Training to Improve Spatial Hearing and Situation Awareness when Wearing Hearing Protection

INTRODUCTION

Hearing protection devices (HPDs) are standard personal protective equipment in military settings, but many service members may choose to not use HPDs because they impair spatial hearing and situation awareness. In an effort to reduce barriers to compliance by improving situation awareness while wearing HPDs, this study investigated whether brief training could counteract spatial hearing deficits when wearing HPDs. Participant's ability to correctly apply the HPDs across days was also examined.

MATERIALS AND METHODS

Young adults were randomly assigned to one of two groups: training or control (n = 25/group). Participants in each group performed a spatial hearing task while wearing HPDs and in an open ear condition without HPDs. Individual targets were battlefield sounds or white noise presented from a speaker array that surrounded the participant in the horizontal plane. After presentation of each target sound, the participant then controlled a white noise "auditory pointer," which they moved to the perceived location of the target. The two primary measures were the percent of trials with very large errors (> 45°), which were usually due to confusing front and back locations, and absolute localization, which is the difference between the pointer location and the true sound location. Both groups were tested on Days 1 (baseline) and 5 (post-test). On Days 2 to 4, the training group wore HPDs while receiving auditory and visual feedback after each trial.

RESULTS

Across all participants on Day 1, wearing HPDs increased the frequency of very large errors by about 3× and impaired localization by about 40%, relative to the open ear condition. When comparing performance at baseline (Day 1) and post-training Day 5, the training group with HPDs had significant reductions in very large errors and improved absolute localization (P values < .001). The training group also had significant improvements from Days 1 to 5 in the open ear condition. When the control group wore HPDs, there were also significant improvements from Days 1 to 5 (fewer very large errors and better localization), with smaller effect sizes vs. the training group. Controls did not have significant improvement in the open ear condition, but had similar trends. Most participants consistently applied the HPDs, but a subset of ∼20% frequently failed to achieve the criterion attenuation of 15 dB (over 0.25-4.0 kHz) in both ears.

CONCLUSIONS

These findings show that simple, relatively brief practice and training can substantially reduce HPD impairments on spatial hearing and situation awareness. The gains from training and practice can inform the development of relatively simple, brief methods to reduce HPD spatial hearing impairments, potentially leading to increased HPD compliance. Longitudinal data show that a subset of participants would not have received the full benefit of hearing protection because of improper application of the HPDs.

A novel non-word speech preparation task to increase stuttering frequency in experimental settings for longitudinal research

PURPOSE

The variable and intermittent nature of stuttering makes it difficult to consistently elicit a sufficient number of stuttered trials for longitudinal experimental research. This study tests the efficacy of using non-word pairs that phonetically mimic English words with no associated meaning, to reliably elicit balanced numbers of stuttering and fluent trials over multiple sessions. The study also evaluated the effect of non-word length on stuttering frequency, the consistency of stuttering frequency across sessions, and potential carry-over effects of increased stuttering frequency in the experimental task to conversational and reading speech after the task.

METHODS

Twelve adults who stutter completed multiple sessions (mean of 4.8 sessions) where they were video-recorded during pre-task reading and conversation, followed by an experimental task where they read 400 non-word pairs randomized for each session, and then a post-task reading and conversation sample.

RESULTS

On average, across sessions and participants, non-word pairs consistently yielded a balanced distribution of fluent (60.7%) and stuttered (39.3%) trials over five sessions. Non-word length had a positive effect on stuttering frequency. No carryover effects from experimental to post-task conversation and reading were found.

CONCLUSIONS

Non-word pairs effectively and consistently elicited balanced proportions of stuttered and fluent trials. This approach can be used to gather longitudinal data to better understand the neurophysiological and behavioral correlates of stuttering.

Auditory spatial attention gradients and cognitive control as a function of vigilance

Selection and effort are central to attention, yet it is unclear whether they draw on a common pool of cognitive resources, and if so, whether there are differences for early versus later stages of cognitive processing. This study assessed effort by quantifying the vigilance decrement, and spatial processing at early and later stages as a function of time-on-task. Participants performed an auditory spatial attention task, with occasional "catch" trials requiring no response. Psychophysiological measures included bilateral cerebral blood flow (transcranial Doppler), pupil dilation, and blink rate. The shape of attention gradients using reaction time indexed early processing, and did not significantly vary over time. Later stimulus-response conflict was comparable over time, except for a reduction to left hemispace stimuli. Target and catch trial accuracy decreased with time, with a more abrupt decrease for catch versus target trials. Diffusion decision modeling found progressive decreases in information accumulation rate and non-decision time, and the adoption of more liberal response criteria. Cerebral blood flow increased from baseline and then decreased over time, particularly in the left hemisphere. Blink rate steadily increased over time, while pupil dilation increased only at the beginning and then returned towards baseline. The findings suggest dissociations between resources for selectivity and effort. Measures of high subjective effort and temporal declines in catch trial accuracy and cerebral blood flow velocity suggest a standard vigilance decrement was evident in parallel with preserved selection. Different attentional systems and classes of computations that may account for dissociations between selectivity versus effort are discussed.

Widespread theta coherence during spatial cognitive control

Cognitive control allows humans to process relevant sensory information while minimizing distractions from irrelevant stimuli. The neural basis of cognitive control is known to involve frontal regions of the brain such as the medial prefrontal cortex (mPFC) and anterior cingulate cortex (ACC), but the temporal dynamics of larger scale networks is unclear. Here we used EEG with source localization to identify how the neural oscillations localized to the mPFC and ACC coordinate with parietal, sensory, and motor areas during spatial cognitive control. Theta coherence (3-8 Hz) between the mPFC and ACC increased with task difficulty and predicted individual differences in reaction time. Individual differences in accuracy were predicted by earlier activation of ACC-motor coherence, highlighting the relationship between processing speed and task performance. Our results provide evidence that successful cognitive control requires dynamic coordination between a widespread network of brain regions. Long range theta coherence may be a key mechanism for efficient cognitive control across the neocortex.

Strategic encoding and retrieval processes in verbal recall among middle-aged and older adults

The authors used an analysis of individual differences to examine the role of executive control in strategic encoding and retrieval in verbal recall. Participants enrolled in the Louisiana Healthy Aging Study completed measures of working memory (WM), cognitive status, vocabulary, and free recall of words. Indices of clustering in free recall were calculated to permit inferences on strategic encoding and retrieval processes. We hypothesized that WM would be more strongly associated with strategic encoding and retrieval metrics than vocabulary based on the assumption that successful remembering requires executive control in WM. Regression analyses, together with a variance portioning procedure, confirmed that WM had comparable levels of unique and shared variance with the strategic encoding and retrieval metrics, and both exceeded vocabulary. Theoretical and clinical implications of these data are considered, with the suggestion of future research in lifespan samples as opposed to exclusively young adult or older adult samples.

