Inverted-U function relating cortical plasticity and task difficulty.

Many psychological and physiological studies with simple stimuli have suggested that perceptual learning specifically enhances the response of primary sensory cortex to task-relevant stimuli. The aim of this study was to determine whether auditory discrimination training on complex tasks enhances primary auditory cortex responses to a target sequence relative to non-target and novel sequences. We collected responses from more than 2000 sites in 31 rats trained on one of six discrimination tasks that differed primarily in the similarity of the target and distractor sequences. Unlike training with simple stimuli, long-term training with complex stimuli did not generate target-specific enhancement in any of the groups. Instead, cortical receptive field size decreased, latency decreased, and paired pulse depression decreased in rats trained on the tasks of intermediate difficulty, whereas tasks that were too easy or too difficult either did not alter or degraded cortical responses. These results suggest an inverted-U function relating neural plasticity and task difficulty.

Environmental enrichment selectively increases glutamatergic responses in layer II/III of the auditory cortex of the rat.

Prolonged exposure to environmental enrichment (EE) induces behavioral adaptation accompanied by detectable morphological and physiological changes. Auditory EE is associated with an increased auditory evoked potential (AEP) and increased auditory gating in the primary auditory cortex. We sought physiological correlates to such changes by comparing synaptic currents in control vs. EE-raised rats, in a primary auditory cortex (AI) slice preparation. Pharmacologically isolated glutamatergic or GABA(A)-receptor-mediated currents were measured using perforated patch whole-cell recordings. Glutamatergic AMPA receptor (AMPAR)-mediated excitatory postsynaptic currents (EPSCs) displayed a large amplitude increase (64+/-11% in EE vs. control) accompanied by a rise-time decrease (-29+/-6% in EE vs. control) and decrease in pair pulse ratio in layer II/III but not in layer V. Changes in glutamatergic signaling were not associated with changes in the ratio between N-methyl-D aspartate-receptor (NMDAR)-mediated vs. AMPAR-mediated components, in amplitude or pair pulse ratio of GABAergic transmission, or in passive neuronal properties. A realistic computational model was used for integrating in vivo and in vitro results, and for determining how EE synapses correct for phase error of the inputs. We found that EE not only increases the mean firing frequency of the responses, but also improves the robustness of auditory processing by decreasing the dependence of the output firing on the phase difference of the input signals. We conclude that behavioral and electrophysiological differences detected in vivo in rats exposed to an auditory EE are accompanied and possibly caused by selective changes in cortical excitatory transmission. Our data suggest that auditory EE selectively enhances excitatory glutamatergic synaptic transmission in layer II/III without greatly altering inhibitory GABAergic transmission.

Evaluation of cardiovascular status in severe leptospirosis.

OBJECTIVES: To note incidence and profile of cardiac involvement in severe leptospirosis in South Gujarat. METHODS: A study was carried out on twenty-five serologically proved leptospirosis patients referred to Government Medical College, New Civil Hospital, Surat between June 2002 to September 2002. In all the patients detailed history, physical examination and specific investigations were done to find out the incidence and profile of cardiac involvement in severe leptospirosis. RESULTS: Out of twenty-five seropositive patients, 14(56%) had cardiovascular manifestations. Electrocardiography abnormalities were seen in 13(52%) patients. The commonest finding was first-degree AV block seen in II(44%) patients followed by ST-segment depression in four (16%) patients, T-wave inversion in leads II, III and avF in two (8%) patients, corrected QT-interval prolongation in three (12%) patients and ventricular premature beats in two (8%) patients. Atrial fibrillation was seen in only one patient. Left ventricular function as assessed by two-dimensional echocardiography was normal in all patients. CONCLUSION: In cardiovascular involvement of leptospirosis, although electrocardiographic abnormalities were commonly seen, there was no left ventricular dysfunction.