Long-duration Stimuli Research Articles

Assessment of local cone on- and off-pathway function using multifocal ERG technique.

There is evidence that recording the photopic ON- and OFF-responses with long-duration stimuli is useful for determining the contribution of the cone ON- and OFF-pathways to the primate photopic electroretinogram (ERG). In this study, the optimal conditions for recording multifocal ON-OFF responses are described, and the technique is applied to normal subjects and two patients with unusual retinal diseases. The results from the normal subjects demonstrated that there were topographical variations of the photopic ERG waveform: when responses were normalized to the ON-response (b-wave) amplitude, the OFF-response (d-wave) amplitude increased with increasing eccentricity. The changes in the waveform in two patients suggested relatively greater defects of the hyperpolarizing or depolarizing bipolar cells. We conclude that the multifocal ERG technique with long-duration stimuli can be a useful tool to assess the function of local cone ON- and OFF-pathways in normal and diseased retinas.

Large repulsion, but not attraction, tilt illusions occur when stimulus parameters selectively favour either transient (M-like) or sustained (P-like) mechanisms

A vertical test grating appears tilted away from a surrounding inducing grating which is 15° from vertical (repulsion effect) but towards an inducer 75° from vertical (attraction effect). This is the tilt illusion (TI) and similar effects occur when inducing and test stimuli are presented successively (tilt after-effect or TAE). When it was reported [Wolfe, J. (1984). Vision Research, 24, 1959–1964] that large repulsion TAEs occurred with short test flashes, Wolfe postulated that either there are distinct mechanisms which process brief and longer duration stimuli; or that there are distinct mechanisms which are not primarily concerned with duration but are differentially responsive to temporal parameters, amongst several others. Other evidence that TI attraction effects are not modulated by test flash duration resulted in an hypothesis that repulsion and attraction effects are mediated by transient and sustained mechanisms, respectively [Wenderoth, P., van der Zwan, R., & Johnstone, S. (1989). Perception, 18, 715–728]. We demonstrate that large repulsion TIs can be induced when parameters other than duration are manipulated, including contrast and spatial frequency but that these parameters fail to modulate attraction TIs. These results are consistent with some previous hypotheses regarding the origin of repulsion and attraction effects and with Wolfe’s latter hypothesis but do not support the view that the two effects are processed, respectively, by transient and sustained mechanisms.

Extraction of depth from opposite-contrast stimuli: transient system can, sustained system can’t

The ability of observers to extract depth from opposite luminance-contrast-polarity stimuli was investigated. The stimuli consisted of two dichoptic-pairs of Gaussians, with one of the Gaussians in each pair having a positive contrast-polarity and the other a negative contrast-polarity. Stimulus durations ranging from 0.2 to 4 s were used. This range of durations was employed to reveal stereo mechanisms that were preferentially sensitive to transient or sustained stimuli. Stimuli were presented in a raised-cosine temporal envelope. Performance with stimuli of the same contrast-polarity was also tested. Observers could easily perceive depth with the same-polarity stimuli, at both long and short durations. Depth could be perceived with low-contrast opposite-polarity stimuli only at short durations. However, depth could be perceived with long-duration stimuli presented within a raised cosine temporal-envelope if a high contrast was used. Depth could also be perceived with low-contrast long-duration stimuli if they were presented within a rectangular temporal-envelope. These findings suggest there are separate sustained and transient mechanisms for stereopsis and that the transient-stereoscopic system can extract depth from opposite-contrast stereograms while the sustained system cannot. Further, it is likely that depth perception with opposite-contrast stereograms found in many previous studies was mediated by the transient-stereopsis system.

Different types of calcium channels and secretion from bovine chromaffin cells.

