PROCESSING OF BINAURAL DISPARITY CUES AND FREQUENCY INFORMATION IN THE OWL’S INFERIOR COLLICULUS

 The barn owl uses interaural differences in intensity (IID) and time (ITD) to localize the vertical and horizontal positions of sound, respectively.  These two cues are independently processed in anatomically separate pathways and this segregation is maintained up to the “core” and the “medial shell” of the central nucleus of the inferior colliculus.  Neurons in the external nucleus(ICx), however, are sensitive to both IID and ITD.  ICx neurons are also distinct from lower-order neurons in their broad tuning to frequency.  IID and ITD information should be combined and inputs from a broad frequency range should converge in the ICx or somewhere in the central nucleus.   The lateral shell of the central nucleus of the inferior colliculus(ICL) is a likely site for these processes to occur, because it receives projections from the core and medial shell and, in turn, projects to the ICx.  Is it in the ICx or ICL that these processes take place?  Which of these processes occurs first, integration of ITD and IID information or convergence of multiple frequency channels?  To answer these questions we made single unit recordings in the ICL and analysed neuronal selectivity to IID, ITD and frequency.  All ICL neurons were sensitive to IID and ITD.  IID functions were either bell-shaped or monotonic.  ICL neurons could be divided into two groups according to their noise ITD curves: one with multiple peaks of identical height (type1, “phase ambiguous”) and the other with a main peak and smaller secondary peaks(type2).  The latency of the auditory responses were shorter in type 1 than in type 2.  The results suggest that IID information is first combined with ITD information in type 1 ICL cells and then multiple frequency inputs begin to converge in type 2 cells to eliminate phase ambiguity.

(日本語 要旨):

藤田一郎(理化学研究所国際フロンティア)

「フクロウの脳が教えること」

 メンフクロウの聴覚空間認識の神経機構は、脊椎動物の感覚情報処理機構の中で最も解析が進んでいる例の一つである。ここでは、聴覚受容野およぴ聴覚空間地図がメンフクロウの脳の中でどのように形成されるかを、ごく簡単に概観し、そこから脳の働き一般に関する問題点を検討してみる。とりあつかう問題は、(1)感覚情報は必ずしもスパイクの発火頻度に含まれていないこと、(2)解剖学的構造が、どの程度、神経細胞におけるコンピュテーションを規定するか、(3)最適刺激が興奮と抑制の両方を同時に引き起こすことの意味、(4)認識細胞仮説("おばあさん細胞仮説")は有効か、などである。


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