Electrophysiological evidence for a postperceptual locus of suppression during the attentional blink

When an observer detects a target in a rapid stream of visual stimuli, there is a brief period of time during which the detection of subsequent targets is impaired. In this study, event-related potentials (ERPs) were recorded from normal adult observers to determine whether this "attentional blink" reflects a suppression of perceptual processes or an impairment in postperceptual processes. No suppression was observed during the attentional blink interval for ERP components corresponding to sensory processing (the P1 and N1 components) or semantic analysis (the N400 component). However, complete suppression was observed for an ERP component that has been hypothesized to reflect the updating of working memory (the P3 component). Results indicate that the attentional blink reflects an impairment in a postperceptual stage of processing.     

Activity staining of protein inhibitors of proteases on gelatin-containing polyacrylamide gel electrophoresis

Stimulation of the supraorbital branch of the trigeminal nerve (SO) elicited eye blinks in the rabbit, but did not decrease the amplitude of visual cortical evoked potential from stimulation of the optic chiasm (OX). In addition, the SO stimulation neither induced an inhibitory postsynaptic potential (IPSP) in LGN cells, nor activated inhibitory interneurons in the thalamic reticular nucleus (TRN), which proved to mediate both recurrent inhibition and saccadic suppression in the dorsal lateral geniculate nucleus (LGN). All these indicate that there is no visual suppression in the rabbit LGN during blink reflex.     

Express saccades elicited during visual scan in the monkey

Monkeys trained to saccade to visual targets can develop separate "express" and "regular" modes in their distribution of saccadic latencies. The purpose of this study was to determine whether this occurs under more natural viewing conditions, when targets are suddenly presented in a structured visual field during visual scan. It was found that scanning saccades stopped appearing 60 msec after a target's onset, and subsequent saccades, which were directed toward the suddenly appearing target, had a bimodal distribution of latencies. Express saccades were more likely to occur as the target was presented later in a fixation. Regular mode saccades were more likely to occur with longer target durations. Scanning saccades made to stimuli of the structured visual field always had unimodal inter-saccadic interval distributions. All these effects were apparent after only 2-3 days of training. These findings, taken together with recent physiological results, suggest that the visuomotor cells of the superior colliculus mediate latency bimodality.     

Evidence for a global scanpath strategy in viewing abstract compared with realistic images

Scanpaths, the repetitive sequences of saccadic eye movements, occurred when subjects viewed slide projections of both realistic and abstract art. Variance analysis demonstrated that global/local eye movement indices were lower for local scanning by professional art viewers who relied on more global viewing, particularly in abstract images. Non-professional, unsophisticated subjects carried their local scanpath patterns from realistic images on to abstract images. The blink rate of professional subjects viewing abstract images was also significantly lower, indicating increased visual effort. Non-professional viewers showed no difference in blink rates.     

The effect of photon noise on the detection of white flashes

Thresholds for detecting brief, white, foveal test flashes drop abruptly within 0.2 sec of the offset of a white adapting field. The magnitude of the abrupt drop is proportional to the square root of field intensity (square root of I) correct for bleaching and dark light. Thresholds are then stable out to 1.6 sec for 200 msec tests, or recover only slightly for 20 msec tests. These results exclude some simple deterministic models in which Weber-like gain controls in the luminance pathway are assumed to recover exponentially in the dark, but can be explained parsimoniously if turning off the field abolishes photon-driven noise, improving the S/N ratio while leaving visual responsivity virtually unaltered. This theory was first put forward by Krauskopf and Reeves [(1980) Vision Research, 20, 193-196] for S-cone thresholds; it implies that the Weber law for increment thresholds is not due to a single gain control, but rather expresses the product of two distinct square root of I factors, adjustment of responsivity and photon-driven noise. Removal of the noise, not recovery of gain, permits thresholds to fall in early dark adaptation.     

Auditory stimulus detection is not suppressed during saccadic eye movements

Auditory detection and pitch discrimination thresholds were measured during saccades and during periods of fixation. The accuracy of auditory localisation under these two conditions was also measured. These thresholds were unaffected by whether the sound was presented during a saccade or during periods of steady fixation: there is no evidence for saccadic suppression of auditory processing.     