Sensorimotor Integration Can Enhance Auditory Perception

Whenever we move, speak, or play musical instruments, our actions generate auditory sensory input. The sensory consequences of our actions are thought to be predicted via sensorimotor integration, which involves anatomical and functional links between auditory and motor brain regions. The physiological connections are relatively well established, but less is known about how sensorimotor integration affects auditory perception. The sensory attenuation hypothesis suggests that the perceived loudness of self-generated sounds is attenuated to help distinguish self-generated sounds from ambient sounds. Sensory attenuation would work for louder ambient sounds, but could lead to less accurate perception if the ambient sounds were quieter. We hypothesize that a key function of sensorimotor integration is the facilitated processing of self-generated sounds, leading to more accurate perception under most conditions. The sensory attenuation hypothesis predicts better performance for higher but not lower intensity comparisons, whereas sensory facilitation predicts improved perception regardless of comparison sound intensity. A series of experiments tested these hypotheses, with results supporting the enhancement hypothesis. Overall, people were more accurate at comparing the loudness of two sounds when making one of the sounds themselves. We propose that the brain selectively modulates the perception of self-generated sounds to enhance representations of action consequences.

Dynamics of auditory spatial attention gradients

Auditory spatial attention faces the conflicting demands of focusing on the current task while also rapidly shifting location to unexpected stimuli. We examined the interplay of sustained focus and intermittent shifts in an auditory spatial attention task. Most trials required a choice response from a standard location in virtual space (L-R: -90°, 0°, +90°), but occasionally the location shifted between 45°-180°. Reaction time curves for angular shifts had a quadratic shape, with slowing for small shifts but faster reaction times for larger shifts. The reaction time curves were maintained at faster stimulus rates and usually scaled to fit the range of stimulus locations. However, focus on the right had an attenuated curve, and did not scale to the range of locations. The findings suggest two mechanisms: a top-down bias centered on standard locations that decreases with distance, and a bottom-up bias that under these conditions increases with distance from the standard location.

Single-Trial Classification of Disfluent Brain States in Adults Who Stutter

Normal human speech requires precise coordination between motor planning and sensory processing. Speech disfluencies are common when children learn to talk, but usually abate with time. About 5% of children experience stuttering. For most, this resolves within a year. However, for approximately 1% of the world population, stuttering continues into adulthood, which is termed 'persistent developmental stuttering'. Most stuttering events occur at the beginning of an utterance. So, in principle, brain activity before speaking should differ between fluent and stuttered speech. Here we present a method for classifying brain network states associated with fluent vs. stuttered speech on a single trial basis. Brain activity was recorded with EEG before people who stutter read aloud pseudo-word pairs. Offline independent component analysis (ICA) was used to identify the independent neural sources that underlie speech preparation. A time window selection algorithm extracted spectral power and coherence data from salient windows specific to each neural source. A stepwise linear discriminant analysis (sLDA) algorithm predicted fluent vs. stuttered speech for 81% of trials in two subjects. These results support the feasibility of developing a brain-computer interface (BCI) system to detect stuttering before it occurs, with potential for therapeutic application.

Cognitive control and midline theta adjust across multiple timescales

Cognitive control of attention in conflict situations is a basic skill that is vital for goal-oriented behavior. Behavioral evidence shows that conflict control occurs over successive trials as well as longer time scales of trial blocks, but the relation among time scales as well as their neural mechanisms are unclear. This study used measures of behavior, EEG, and a simple quantitative model to test the hypothesis that conflict control at the block level is not exclusively driven by control adjustments over successive trials. Young adults performed an auditory Simon task, and the base rate of compatible vs. incompatible trials was manipulated in separate blocks (25, 50, 75% compatible). EEG data were analyzed using independent component analysis (ICA) to define cortical mechanisms of any base rate and trial-by-trial sequence effects. Reaction time measures had both sequence and base rate effects. Two fronto-medial ICA components indexed sequence and base rate effects, with specific profiles for evoked potentials and oscillations in the theta and alpha frequency bands. Predictive modeling showed that sequence effects accounted for a minority of the variance on behavioral and ICA measures (all < 36%). The results strongly suggest that the base rate manipulation affected behavior and many neural measures beyond the influence of sequence effects.

Numerical value biases sound localization

Speech recognition starts with representations of basic acoustic perceptual features and ends by categorizing the sound based on long-term memory for word meaning. However, little is known about whether the reverse pattern of lexical influences on basic perception can occur. We tested for a lexical influence on auditory spatial perception by having subjects make spatial judgments of number stimuli. Four experiments used pointing or left/right 2-alternative forced choice tasks to examine perceptual judgments of sound location as a function of digit magnitude (1–9). The main finding was that for stimuli presented near the median plane there was a linear left-to-right bias for localizing smaller-to-larger numbers. At lateral locations there was a central-eccentric location bias in the pointing task, and either a bias restricted to the smaller numbers (left side) or no significant number bias (right side). Prior number location also biased subsequent number judgments towards the opposite side. Findings support a lexical influence on auditory spatial perception, with a linear mapping near midline and more complex relations at lateral locations. Results may reflect coding of dedicated spatial channels, with two representing lateral positions in each hemispace, and the midline area represented by either their overlap or a separate third channel.

Impact of Spatial and Verbal Short-Term Memory Load on Auditory Spatial Attention Gradients

Short-term memory load can impair attentional control, but prior work shows that the extent of the effect ranges from being very general to very specific. One factor for the mixed results may be reliance on point estimates of memory load effects on attention. Here we used auditory attention gradients as an analog measure to map-out the impact of short-term memory load over space. Verbal or spatial information was maintained during an auditory spatial attention task and compared to no-load. Stimuli were presented from five virtual locations in the frontal azimuth plane, and subjects focused on the midline. Reaction times progressively increased for lateral stimuli, indicating an attention gradient. Spatial load further slowed responses at lateral locations, particularly in the left hemispace, but had little effect at midline. Verbal memory load had no (Experiment 1), or a minimal (Experiment 2) influence on reaction times. Spatial and verbal load increased switch costs between memory encoding and attention tasks relative to the no load condition. The findings show that short-term memory influences the distribution of auditory attention over space; and that the specific pattern depends on the type of information in short-term memory.

Cortical activity during cued picture naming predicts individual differences in stuttering frequency

OBJECTIVE

Developmental stuttering is characterized by fluent speech punctuated by stuttering events, the frequency of which varies among individuals and contexts. Most stuttering events occur at the beginning of an utterance, suggesting neural dynamics associated with stuttering may be evident during speech preparation.

METHODS

This study used EEG to measure cortical activity during speech preparation in men who stutter, and compared the EEG measures to individual differences in stuttering rate as well as to a fluent control group. Each trial contained a cue followed by an acoustic probe at one of two onset times (early or late), and then a picture. There were two conditions: a speech condition where cues induced speech preparation of the picture's name and a control condition that minimized speech preparation.

