Orienting, Habituation, and Short-Term Memory

Two experiments investigated the relationship between orienting activity and proactive inhibition in the Brown-Peterson short-term memory paradigm. In both experiments, different groups of 4 words were presented on each of 6 trials, and recall was tested on each trial following a rehearsal-prevention period. Electrodermal activity was monitored throughout. The taxonomic category of words was the same throughout Experiment 1 (N = 96) for control groups, whereas for experimental groups, trial 5 consisted of words from a different category. In Experiment 2 (N = 96), taxonomic category was the same on all trials for all groups. However, intertrial intervals (ITIs) were either short or long, and for experimental groups, the trial 5–trial 6 ITI was either shortened or lengthened. The results of both experiments indicated that decline in recall performance across trials 1–5 was accompanied by a decrease in skin conductance response (SCR) magnitude. The change in taxonomic category in Experiment 1 produced both improved recall and increased SCR magnitude. The results of Experiment 2 indicated that both recall performance and SCR magnitude declined more slowly with long than with short ITIs. Moreover, an increase in ITI produced improved recall and increased SCR magnitude. However, although a decrease in ITI produced a decrement in recall performance, it had no effect on SCR magnitude. The results are discussed in terms of current theories of habituation and proactive inhibition in short-term memory.     

Habituation During Sleep

In some recent reports, authors have found no evidence for habituation during sleep, and have concluded that habituation and sleep are incompatible. This finding was tested using skin potential, heart rate, and EEG responses to a 70 dB, 1 sec, 1000 Hz tone, in sleep stages 2, 4, and REM, with daytime controls. Interstimulus intervals used ranged from 10 to 30 sec. Autonomic responses analyzed separately for each series of stimuli showed significant habituation, except for heart rate responses in REM and in the daytime (drowsy) controls. EEG responses also demonstrated habituation. An interstimulus interval effect was found with EEG K-complexes, habituation occurring more slowly or not at all with the longer, irregular interstimulus intervals. It is suggested that previous failure to detect habituation has been due to a technique of averaging responses over hourly periods and/or the use of long irregular interstimulus intervals. It is concluded that habituation is indeed possible in human sleep if stimulus intervals are not long.     

Effects of early and late nocturnal sleep on priming and spatial memory.

A wordstem priming task (nondeclarative memory), and a mental spatial rotation task (declarative memory) were presented to subjects of an experimental "sleep" group (n = 11) and of a "wake" control group (n = 10). Repetition priming effects and recall of spatial memory were tested after 3-hr retention intervals, which followed learning and were placed either in the early or in the late half of the night. Sleep group subjects slept during the retention intervals while subjects of the wake group stayed awake. As expected, early retention sleep was dominated by slow wave sleep (SWS), whereas rapid eye movement (REM) sleep prevailed during late retention sleep. After early retention sleep, recall of spatial memory was superior to that after late retention sleep (p < 0.01), and also to that after retention intervals of wakefulness (p < 0.05). In contrast, priming was more effective after late than early retention sleep (p < 0.05). It appears that early sleep dominated by SWS facilitates consolidation of declarative memory whereas late sleep dominated by REM sleep facilitates consolidation of nondeclarative memory.     

Laugh yourself to sleep: memory consolidation for humorous information

There is extensive evidence that emotional information is better remembered than neutral information across long delays, especially if the delay interval contains an opportunity for sleep. However, as prior studies have focused on memory for negative stimuli, it is unclear whether positive memories benefit from time and sleep as well. To investigate the consolidation of positive memories, the current study examined differences in memory for humorous and non-humorous cartoons. While prior evidence demonstrates that humorous information is preferentially remembered relative to non-humorous information over brief delays, it is unknown whether this benefit lasts across longer delay intervals or whether sleep is important for lasting humor memories to form. Thus, we tested memory for 27 cartoons across 12-h delay periods containing either sleep or wakefulness. Results indicate that humor’s enhancing effect on recall memory is robust across a 12-h delay and that a period of sleep facilitates this effect over wakefulness when cartoons are novel to participants and ranked based on subjective emotional ratings. Further, in accordance with previous studies that reveal diminished emotional reactivity to stimuli following sleep, in a supplemental experiment, we found that sleep reduced subjective ratings of humor, arousal, and positivity of humorous cartoons. These findings provide preliminary evidence that sleep’s impact on negative emotional memory consolidation and emotional reactivity can be extended to positive stimuli as well.     

