Hippocampal neurogenesis is not required for behavioral effects of environmental enrichment.

Environmental enrichment increases adult hippocampal neurogenesis and alters hippocampal-dependent behavior in rodents. To investigate a causal link between these two observations, we analyzed the effect of enrichment on spatial learning and anxiety-like behavior while blocking adult hippocampal neurogenesis. We report that environmental enrichment alters behavior in mice regardless of their hippocampal neurogenic capability, providing evidence that the newborn cells do not mediate these effects of enrichment.

Intact and impaired cognitive-control processes in schizophrenia.

Deficits of cognitive-control in schizophrenia have been assumed to result from a single impairment that leads to widespread consequences. Contrary to this view, we hypothesized that different control processes operate at different stages of processing, and that only some of these processes may be impaired. We employed two selection tasks to test the hypothesis that patients with schizophrenia have deficits in selecting information in working memory (WM), but not in selecting perceptual information. In the "Ignore" task, which fosters perceptual selection, participants saw a cue to remember either red or blue words, followed by a memory-set (2 red, 2 blue), a brief delay, and then a probe. The "Suppress" task was similar, except the memory-set came before the instruction-cue, and hence selection had to occur in WM. We recorded reaction time and percentage errors for positive probes ("Valid"), and two kinds of negative probes, those that were supposed to have been dropped from WM ("Lures") and those that had not appeared in the memory-set ("Controls"). Compared to healthy controls, patients were impaired in the Suppress but not the Ignore task. This dissociation implies that there are two different selection mechanisms.

Single-dose levodopa administration and aging independently disrupt time production.

We tested the hypothesis that age-related time production deficits are dopamine-mediated. The experiment was conducted double-blind, and with random assignment of 32 healthy aged and 32 healthy young participants to either inert placebo or levodopa (200 mg) groups. The procedure included training participants to produce two target time intervals (6 and 17 sec) in separate blocks, drug/placebo administration, a 1-hr delay, and then delayed free-recall time production retesting without feedback. Participants also performed a speeded choice reaction time (RT) task, as a control for potential dopaminergic and aging effects on attention and psychomotor speed. Results indicate that during retesting, aged participants show duration-dependent timing errors that are larger than those shown by the young participants. Levodopa administration yielded lengthened time production of both target intervals. The aging and levodopa effects did not interact. Also, aging slowed RT and increased RT variability, but levodopa had no effect on the RT. These results suggest that at this dosage and under these specific conditions, timing is dopamine-mediated but the effect of aging on time production is not. Moreover, the levodopa timing effect cannot be attributed to the effects of dopaminergic function on psychomotor speed.

The effects of aging on time reproduction in delayed free-recall.

The experiments presented here demonstrate that normal aging amplifies differences in time production occurring in delayed free-recall testing. Experiment 1 compared the time production ability of two healthy aged groups as well as college-aged participants. During the test session, which followed a 24-h delay and omitted all feedback and examples of the two target intervals, the two groups of aged participants' over-produced a 6s interval. This effect is similar in form to errors shown by young participants, but twice the magnitude. Productions of a 17 s interval were generally accurate overall. However, further analysis indicated that the majority of aged participants over-produced the 17 s interval while a minority greatly under-produced the 17 s interval. Furthermore, aged participants showed violations of the scalar property of timing variability in the training session, while in the test session, only those who under-produced the 17 s interval showed this tendency. In contrast, training session performance was good for all participants. Experiments 2 and 3 investigated the ability of the participants in Experiment 1 to reproduce the length of a line from memory, under conditions analogous to those of the time production experiments. These experiments provided tests of the specificity of the errors observed in Experiment 1. Performance in the older participants was accurate, if more variable, compared to the young participants, in contrast to the time production results, indicating that production inaccuracy in free-recall is specific to interval timing in the current context.