RESULTS

Across conditions stuttering frequency correlated to cue-related EEG beta power and auditory ERP slow waves from early onset acoustic probes.

CONCLUSIONS

The findings reveal two new cortical markers of stuttering frequency that were present in both conditions, manifest at different times, are elicited by different stimuli (visual cue, auditory probe), and have different EEG responses (beta power, ERP slow wave).

SIGNIFICANCE

The cue-target paradigm evoked brain responses that correlated to pre-experimental stuttering rate.

Atypical sensory reactivity influences auditory attentional control in adults with autism spectrum disorders

Frequent observations of atypical sensory reactivity in people with autism spectrum disorders (ASD) suggest that the perceptual experience of those on the Spectrum is dissimilar to neurotypicals. Moreover, variable attention abilities in people with ASD, ranging from good control to periods of high distractibility, may be related to atypical sensory reactivity. This study used auditory event-related potential (ERP) measures to evaluate top-down and bottom-up attentional processes as a function of perceptual load, and examined these factors with respect to sensory reactivity. Twenty-five age and IQ-matched participants (ASD: 22.5 year, SD = 4.1 year; Controls: 22.8 year, SD = 5.1 year) completed the Adolescent/Adult Sensory Profile prior to performing a modified 3-stimulus (target, non-target, and distractor) auditory oddball target detection task EEG was recorded during task completion. ERP analysis assessed early sensory processing (P50, ∼50 ms latency; N100, ∼100 ms latency), cognitive control (N200, ∼200 ms latency), and attentional processing (P3a and P3b, ∼300 ms latency). Behavioral data demonstrates participants with ASD and neurotypical performed similarly on auditory target detection, but diverged on sensory profiles. Target ERP measures associated with top-down control (P3b latency) significantly increased under greater load in controls, but not in participants with ASD. Early ERP responses associated with bottom-up attention (P50 amplitude) were positively correlated to increased sensory sensitivity. Findings suggest specific neural mechanisms for increased perceptual capacity and enhanced bottom-up processing of sensory stimuli in people with autism. Results from participants with ASD are consistent with load theory and enhanced perceptual functioning. Autism Res 2016, 9: 1079-1092. © 2016 International Society for Autism Research, Wiley Periodicals, Inc.

Interaction of Number Magnitude and Auditory Localization

The interplay of perception and memory is very evident when we perceive and then recognize familiar stimuli. Conversely, information in long-term memory may also influence how a stimulus is perceived. Prior work on number cognition in the visual modality has shown that in Western number systems long-term memory for the magnitude of smaller numbers can influence performance involving the left side of space, while larger numbers have an influence toward the right. Here, we investigated in the auditory modality whether a related effect may bias the perception of sound location. Subjects (n = 28) used a swivel pointer to localize noise bursts presented from various azimuth positions. The noise bursts were preceded by a spoken number (1-9) or, as a nonsemantic control condition, numbers that were played in reverse. The relative constant error in noise localization (forward minus reversed speech) indicated a systematic shift in localization toward more central locations when the number was smaller and toward more peripheral positions when the preceding number magnitude was larger. These findings do not support the traditional left-right number mapping. Instead, the results may reflect an overlap between codes for number magnitude and codes for sound location as implemented by two channel models of sound localization, or possibly a categorical mapping stage of small versus large magnitudes.

Speech preparation in adults with persistent developmental stuttering

Motor efference copy conveys movement information to sensory areas before and during vocalization. We hypothesized speech preparation would modulate auditory processing, via motor efference copy, differently in men who stutter (MWS) vs. fluent adults. Participants (n=12/group) had EEG recorded during a cue-target paradigm with two conditions: speech which allowed for speech preparation, while a control condition did not. Acoustic stimuli probed auditory responsiveness between the cue and target. MWS had longer vocal reaction times (p<0.01) when the cue-target differed (10% of trials), suggesting a difficulty of rapidly updating their speech plans. Acoustic probes elicited a negative slow wave indexing motor efference copy that was smaller in MWS vs. fluent adults (p<0.03). Current density responses in MWS showed smaller left prefrontal responses and auditory responses that were delayed and correlated to stuttering rate. Taken together, the results provide insight into the cortical mechanisms underlying atypical speech planning and dysfluencies in MWS.

Rapid cortical dynamics associated with auditory spatial attention gradients

Behavioral and EEG studies suggest spatial attention is allocated as a gradient in which processing benefits decrease away from an attended location. Yet the spatiotemporal dynamics of cortical processes that contribute to attentional gradients are unclear. We measured EEG while participants (n = 35) performed an auditory spatial attention task that required a button press to sounds at one target location on either the left or right. Distractor sounds were randomly presented at four non-target locations evenly spaced up to 180° from the target location. Attentional gradients were quantified by regressing ERP amplitudes elicited by distractors against their spatial location relative to the target. Independent component analysis was applied to each subject's scalp channel data, allowing isolation of distinct cortical sources. Results from scalp ERPs showed a tri-phasic response with gradient slope peaks at ~300 ms (frontal, positive), ~430 ms (posterior, negative), and a plateau starting at ~550 ms (frontal, positive). Corresponding to the first slope peak, a positive gradient was found within a central component when attending to both target locations and for two lateral frontal components when contralateral to the target location. Similarly, a central posterior component had a negative gradient that corresponded to the second slope peak regardless of target location. A right posterior component had both an ipsilateral followed by a contralateral gradient. Lateral posterior clusters also had decreases in α and β oscillatory power with a negative slope and contralateral tuning. Only the left posterior component (120-200 ms) corresponded to absolute sound location. The findings indicate a rapid, temporally-organized sequence of gradients thought to reflect interplay between frontal and parietal regions. We conclude these gradients support a target-based saliency map exhibiting aspects of both right-hemisphere dominance and opponent process models.

Cortical potentials in an auditory oddball task reflect individual differences in working memory capacity

This study determined whether auditory cortical responses associated with mechanisms of attention vary with individual differences in working memory capacity (WMC) and perceptual load. The operation span test defined subjects with low versus high WMC, who then discriminated target/nontarget tones while EEG was recorded. Infrequent white noise distracters were presented at midline or ±90° locations, and perceptual load was manipulated by varying nontarget frequency. Amplitude of the N100 to distracters was negatively correlated with WMC. Relative to targets, only high WMC subjects showed attenuated N100 amplitudes to nontargets. In the higher WMC group, increased perceptual load was associated with decreased P3a amplitudes to distracters and longer-lasting negative slow wave to nontargets. Results show that auditory cortical processing is associated with multiple facets of attention related to WMC and possibly higher-level cognition.

