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.

Spectral features control temporal plasticity in auditory cortex.

Cortical responses are adjusted and optimized throughout life to meet changing behavioral demands and to compensate for peripheral damage. The cholinergic nucleus basalis (NB) gates cortical plasticity and focuses learning on behaviorally meaningful stimuli. By systematically varying the acoustic parameters of the sound paired with NB activation, we have previously shown that tone frequency and amplitude modulation rate alter the topography and selectivity of frequency tuning in primary auditory cortex. This result suggests that network-level rules operate in the cortex to guide reorganization based on specific features of the sensory input associated with NB activity. This report summarizes recent evidence that temporal response properties of cortical neurons are influenced by the spectral characteristics of sounds associated with cholinergic modulation. For example, repeated pairing of a spectrally complex (ripple) stimulus decreased the minimum response latency for the ripple, but lengthened the minimum latency for tones. Pairing a rapid train of tones with NB activation only increased the maximum following rate of cortical neurons when the carrier frequency of each train was randomly varied. These results suggest that spectral and temporal parameters of acoustic experiences interact to shape spectrotemporal selectivity in the cortex. Additional experiments with more complex stimuli are needed to clarify how the cortex learns natural sounds such as speech.

Rubella-specific immune complexes after congenital infection and vaccination.

Circulating immune complexes which contained rubella-specific immunoglobulins were detected in 21 out of 63 subjects with congenital rubella and in 39 out of 65 subjects vaccinated with attenuated rubella virus, but in none of 43 subjects susceptible to rubella or 87 subjects with remote naturally acquired immunity to rubella. The presence or level of circulating immune complexes and the presence of rubella-specific complexes did not correlate with conventional serum rubella hemagglutination inhibition antibody titers. In the group with congenital infection, the presence of specific complexes many years after birth was associated with late-emerging clinical problems involving several organ systems. In vaccinates, the presence of specific complexes was associated with a higher incidence of side reactions. Two-thirds of the vaccinates and all of those revaccinated showed specific immune complexes as late as 8 months after immunization.

Efficacy of a virion envelope herpes simplex virus vaccine against experimental skin infections in hairless mice.

Hairless mice were immunized with herpes simplex virus type 1 (HSV-1) envelope antigen (EAG), EAG in association with polyriboinosinic . polyribocytidylic acid-poly-L-lysine complexed with carboxymethylcellulose (PICLC), and inactivated purified HSV-1 (VAG). After 2 weeks the mice were challenged by a percutaneous HSV-1 infection in the orofacial (OF) or lumbosacral (LS) skin area. Following immunization a consistent cell-mediated immune response was observed in all immunized mice, although the humoral immune response was very low, or not detectable. After challenge, a marked secondary humoral and cell-mediated immune response developed in all immunized mice, and the animals were protected against the development of skin lesions and the fatal outcome of infection. However, the establishment of latent infections in the sensory ganglia was not prevented by the immunization procedure.