Saccadic suppression: A review and an analysis.

Reviews the history of saccadic suppression and describes the suggested causes of the phenomenon. Primary emphasis is placed on "ordinary" saccadic suppression (i.e., decreased sensitivity for stimulation received from the outside environment either during or within a few hundred milliseconds of the time of occurrence of a saccade). However, the suppression of afterimages and entopic images is also considered. Some suggestions are made about the role of suppression in maintaining a stable visual world (constancy of visual direction) when voluntary saccades occur. (98 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Binocular coordination in reading.

Measured the differences in time between the 2 eyes, using laboratory real-time computer methods to detect and measure the time between velocity peaks of binocular saccadic movements in reading. The hypothesis was that instead of being completely conjugate as indicated by prior methods of ocular measurement, the eyes must be coordinated in directional motion by small time differences that govern their feedback guidance and relative velocity. Results with 3 Ss indicate that the time differences between the eyes clustered around 3 values: (a) near synchrony, including no difference and left eye leading by 1 msec.; (b) left eye leading by 7-9 msec.; and (c) left eye leading by 14 msec. These time differences were not related to the difficulty of the reading material, but were changed significantly by 15– horizontal rotation of the reading display. Results change the established views that the eyes are perfectly conjugate in saccadic motion and provide initial suggestive data toward a dynamic feedback doctrine of coordinate eye motion and functional disabilities in visual perception. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Attentional and perceptual factors affecting the attentional blink for faces and objects.

When 2 different visual targets presented among different distracters in a rapid serial visual presentation (RSVP) are separated by 400 ms or less, detection and identification of the 2nd targets are reduced relative to longer time intervals. This phenomenon, termed the attentional blink (AB), is attributed to the temporary engagement of a limited-capacity attentional system by the 1st target, which reduces resources available for processing the 2nd target. Although AB has been reliably obtained with many stimulus types, it has not been found for faces (E. Awh et al., 2004). In the present study, the authors investigate the underpinnings of this immunity. Unveiling circumstances in which AB occurs within and across faces and other categories, the authors demonstrate that a multichannel model cannot account for the absence of AB effects on faces. The authors suggest instead that perceptual salience of the face within the distracters' series as well as the available resources determine whether or not faces are blinked in RSVP. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

An earth-stationary perceived visual scene during roll and yaw motions in a flight simulator

A flight simulator was used for two experiments to determine the amplitude combinations of visual scene motion (with respect to the observer) and inertial body motion (with respect to an earth-fixed frame) that provide the perception of an earth-stationary visual scene and realistic simulated self-motion. In the first experiment, this range was determined for simulated self-motion about the longitudinal body axis, while in the second, self-motion about the vertical body axis was considered. Both the inertial and the visual motions consisted of 0.75 a accelerations, followed by 1.50 s decelerations, and 0.75 s accelerations. The visual scene acceleration amplitude, W, was fixed at either 0, 2, 4, 8, or 12 degrees/s2 while the inertial acceleration amplitude, I, was varied by a staircase procedure. Following the visual and inertial motions, the subjects pushed a button when they perceived the scene to be not earth-stationary. At each visual scene acceleration amplitude, the lower and upper inertial threshold amplitudes were determined, which bounded the range in which the visual scene was perceived to be earth-stationary. The lower and upper inertial thresholds were defined as the inertial motion amplitudes for which the inertial stimulations were too small or too large, respectively, to provide the perception of an earth-stationary visual scene. The lower inertial thresholds were determined for W = 2 through W = 12 degrees/s2 and were found to be well approximated by the linear relation I = -0.37 + 0.60 W for the roll motions tested, and I = 1.1 + 0.33 W for the yaw motions tested. The upper inertial thresholds were determined for W = 0 through W = 12 degrees/s2 and were found to be well approximated by the linear relation I = 2.7 + 1.7 W for roll and I = 2.2 + 1.4 W for yaw. With the assumption that the lower and upper inertial threshold amplitudes are symmetric about the W = 0 condition, the present results infer a strong nonlinearity of the thresholds near W = 0.     