Bovine chromaffin cells possess several types of Ca2+ channels, and influx of Ca2+ is known to trigger secretion. However, discrepant information about the relative importance of the individual subtypes in secretion has been reported. We used whole-cell patch-clamp measurements in isolated cells in culture combined with fura-2 microfluorimetry and pharmacological manipulation to determine the dependence of secretion on different types of Ca2+ channels. We stimulated cells with relatively long depolarizing voltage-clamp pulses in a medium containing 60 mM CaCl2. We found that, within a certain range of pulse parameters, secretion as measured by membrane capacitance changes was mainly determined by the total cumulative charge of Ca2+ inflow and the basal [Ca2+] level preceding a stimulus. Blocking or reducing the contribution of specific types of Ca2+ channels using either 20 microM nifedipine plus 10 microM nimodipine or 1 microM omegaCTxGVIA (omega-conotoxin GVIA) or 2 microM omegaCTxMVIIC (omega-conotoxin MVIIC) reduced secretion in proportion to Ca2+ charge, irrespective of the toxin used. We conclude that for long-duration stimuli, which release a large fraction of the readily releasable pool of vesicles, it is not so important through which type of channels Ca2+ enters the cell. Release is determined by the total amount of Ca2+ entering and by the filling state of the readily releasable pool, which depends on basal [Ca2+] before the stimulus. This result does not preclude that other stimulation patterns may lead to responses in which subtype specificity of Ca2+ channels matters.

Kinematics of eye movements of cats to broadband acoustic targets.

Operant conditioning was used to train cats with their heads immobilized to localize sound by directing their eyes to the location of the sources. The kinematics of those eye movements were studied and compared with eye movements to visual targets at the same locations. The main finding of this study is that eye movements to broadband long-duration acoustic targets have two components: an initial slow phase of variable duration and a fast, normal saccade. The slow component is characterized by a persistent, shallow velocity ramp, while the saccadic component of the response falls on the main sequence computed from eye movements to visual targets. The slow component was shorter before saccades to long-duration stimuli performed under the delayed-saccade task and practically absent before saccades to transient acoustic stimuli. The results suggest that the initial slow component is used by cats to deal with uncertainty associated with the location of long-duration broadband targets and that the input to the saccade integrator(s) is similar under both visual and acoustic conditions.

Peripheral encoding of behaviorally relevant acoustic signals in a vocal fish: single tones.

The midshipman fish, Porichthys notatus, generates acoustic signals for intraspecific communication. Nesting males produce long-duration "hums" which attract gravid females and can be effectively mimicked by pure tones. In this study we examine the encoding of tonal signals by the midshipman peripheral auditory system. Single-unit recordings were made from afferents innervating the sacculus while presenting sounds via an underwater loudspeaker. Units were characterized by iso-intensity spike rate and vector strength of synchronization curves, as well as by peristimulus time histograms. Additionally, response-intensity curves and responses to long-duration (up to 10 s) stimuli were obtained. As has been seen in other teleosts, afferents had highly variable activity profiles. Excitatory frequencies ranged from 60 to over 300 Hz with most units responding best around 70 or 140 Hz. Thresholds at 90 Hz ranged from 95 to 145 dB re 1 microPa. Strong synchronization provided a robust temporal code of frequency, comparable to that described for goldfish. Spike rate showed varying degrees of adaptation but high rates were generally maintained even for 10-s stimuli. The midshipman peripheral auditory system is well suited to encoding conspecific communication signals, but nonetheless shares many response patterns with the auditory system of other teleosts.

Neural substrates of the tilt illusion.

It has been suggested that direct and indirect tilt illusions and after-effects have different mechanisms, namely that the direct effects arise in VI and are sensitive to differences in spatial and temporal parameters between test and inducing stimuli, whereas indirect effects arise in extrastriate cortex and are insensitive to such parameters. When Wolfe (Vision Research 1984; 24: 1959-64) reported that large direct tilt after-effects occurred with short test flashes, he postulated that either there are distinct mechanisms which process brief and longer duration stimuli or that there are distinct mechanisms that are not primarily concerned with duration but are differentially responsive to temporal parameters amongst several others. In three experiments we demonstrate that large direct tilt illusions can be induced when parameters other than duration are manipulated, including contrast and spatial frequency, and that such large effects can occur when stimulus parameters are chosen to favour preferentially either the transient (magnocellular-like) system or the sustained (parvocellular-like) system. These results are thus consistent with Wolfe's second hypothesis. None of these stimulus manipulations had any effect on indirect tilt illusions, consistent with previous findings and hypotheses about the different mechanisms of the direct and indirect effects.

