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El cerebro desactiva el ruido que le molesta

Para asegurarse de que un ratón escucha los sonidos de un gato que se acerca, mejor que escuchar los sonidos que producen sus propios pasos, el cerebro del ratón tiene un circuito de cancelación de ruido incorporado. Es una conexión directa de la corteza motora del cerebro a la corteza auditiva que dice esencialmente: “estoy corriendo ahora, no debo prestar atención al sonido de mis pasos”.

“Lo especial de este proceso de cancelación es que el cerebro aprende a desactivar las respuestas a sonidos autogenerados predecibles”, afirma el autor principal de esta investigación, Richard Mooney, profesor de Neurobiología de la Universidad Duke, en Durham, en Carolina del Norte, en Estados Unidos. “Puedes ver cómo estas respuestas desaparecen en función del tiempo y la experiencia”, añade este experto, cuyo trabajo se publica en la edición digital de este miércoles de ‘Nature’.

Este circuito cerebral funciona de manera diferente a los auriculares con cancelación de ruido, pero los resultados son similares. Los auriculares monitorizan el ruido ambiental alrededor del oyente y luego producen sonidos que son imágenes especulares de esas ondas de sonido para cancelarlas. Del mismo modo, la corteza auditiva del cerebro recibe una señal directamente de la corteza motora que dice a sus neuronas inhibidoras que cancelen selectivamente los sonidos que aprendió que surgirán de un movimiento particular.

Para que este sistema funcione, no puede depender únicamente de la entrada de los oídos, dice Mooney, “porque para cuando la señal auditiva del oído es procesada por el cerebro, son noticias viejas”. De hecho, la corteza motora envía la señal de cancelación a la corteza auditiva en paralelo al comando de un movimiento, un proceso tan rápido que la cancelación en la corteza auditiva es realmente predictiva.

“No se escucha el sonido de la primera pisada”, subraya David Schneider, exinvestigador postdoctoral de Duke en el Laboratorio de Mooney, que ahora es profesor asistente en la Universidad de Nueva York. “Tendríamos dificultades para operar en el mundo natural, si no pudiéramos predecir las consecuencias sensoriales de moverse en él”, explica Mooney, quien también estudió la conexión entre la corteza auditiva y la corteza motora a medida que las aves aprenden a cantar.

Experimentos de “realidad virtual”

Para monitorizar el circuito, Schneider y la estudiante de posgrado de Duke Janani Sundararajan entrenaron a los ratones para asociar un tono artificial con sus pisadas. A medida que los ratones caminaron o corrieron en una cinta rodante en este experimento de “realidad virtual”, el tempo del tono se correspondía con cada golpeteo. “Decidimos hacer el sonido lo más artificial posible para empujar el cerebro del ratón más allá de lo que se había desarrollado”, dice Schneider.

Schneider y Sundararajan observaron el cerebro del ratón como sinapsis que la corteza motora produce en la corteza auditiva cambiada a medida que aprendía a cancelar un ruido predecible relacionado con el movimiento. Fueron capaces de identificar las neuronas inhibidoras que respondieron al tono artificial para cancelar su señal, “exactamente igual a la cancelación de ruido”, resume Schneider.

Para confirmar lo que estaban viendo, Sundararajan realizó una serie de experimentos de comportamiento en los que se enseñó a los roedores a buscar una recompensa después de escuchar dos tonos diferentes. Luego, los entrenó en la cinta de correr como antes para asociar uno de esos tonos con caminar. Después del entrenamiento, los ratones detectaron el tono no vinculado mejor que el tono de “caminar” cuando estaban caminando, a pesar de que detectaron ambos tonos igualmente bien cuando estaban quietos.

“El cerebro preferiría ser más sensible a los ruidos que no sean los que hacemos”, dice Sundararajan. Para un ratón acosado por un gato cercano, es una cuestión de supervivencia.

 

Fuente: Instituto Integral Audiológico

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