Biomedical subjects

L Nielsen-Bohlman

Publications and source records attributed to L Nielsen-Bohlman.

7 recordsLinked to original sources

PubMed1999

Prefrontal cortical involvement in visual working memory.

Studies of human amnesia provide evidence for a short-term memory store with information transfer to long term memory occurring within 60 s of sensory encoding. Human and nonhuman primate research has shown that maintenance of this short-term or working memory store is dependent upon frontal cortical activation, although the critical temporal parameters of frontal involvement throughout this 60-s window are undetermined. We examined prefrontal contributions to rapid (under 2 s) and sustained (over 4 s) visual working memory by recording behavioral performance and event-related potentials (ERPs) in patients with lesions in dorsolateral frontal cortex and age-matched control subjects. Prefrontal lesioned patients generated a reduced sustained frontal positivity at all delays. At short delays, patients generated reduced performance to stimuli presented in the contralesional field. Patients generated a negative potential (N400), greatest to contralesionally presented stimuli, that was observed in the control subjects only at long delays. The results indicate that prefrontal lesions impair the frontal component of an anterior-posterior working memory network activated during rapid and sustained visual memory processing. Frontal patients may require activation of limbic cortex, indexed by N400, for maintenance of both rapid and sustained working memory.

Aging↗

PubMed1997

Impaired word-stem priming in patients with temporal-occipital lesions.

In the word-stem priming test, words are presented (e.g., MOTEL, PARADE), and later subjects are shown three-letter word stems (e.g., MOT, PAR) and asked to complete each stem with the first word that comes to mind. Word-stem priming, as well as other aspects of implicit memory, are intact in amnesic patients with medial temporal lesions. However, this form of priming has been shown to be impaired in patients with Alzheimer's disease, suggesting that damage to neocortical areas outside the medial temporal lobe contributes to impaired priming in these patients. To examine the role of posterior cortical areas on word-stem priming, we administered the test to patients with unilateral temporal-occipital lesions. Patients with temporal-occipital lesions exhibited significantly impaired priming on this test. The findings suggest a critical role of the inferior posterior neocortex in the expression of this form of implicit memory.

Aged↗

PubMed1997

Semantic priming impairment in HIV.

HIV+ subjects have shown impairment on tests of executive function including automatic attention and verbal tasks. Impairment of semantic priming in HIV patients would suggest a disruption of automatic semantic activation. We examined semantic priming in HIV+ individuals and HIV- control participants with no history of substance abuse, neurologic or psychiatric disorder unrelated to HIV. HIV+ participants were divided into cognitively normal and cognitively impaired subgroups on the basis of a neuropsychological battery of 15 tests. Participants were presented with English words and nonword letter strings and indicated if the stimulus was a word or nonword. The nonwords were orthographically and phonologically correct and were created by rearranging the letter sequence of words ("ulpit"). All words had an obvious antonym ("deep"); two-thirds were presented as sequential antonym pairs ("enter"-"exit"). There were no group differences in speed of response to nonwords, indicating no generalized reaction time deficit. While control and cognitively normal HIV+ participants showed an effect of priming on reaction time to correctly detected words, cognitively impaired HIV+ participants did not. The lack of semantic priming demonstrated by cognitively impaired HIV+ participants suggests that they have lessened activation of automatic semantic networks.

Adult↗

PubMed1995

Auditory event-related potentials dissociate early and late memory processes.

Event-related potentials (ERPs) to environmental sounds were recorded from 15 young control subjects in an auditory recognition memory task. Subjects listened to a continuous string of binaurally presented sounds, 20% of which were presented once and 80% were repeated. Of the repeated sounds, some repeated immediately after the initial presentation (2 sec; short delay repetition) while others repeated after 2-6 intervening sounds (4-12 sec; long delay repetition). Subjects were instructed to indicate whether they had heard the sounds before by pressing a "yes" or "no" button. The initial stimulus presentation and long delay repetition stimuli generated both an N4 component and a prolonged latency P3 component while the short delay repetition stimuli elicited no N4 component and an earlier latency P3 component. Subjects' responses were faster and more accurate for short delay repetition. All stimuli generated a sustained frontal negative component (SFN). These data indicate that auditory recognition memory for environmental sounds may involve two processes. The P3 generated by both short and long delay repetition stimuli may index activation of a neocortical template matching system. The N4 generated by initial stimulus presentations and long delay repetition is proposed to measure additional activation of limbic memory systems at long retention intervals.