A neurophysiologic correlate of visual short-term memory in humans

Neurophysiological investigations in non-human primates and neuropsychological studies in patients with lesions indicate that the inferotemporal cortex is critically involved in visual object recognition. We have recorded event-related potentials from 31 electrodes in a group of healthy normal individuals performing a modified delayed matching to sample task. We have identified a visual memory potential (VMP) which indexes visual short-term memory in humans. This component of the event-related brain potential occurs as early as 170 msec, is maximal at 240 msec, and is generally located in the temporal region. Both the temporal and spatial characteristics of the VMP in humans are in keeping with single cell studies in monkeys.     

Peripheral shift reduces visual sensitivity in cat geniculate neurones

The sudden displacement of the retinal image during a saccade raises the visual threshold of human observers to foveal stimuli. The fall in visual sensitivity observed during this phenomenon, known as saccadic suppression, seems to occur very early in the visual processing chain. The lateral geniculate nucleus (LGN) is a likely locus for the multiple retinal and extraretinal interactions occurring during saccadic eye movements, therefore we used the responses of relay cells of adult cats to simulate a psychophysical experiment. We first measured the responses of X and Y relay cells (27 X and 13 Y) to central spots of optimal size and different contrasts. The spots were presented either alone or time locked with the rapid movement of a large, high-contrast peripheral pattern, referred to as shift. We measured the percentage of trials on which the relay cell fired more spikes when the spot (contrast: 0.03-1.0) was present than when it was absent. In experiments with human observers the task was to indicate, by a keypress, which of two otherwise identical temporal intervals contained the spot. The shift reduces the sensitivity (raises the contrast threshold) of neurones in the cat relay cells to brief, stationary targets presented to the receptive-field center. The suppression of visual sensitivity is significantly greater in Y cells than in X cells (average sensitivity ratios 5.6 +/- 5.4 in Y cells, 1.59 +/- 0.9 in X cells: P < 0.001, U test). The shift also reduces the sensitivity of human observers to the same target. This suggests that the LGN is a potential locus for the modulation of visual responses that leads to saccadic suppression.     

Echo suppression and discrimination suppression aspects of the precedence effect

The precedence effect is a phenomenon that may occur when a sound from one direction (the lead) is followed within a few milliseconds by the same or a similar sound from another direction (the lag, or the echo). Typically, the lag sound is not heard as a separate event, and changes in the lag sound's direction cannot be discriminated. The hypothesis is proposed in this study that these two aspects of precedence (echo suppression and discrimination suppression) are at least partially independent phenomena. Two experiments were conducted in which pairs of noise bursts were presented to subjects from two loudspeakers in the horizontal plane to simulate a lead sound and a lag sound (the echo). Echo suppression threshold was measured as the minimum echo delay at which subjects reported hearing two sounds rather than one sound; discrimination suppression threshold was measured as the minimum echo delay at which subjects could reliably discriminate between two positions of the echo. In Experiment 1, it was found that echo suppression threshold was the same as discrimination suppression threshold when measured with a single burst pair (average 5.4 msec). However, when measured after presentation of a train of burst pairs (a condition that may produce "buildup of suppression"), discrimination suppression threshold increased to 10.4 msec, while echo suppression threshold increased to 26.4 msec. The greater buildup of echo suppression than of discrimination suppression indicates that the two phenomena are distinct under buildup conditions and may be the reflection of different underlying mechanisms. Experiment 2 investigated the effect of the directional properties of the lead and lag sounds on discrimination suppression and echo suppression. There was no consistent effect of the spatial separation between lead and lag sources on discrimination suppression or echo suppression, nor was there any consistent difference between the two types of thresholds (overall average threshold was 5.9 msec). The negative result in Experiment 2 may have been due to the measurements being obtained only for single-stimulus conditions and not for buildup conditions that may involve more central processing by the auditory system.     

