Support for a structural model of aural asymmetries.

Right ear (RE) advantages for the recognition of speech can be explained by structural or attentional mechanisms. Structural mechanisms focus on biases in the neural access the ears have to the contralateral and ipsilateral cerebral hemispheres. Attentional mechanisms focus on biases in hemispatial attention. The contribution of structural and attentional mechanisms for a monaural lexical decision task was examined in a group of 26 dextral adults. Trials requiring a lexical decision were randomly intermixed with a tone discrimination task. A pre-test demonstrated that the tone discrimination task produced no ear asymmetry, but could be affected by shifts in spatial attention. Responses were faster in the RE for the lexical decision task. No asymmetry emerged for the tone discrimination task. If the RE advantage for lexical decisions was the result of an attentional bias, a RE advantage should have been evident for the tone discrimination task. The independence of the two tasks supports a structural model of perceptual asymmetry. A structural model which takes into account the dynamic functional organization of the hemispheres is proposed.