The SpeechEasy device in stuttering and nonstuttering adults: fluency effects while speaking and reading

The SpeechEasy is an electronic device designed to alleviate stuttering by manipulating auditory feedback via time delays and frequency shifts. Device settings (control, default, custom), ear-placement (left, right), speaking task, and cognitive variables were examined in people who stutter (PWS) (n=14) compared to controls (n=10). Among the PWS there was a significantly greater reduction in stuttering (compared to baseline) with custom device settings compared to the non-altered feedback (control) condition. Stuttering was reduced the most during reading, followed by narrative and conversation. For the conversation task, stuttering was reduced more when the device was worn in the left ear. Those individuals with a more severe stuttering rate at baseline had a greater benefit from the use of the device compared to individuals with less severe stuttering. Our results support the view that overt stuttering is associated with defective speech-language monitoring that can be influenced by manipulating auditory feedback.

Evidence that the Lombard effect is frequency-specific in humans

Recent perspectives suggest that the Lombard effect is an increase in the suprasegmental speech parameters of vocal intensity, duration, and fundamental frequency in the presence of noise. It has been viewed as a non-specific response to ambient noise, but this assumption has not been thoroughly tested. Two experiments using healthy adults measured intensity, duration, and F0 changes in broadband (0.2-20 kHz) and notched noise (0.05-4 kHz removed) during a picture naming task. The pilot experiment showed that broadband noise containing speech-similar frequencies significantly increased intensity, duration, and F0 while notched noise, which removed the majority of speech-similar frequencies, had no effect. The main experiment added bandpass noise (0.05-4.0 kHz) which contained a major portion of speech-similar frequencies and was the mirror image of the notched noise. Broadband and notched noise results were replicated. Bandpass noise increased intensity and duration, but to a lesser degree than did broadband noise, and had no effect on F0. Findings show that the Lombard effect is sensitive to frequencies vital for speech and is not a general response to any competing sound in the environment. Implications for suprasegmental control of speech are discussed.

Cortical mechanisms of auditory spatial attention in a target detection task

The benefits of spatial attention on stimulus processing are thought to diminish with increased distance from the attended location, indicating an attention gradient. Evidence for attention gradients is provided by spatial attention effects on event-related potentials (ERPs) under conditions of rapid stimulus presentation from closely spaced locations. This study was motivated by ecological considerations which suggest that auditory attention is particularly useful for panoramic orienting to intermittent sounds. Auditory ERPs were recorded from a wide range of horizontal locations (180°) while subjects pressed a button to occasional targets at one attended location. Results showed that an ERP component associated with automatic orienting, the P3a, had linear amplitude increases to non-targets as a function of distance from the attended location. A component prior to the P3a with a latency of ~200ms, the P200, showed a similar pattern but only when subjects attended to the left hemifield. When attending to lateral targets frontal slow waves contralateral to the attended location followed the P3a and were attenuated at greater distances from the target location for at least 1s. Results suggest that auditory spatial attention under low cognitive loads modulates orienting responses as a function of distance from the attended location. The slow wave findings show that information about the relation between a stimulus and the attended location persists well beyond the time of initial sensory processing and may involve frontal regions important for maintaining online representations of task set.

Modulation of sensory and motor cortex activity during speech preparation

Previous studies have shown that speaking affects auditory and motor cortex responsiveness, which may reflect the influence of motor efference copy. If motor efference copy is involved, it would also likely influence auditory and motor cortical activity when preparing to speak. We tested this hypothesis by using auditory event-related potentials and transcranial magnetic stimulation (TMS) of the motor cortex. In the speech condition subjects were visually cued to prepare a vocal response to a subsequent target, which was compared to a control condition without speech preparation. Auditory and motor cortex responsiveness at variable times between the cue and target were probed with an acoustic stimulus (Experiment 1, tone or consonant-vowels) or motor cortical TMS (Experiment 2). Acoustic probes delivered shortly before targets elicited a fronto-central negative potential in the speech condition. Current density analysis showed that auditory cortical activity was attenuated at the beginning of the slow potential in the speech condition. Sensory potentials in response to probes had shorter latencies (N100) and larger amplitudes (P200) when consonant-vowels matched the sound of cue words. Motor cortex excitability was greater in the speech than in the control condition at all time points before picture onset. The results suggest that speech preparation induces top-down regulation of sensory and motor cortex responsiveness, with different time courses for auditory and motor systems.

Neural activity before and after conscious perception in dichotic listening

The neural basis of conscious perception can be studied using stimuli that elicit different percepts on different occasions (multistable perception). Multistable perception allows direct comparisons between brain activity and conscious perception that control for sensory input, and also serves as a model for attentional competition, with the winning perceptual outcome varying across trials. Dichotic listening tasks present multistable stimuli consisting of two different consonant-vowels (CVs, one/ear). For each trial one ear usually conveys the dominant percept. We used EEG to measure neural activity before and after dichotic stimulus presentation to compare activity among left vs. right ear percepts and a control task. Consonant-vowels were perceived more often to the right vs. left ear. Pre-stimulus EEG power in the beta band (16-20 Hz) increased for left compared to right ear percepts and control trials. Event-related potentials after stimulus onset showed smaller P50 amplitudes ( approximately 50 ms latency) for left ear compared to right ear and control trials. Results indicate that neural activity for right ear percepts is comparable to control conditions, while activity for the atypical left ear percept differs before and after stimulus onset. Pre-stimulus EEG changes for left ear percepts may indicate a mechanism of spontaneous fluctuations in cortical networks that bias attentional competition during subsequent sensory processing. The P50 amplitude differences among perceived ears suggests that rapid sensory and/or arousal-related activities contribute to the content of conscious perception, possibly by biasing attentional competition away from the dominant right ear channel.

Cortical event-related potentials in preclinical familial Alzheimer disease

OBJECTIVE

To define changes in cortical function in persons inheriting familial Alzheimer disease (FAD) mutations before the onset of cognitive decline.

METHODS

Twenty-six subjects with a family history of FAD were divided into 2 subgroups according to genotype (FAD mutation carriers, n = 15; FAD noncarriers, n = 11). Subjects were given standardized tests of cognitive function and the Clinical Dementia Rating scale (CDR). Sensory (P50, N100, P200) and cognitive (N200, P300) event-related potentials were recorded during an auditory discrimination task. Amplitudes and latencies of cortical potentials were compared among FAD mutation carriers and noncarriers.

RESULTS

FAD mutation carriers and noncarriers did not significantly differ in age or on measures of cognitive function, but FAD carriers had a greater incidence of 0.5 CDR scores (1/10 noncarriers, 5/15 carriers). Relative to noncarriers, FAD mutation carriers had significantly longer latencies of the N100, P200, N200, and P300 components, and smaller slow wave amplitudes. Subanalyses of subjects having CDR scores of 0.0 also showed latency increases in FAD mutation carriers.

CONCLUSIONS

Auditory sensory and cognitive cortical potentials in persons with familial Alzheimer disease (FAD) mutations are abnormal approximately 10 years before dementia will be manifest. Longer event-related potential latencies suggest slowing of cortical information processing in FAD mutation carriers.