Effects of stimulus modality and task condition on blink startle modification and on electrodermal responses

Participants in Experiments 1 and 2 performed a discrimination and counting task to assess the effect of lead stimulus modality on attentional modification of the acoustic startle reflex. Modality of the discrimination stimuli was changed across subjects. Electrodermal responses were larger during task-relevant stimuli than during task-irrelevant stimuli in all conditions. Larger blink magnitude facilitation was found during auditory and visual task-relevant stimuli, but not for tactile stimuli. Experiment 3 used acoustic, visual, and tactile conditioned stimuli (CSs) in differential conditioning with an aversive unconditioned stimulus (US). Startle magnitude facilitation and electrodermal responses were larger during a CS that preceded the US than during a CS that was presented alone regardless of lead stimulus modality. Although not unequivocal, the present data pose problems for attentional accounts of blink modification that emphasize the importance of lead stimulus modality.     

Saccadic reaction times: a statistical analysis of multimodal distributions

The distributions of saccadic reaction times (SRT) often deviate from unimodal normal distributions. An excess-mass procedure was used to detect peaks in 963 data sets containing 90,927 reaction times from 170 subjects. About 55% showed one, 30% two, 12% three and 3% four peaks. According to their clustering along the reaction time scale the modes could be classified into express (90-120 msec), fast regular (135-170 msec) and slow regular (200-220 msec) modes. Among the unimodal distributions 29% had peaks in the range of the express mode and 46% had peaks in the range of the fast regular mode. Therefore, 87% of the data sets support the notion of saccadic reaction time distributions being the superposition of three modes. All experimental distributions were fitted by as many gamma distributions as determined by the excess-mass test. The significance of the multimodality for saccade generation processes is discussed.     

Tracking of small objects in front of a textured background by insects and vertebrates: phenomena and neuronal basis

To follow visually a small object moving in front of a textured background, insects and vertebrates can employ a similar strategy: saccadic tracking. In the case of vertebrates, the neural components that generate this behavior are not known in detail. The neural substrate of optomotor behavior in Diptera is relatively well understood. Here a model developed from the dipteran data is found to be capable of saccadic tracking. It is characterized by the following components and functions: (1) Two subsystems contribute to the response, a small-field tracking system and a large-field compensatory optomotor system, as suggested previously (Egelhaaf et al. 1988). (2) Both systems need to be suppressed during saccadic rotation. In the small-field system, the suppression, close to the visual input, is mediated by the activity of the large-field system. In the large-field system, suppression, close to the motor output, is due to efferent signals from the saccade generator. A similar model could also apply to vertebrates. Two implications of the present model are that saccadic tracking does not require object identification, and under saccadic tracking it is the background rather than the object that is stabilized on the retina. If objects are identified under these conditions, this must occur even though their image is not stabilized on the retina.     

Spatiotemporal effects of syncytial heterogeneities on cardiac far-field excitations during monophasic and biphasic shocks

INTRODUCTION: It has recently been postulated that syncytial (anatomic) heterogeneities inherent within cardiac tissue might represent a significant mechanism underlying field-induced polarization of the bulk myocardium. This simulation study examines and characterizes the spatiotemporal excitatory dynamics associated with this newly hypothesized mechanism. METHODS AND RESULTS: Two-dimensional regions of syncytially heterogeneous cardiac tissue were simulated with active membrane kinetics. Heterogeneities were manifested via random spatial variations of intracellular volume fractions over multiple length scales. Excitation thresholds were determined for uniform rectangular monophasic (M) and symmetric biphasic (B) far-field stimuli, from which strength-duration and strength-interval relationships were constructed. For regions measuring 5.4 x 5.4 mm, baseline diastolic thresholds for longitudinal (L) and transverse (T) shocks of 5-msec total duration averaged (in V/cm, n = 10) M-L = 2.87+/-0.26, M-T = 6.71+/-0.83, B-L = 3.22+/-0.25, and B-T = 7.93+/-0.51. These thresholds decreased by 15% to 25% when the region sizes were increased to 10.8 x 10.8 mm. Strength-duration relationships correlated strongly with the Weiss-Lapicque hyperbolic relationship, with rheobases and chronaxies of 2.33 V/cm and 1.15 msec for M-L stimuli, and 2.28 V/cm and 2.04 msec for B-L stimuli. Strength-interval relationships for M-L and B-L stimuli decreased monotonically with increasing coupling intervals, with similar minimum coupling intervals at absolute refractoriness. However, the B-L thresholds were substantially less sensitive to changes in coupling intervals than their M-L counterparts. CONCLUSION: This study provides strong additional support for and understanding of the syncytial heterogeneity hypothesis and its manifested properties. Furthermore, these results predict that syncytial heterogeneities of even modest proportions could represent a significant mechanism contributing to the far-field excitation process.     