The mesograde amnesia of sleep may be attenuated in subjects with primary insomnia

In this study, we pilot tested one of the more controversial components of the Neurocognitive Model of Insomnia; the proposition that subjects with chronic primary insomnia are better able to recall and/or recognize information from sleep onset intervals than good sleeper controls. Nine subjects participated in this pilot study, five of whom had a complaint of insomnia. The remaining four subjects were self-reported good sleeper controls. Subjects were matched for age, sex, and body mass. All subjects spent two nights in the sleep laboratory. The first night served as an adaptation night. The second night served as the experimental night during which a forced awakening and memory task was deployed. In this procedure, subjects were played single-word stimuli across four time periods: at natural sleep onset (Trial 1) and at the sleep onset transitions following three forced awakenings (Trials 2-4 from Stage 2 sleep). All subjects were awakened after about 6 h had elapsed from lights out and were tested for free recall and recognition memory for the word stimuli. The insomnia subjects, tended to identify more of the word stimuli on the recognition task (average for the four trials) and recognized significantly more of the words that were presented at sleep onset proper (Trial 1). This finding suggests that the natural mesograde amnesia of sleep may be attenuated in subjects with insomnia.     

P300, Probability, and Interstimulus Interval

The relationship between target stimulus probability and interstimulus interval on the P300 (P3) component of the event-related potential was assessed in three experiments. In each study an auditory discrimination paradigm was employed wherein subjects indicated with a finger tap response the occurrence of a randomly presented 2000 Hz target tone embedded in a series of 1,000 Hz tones. Target stimuli were presented with a probability of either .20 or .80 in different conditions which were combined factorially with different interstimulus intervals. Experiment 1 presented stimuli at 2.0, 2.5, or 3.0 second intervals; Experiment 2 presented stimuli at 2.0, 3.0, or 4.0 second intervals; Experiment 3 presented stimuli at 4.0, 6.0, or 10.0 second intervals. P3 amplitude was larger for the .20 relative to the .80 target probability conditions for the shorter interstimulus intervals but not for the longer intervals. P3 latency was consistently longer for the .20 relative to the .80 target probability conditions, with generally little effect observed for interstimulus interval changes. The results suggest that interstimulus interval affects component amplitude by determining the amount of processing resources available when the P3 is produced.     

Spectral analysis of event-related EEG responses during short-term memory performance

Summary Averaged event-related EEG frequency response profiles were generated from normalized spectral analysis data obtained in 2 studies of short-term memory. A continuous performance task was used requiring a keyboard response to targeted letter sequences presented on a video screen at 2 sec. intervals. Seven 2 Hz frequency bands between 5–15 Hz were evaluated. In the first study (n=14) response profiles from control and target stimuli were compared. This analysis disclosed 1) a short latency increase at 5–7 and 7–9 Hz in posterior cortex that was identical in both conditions and was attributed to frequency manifestations of the extrinsic visual evoked response (VEP), 2) a separate long-latency increase of uncertain origin at 5–7 Hz in anterior cortex only in the task condition, and 3) a generalized mid-latency alpha frequency suppression and recovery pattern (event-related desynchronization, or ERD) in all bands during both conditions. The ERD in bands between 9–13 Hz was significantly increased at left central and bilateral parietal cortex during target recall. The second study (n=26) compared good vs. poor performance in the target recall task. Findings disclosed significant differences at 7–9 Hz localized to posterior temporal cortex bilaterally. These differences included reduced magnitude in the VEP component and increased suppression in the ERD component among good performers.This work was supported by the Veterans Administration, the Northrop-Grumman Corporation, and the United States Air Force.     

Time-dependent effects of post-trial reinforcement,punishment or ECS on passive avoidance learning

Three studies were performed to assess the effects of posttrial food reinforcement, ice-water punishment, and ECS on step-down passive avoidance learning in mice. The treatments were each administered at <5, 30, 60 and 90 sec after the footshock. One-half min of feeding beginning either 10 or 30 sec after the footshock increased the step-down latencies 24 hr later. This was considered to support the hypothesis that reinforcement facilitates learning by a direct influence on short-term memory processes. Posttrial ice-water punishment (10 sec immersion) disrupted learning only in the 60 sec postshock delay group. ECS (5.4 mA, 0.35 sec, across the ears) disrupted learning at all posttrial delays except 90 sec, with maximal effects immediately and at 60 sec, and minimal disruption at 30 sec. Similarities in the topography of the ECS and punishment curves suggested that some of the effects of posttrial ECS could be due to aversive properties of the ECS (acting directly on memory processes).     

