AUDITORY NEURONS WITH TRANSITORY AXONS TO VISUAL AREAS FORM SHORT PERMANENT PROJECTIONS

The kitten's auditory cortex (including the first and second auditory fields Al and All) is known to send transient axons to either ipsi‐ or contralateral visual areas 17 and 18. By the end of the first postnatal month the transitory axons, but not their neurons of origin, are eliminated. Here we investigated where these neurons project after the elimination of the transitory axon. Eighteen kittens received early (postnatal day (pd) 2–5) injections of long lasting retrograde fluorescent traces in visual areas 17 and 18 and late (pd 35–64) injections of other retrograde fluorescent tracers in either hemisphere, mostly in areas known to receive projections from Al and All in the adult cat. The middle ectosylvian gyrus was analysed for double‐labelled neurons in the region corresponding approximately to Al and All. Late injections in the contralateral (to the analysed Al, All) hemisphere including all of the known auditory areas, as well as some visual and ‘association’ areas, did not relabel neurons which had had transient projections to either ipsi‐ or contralateral visual areas 17–18. Thus, Al and All neurons after eliminating their transient juvenile projections to visual areas 17 and 18 do not project to the other hemisphere. In contrast, relabelling was obtained with late injections in several locations in the ipsilateral hemisphere; it was expressed as per cent of the population labelled by the early injections. Few neurons (0–2.5%) were relabelled by large injections in the caudal part of the posterior ectosylvian gyrus and the adjacent posterior suprasylvian sulcus (areas DP, P, VP). Multiple injections in the middle ectosylvian gyrus relabelled a considerably larger percentage of neurons (13%). Single small injections in the middle ectosylvian gyrus (areas Al, All), the caudal part of the anterior ectosylvian gyrus and the rostral part of the posterior ectosylvian gyrus relabelled 3.1–7.0% of neurons. These neurons were generally near (< 2.0 mm) the outer border of the late injection sites. Neurons with transient projections to ipsi‐ or contralateral visual areas 17 and 18 were relabelled in similar proportions by late injections at any given location. Thus, Al or All neurons which send a transitory axon to ipsi‐ or contralateral visual areas 17 and 18 are most likely to form short permanent cortical connections. In that respect, they are similar to medial area 17 neurons that form transitory callosal axons and short permanent axons to ipsilateral visual areas 17 and 18. Copyright © 1990, Wiley Blackwell. All rights reserved