Temporal integration of acoustic stimulation obtained in reflex inhibition in rats and humans.

Investigated whether inhibition provided by initial stimuli of various durations conforms to established temporal integration functions. Initial stimuli (S1) were noise bursts varying in duration (2, 20, or 200 msec) and intensity (55 or 85 db). Eliciting stimuli (S2) for 6 Holtzman rats were intense tone bursts, which elicited the acoustic startle reflex, and for 9 19–24 yr old humans were electrotactile stimuli to the forehead, which elicited the eye blink. Findings reveal that inhibition was greater with the 85-db S1 stimulus and increased linearly with log increases in duration. Data suggest that the acoustic substrate for reflex inhibition has a long-time constant. There was one exception to this general finding. For 7 human Ss, inhibition declined when the duration of the 85-db S1 was increased from 20 to 200 msec. Postexperimental questioning and video monitoring suggest that this anomaly resulted from a reflex-enhancing arousal process. (38 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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)     

Temporal acuity in auditory function in the rat: Reflex inhibition by brief gaps in noise.

Previous results show that the acoustic startle reflex in the rat is inhibited if a relatively weak stimulus precedes the startle-eliciting tone burst. The present 5 experiments explored the effect of brief silent periods (gaps) in white noise on the startle reflex in order to describe the limits of temporal resolution in the auditory system of 12 Long-Evans hooded rats. Brief silent periods did depress reflex behavior, and 2 responsible processes were identified. One was most evident at a 190-msec lead time between gap and startle tone. It yielded a linear decrement in reflex expression over a dynamic range of 0–7 msec and an estimate for the threshold of temporal acuity of 3.5 msec. The 2nd was evident primarily at a 40-msec interstimulus interval and had a linear effect over a dynamic range of at least 40 msec. Brief gaps had a greater inhibitory effect at the 190-msec interval between gap and startle stimulus; prolonged gaps had their greater effect at the 40-msec interval. The 1st process was identified as reflex inhibition, which is sensitive to the sensory properties of the lead stimulus. The 2nd process was identified as sensory adaptation, produced by noise exposure but unmasked by silence. (47 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential conditioning of the rabbit's nictitating membrane response to serial compound stimuli.

Conducted 3 experiments with 213 male and female albino rabbits (Orytolagus cuniculus) to determine the time course and contents of conditioned stimulus/stimuli (CS) representations through an examination of differential conditioning of the S"s nictitating membrane response to 2 serial compounds. One compound (A-X+) was always paired with the unconditioned stimulus/stimuli (UCS), and the other (B-X–) was always presented alone. All 3 experiments entailed manipulation of the interstimulus interval between the initial distinctive element of each compound (A and B) and the 2nd shared element (X). The joint results reveal that (1) conditioned response (CR) acquisition to the initial elements depended on the presence of X in the A-X+ compound; (2) differentiation between A and B appeared across interstimulus intervals up to 4,600 msec; and (c) conditional control over responding following A and B appeared at interstimulus intervals of at least 4,600 msec and perhaps up to 12,600 msec. (51 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Prepulse inhibition of acoustic or trigeminal startle of rats by unilateral electrical stimulation of the inferior colliculus.

Transient electrical stimulation of the inferior colliculus (IC) of adult male rats had a strong and long-lasting inhibitory effect on startle responses elicited by either intense noise bursts or unilateral electrical stimulation of the principal trigeminal nucleus. Startle elicited by noise bursts was inhibited over a wide range of interstimulus intervals (ISIs) with the maximum inhibition at ISIs between 15 and 30 msec. Startle elicited by trigeminal stimulation was inhibited more sharply than acoustic startle, with the maximum inhibition at ISIs between 20 and 35 msec. These data support the view that the IC is a critical part of the pathway mediating prepulse inhibition (D. S. Leitner & M. E. Cohen, 1985). More important, the data reveal the time course of the inhibitory influence of the IC on startle and indicate that the inhibitory effects of IC stimulation have higher temporal resolution on trigeminal startle than on acoustic startle. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Plasticity of the acoustic startle response in the acutely decerebrate rat.