Temporal resolution and temporal masking properties of transient stimuli: data and an auditory model.

Temporal resolution is often measured using the detection of temporal gaps or signals in temporal gaps embedded in long-duration stimuli. In this study, psychoacoustical paradigms are developed for measuring the temporal encoding of transient stimuli. The stimuli consisted of very short pips which, in two experiments, contained a steady state portion. The carrier was high-pass filtered, dynamically compressed noise, refreshed for every stimulus presentation. The first experiment shows that, with these very short stimuli, gap detection thresholds are about the same as obtained in previous investigations. Experiments II and III show that, using the same stimuli, temporal-separation thresholds and duration-discrimination thresholds are better than gap-detection thresholds. Experiment IV investigates the significance of residual spectral cues for the listeners' performance. In experiment V, temporal separation thresholds were measured as a function of the signal-pip sensation level (SL) in both forward- and backward-masking conditions. The separation thresholds show a strong temporal asymmetry with good separation thresholds independent of signal-pip SL in backward-masking conditions and increasing separation thresholds with decreasing signal-pip SL in forward-masking conditions. A model of the auditory periphery is used to stimulate the gap-detection and temporal-separation thresholds quantitatively. By varying parameters like auditory-filter width and transduction time constants, the model provides some insight into how the peripheral auditory system may cope with temporal processing tasks and thus represents a more physiology-related complement to current models of temporal processing.

Impairment in preattentive visual processing in patients with Parkinson's disease.

We explored the possibility of whether preattentive visual processing is impaired in Parkinson's disease. With this aim, visual discrimination thresholds for orientation texture stimuli were determined in two separate measurement sessions in 16 patients with idiopathic Parkinson's disease. The results were compared with those of 16 control subjects age-matched and 16 young healthy volunteers. Discrimination thresholds were measured in a four-alternative spatial forced-choice paradigm, in which subjects judged the location of a target embedded in a background of distractors. Four different stimulus configurations were employed: (i) a group of vertical targets among horizontal distractors ('vertical line targets'); (ii) targets with varying levels of orientation difference on a background of spatially filtered vertically oriented noise ('Gaussian filtered noise'); (iii) one 'L' among 43 '+' signs ('texton'), all of which assess preattentive visual processing; and (iv) control condition, of one 'L' among 43 'T' distractors ('non-texton' search target), which reflects attentive visual processing. In two of the preattentive tasks (filtered noise and texton), patients with Parkinson's disease required significantly greater orientation differences and longer stimulus durations, respectively. In contrast, their performance in the vertical line target and non-texton search target was comparable to that of the matched control subjects. These differences were more pronounced in the first compared with the second session. Duration of illness and age within the patient group correlated significantly with test performance. In all conditions tested, the young control subjects performed significantly better than the more elderly control group, further indicating an effect of age on this form of visual processing. The results suggest that, in addition to the well documented impairment in retinal processing, idiopathic Parkinson's disease is associated with a deficit in preattentive cortical visual processing.