Acoustic Stimulation↗

PubMed1994

Electrophysiological dissociation of rapid memory mechanisms in humans.

Human memory involves an interaction between transient working memory and a long-term store. We found electrophysiological evidence supporting the existence of two distinct mechanisms subserving these processes. Stimuli held in memory for less than 4 s generate large, early latency P300 potentials which may index activation of a frontally-mediated rapid working memory system. Stimuli held in memory for over 4 s selectively generate N400 potentials which may reflect activation of mesial temporal cortices involved in access to the long-term store. These results suggest that memory processing in the initial 10 s after stimulus detection involves at least two distinct distributed cortical-limbic systems.

Adult↗

PubMed1991

Differential auditory processing continues during sleep.

Auditory evoked potentials (AEPs) were used to examine selective stimulus processing in sleep. In waking, repetitive stimuli generate exogenous P1, N1 and P2 components of the auditory evoked potential (AEP). Deviant stimuli generate endogenous cognitive components including the mismatch negativity (MMN), N2 and P3 components. We examined long-latency auditory evoked potentials elicited by repetitive and deviant stimuli during waking and stage II-IV sleep to assess whether stimulus deviance is detected during sleep. The waking P1, N1b and P2 had maximal amplitudes at fronto-central scalp sites, with additional peaks (N1a, N1c) at temporal sites. Deviant tones generated a frontal maximal MMN, and complex novel tones generated an additional P3 component maximal at centro-parietal sites. During stages II-IV sleep N1a, b, c amplitudes were reduced. During stage II sleep all stimuli generated increased P2 amplitudes and a late negative component (N340). Deviant stimuli generated greater P2 and N340 amplitudes than frequent stimuli in stage II sleep, as well as an additional P420 component. In stage III-IV sleep the P420 was absent and the AEP was dominated by a negativity of long duration whose amplitude increased in response to deviant stimuli. These data indicate that auditory evoked activity changes from wakefulness to sleep. The differential response to deviant sounds observed during waking and all sleep stages supports the theory that selective processing of auditory stimuli persists during sleep.

Adolescent↗

PubMedDate not supplied

Prefrontal alterations during memory processing in aging.

Studies of human amnesia provide evidence for a short-term memory store with information transfer to long-term memory occurring within 60 sec of encoding. Frontal cortical activation is critical for maintenance of the short-term store, and limbic structure are necessary for access to the long-term store. The P3 and N4 components of the event-related potential (ERP) are generated during memory processes mediated by these brain regions. The current study examines the effects of age on ERPs generated to correctly identified stimuli presented at delays of 1-150 sec in a visual recognition memory task. Consistently different evoked potentials and performance were obtained to stimuli repeated at 1.2 sec delay as opposed to stimuli repeated at delays of over 4 sec in all subjects. At the 1.2 sec delay, the performance and posterior P3 amplitudes generated by old and young subjects were comparable. At longer delays, the older subjects had impaired performance and decreased P3 amplitude at posterior scalp sites. In addition, fronto-central N4 activity was reduced at long delays in the elderly. Older subjects generated a sustained frontal positivity (50-800 msec) to both short and long delay stimuli that was not observed in the younger group. The late phase of the frontal positivity was enhanced at long delays in the elderly. The data provide evidence of intact rapid and impaired delayed recognition memory in aging. Alternations in frontal cortical control of posterior and limbic regions may contribute to the memory changes observed in aging.

Adult↗