Electrophysiological investigations of the somatosensory thalamus of the pigeon

Somatosensory areas in the thalamus of the pigeon were studied electrophysiologically at the single-unit level. Of 459 units, 429 responded to somatosensory stimuli. Of these, 394 units responded specifically to somatosensory stimuli, whereas 8 responded in addition to visual stimuli and 27 to auditory stimuli also (bimodal neurons). Seven units were exclusively driven by visual stimuli and 23 units by auditory stimuli. Recording sites included nucleus dorsalis intermedius ventralis anterior (DIVA) and nucleus dorsolateralis posterior (DLP). The number of bimodal or visual and auditory neurons was much larger in DLP (40%) than in DIVA (7%). This points to a specific somatosensory function of DIVA. There was only a poor somatotopic organization in both nuclei. However, in vertical penetrations, wing representation was usually dorsal to leg representation. Distal parts of the limbs had smaller receptive fields (RFs) than proximal parts, and the smallest RFs were found on the toes, which seem to be represented in most detail.