Excitation of Central Nervous System Neurons by Nonuniform Electric Fields

The goal of this study was to determine which neural elements are excited by microstimulation of the central nervous system. A cable model of a neuron including an axon, initial segment, axon hillock, soma, and simplified dendritic tree was used to study excitation with an extracellular point source electrode. The model reproduced a wide range of experimentally documented extracellular excitation patterns. The site of action potential initiation (API) was a function of the electrode position, stimulus duration, and stimulus polarity. The axon or initial segment was always the site of API at threshold. When the electrode was positioned near the cell body, the site of excitation was dependent on the stimulus amplitude. With the electrode in close proximity to the neuron, short-duration cathodic pulses produced lower thresholds with the electrode positioned over the axon than over the cell body, and long-duration stimuli produced opposite relative thresholds. This result was robust to alterations in either the maximum conductances or the intracellular resistivities of the model. The site of maximum depolarization was not always an accurate predictor of the site of API, and the temporal evolution of the changes in membrane potential played a strong role in determining the site of excitation.

Physiological studies of the precedence effect in the inferior colliculus of the cat. I. Correlates of psychophysics.

The precedence effect (PE) is experienced when two spatially separated sounds are presented with such a brief delay that only a single auditory image at or toward the location of the leading source is perceived. The responses of neurons in the central nucleus of the inferior colliculus (ICC) of cats were studied using stimuli that are known to elicit the PE, focusing on the effects of changes in stimulus conditions that a listener might encounter in a natural situation. Experiments were conducted under both free-field (anechoic chamber) and dichotic (headphones) conditions. In free field, the PE was simulated by presenting two sounds from different loudspeakers with one sound delayed relative to the other. Either click or noise stimuli (2- to 10-ms duration) were used. Dichotically, the same conditions were simulated by presenting two click or noise pairs separated by an interstimulus delay (ISD) with interaural time differences (ITDs) imposed separately for each pair. At long ISDs, all neurons responded to both leading and lagging sources as if they were delivered alone. As the ISDs were shortened, the lagging response became suppressed. The ISD of half-maximal suppression varied considerably within the population of neurons studied, ranging from 2 to 100 ms, with means of 35 and 38 ms for free field and dichotic conditions, respectively. Several correlates of psychophysical findings were observed in ICC neurons: suppression was usually stronger with lower overall stimulus level and longer duration stimuli. Suppression also was compared along the azimuth and elevation in free field by placing the lagging source at (0 degree,0 degree), which is common to both axes, and the leading sources at locations along either plane that generated similar discharge rates. All neurons that showed suppression along the azimuth also did so in the elevation. In addition, there was a high correlation in the ISD of half-maximal suppression along the two planes (r = 0.87). These findings suggest that interaural difference cues, which are robust along the horizontal axis but minimal in the median plane, are not necessary for neural correlates of the PE to be manifested. Finally, single-neuron responses did not demonstrate a correlate of build-up of suppression, a phenomenon whereby echo suppression accumulates with ongoing stimulation. This finding adds credibility to theories about the PE that argue for a "higher order" component of the PE.

Extraversion and the psychological refractory period

On the grounds that extraverts have a strong tendency to organize responses rather than analyze stimulus information (Brebner and Cooper, 1978) it was predicted that extraverts would respond more rapidly to the first of two rapidly successive stimuli than introverts. It was also hypothesized that the delay in responding to the second stimulus, which is termed the ‘psychological refractory period’, would be shorter for extraverts, and that using longer stimulus durations would lengthen reaction times but that extraverts would again be less affected than introverts. The results support the predictions and point to the importance of including individual differences in studies of the speed of performance.

Behavioral Studies of Sound Localization in the Cat

Using the magnetic search coil technique to measure eye and ear movements, we trained cats by operant conditioning to look in the direction of light and sound sources with their heads fixed. Cats were able to localize noise bursts, single clicks, or click trains presented from sources located on the horizontal and vertical meridians within their oculomotor range. Saccades to auditory targets were less accurate and more variable than saccades to visual targets at the same spatial positions. Localization accuracy of single clicks was diminished compared with the long-duration stimuli presented from the same sources. Control experiments with novel auditory targets, never associated with visual targets, demonstrated that the cats localized the sound sources using acoustic cues and not from memory. The role of spectral features imposed by the pinna for vertical sound localization was shown by the breakdown in localization of narrow-band (one-sixth of an octave) noise bursts presented from sources along the midsagittal plane. In addition, we show that cats experience summing localization, an illusion associated with the precedence effect. Pairs of clicks presented from speakers at (+/-18 degrees,0 degrees ) with interclick delays of +/-300 microsec were perceived by the cat as originating from phantom sources extending from the midline to approximately +/-10 degrees.