In 4 experiments, plasticity of the acoustic startle reflex was measured in male albino Sprague-Dawley rats in which a complete transection between the forebrain and midbrain was made. During a period from 60 to 100 min after surgery, startle amplitude in the transected Ss was relatively stable and comparable with that of controls (anesthetized with halothane and placed in a stereotaxic instrument). During this period the transection did not alter the temporal recovery process (with intervals of 2, 4, 8, or 16 sec) or auditory prepulse inhibition (with intervals of 25, 50, 100, 500, or 1,000 msec) or the normal reduction in startle caused by high levels of background noise. The transection did prevent the normal increase in startle caused by moderate levels of background noise and eliminated within-session habituation. The effect on habituation was particularly noticeable since the curves of the transected and nontransected Ss actually crossed. Results are dicussed in terms of how the transection procedure can be used to evaluate various hypotheses about underlying mechanisms of startle plasticity. (50 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal integration in the acoustic startle reflex of the rat.

Exposed 6 male albino rats to 6,900 Hz tone pulses with durations of .25-64 msec. and intensities of 80-125 db. re .0002 dyne/cm2. Startle amplitudes increased with duration in a manner suggestive of an exponential function distorted by the failure of the auditory system to reflect faithfully prolonged intense sounds. 5 other male albino rats were exposed to pairs of 1-msec pulses at 125 db., having interpulse intervals ranging 1-11 msec. For pulses separated by 3 msec. or more, response amplitudes demonstrated a decaying exponential summation function with a time constant of 3 msec. Results are in accord with established principles of reflex function and organization. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

"Temporal summation decay" in hypothalamic self-stimulation: Threshold changes at long intrapair intervals due to axonal subnormal periods.

Rats were trained to bar press for trains of conditioning (C) and test (T) pulses delivered via lateral hypothalamic electrodes. As intrapair (C–T) intervals increased from 10 msec to 100 msec, the frequency of pulses required for self-stimulation increased, similar to results of N. S. Smith and E. E. Coons (1970). This effect was observed only for electrode placements where self-stimulation was obtained at frequencies below 16 Hz and currents of 600 μA and higher. The effect was larger when the train duration was increased from 0.5 sec to 2.0 sec. The threshold increase was abolished when the T pulses were greater in current than the C pulses but not when C pulses were larger than T pulses. The larger T pulses also removed relative refractory period effects at a C–T interval of 1.0 msec. Therefore, the increase in required current or frequency at long C–T intervals appears to be due to a decline in axonal excitability (i.e., the subnormal period) rather than a decay in synaptic temporal summation. Possible flaws in other reports of paired-pulse "temporal summation decay" at long C–T intervals using 2 electrodes are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The paired-pulse index: a measure of hippocampal dentate granule cell modulation

This study was undertaken to assess whether the paired-pulse index (PPI) is an effective measure of the modulation of dentate granule cell excitability during normal development. Paired-pulse stimulations of the perforant path were, therefore, used to construct a PPI for 15-, 30-, and 90-day old, freely moving male rats. Significant age-dependent differences in the PPI were obtained. Fifteen-day old rats showed significantly less inhibition at short interpulse intervals [interpulse interval (IPI): 20 to 30 msec), a lack of facilitation at intermediate IPIs (50 to 150 msec), and significantly less inhibition at longer IPIs (300 to 1,000 msec) than adults.     

Very short-term plasticity in hippocampal synapses

Hippocampal pyramidal neurons often fire in bursts of action potentials with short interspike intervals (2-10 msec). These high-frequency bursts may play a critical role in the functional behavior of hippocampal neurons, but synaptic plasticity at such short times has not been carefully studied. To study synaptic modulation at very short time intervals, we applied pairs of stimuli with interpulse intervals ranging from 7 to 50 msec to CA1 synapses isolated by the method of minimal stimulation in hippocampal slices. We have identified three components of short-term paired-pulse modulation, including (i) a form of synaptic depression manifested after a prior exocytotic event, (ii) a form of synaptic depression that does not depend on a prior exocytotic event and that we postulate is based on inactivation of presynaptic N-type Ca2+ channels, and (iii) a dependence of paired-pulse facilitation on the exocytotic history of the synapse.     