Functional significance of the hippocampus

The alimentary conditioned reflex to time was manifested in a motor reaction of the unrestrained dog to the food cup. Latencies of responding and running speeds were measured to within hundredths of a second. The conditioned stimuli were presented in a stereotype. Before the lesions the dogs were performing to the time factor alone, i.e. without presentation of the stimuli. Hippocampal lesions resulted in a complete disappearance of the well established conditioned reflexes to time with intervals of 30 sec, 1, 2 and 5 min. In spite of the long training (over two years) the hippocampectomized dogs failed to recover the conditioned reflexes to time. At the same time hippocampal lesions hardly affected the positive conditioned reflexes and discrimination of single and complex conditioned stimuli. The hippocampus appears to be involved both in the mechanisms of remembering the time count and in trace processes underlying memory. Lesions in the parietal cortex and underlying white matter impaired neither reflexes to time, nor those to the stimuli.     

The representation of unseen objects in visual neglect: effects of view and object identity

We provide evidence for long-term priming based on view-specific representations of neglected stimuli. A patient with visual neglect, M.P., was asked to search for a target presented amongst other objects on a table. Subsequently recognition memory was tested for items that were identified and for items missed in search. Items that were missed were rejected more slowly than novel items in the recognition memory task, providing evidence for implicit processing (Experiment 1). Implicit memory for missed items was both item-specific (Experiment 2) and view-specific (Experiment 3), and it was eliminated when there were intervening activities lasting about 1 hour (Experiment 4). There was also an implicit memory for distractors in the search task, which was item- but not view-specific (Experiments 2 and 3) and it lasted for at least an hour, even with other intervening activities (Experiment 4). The data suggest that the representations of neglected stimuli may differ qualitatively from those of nonneglected items, with representations of neglected objects being both view-specific and vulnerable across extended retention intervals. The results support the argument that attention is needed in order to encode object representations that are robust to view transformations and temporal decay or interference.     

Differential extinction of 2-way active avoidance in young and adult rats.

Three experiments examined age-related differences in extinction of active avoidance in rats through manipulation of the stimulus context. In Experiment I, 36 weanlings and 36 adults acquired comparable avoidance levels to a tonal conditioned stimulus (CS), and novel stimuli consisting of either the presence of a naive rat or a light were presented during 2 tests in extinction. Results indicated more pronounced disruption of extinction in the pups, compared to adults, with the animate novel stimulus having the greater effect. Eighteen pups of 22 days of age and 18 adults in Experiment II were trained to avoid shock with a CS consisting of simultaneous tone and light onset. During extinction responses did not terminate 1 of the CS elements, but rather produced continuation of either the tone or light for an additional 5 sec. Continuation of the tonal element resulted in greater disruption of extinction trials than the light in the adults, although the pups had faster, but nondifferential, extinction rates. A 3rd experiment presented 36 pups and adults with either 0-, 5-, or 10-sec delays of tonal CS reactivation after extinction responses. Both 5- and 10-sec delays resulted in increased numbers of extinction trials in the adults, but the pups failed to respond differentially to any of the delay intervals. Results of all of the experiments were considered in light of species-specific behaviors in avoidance extinction relative to environmental and associative saliencies between ages.     

Effects of scopolamine on performance of rats in a delayed-response radial maze task

The effects of the cholinergic blocker scopolamine on performance of rats in an 8-arm radial maze were studied. In Experiment 1, rats received injections of scopolamine-HBr (0.2 mg/kg, IP) or saline, 20 min before a trial. The drug impaired performance only when midtrial delays were introduced (10 sec, 5, 20, 60 or 120 min), during which the animals were removed from the apparatus, and to a similar extent at all delays. Injection of scopolamine directly after choice 4 in a 20 min delayed trial affected performance at a dose of 0.4 mg/kg, but not at 0.2 mg/kg. In Experiment 2, the rats were kept in the apparatus during the delay. There was a significant effect of scopolamine at 0.1 and at 0.2 mg/kg, initially irrespective of the length of the delay (10 sec, 2.5 or 5 min). After considerable training, administration of scopolamine (0.2 mg/kg) had no significant effect at the 10 sec delay but did impair performance after longer (5 or 10 min) midtrial intervals. These results suggest that scopolamine has differential effects, depending on dose and degree of training, and that an effect on memory storage may be one of them.     