The c-wave of the electroretinogram recorded under clinical conditions from rabbits.

To determine whether large and repeatable c-waves can be recorded from rabbits with equipment already in use in clinical electroretinographic laboratories, the Burian-Allen electrode, connected bipolarly or monopolarly, was used to record electroretinograms from pigmented rabbits. The Jet electrode was also used. The c-waves elicited by long-duration (4-second) stimuli were compared to those elicited by stroboscopic stimuli. In addition, the c-waves recorded with direct-coupled amplification were compared to those recorded with condenser-coupled amplification (one-half-amplitude bandpass=0.1 Hz). The b-wave amplitude was not altered by the amplifier coupling or by the two stimulus durations. The largest c-waves were elicited by 4-second-duration stimuli and recorded with direct-coupled amplification. Although the c-wave amplitude was reduced by stroboscopic stimuli and by condenser coupling, large and repeatable c-waves were elicited by stroboscopic stimuli and recorded with condenser-coupled amplification. A comparison of stimulus duration and amplifier coupling showed that the stimulus duration was more important in recording large-amplitude c-waves. Similar results were obtained with the Jet electrode. We conclude that repeatable and large c-waves can be elicited by a stroboscopic stimuli and can be recorded with condenser-coupled amplification with good low-frequency response from rabbits.

Prevention of action potentials during extracellular electrical stimulation of long duration.

This study investigated if action potentials can be prevented by electrical field stimuli of long duration. The transmembrane potential was recorded by a double-barrel micro-electrode during field stimulation given across a papillary muscle from 10 guinea pigs. After 10 stimuli (S) with a 200-msec S-S interval, a 400-msec square wave shock was given just before or after the end of the effective refractory period following the 10th stimulus through electrodes 1 cm on either side of the papillary muscles. Another two stimuli (S' and S") having the same 200-msec S-S interval were given during the shock pulse to test if the action potentials induced by these two stimuli could be prevented by the shock. The shock strength was increased until the shock field prevented the action potentials induced by the S' and S" stimuli. The resting membrane potential was -85.5 +/- 2.9 mV. For shocks causing depolarization at the recording site, the field strength required to prevent S'- and S"-induced action potentials was 1.5 +/- 0.4 V/cm, which depolarized the transmembrane potential to -55.3 +/- 8.9 mV and -58.1 +/- 7.2 mV from the resting membrane potential at the time of the S' and S" stimuli, respectively. The strength of shocks causing hyperpolarization required to prevent S'- and S" -induced action potentials was 5.0 +/- 0.8 V/cm, which hyperpolarized the transmembrane potential to -105 +/- 6.5 mV and -115.6 +/- 6.9 mV from the resting membrane potential at the time of the S' and S" stimuli, respectively. Both depolarization and hyperpolarization caused by an electrical field can prevent action potentials.

The Effects of Selective Cholinergic Basal Forebrain Lesions and Aging upon Expectancy in the Rat

The effects of selective cholinergic cell loss within the basal forebrain (BF) were determined using a task that requires shifting of attention between two visual stimuli. Discriminability between two stimuli and response bias were determined in young and old F-344 rats given BF injections of IgG-192 saporin (100 ng). The lesion reduced ChAT activity in the frontal and parietal cortices, hippocampus, and olfactory bulbs. The lesion did not significantly alter Na+/K+-ATPase activity in cortex, hippocampus, or olfactory bulbs, or endogenous levels of neuropeptide Y and neurokinin B within the BF. The BF lesions impaired both stimulus discriminability and response bias in young and old rats. The BF lesions had a significantly greater effect upon stimulus discriminability and response bias in aged rats, compared to young rats, only when the stimulus duration was very brief, i.e., when the task was most difficult to solve. At longer stimulus durations, aging and lesions showed no interaction. The results suggest that the selective loss of cholinergic cells in the BF, but not normal aging, impairs the ability to discriminate between independent sensory stimuli. The loss of these cells confers a response bias in simple operant tasks involving motor responses to reward-related visual stimuli.