Cholinesterase inhibitors affect brain potentials in amnestic mild cognitive impairment

Amnestic mild cognitive impairment (MCI) is an isolated episodic memory disorder that has a high likelihood of progressing to Alzheimer's disease. Auditory sensory cortical responses (P50, N100) have been shown to be increased in amplitude in MCI compared to older controls. We tested whether (1) cortical potentials to other sensory modalities (somatosensory and visual) were also affected in MCI and (2) cholinesterase inhibitors (ChEIs), one of the therapies used in this disorder, modulated sensory cortical potentials in MCI. Somatosensory cortical potentials to median nerve stimulation and visual cortical potentials to reversing checkerboard stimulation were recorded from 15 older controls and 15 amnestic MCI subjects (single domain). Results were analyzed as a function of diagnosis (Control, MCI) and ChEIs treatment (Treated MCI, Untreated MCI). Somatosensory and visual potentials did not differ significantly in amplitude in MCI subjects compared to controls. When ChEIs use was considered, somatosensory potentials (N20, P50) but not visual potentials (N70, P100, N150) were of larger amplitude in untreated MCI subjects compared to treated MCI subjects. Three individual MCI subjects showed increased N20 amplitude while off ChEIs compared to while on ChEIs. An enhancement of N20 somatosensory cortical activity occurs in amnestic single-domain MCI and is sensitive to modulation by ChEIs.

Memory Evaluation in Mild Cognitive Impairment using Recall and Recognition Tests

Amnestic mild cognitive impairment (MCI) is a selective episodic memory deficit that often indicates early Alzheimer's disease. Episodic memory function in MCI is typically defined by deficits in free recall, but can also be tested using recognition procedures. To assess both recall and recognition in MCI, MCI (n = 21) and older comparison (n = 30) groups completed the USC-Repeatable Episodic Memory Test. Subjects memorized two verbally presented 15-item lists. One list was used for three free recall trials, immediately followed by yes/no recognition. The second list was used for three-alternative forced-choice recognition. Relative to the comparison group, MCI had significantly fewer hits and more false alarms in yes/no recognition, and were less accurate in forced-choice recognition. Signal detection analysis showed that group differences were not due to response bias. Discriminant function analysis showed that yes/no recognition was a better predictor of group membership than free recall or forced-choice measures. MCI subjects recalled fewer items than comparison subjects, with no group differences in repetitions, intrusions, serial position effects, or measures of recall strategy (subjective organization, recall consistency). Performance deficits on free recall and recognition in MCI suggest a combination of both tests may be useful for defining episodic memory impairment associated with MCI and early Alzheimer's disease.

Auditory cortical activity in amnestic mild cognitive impairment: relationship to subtype and conversion to dementia

Mild cognitive impairment (MCI) patients have a high risk of converting to Alzheimer's disease. The most common diagnostic subtypes of MCI have an episodic memory disorder (amnestic MCI) occurring either alone [single domain (SD)] or with other cognitive impairments [multiple domain (MD)]. Previous studies report increased amplitudes of auditory cortical potentials in MCI, but their relationships to MCI subtypes and clinical outcomes were not defined. We studied subjects with amnestic MCI (n = 41: 28 SD, 13 MD), Alzheimer's disease (n = 14), and both younger (n = 22) and age-matched older controls (n = 44). Baseline auditory sensory (P50, N100) and cognitive potentials (P300) were recorded during an auditory discrimination task. MCI patients were followed for up to 5 years, and outcomes were classified as (i) continued diagnosis of MCI (MCI-stable, n = 16), (ii) probable Alzheimer's disease (MCI-convert, n = 18), or other outcomes (n = 7). Auditory potentials were analysed as a function of MCI diagnosis and outcomes, and compared with young, older controls, and mild Alzheimer's disease subjects. P50 amplitude increased with normal ageing, and had additional increases in MCI as a function of both initial diagnosis (MD > than SD) and outcome (MCI-convert > MCI-stable). P300 latency increased with normal ageing, and had additional increases in MCI but did not differ among outcomes. We conclude that auditory cortical sensory potentials differ among amnestic MCI subtypes and outcomes occurring up to 5 years later.

Vascular volume and blood-brain barrier permeability measured by dynamic contrast enhanced MRI in hippocampus and cerebellum of patients with MCI and normal controls

PURPOSE

To measure the cerebrovascular volume and blood-brain barrier (BBB) permeability indices in hippocampus and cerebellum of patients with mild cognitive impairment (MCI) using dynamic contrast-enhanced MRI (DCE-MRI), and compare to that of normal controls.

MATERIALS AND METHODS

A total of 11 MCI subjects and 11 healthy elderly controls participated in this prospective study. DCE-MRI was performed to measure the contrast enhancement kinetics. The early enhancement percentage (at 50 seconds after injection) was defined as the vascular volume index, and the ratio between the four to five-minute enhancement relative to the 50-second enhancement was defined as the BBB permeability index.

RESULTS

The enhancement kinetics measured from hippocampus of MCI individuals demonstrated a lower magnitude and slower decay than healthy controls, suggesting that they had a smaller vascular volume (significant in the right side; P <0.001) and a higher BBB permeability (not reaching significance level). The vascular volume index was significantly correlated with naming ability (P 0.05).

CONCLUSION

These results suggest that changes in cerebrovasculature may occur in hippocampus of MCI. DCE-MRI may provide a noninvasive means to measure the subtle BBB leakage associated with the cerebrovascular pathology commonly found in Alzheimer's disease.

Absence of postexercise and delayed facilitation of motor cortex excitability in restless legs syndrome: evidence of altered cortical plasticity?

STUDY OBJECTIVE

Restless legs syndrome (RLS) is a neurologic disorder with well-defined clinical and diagnostic criteria but the pathophysiology of which is unclear. Previous studies have suggested alterations in motor cortex function in RLS. We aimed to compare motor cortex plasticity in subjects with RLS versus healthy controls.

DESIGN

Biphasic single-pulse transcranial magnetic stimulation (TMS) of the nondominant hemisphere was used to define motor evoked potential (MEP) amplitude, motor threshold, and silent period. Subjects also performed 3 blocks (30-s, 60-s, and 90-s duration) of a bimanual motor task (exercise condition). Amplitude of MEPs elicited immediately after each block, and then after a 15-minute rest period were compared with baseline. The time course of intracortical inhibition was also tested using paired-pulse TMS at 1- to 6-millisecond interstimulus intervals.

SETTING

Clinical neurophysiology laboratory in a General Hospital.

STUDY PARTICIPANTS

For the single-pulse TMS procedures, the RLS group included 11 patients affected by primary RLS and the control group included 11 age- and sex-matched normal subjects. For the paired-pulse TMS procedures, there were 9 patients and 6 controls.