Isolated REM Sleep Facilitates Recall of Complex Associative Information

This study directly tested the beneficial effect of isolated REM and isolated NREM sleep on the recall of narcoleptics. In a within subject design, 10 narcoleptics were instructed to sleep for a certain optimal duration and at a certain optimal time before each session, and were given 12–14 sessions, one a day, on different lists of a complex associative memory task and a minimally associative memory task. Following the 10-min task, the subject either had 20 min of polygraphically recorded napping or card playing, followed by a free recall test. The results for the complex associative task indicated significant differences between the three conditions for free recall. Recall was significantly better after isolated REM than after isolated NREM sleep or wakefulness and was significantly better after NREM than after wakefulness. The results from the minimally associative task were inconclusive. The results are consistent with the proposed neuronal activity correlates (NAC) theory that REM sleep actively consolidates and/or integrates complex associative information and that NREM sleep passively prevents retroactive interference of recently acquired complex associative information.     

Delay-dependent modulation of memory retrieval by infusion of a dopamine D1 agonist into the rat medial prefrontal cortex

Dopamine (DA) in the medial prefrontal cortex (PFC) can modulate the short-term retention of information and other executive functions. The present study examined whether administration of a DA D1 agonist into the PFC could have differential effects on memory retrieval in circumstances in which memory was either excellent or poor. Separate groups of rats were trained on a delayed version of the radial maze task. On the test day, the delay between the phases was either 30 min or 12 hr. Infusions of the D1 receptor agonist SKF 81297 (0.05, 0.10, or 0.20 microg/0.5 microl) into the PFC before the test phase improved memory retrieval after a 12-hr delay but disrupted performance after a 30-min delay. These data suggest that D1 receptor activity can exert differential effects over PFC function, depending on the strength of the memory trace. When memory is decremented by an extended delay, activation of PFC DA D1 receptors by an agonist can improve cognitive function.     

Heart Rate and Skin Conductance in Anticipation of Shocks With Varying Probability of Occurrence

In Experiment I,Ss in each of four groups were given 8 shocks in either 8, 11, 16, or 32 trials, confounding probability with number of trials. Each trial was a countdown period of 30 sec, with an ITI of 30 sec. There were no shock probability effects using either heart rate (HR) or skin conductance (SC), with respect either to average within trial changes or to average pre-shock levels relative to rest. In every group after a large acceleration in Trial 1, HR subsided and stabilized; SC rose steadily throughout the session. In Experiment II, Ss in each of four groups were given either 5, 10, 15, or 20 shocks in 20 trials, confounding probability with number of shocks. In addition to the large HR acceleration in the first trial, there were in this case between-group differences in HR, which were a direct function of number and probability of shocks. Skin conductance did not discriminate the groups, though it rose in all groups both within and across trials. Experiment III, a partial replication of Experiment II, showed that the HR differences in Experiment II failed to repeat when the shock intensity was reduced; and SC no longer increased over trials.     

Electrical Stimulation of a Human Ventrolateral-Subventrolateral Thalamic Target Area

A method is described for electrical stimulation of human subcortical brain structures. The necessity is shown for monitoring both the voltage across and also the current through the stimulated brain tissue, as well as the duration of the stimuli applied. Stimulus trains of constant composition were repeatedly applied to a ventrolateral-subventrolateral thalamic target area in 12 alert patients undergoing stereotaxic thalamotomy. Stimulation at the clinical threshold resulted in motor and “sensory” responses projected to contralateral parts of the body. In spite of small temporary variations, no distinct change of response from one stimulus train to the next in the same patient could be detected when the stimulation was applied at intervals of either 15, 30, or 60 sec. The current thresholds, on the other hand, showed a tendency to decrease if the intervals were less than 60 sec. The stimulation method and the results of stimulation are discussed.     