Neural activity in the medial geniculate nucleus during auditory trace conditioning.

In classical trace conditioning the acquisition of a conditioned response (CR) is possible even though an interval (the trace interval) elapses between the conditioned stimulus (CS) and unconditioned stimulus (US). This implies that some neural representation of the CS (the stimulus trace) is able to support association between the two stimuli. The medial geniculate nucleus (MGN), particularly the medial division (mMGN), has been identified as one site in the auditory pathway where associative related changes in neural activity occur. If neurons in the MGN are involved in such a sensory trace and in acquisition of a CR, then it is expected that activity following an acoustic CS should be related to both stimulus and response. This study examined the extracellular activity of single units in the MGN during differential auditory trace conditioning of the rabbit nictitating membrane response (NMR). Two 150-ms tones (600 Hz and 1200 Hz) served as CS+ and CS-, and the US was periorbital electrostimulation. Changes in activity during the stimulus and trace interval were largest in the medial and dorsal MGN divisions on CS+ trials and on trials in which a CR was made. Examination of probe stimuli of short (50 ms) and long (600 ms) duration suggested that both CR latency and activity changes in the trace interval were related to stimulus duration and time-locked to stimulus offset. Comparisons of neural activity on the basis of fast or slow CR responses revealed different patterns of response--activity on fast CR trials was generally greater and tended to occur earlier. These results suggest that MGN neurons are involved in the maintenance of a sensory memory trace and possibly play a part in CR generation and timing.

Scalar timing in temporal generalization in humans with longer stimulus durations.

Three experiments investigated temporal generalization performance in humans by using stimulus durations similar to those previously used with rats. In most conditions, chronometric counting was prevented by concurrent shadowing of temporally irregular numbers. Experiment 1 examined performance with visual stimuli, when the standard was 4.0 s long and nonstandard stimuli were spaced either linearly or logarithmically around the standard. Generalization gradients were asymmetrical with linear spacing but symmetrical with logarithmic spacing, a result obtained previously with humans. Experiment 2 used auditory stimuli and varied the standard across values of 2.0, 4.0, 6.0, and 8.0 s. All gradients were asymmetrical, and good superposition was obtained, indicating conformity to scalar timing. Experiment 3 prevented or encouraged chronometric counting by changing instructions, and temporal generalization gradients differed when counting was and was not used.

Scalar timing in temporal generalization in humans with longer stimulus durations.

Scalar timing in temporal generalization in humans with longer stimulus durations.

Effect of racemic mixture and the (S+)-isomer of ketamine on temporal and spatial summation of pain

We have compared the analgesic efficacy of the racemic mixture and the stereoisomer (S+) of the NMDA antagonist ketamine. In a double-blind, three-way crossover, placebo-controlled study, we assessed the following: pain evoked by small/large area pressure stimuli, pain detection threshold and pain ratings to small/large area of heat stimuli, pain detection threshold and pain rating to heat stimuli of brief/long duration, summation pain threshold and pain ratings to repeated heat/electrical stimuli, side effects and reaction time. Plasma concentrations of 350 ng ml-1 for ketamine (racemic) and 180 ng ml-1 for ketamine (S+) were tried. We found that ketamine (racemic) prolonged the reaction time more than ketamine (S+). Both drugs affected pain caused by repeated stimuli or stimuli of long duration equally or more than a single stimulus of short duration. They also affected pain evoked from large areas equally or more than pain evoked from small areas. The (S+)-isomer was approximately twice as potent as the racemic mixture of ketamine in inhibiting central summation.