Two columnar systems in the auditory neostriatum of the chick: evidence from 2-deoxyglucose

In chicks, monaurally deafened at 3 weeks of age, 2-deoxyglucose labeling of the input layer L2 in the auditory neostriatum (field L) was analyzed after acoustic stimulation. Two types of stimuli were used: narrow band frequency modulations and white noise. Both stimuli provide evidence that there are bands or columns with different inputs from the contra- and ipsilateral ear, a rostral, intermediate, and caudal band, all oriented orthogonal to the isofrequency contours. The intermediate band, in addition, shows multiple alternating columns of ipsi-vs. contralateral dominant input, parallel to the isofrequency contours. Consequently, the tonotopic gradient is interrupted several times by these columns. The organization of field L bears interesting parallels to the mammalian auditory cortex and is the first evidence of columnar organization in a submammalian forebrain. The multiple interruptions of the tonotopic gradient between low and high frequencies in the intermediate band by alternating aural dominance has possible implications for understanding directional hearing, as well as for understanding formant analysis of species-specific sounds.