among all allowable directions, a stress fiber aligns in the direction which minimizes the stimulus, i.e., the summation of the changes in length of the stress fiber over one stretch cycle; and (2) there is a limit in the sensitivity of the cellular response to the mechanical stimulus. Due to this sensing limit, the orientation angle in stress fibers is distributed around the angle corresponding to the minimum stimulus. To validate this hypothesis, we approximated an anisotropic deformation of the membrane on which cells were to be cultured. We then obtained the relationships between the stretch range and the fiber angle in the undeformed state which minimize the mechanical stimuli, assuming that the membrane on which stress fibers and cells adhered was homogeneous and incompressible. Numerical simulation results showed that the proposed hypothesis described our previous experimental results well and was consistent with the experimental results in the literature. The simulation results, taking account of the second part of the hypothesis with a small value for the limit in sensitivity to the mechanical stimulus, could explain why cell orientation is distributed so widely with cyclic stretch ranges of <10%. The proposed hypothesis can be applied to various types of deformation because the mechanical stimulus is always sensed and accumulates under cyclic deformation without the necessity of a reference state to measure the stimulus.

Stochastic Designs in Event-Related fMRI

This article considers the efficiency of event-related fMRI designs in terms of the optimum temporal pattern of stimulus or trial presentations. The distinction between “stochastic” and “deterministic” is used to distinguish between designs that are specified in terms of the probability that an event will occur at a series of time points (stochastic) and those in which events always occur at prespecified time (deterministic). Stochastic designs may be “stationary,” in which the probability is constant, or nonstationary, in which the probabilities change with time. All these designs can be parameterized in terms of a vector of occurrence probabilities and a prototypic design matrix that embodies constraints (such as the minimum stimulus onset asynchrony) and the model of hemodynamic responses. A simple function of these parameters is presented and used to compare the relative efficiency of different designs. Designs with slow modulation of occurrence probabilities are generally more efficient than stationary designs. Interestingly the most efficient design is a conventional block design. A critical point, made in this article, is that the most efficient design for one effect may not be the most efficient for another. This is particularly important when considering evoked responses and the differences among responses. The most efficient designs for evoked responses, as opposed to differential responses, require trial-free periods during which baseline levels can be attained. In the context of stochastic, rapid-presentation designs this is equivalent to the inclusion of “null events.”