Attentional capture by emotional stimuli is modulated by semantic processing.

The attentional blink paradigm was used to examine whether emotional stimuli always capture attention. The processing requirement for emotional stimuli in a rapid sequential visual presentation stream was manipulated to investigate the circumstances under which emotional distractors capture attention, as reflected in an enhanced attentional blink effect. Emotional distractors did not cause more interference than neutral distractors on target identification when perceptual or phonological processing of stimuli was required, showing that emotional processing is not as automatic as previously hypothesized. Only when semantic processing of stimuli was required did emotional distractors capture more attention than neutral distractors and increase attentional blink magnitude. Combining the results from 5 experiments, the authors conclude that semantic processing can modulate the attentional capture effect of emotional stimuli. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of co-stimulation in CD8+ T cell activation

This study was undertaken to determine whether dl-sotalol can prevent ventricular tachyarrhythmia inducibility that can be predicted from electrophysiologic parameters. The effects of dl-sotalol in 16 patients (ventricular tachycardia (VT) in 11 and fibrillation (VF) in 5) were determined in electrophysiologic studies before and after dl-sotalol (320 mg/day). In 9 of 16 patients (56%) after dl-sotalol, ventricular tachyarrhythmia could not be induced by the entire stimulation protocol (responders). There were significant differences in QT interval (462 +/- 52 vs. 415 +/- 34 msec; p < 0.05) and ventricular effective refractory period (VERP) at 600, 400 and 300 msec (302 +/- 28 vs. 262 +/- 20 msec; p < 0.001, 280 +/- 23 vs. 240 +/- 21 msec; p < 0.001, 256 +/- 24 vs. 222 +/- 12 msec; p < 0.005, respectively) between responders and non-responders. The percentile increases in VERP (% VERP) at 600, 400, and 300 msec in responders were 25%, 26%, and 27%, whereas those in non-responders was 9%, 7%, and 7%, respectively. Isoproterenol administered to responders did not fully reverse the dl-sotalol-induced prolongation of VERP (delta VERP) at 600, 400, and 300 msec, which remained significantly prolonged compared to the baseline (281 +/- 18 vs. 241 +/- 16 msec; p < 0.01, 258 +/- 20 vs. 223 +/- 21 msec; p < 0.01, 247 +/- 22 vs. 202 +/- 16 msec; p < 0.01, respectively). % VERP did not exhibit significant differences at 600 (16%), 400 (15%), and 300 (20%) msec, indicating the lack of a reverse use-dependency. The results suggest that delta VERP in responders did not show reverse use-dependency, and that the phenomenon may account for the efficacy of dl-sotalol.     

A behavioral study of temporal processing and visual persistence in young and aged rats.

Young rats were presented with light flash prepulses varying in duration from 1 to 128 msec, with light offset or light onset fixed at 70 msec prior to an acoustic startle stimulus (Experiment 1A), and, with single or paired 1-msec flashes, the 2nd (or only) flash given 100 to 500 msec before the startle, and 1 msec to 400 msec interflash intervals (Experiment 1B). Older rats (10 and 20 mo old) received the same single and double flashes but with the maximum interflash interval extended to 1,500 msec (Experiment 2). Reflex inhibition increased with increased duration from 1 to 8 msec and decreased as light onset progressively exceeded 100 msec. Inhibition for both single and double flashes also declined for onset lead times beyond 100 msec, then increased for a double flash once the interflash interval exceeded 100 msec in young and middle-aged rats and 1,500 msec in the oldest rats. Peak inhibition was much reduced in the oldest rats at short lead times but was greater than that of younger rats at long lead times. These data suggest that aged rats process visual stimuli more slowly than younger rats and show poorer temporal acuity coupled with greater visual persistence. (PsycINFO Database Record (c) 2010 APA, all rights reserved)