Reflex-inhibiting stimuli and the refractory period of the acoustic startle reflex in the rat.

Previous studies show that if acoustic startle stimuli are presented in pairs, then the reaction to the 2nd stimulus (S2) is reduced, with the size of the refractory decrement determined by the interstimulus interval and the relative intensity of the 1st stimulus (S1). If a neutral stimulus (p) is presented just prior to S1, then the reaction to S1 is similarly inhibited, revealing the phenomenon of prestimulus inhibition. In 2 experiments with male albino Holtzman rats (N = 24) it was found that suppression of the reflex to S2 by S1 was unaffected by prestimulus inhibition of S1 (i.e., reflex amplitudes associated with S2 were identical in pS1-S2 series and S1-S2 series). In contrast, a reduction in the intensity of S1 relative to S2 did reduce the effect of S1 on S2. These data indicate that prestimulus inhibition of the reflex to S1 does not result because the preliminary stimulus attenuates the sensory impact of S1. The inhibitory process may be presumed to have a central locus. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Where and when to pay attention: the neural systems for directing attention to spatial locations and to time intervals as revealed by both PET and fMRI

Although attention is distributed across time as well as space, the temporal allocation of attention has been less well researched than its spatial counterpart. A temporal analog of the covert spatial orientation task [Posner MI, Snyder CRR, Davidson BJ (1980) Attention and the detection of signals. J Exp Psychol Gen 109:160-174] was developed to compare the neural systems involved in directing attention to spatial locations versus time intervals. We asked whether there exists a general system for allocating attentional resources, independent of stimulus dimension, or whether functionally specialized brain regions are recruited for directing attention toward spatial versus temporal aspects of the environment. We measured brain activity in seven healthy volunteers by using positron emission tomography (PET) and in eight healthy volunteers by using functional magnetic resonance imaging (fMRI). The task manipulated cued attention to spatial locations (S) and temporal intervals (T) in a factorial design. Symbolic central cues oriented subjects toward S only (left or right), toward T only (300 msec or 1500 msec), toward both S and T simultaneously, or provided no information regarding S or T. Subjects also were scanned during a resting baseline condition. Behavioral data showed benefits and costs for performance during temporal attention similar to those established for spatial attention. Brain-imaging data revealed a partial overlap between neural systems involved in the performance of spatial versus temporal orientation of attention tasks. Additionally, hemispheric asymmetries revealed preferential right and left parietal activation for spatial and temporal attention, respectively. Parietal cortex was activated bilaterally by attending to both dimensions simultaneously. This is the first direct comparison of the neural correlates of attending to spatial versus temporal cues.     

Stimulus selection as a function of CS1-CS2 interval in compound classical conditioning of cats.

Investigated stimulus selection as a function of the time between the onsets of the longer and shorter elements of a compound stimulus (tone and light). The paw flexions and galvanic skin responses of 48 male and spayed female cats to shock were classically conditioned. With modality counterbalanced, each of 3 groups of Ss experienced a different CS1-CS2 interval: 2,000, 500, and 150 msec. The CS2 always preceded shock by 500 msec; all stimuli terminated together. After 8 wks of conditioning, Ss were tested to either the long or the short stimulus alone. Response strengths of the elements were clearly functions of the CS1-CS2 intervals, the shorter stimulus being more effective at the 2,000 and 150 msec intervals, the longer stimulus at the 500. Redundancy interpretation cannot account for all results. (22 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Suppression of an inhibitory jaw reflex by the anticipation of pain in man

Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 15 human subjects. In control sequences, the reflex had a mean latency and duration (+/- S.E.M.) of 45.4 +/- 1.3 msec and 47.9 +/- 2.8 msec, respectively. Significant decreases in the reflex as well as increases in heart rate and anxiety levels assessed by a visual analogue scale, occurred when the subjects were stressed by the anticipation of receiving painful electrical stimuli above the ankle (P < 0.00005; Student's t-tests). During such sequences, the magnitude of the reflex measured by integration of the EMG, was reduced by 47.7 +/- 5.6%. This effect involved a reduction in both the duration and depth of the inhibitory wave. It occurred regardless of whether the painful stimuli were applied during or after the recording of the reflex and of whether the baseline activity in the muscle was inadvertently raised or lowered during the stressful sequences. It is concluded that stress induced by the anticipation of pain, can markedly reduce an inhibitory jaw reflex in man by exerting an influence on the reflex pathway prior to the motoneurones.     