INTERVENTIONS

None.

RESULTS

There were no group differences in motor threshold and MEP amplitudes, but the silent period was significantly shorter in subjects with RLS. Compared with baseline, control subjects had larger MEP amplitudes after 30 and 60 seconds of exercise and also after the rest period. In contrast, MEP amplitudes in patients with RLS were not significantly different from baseline after any of the exercise conditions or following the rest period. Patients with RLS also had decreased short-latency paired-pulse inhibition.

CONCLUSIONS

Findings show abnormal motor cortex inhibition and cortical excitability differences in RLS. We suggest the possibility of alterations in movement-related cortical plasticity in RLS.

Auditory brain-stem, middle- and long-latency evoked potentials in mild cognitive impairment

OBJECTIVE

Mild cognitive impairment (MCI) is a selective episodic memory deficit in the elderly with a high risk of Alzheimer's disease. The amplitudes of a long-latency auditory evoked potential (P50) are larger in MCI compared to age-matched controls. We tested whether increased P50 amplitudes in MCI were accompanied by changes of middle-latency potentials occurring around 50 ms and/or auditory brain-stem potentials.

METHODS

Auditory evoked potentials were recorded from age-matched controls (n = 16) and MCI (n = 17) in a passive listening paradigm at two stimulus presentation rates (2/s, 1/1.5 s). A subset of subjects also received stimuli at a rate of 1/3 s.

RESULTS

Relative to controls, MCI subjects had larger long-latency P50 amplitudes at all stimulus rates. Significant group differences in N100 amplitude were dependent on stimulus rate. Amplitudes of the middle-latency components (Pa, Nb, P1 peaking at approximately 30, 40, and 50 ms, respectively) did not differ between groups, but a slow wave between 30 and 49 ms on which the middle-latency components arose was significantly increased in MCI. ABR Wave V latency and amplitude did not differ significantly between groups.

CONCLUSIONS

The increase of long-latency P50 amplitudes in MCI reflects changes of a middle-latency slow wave, but not of transient middle-latency components. There was no evidence of group difference at the brain-stem level.

SIGNIFICANCE

Increased slow wave occurring as early as 50 ms may reflect neurophysiological consequences of neuropathology in MCI.

Event-related potentials accompanying motor preparation and stimulus expectancy in the young, young-old and oldest-old

Although aging is accompanied by neurobiological changes and increased susceptibility to many neurological disorders, little is known about neurophysiological changes that start in old age. Here, neurophysiological changes during old age were assessed by recording brain potentials associated with motor preparation and stimulus expectancy (contingent negative variation, CNV) in young-old (60-69), oldest-old (85-98), and young (17-23) subjects. Individual trials began by a button press, followed 2.5 s later by either a low or high pitch tone. In the "motor" condition subjects responded following high pitch tones (P=0.20); in the "non-motor" condition subjects did not respond. Motor condition CNV amplitudes in the oldest old were more positive than the young and young-old groups, which were similar. In the non-motor condition, the young-old and oldest-old had similar CNV amplitudes that were positive in polarity, and were significantly different from young subjects. Motor potentials before button presses that started the trials were comparable among groups. Results show that neural activity associated with motor preparation and stimulus expectancy changes during advanced age, and that group differences can be modulated by task requirements.

Age-related differences in auditory event-related potentials during a cued attention task

OBJECTIVE

To determine if aging is associated with differences in attentional regulation using behavioral and event-related potential (ERP) measures.

METHODS

Younger (n=13;M=20 years) and older (n=12;M=76 years) subjects performed an auditory cued attention task. Verbal cues correctly (valid) or incorrectly (invalid) predicted the ear receiving a target tone 1.5 s later, or were uninformative (neutral). Targets were either 'high' (2000 Hz) or 'low' (1000 Hz) pitch monaural tones. Subjects pressed one of 4 buttons to indicate target ear and pitch. ERPs following cues and targets (P50, N100, P200, slow waves), and negative slow potentials (CNV) between cues and targets were assessed.

RESULTS

Cue information had significant effects on reaction time for both groups (valid<neutral<invalid). Target N100 amplitude was significantly affected by cueing in younger (invalid>valid) but not older subjects. Target slow waves were also affected by cue information (invalid>valid), and the difference was larger and lasted longer in older subjects. Slow waves following cues were significantly larger in older subjects, but the subsequent CNV amplitudes were comparable among groups.

CONCLUSIONS

When performing a cued attention task, age differences are present in transient ERPs following cues and targets.

SIGNIFICANCE

Age differences in ERPs associated with attentional regulation support the hypothesis that attentional changes contribute to cognitive aging.

Passive movements of the head do not abolish anticipatory firing properties of head direction cells

Neurons in the anterior dorsal thalamic nucleus (ADN) of the rat selectively discharge in relation to the animal's head direction (HD) in the horizontal plane. Temporal analyses of cell firing properties reveal that their discharge is optimally correlated with the animal's future directional heading by approximately 24 ms. Among the hypotheses proposed to explain this property is that ADN HD cells are informed of future head movement via motor efference copy signals. One prediction of this hypothesis is that when the rat's head is moved passively, the anticipatory time interval (ATI) will be attenuated because the motor efference signal reflects only the active contribution to the movement. The present study tested this hypothesis by loosely restraining the animal and passively rotating it through the cell's preferred direction. Contrary to our prediction, we found that ATI values did not decrease during passive movement but in fact increased significantly. HD cells in the postsubiculum did not show the same effect, suggesting independence between the two sites with respect to anticipatory firing. We conclude that it is unlikely that a motor efference copy signal alone is responsible for generating anticipatory firing in ADN HD cells.

Visual encoding differentially affects auditory event-related potentials during working memory retrieval

Previous working memory studies using auditory stimuli at both encoding and retrieval show amplitude decreases in event-related potentials (N100 and late positive wave, LPW) at retrieval as a function of memory load. This study tested if these effects are associated with phonological or semantic coding by presenting visual stimuli at encoding and auditory stimuli at retrieval. We hypothesized that event-related potentials associated with phonological but not semantic coding would be affected by modality differences at encoding and retrieval. Memory sets having one, three, or five visual digits were followed by auditory probes that subjects classified as present or absent from the set. Reaction time increased and LPW amplitudes decreased with increases in memory load, but there were no significant effects of memory load on N100 amplitude. Results suggest that with respect to brain activity that covaries with memory load, probe N100 amplitude is associated with phonological coding and LPW amplitude is associated with semantic coding.

