COMPARATIVE PHYSIOLOGY OF ACOUSTIC AND ALLIED CENTRAL ANALYZERS

To exploit comparisons among classes of vertebrates and invertebrates, and between higher and lower levels of the brain, and between modalit ies, some important needs and opportunities for new research into the way central processing of acoustic input takes place are pointed out. Most of these are suggested by unfamiliar results on fish and reptiles that call for new controls in mammalian experiments as well as more s ystematic study of nonmammalian taxa. Three frameworks or basic agenda s are outlined: i) systematic comparison of dynamical properties to ac oustic variables, including especially repetition at different rates a nd the related states of expectation; ii) comparison of response measu res, including especially sequences such as oscillations and measures of assembly cooperativity such as synchrony, coherence and bicoherence ; and i) comparison of auditory subsystems, including especially modal categories such as complex feature selective regions and small sets. Some recent and some new results are summarized on human acoustic and non-mammalian event related potentials (ERPs) in response to expectati ons. When a regular and frequent standard stimulus is omitted, the omi tted stimulus potential (OSP) after conditioning with low repetition r ates (long ISIs-1-3 s) is a slow, broad positivity (P600-900), previou sly known. With high rates (ISI < 1 s), a new form of response appears , with fast components (P22), different dynamics and less dependence o n attention. Slow and fast OSPs each show a constant peak latency afte r the due-lime of the missing stimulus, as though a temporal expectati on has been learned. Unlike the visual OSP we have reported earlier, b oth fast and slow can occur together in the 1-2 Hz range. Very few con ditioning stimuli suffice to create the ''expectation'' that causes an OSP-only two for the slow type. These and more familiar ERPs, conside red in human subjects to index cognitive events, need to be compared i n other classes of vertebrates and invertebrates.