Perceived numerosity: An information processing analysis.

Presented 3 university students with matrices of from 1 to 16 dots and asked them to count the dots they could see. In Condition 1 a visual noise pattern followed the display of dots by one of several intervals in order to control the time available to process them. In Condition 2 no mask was used, but the exposure duration was varied. In the processing time condition, dots were counted at a rate of 4 msec/dot when less than 6 dots were presented, and at a rate of 60 msec/dot for all dots in excess of 6. If enough time was given to process all the dots presented, virtually all were reported, whereas, if enough time was given to count only some of the dots, virtually none of the excess were counted. In the duration condition, in which processing time was not restricted, counting appeared to be a function of the visibility of the dots, as shown by a family of more linear functions between number of dots presented and number counted, with the slope determined by the duration of exposure. Data are consistent with a serial processing interpretation of dot counting occurring at a very early stage of information extraction, in which there is a serial scanning mechanism which extracts information from an initial brief store and transfers it to a 2nd store for actual counting. (French summary) (18 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Persistence of negative priming: Steady state or decay?

Responses to recently ignored stimuli may be slower and less accurate than responses to new stimuli. W. T. Neill and R. L. Westberry (1987) found that such negative priming effects dissipated within a 2-sec interval between response and the next stimulus, the response–stimulus interval (RSI). However, experiments by S. P. Tipper et al, (1991) found negative priming persisted unchanged over RSIs from 1,350 to 6,600 msec. The current experiments used a letter-matching procedure in which target letters were flanked by irrelevant letters. Negative priming was manifested by longer reaction times (RTs) and more errors to letters that had appeared as flankers in the preceding trial. RSI was varied from 500 to 8,000 msec. Negative priming diminished over the RSI, particularly within the 1st sec. This effect did not depend on the presence or absence of temporal uncertainty or on Ss' awareness of intertrial relations. An episode retrieval theory to account for negative priming phenomena is proposed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Activity of motoneurons during fictitious scratch reflex in the cat

(1) Intracellular recording of motoneurons of different hindlimb muscles: tibialis anterior (TA), gastrocnemius and soleus (GS), vastus crureus (Vast), posterior biceps and semitendinosus (PBSt), was carried out during the fictitious scratch reflex in decerebrate cats. (2) During the postural stage of the reflex, a depolarizaiton (3.8 mV on average) was observed in TA motoneurons accompanied by tonic discharge. No change of the membrane potential (MP) and no discharge were observed during this stage in GS, Vast and PBSt motoneurons. (3) In the rhythmical stage of the reflex, the MP of TA motoneurons changed only slightly during the 'long' (L) phase of the scratch cycle and remained at approximately the same level as during the postural stage. In this phase, motoneurons discharged at frequencies of 20-100 pps. In the 'short' (S) phase of the scratch cycle a strong repolarization occurred, the MP reached the same level as observed during resting conditons (MP0), and the discharge discontinued. (4) GS motoneurons were gradually depolarized during the second half of the L-phase. The depolarization reached its maximum (5.5 mV on average in relation to the MP0) on average in relation to the MP0) in the S-phase, and several action potentials were generated with intervals of 5-10 msec. Then, at the beginning of the L-phase, the motoneurons were repolarized and the MP reached the level of the MP0. The behavior of Vast motoneurons was essentially similar to that of GS motoneurons. (5) The PBSt motoneurons usually had two peaks of depolarization per cycle--in the S-phase and at the beginning of the L-phase. The maximal depolarization was 3.5 mV (on average). The motoneurons generated action potentials at one or both peaks of depolarization. (6) The possible organization of the central influences upon motoneurons of different muscles during scratching is discussed.     

