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Biomedical subjects

Eduardo Mercado

Publications and source records attributed to Eduardo Mercado.

6 recordsLinked to original sources
PubMed2005

Auditory categorization of complex sounds by rats (Rattus norvegicus).

Little research has explored the auditory categorization abilities of mammals. To better understand these processes, the authors tested the abilities of rats (Rattus norvegicus) to categorize multidimensional acoustic stimuli by using a classic category-learning task developed by R. N. Shepard, C. I. Hovland, and H. M. Jenkins (1961). Rats proved to be able to categorize 8 complex sounds on the basis of either the direction or rate of frequency modulation but not on the basis of the range of frequency modulation. Rats' categorization abilities were limited but improved slowly and incrementally, suggesting that learning was not facilitated by selective attention to acoustic dimensions.

Animals↗
PubMed2005

Cortical responses in rats predict perceptual sensitivities to complex sounds.

The common assumption that perceptual sensitivities are related to neural representations of sensory stimuli has seldom been directly demonstrated. The authors analyzed the similarity of spike trains evoked by complex sounds in the rat auditory cortex and related cortical responses to performance in an auditory task. Rats initially learned to identify 2 highly different periodic, frequency-modulated sounds and then were tested with increasingly similar sounds. Rats correctly classified most novel sounds; their accuracy was negatively correlated with acoustic similarity. Rats discriminated novel sounds with slower modulation more accurately than sounds with faster modulation. This asymmetry was consistent with similarities in cortical representations of the sounds, demonstrating that perceptual sensitivities to complex sounds can be predicted from the cortical responses they evoke.

Acoustics↗
PubMed2004

Changes in NMDA receptor expression in auditory cortex after learning.

Extensive practice on auditory learning tasks dramatically alters the functional organization and response properties of neurons in the auditory cortex. The cellular mechanisms responsible for this auditory learning-induced cortical plasticity are unclear; however, changes in synaptic function involving NMDA receptors have been strongly implicated. To test this hypothesis, we measured the change in gene expression of NMDA receptors and associated proteins in the auditory cortex of adult rats trained to perform an auditory identification task. NMDA receptor 2A and 2B gene expression in auditory cortex decreased significantly as auditory discrimination improved whereas expression of Arc, an immediate early gene involved in memory stabilization, increased. These results suggest that changes in NMDA receptors 2A and 2B and Arc enhance synaptic plasticity, thereby facilitating experience-dependent cortical remodeling and auditory learning.

Adaptation, Physiological↗
PubMed2004

Song copying by humpback whales: themes and variations.

Male humpback whales (Megaptera novaeangliae) produce long, structured sequences of sound underwater, commonly called "songs." Humpbacks progressively modify their songs over time in ways that suggest that individuals are copying song elements that they hear being used by other singers. Little is known about the factors that determine how whales learn from their auditory experiences. Song learning in birds is better understood and appears to be constrained by stable core attributes such as species-specific sound repertoires and song syntax. To clarify whether similar constraints exist for song learning by humpbacks, we analyzed changes over 14 years in the sounds used by humpback whales singing in Hawaiian waters. We found that although the properties of individual sounds within songs are quite variable over time, the overall distribution of certain acoustic features within the repertoire appears to be stable. In particular, our findings suggest that species-specific constraints on temporal features of song sounds determine song form, whereas spectral variability allows whales to flexibly adapt song elements.

Acoustics↗
PubMed2002

A connectionist model of septohippocampal dynamics during conditioning: closing the loop.

Septohippocampal interactions determine how stimuli are encoded during conditioning. This study extends a previous neurocomputational model of corticohippocampal processing to incorporate hippocamposeptal feedback and examines how the presence or absence of such feedback affects learning in the model. The effects of septal modulation in conditioning were simulated by dynamically adjusting the hippocampal learning rate on the basis of how well the hippocampal system encoded stimuli. The model successfully accounts for changes in behavior and septohippocampal activity observed in studies of the acquisition, retention, and generalization of conditioned responses and accounts for the effects of septal disruption on conditioning. The model provides a computational, neurally based synthesis of prior learning theories that predicts changes in medial septal activity based on the novelty of stimulus events.

Animals↗