Serial Position Effects in Auditory Event-related Potentials during Working Memory Retrieval

It is established that recall of an item from a list of sequentially presented items is sensitive to the item's position in the memorized list. However, little is known about the brain mechanisms that mediate these serial position effects. Studies of working memory retrieval using event-related potentials report amplitude reductions during retrieval (auditory cortical N100, neocortical late positive wave [LPW]) as memory load increases. We tested the hypothesis that N100 and LPW amplitudes to probes are also affected by serial position. Eventrelated potentials were recorded from subjects performing an auditory working memory task. A set of one or five digits was memorized, then subjects classified a probe digit as either present or absent from the memory set. A control task was also given. Amplitudes of the N100 and LPW were reduced in the 5-item versus the 1-item set. In the 5-item set N100 amplitude was significantly larger for the initial (1st) serial position, relative to Positions 2–5, while linear increases in LPW amplitude were seen across serial positions (5th > 1st position). A control task without memorization showed no N100 or LPW amplitude changes with set size or serial position. The findings reveal that the N100 and LPW are influenced differently by serial position during working memory retrieval: N100 shows a primacy effect and LPW demonstrates a recency effect. The results suggest that primacy and recency effects may be mediated by different brain regions at different times during memory retrieval.

Passive transport disrupts directional path integration by rat head direction cells

A subset of neurons in the rat limbic system encodes head direction (HD) by selectively discharging when the rat points its head in a preferred direction in the horizontal plane. The preferred firing direction is sensitive to the location of landmark cues, as well as idiothetic or self-motion cues (i.e., vestibular, motor efference copy, proprioception, and optic flow). Previous studies have shown that the preferred firing direction remains relatively stable (average shift +/- 18 degrees ) after the rat walks from a familiar environment into a novel one, suggesting that without familiar landmarks, the preferred firing direction can be maintained using idiothetic cues, a process called directional path integration. This study repeated this experiment and manipulated the idiothetic cues available to the rat as it moved between the familiar and novel environment. Motor efference copy/proprioceptive cues were disrupted by passively transporting the animal between the familiar and novel environment. Darkening the room as the animal moved to the novel environment eliminated optic flow cues. HD cell preferred firing directions shifted in the novel environment by an average of 30 degrees after locomotion from the familiar environment with the room lights off; by an average of 70 degrees after passive transport from the familiar environment with the room lights on; and by an average of 67 degrees after passive transport with the room lights off. These findings are consistent with the view that motor efference copy/proprioception cues are important for maintaining the preferred firing direction of HD cells under conditions requiring path integration.

Differences between appetitive and aversive reinforcement on reorientation in a spatial working memory task

Tasks using appetitive reinforcers show that following disorientation rats use the shape of an arena to reorient, and cannot distinguish two geometrically similar corners to obtain a reward, despite the presence of a prominent visual cue that provides information to differentiate the two corners. Other studies show that disorientation impairs performance on certain appetitive, but not aversive, tasks. This study evaluated whether rats would make similar geometric errors in a working memory task that used aversive reinforcement. We hypothesized that in a task that used aversive reinforcement rats that were initially disoriented would not reorient by arena shape and thus make similar geometric errors. Tests were performed in a rectangular arena having one polarizing cue. In the appetitive condition water consumption was the reward. The aversive condition was a water maze task with reinforcement provided by escape to a hidden platform. In the aversive condition rats returned to the reinforced corner significantly more often than in the dry condition, and did not favor the diagonally opposite corner. Results show that rats can use cues besides arena shape to reorient in an aversive reinforcement condition. These findings may also reflect different strategies, with an escape/homing strategy in the wet condition and a foraging strategy in the dry condition.

Preparatory slow potentials and event-related potentials in an auditory cued attention task

OBJECTIVES

To examine reaction times and event-related potentials (ERPs) in an auditory cued attention task varying motor requirements, cue validity, and cue location.

METHODS

Subjects (n=13) listened to cue-target stimulus pairs. Verbal cues (monaural, binaural) indicated the ear to receive a target tone 1.5s later. Cues correctly (valid) or incorrectly (invalid) predicted target ear, or were uninformative (neutral). In separate conditions subjects either responded by pressing one of two buttons, or did not respond to targets. ERPs for cues and targets (P50, N100, P200, late slow wave), and negative slow potentials between cues and targets were assessed.

RESULTS

Target reaction times for valid cues were significantly shorter than for invalid cues, with intermediate values for neutral cues. When no motor response was required larger ERPs were seen to both cues and targets. Negative slow potentials had larger amplitudes before target presentation when subjects responded to targets; and were larger following neutral, vs. valid/invalid, cues. ERPs (N100, P200) to invalidly cued targets were significantly larger and a subsequent late slow wave was more positive, relative to validly cued targets.

CONCLUSIONS

Expectancy for targets begins shortly after cue presentation, and is affected by both motor requirements and the information content of the cue. ERP amplitudes to targets are modulated by the correspondence between cue information and actual target location.

Auditory event-related potentials during target detection are abnormal in mild cognitive impairment

OBJECTIVE

To define brain activity and behavioral changes in mild cognitive impairment (MCI), an isolated memory deficit in the elderly that is a major risk factor for Alzheimer's disease.

METHODS

Brain potentials and reaction time were examined in elderly controls (n=12) and MCI (n=15) using a target detection paradigm. Subjects listened to a sequence of tones and responded to high-pitched target tones (P=0.20) that were randomly mixed with low-pitched tones (P=0.80). Measures were a pre-stimulus readiness potential (RP), post-stimulus potentials (P50, N100, P200, N200, P300), and reaction time.

RESULTS

Accuracy was equivalent between groups, but there was a trend for longer reaction times in MCI (P=0.08). Two potentials differed between groups: (1) P50 amplitude and latency were significantly increased in MCI, and (2) P300 latency was significantly longer in MCI. Results from two MCI subjects that converted to Alzheimer's disease are also discussed.

CONCLUSIONS

Brain potentials in MCI subjects during target detection have certain features similar to healthy aging (RP, N100, P200, N200), and other features similar to Alzheimer's disease (delayed P300 latency, slower reaction time). P50 differences in MCI may reflect pathophysiological changes in the modulation of auditory cortex by association cortical regions having neuropathological changes in early Alzheimer's disease.

Sensory cortical interactions in aging, mild cognitive impairment, and Alzheimer's disease

Progressive declines in memory function accompany normal aging, mild cognitive impairment (MCI), and Alzheimer's disease (AD). Neuropathological studies suggest that damage to neurons providing connections between cortical areas may contribute to memory impairments in AD. Because AD develops slowly, similar neuropathological changes, to a lesser degree, may be present in MCI and some asymptomatic elderly subjects. In this study we tested the hypothesis that corticocortical interactions between sensory regions are impaired in aging, MCI, and AD, as compared with young subjects. When sensory cortical evoked potentials are elicited by pairs of stimuli the amplitudes of potentials to the second stimulus are attenuated. Corticocortical interactions were assessed by presenting stimulus pairs in different modalities (auditory/visual). There were significant group differences in the degree that a visual stimulus attenuated subsequent auditory potentials (young > healthy elderly > MCI > AD). Control experiments indicated equivalent amplitude reductions for all groups to the second stimulus for stimulus pairs having the same modality. These findings are compatible with progressive declines in corticocortical processing in aging, MCI, and AD.

