Developing a hippocampal neural prosthetic to facilitate human memory encoding and recall

OBJECTIVE

We demonstrate here the first successful implementation in humans of a proof-of-concept system for restoring and improving memory function via facilitation of memory encoding using the patient's own hippocampal spatiotemporal neural codes for memory. Memory in humans is subject to disruption by drugs, disease and brain injury, yet previous attempts to restore or rescue memory function in humans typically involved only nonspecific, modulation of brain areas and neural systems related to memory retrieval.

APPROACH

We have constructed a model of processes by which the hippocampus encodes memory items via spatiotemporal firing of neural ensembles that underlie the successful encoding of short-term memory. A nonlinear multi-input, multi-output (MIMO) model of hippocampal CA3 and CA1 neural firing is computed that predicts activation patterns of CA1 neurons during the encoding (sample) phase of a delayed match-to-sample (DMS) human short-term memory task.

MAIN RESULTS

MIMO model-derived electrical stimulation delivered to the same CA1 locations during the sample phase of DMS trials facilitated short-term/working memory by 37% during the task. Longer term memory retention was also tested in the same human subjects with a delayed recognition (DR) task that utilized images from the DMS task, along with images that were not from the task. Across the subjects, the stimulated trials exhibited significant improvement (35%) in both short-term and long-term retention of visual information.

SIGNIFICANCE

These results demonstrate the facilitation of memory encoding which is an important feature for the construction of an implantable neural prosthetic to improve human memory.

Detection of feigned ADHD in college students

Significant motivations and incentives exist for young-adult students to seek a diagnosis of attention-deficit/hyperactivity disorder (ADHD). With ADHD information readily accessible on the Internet, today's students are likely to be symptom educated prior to evaluation. This may result in false-positive diagnoses, particularly when students are motivated to convey symptoms. We evaluated the utility of ADHD symptom checklists, neurocognitive tests, and measures initially developed to detect feigned neurocognitive or psychiatric dysfunction (symptom validity tests [SVTs]). The performance of 31 undergraduates financially motivated and coached about ADHD via Internet-derived information was compared to that of 29 ADHD undergraduates following medication washout and 14 students not endorsing symptomatology. Results indicated malingerers readily produced ADHD-consistent profiles. Symptom checklists, including the ADHD Rating Scale and Conners's Adult ADHD Rating Scale-Self-Rating Form: Long, were particularly susceptible to faking. Conners's Continuous Performance Test-II findings appeared more related to motivation than condition. Promising results were seen with all cognitive SVTs (Test of Memory Malingering [TOMM], Digit Memory Test, Letter Memory Test, and Nonverbal-Medical Symptom Validity Test), particularly TOMM Trial 1 when scored using Trial 2 criteria. All SVTs demonstrated very high specificity for the ADHD condition and moderate sensitivity to faking, which translated into high positive predictive values at rising base rates of feigning. Combining 2 or more failures resulted in only modest declines in sensitivity but robust specificity. Results point to the need for a thorough evaluation of history, cognitive and emotional functioning, and the consideration of exaggerated symptomatology in the diagnosis of ADHD.

Identification of Feigned Mental Retardation Using the New Generation of Malingering Detection Instruments: Preliminary Findings

A recent Supreme Court decision--Atkins v. Virginia, 536 U.S. 304 (2002)--prohibiting the execution of mentally retarded (MR) defendants may have raised the attractiveness of feigning this condition in the criminal justice system. Unfortunately, very few published studies have addressed the detection of feigned MR. The present report compared results from tests of intelligence, psychiatric feigning, and neurocognitive faking in a group of 26 mild MR participants (MR) and 25 demographically matched community volunteers asked to feign MR (CVM). Results showed that the CVM suppressed their IQ scores to approximate closely the level of MR participants. WAIS-III and psychiatric malingering measures were relatively ineffective at discriminating feigned from genuine MR. Although neurocognitive malingering tests were more accurate, their reduced specificity in MR participants was of potential concern. Revised cutting scores, set to maintain a Specificity rate of about .95 in MR clients, were identified, although they require cross-validation. Overall, these results suggest that new cutting scores will likely need to be validated to detect feigned MR using current malingering instruments.

Known-Groups Cross-Validation of the Letter Memory Test in a Compensation-Seeking Mixed Neurologic Sample

Compensation-seeking neuropsychological evaluees were classified into Honest (HON; n = 37) or Probable Cognitive Feigning (PCF; n = 53) groups based on results from the Victoria Symptom Validity Test, the Test of Memory Malingering, and the Digit Span subtest of the Wechsler Adult Intelligence Scale--3rd ed. The groups were generally comparable on demographic, background, and injury severity characteristics, although HON TBI participants were significantly more likely to have a documented loss of consciousness, whereas PCF participants were significantly more likely to be currently on disability. PCF participants scored significantly lower on many neuropsychological test, particularly of memory, as well as higher on most MMPI-2 clinical scales. The PCF group also had significantly higher scores on multiple indices of feigning of psychiatric symptoms. Results from the Letter Memory Test (LMT) were significantly lower for the PCF group, and using the recommended cutting score, specificity was .984, whereas sensitivity was .640, suggesting adequate performance on cross-validation.