Four new mutations in the apolipoprotein B gene causing hypobetalipoproteinemia, including two different frameshift mutations that yield truncated apolipoprotein B proteins of identical length

Magnetic resonance imaging is frequently complicated by the presence of motion and susceptibility gradients. Also, some biologic tissues have short T2s. These problems are particularly troublesome in fast spin-echo (FSE) imaging, in which T2 decay and motion between echoes result in image blurring and ghost artifacts. The authors reduced TE in conventional spin-echo (SE) imaging to 5 msec and echo spacing (E-space) in FSE imaging to 6 msec. All magnetic gradients (except readout) were kept at a maximum, with data sampling as fast as 125 kHz and only ramp waveforms used. Truncated sinc radio-frequency pulses and asymmetric echo sampling were also used in SE imaging. Short TE (5.8 msec) SE images of the upper abdomen were compared with conventional SE images (TE = 11 msec). Also, FSE images with short E-space were compared with conventional FSE images in multiple body sites. Short TE significantly improved the liver-spleen contrast-to-total noise ratio (C/N) (7.9 vs 4.1, n = 9, P < .01) on T1-weighted SE images, reduced the intensity of ghost artifacts (by 34%, P < .02), and increased the number of available imaging planes by 30%. It also improved delineation of cranial nerves and reduced susceptibility artifacts. On short E-space FSE images, spine, lung, upper abdomen, and musculoskeletal tissues appeared crisper and measured spleen-liver C/N increased significantly (6.9 vs 4.0, n = 12, P < .01). The delineation of tissues with short T2 (eg, cartilage) and motion artifact suppression were also improved. Short TE methods can improve image quality in both SE and FSE imaging and merit further clinical evaluation.     

Negative priming for spatial location?

Examined negative priming for spatial location in 2 studies. Study 1 involved combinations of target, distractor, or both, across prime and probe, being presented once to each S in a negative priming for spatial location procedure. Specifically, stimuli were presented using an oscilloscope controlled by a computer system, and the fixation display appeared immediately after a foot pedal was depressed. After 500 msec, the prime array was added to the fixation display until the S responded (depressing the key corresponding to the location of the target). In the 1st Exp, the procedure was examined across a number of Ss (12 university students; aged 20–30 yrs). In the 2nd Exp, the procedure was tested over repeated sessions with 1 S (university student; aged 23 yrs) on consecutive days. Study 2 verified the results in 13 university students. The findings suggest that negative priming in the spatial location procedure may be more closely related to inhibition of return, or to the automatic attraction of attention by new objects, than to the concepts of distractor inhibition, episodic retrieval, and feature mismatch which have traditionally been used to explain negative priming for spatial location. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dispersion of the Q-T interval after myocardial infarct

Non-homogenity of ventricular myocardial repolarization is a substrate for the reentry mechanism of ventricular arrhythmias. It is manifestant by dispersion of Q-T and Q-Tc intervals on the standard ECG curve. The authors studied the possibility of using the dispersity of Q-T and Q-Tc intervals in clinical practice. They evaluated the dispersion of these intervals within the set of 21 patients after myocardial infarction with sustained ventricular tachycardia, and compared it with the dispersion within the control set of 17 patients after myocardial infarction without an arrhythmic episode. By means of comparison, they have discovered that: 1) the dispersion of Q-T and Q-Tc intervals is significantly higher in patients with ventricular tachycardia: Q-T (mean +/- SE) 82.8 +/- 7.8 msec vs 42.2 +/- 4.8 msec, Q-Tc 93.0 +/- 10.2 msec vs 47.1 +/- 4.8 msec, p > 0.001, 2) the dispersion of Q-Tc when higher than 60 msec is an optimum discrimination value for the prognosis of sudden arrhythmic death after myocardial infarction (sensitivity 81%, specificity 76%) and 3) the dispersion of Q-T and Q-Tc intervals has no relation to the function of the left ventricle. Therefore the authors consider the dispersion of Q-T and Q-Tc intervals as being a useful marker of malignant ventricular arrhythmia which could be included into the algorithm of assessment of the risk of sudden arrhythmic death after myocardial infarction.