On the behavioral significance of head direction cells: neural and behavioral dynamics during spatial memory tasks

Current theories assume that rats use the directional information reflected by head direction (HD) cells when performing spatial tasks. This assumption was assessed by monitoring anterior thalamic HD cell activity and relating it to the subject's behavioral response on 2 spatial memory tasks that tested either reference memory or working memory. In both tasks, there was a significant number of trials where there was not a tight coupling between the preferred firing direction of HD cells and the direction of the behavioral response. In addition, it was possible to intentionally change the preferred direction of HD cells without affecting performance accuracy. An additional experiment showed that manipulations that affected internal, but not external, cues impaired performance on the reference memory task. These findings suggest that HD cell activity was not consistently guiding the subjects' behavior on these 2 spatial tasks.

Age-related qualitative differences in auditory cortical responses during short-term memory

OBJECTIVE

To examine the affects of aging on auditory cortical activity during a short-term memory task.

METHODS

Young and elderly subjects performed a working memory task using acoustically presented digits while evoked potential components (N100, P200) generated by auditory cortex were recorded. Reaction time and N100/P200 amplitudes and latency were analyzed as a function of memory load.

RESULTS

N100 amplitude to probes decreased as a function of memory load in young subjects, but increased as a function of memory load in the elderly. Young subjects also exhibited changes in N100 latency during memorization of list items, a result not found in elderly subjects.

CONCLUSIONS

We conclude that normal aging is associated with a qualitatively different pattern of N100 responses during memory retrieval, and a static N100 response during encoding. The findings suggest that aging is accompanied by functional reorganization of the neural network that supports retrieval in auditory working memory.

Effects of stimulus sequence on event-related potentials and reaction time during target detection in Alzheimer's disease

OBJECTIVES

To examine evoked potentials and behavior as a function of stimulus sequence in an auditory target detection paradigm in Alzheimer's disease (AD).

METHODS

Evoked potentials and reaction times were collected from 12 healthy elderly controls and 10 patients with mild AD. Subjects pressed a response button to high-pitched target tones (P=0.20) that were randomly intermixed with low-pitched frequent tones. We measured pre-stimulus readiness potential (RP), event-related potentials (P50, N100, P200, N200 and P300), and reaction time as a function of the stimulus sequence.

RESULTS

AD subjects performed at comparable levels of accuracy as controls, but had significantly increased reaction times. Grand averaged potentials in AD showed a significant reduction of the amplitude of the RP, and an increase of P300 latency. Both controls and AD showed speeding of reaction time, increases in RP amplitude, and decreases in P300 latency as a function of the number of frequents preceding the target. Sequential changes of other components (P200 and N200) were found in controls but not AD.

CONCLUSIONS

AD patients have systematic changes of both RT and certain of the evoked potential components as a function of stimulus sequence. Moment-by-moment changes in target expectancy are largely preserved in AD, even though overall performance and evoked potential measures of expectancy (RP) and stimulus classification (P300 latency) are abnormal.

Head direction cells in rats with hippocampal or overlying neocortical lesions: evidence for impaired angular path integration

Rodents use two distinct navigation strategies that are based on environmental cues (landmark navigation) or internal cues (path integration). Head direction (HD) cells are neurons that discharge when the animal points its head in a particular direction and are responsive to the same cues that support path integration and landmark navigation. Experiment 1 examined whether HD cells in rats with lesions to the hippocampus plus the overlying neocortex or to just the overlying neocortex could maintain a stable preferred firing direction when the rats locomoted from a familiar to a novel environment, a process thought to require path integration. HD cells from both lesion groups were unable to maintain a similar preferred direction between environments, with cells from hippocampal rats showing larger shifts than cells from rats sustaining only cortical damage. When the rats first explored the novel environment, the preferred directions of the cells drifted for up to 4 min before establishing a consistent firing orientation. The preferred direction was usually maintained during subsequent visits to the novel environment but not across longer time periods (days to weeks). Experiment 2 demonstrated that a novel landmark cue was able to establish control over HD cell preferred directions in rats from both lesion groups, showing that the impairment observed in experiment 1 cannot be attributed to an impairment in establishing cue control. Experiment 3 showed that the preferred direction drifted when HD cells in lesioned animals were recorded in the dark. It was also shown that the anticipatory property of anterodorsal thalamic nucleus HD cells was still present in lesioned animals; thus, this property cannot be attributed to an intact hippocampus. These findings suggest that the hippocampus and the overlying neocortex are involved in path integration mechanisms, which enable an animal to maintain an accurate representation of its directional heading when exploring a novel environment.

Cue control and head direction cells

Previous research has shown that head direction (HD) cells in both the anterior dorsal thalamus (ADN) and the postsubiculum (PoS) in rats discharge in relation to familiar, visual landmarks in the environment. This study assessed whether PoS and ADN HD cells would be similarly responsive to nonvisual or unfamiliar environmental cues. After visual input was eliminated by blindfolding the rats, HD cells maintained direction-specific discharge, but their preferred firing directions became less stable. In addition, rotations of the behavioral apparatus indicated that some nonvisual cues (presumably tactile, olfactory, or both) exerted above chance stimulus control over a cell's preferred firing direction. However, a prominent auditory cue was not effective in exerting stimulus control over a cell's preferred direction. HD cell activity also was assessed after rotation of a novel visual cue exposed to the rat for 1, 3, or 8 min. An 8-min exposure was enough time for a novel visual cue to gain control over a cell's preferred direction, whereas an exposure of 1 or 3 min led to control in only about half the sessions. These latter results indicate that HD cells rely on a rapid learning mechanism to develop associations with landmark cues.

Cue control and head direction cells

Previous research has shown that head direction (HD) cells in both the anterior dorsal thalamus (ADN) and the postsubiculum (PoS) in rats discharge in relation to familiar, visual landmarks in the environment. This study assessed whether PoS and ADN HD cells would be similarly responsive to nonvisual or unfamiliar environmental cues. After visual input was eliminated by blindfolding the rats, HD cells maintained direction-specific discharge, but their preferred firing directions became less stable. In addition, rotations of the behavioral apparatus indicated that some nonvisual cues (presumably tactile, olfactory, or both) exerted above chance stimulus control over a cell's preferred firing direction. However, a prominent auditory cue was not effective in exerting stimulus control over a cell's preferred direction. HD cell activity also was assessed after rotation of a novel visual cue exposed to the rat for 1, 3, or 8 min. An 8-min exposure was enough time for a novel visual cue to gain control over a cell's preferred direction, whereas an exposure of 1 or 3 min led to control in only about half the sessions. These latter results indicate that HD cells rely on a rapid learning mechanism to develop associations with landmark cues.