Sleep restriction can attenuate prioritization benefits on declarative memory consolidation

As chronic sleep restriction is a widespread problem among adolescents, the present study investigated the effects of a 1‐week sleep restriction (SR) versus control period on the consolidation of long‐term memory for prose passages. We also determined whether the benefit of prioritization on memory is modulated by adequate sleep occurring during consolidation. Fifty‐six healthy adolescents (25 male, aged 15–19 years) were instructed to remember a prose passage in which half of the content was highlighted (prioritized), and were told that they would receive an additional bonus for remembering highlighted content. Following an initial free recall test, participants underwent a 7‐night period in which they received either a 5‐h (SR) or 9‐h (control) nightly sleep opportunity, monitored by polysomnography on selected nights. Free recall of the passage was tested at the end of the sleep manipulation period (1 week after encoding), and again 6 weeks after encoding. Recall of highlighted content was superior to that of non‐highlighted content at all three time‐points (initial, 1 week, 6 weeks). This beneficial effect of prioritization on memory was stronger 1 week relative to a few minutes after encoding for the control, but not the SR group. N3 duration was similar in the control and SR groups. Overall, the present study shows that the benefits of prioritization on memory are enhanced over time, requiring time and sleep to unfold fully. Partial sleep deprivation (i.e. 5‐h nocturnal sleep opportunity) may attenuate such benefits, but this may be offset by preservation of N3 sleep duration.     

Electrophysiological correlates of the maintenance of the representation of pitch objects in acoustic short-term memory

We studied the neuronal mechanisms that implement acoustic short-term memory (ASTM) for pitch using event-related potentials (ERP). Experiment 1 isolated an ERP component, the sustained anterior negativity (SAN), that increased in amplitude with increasing memory load in ASTM using stimuli with equal duration at all memory loads. The SAN load effect found in Experiment 1, when pitch had to be remembered to perform the task, was absent in Experiment 2 using the same sounds when memory was not required. In Experiment 3, the memory task was performed without or with concurrent articulatory suppression during the retention interval to prevent rehearsal via an articulatory loop. Load-related effects observed in Experiment 1 were found again, whether participants engaged in concurrent suppression or not. The results suggest that the SAN reflects activity required to maintain pitch objects in an ASTM system that is distinct from articulatory rehearsal.     

CER to discrete and contextual stimuli: effects of stimulus modality depend on strain of rat

Strain differences in perception, cognition and affect have been found to interact with the effect of experimental treatments on learning. The present study tested Dark Agouti (DA) and Wistar rats in conditioning to discrete and contextual stimuli, for strain differences in conditioned emotional response (CER) and effects of stimulus modality. A Pavlovian trace-conditioning procedure was used in which a discrete target stimulus (flashing light or tone, counterbalanced across groups) was paired either contiguously or at a 30-s trace with footshock. Contextual conditioning was assessed using a background (continuously presented) experimental stimulus (alternate to that used as target). Experiment 1 used 5 x 0.5 mA, 0.5-s footshock and Experiment 2 used 2 x 0.5 mA, 1-s footshock. In both experiments, conditioning to the discrete (target) and background stimuli interacted with strain of rat and stimulus modality. For conditioning to the target stimulus, the trace-conditioning effect (of relatively greater suppression in contiguously conditioned than trace-conditioned groups) differed by stimulus and strain. It was greater for the Wistar strain with the flashing light stimulus and for the DA strain with the click stimulus (although the latter was not significant in Experiment 1). In addition, suppression to the background stimulus was affected by stimulus modality for the DA but not Wistar strain in both experiments. DAs conditioned more to the click than the light as background, whereas there was no difference by stimulus modality for Wistars, although the exact pattern of how this stimulus modality effect was mediated differed between experiments. These results demonstrate that stimulus modality can be an issue when considering apparent strain differences in conditioning.     

Semantic and phonological information in sentence recall: Converging psycholinguistic and neuropsychological evidence

We present one experiment and a neuropsychological case study to investigate to what extent phonological and semantic representations contribute to short-term sentence recall. We modified Potter and Lombardi's (1990) intrusion paradigm, in which retention of a list interferes with sentence recall such that on the list a semantically related lure is presented, which is expected to intrude into sentence recall. In our version, lure words are either semantically related to target words in the sentence or semantically plus phonologically related. With healthy participants, intrusions are more frequent when lure and target overlap phonologically in addition to semantically than when they solely overlap semantically. When this paradigm is applied to a patient with a phonological short-term memory impairment, both lure types induce the same amount of intrusions. These findings indicate that usually phonological information is retained in sentence recall in addition to semantic information.