THE AUDITORY-EVOKED OFF RESPONSE - SOURCES AND COMPARISON WITH THE ONAND THE SUSTAINED RESPONSES

Objective: It is well known that tone bursts elicit a prominent N1/P2 complex in the auditory evoked potential (the on-response), but less i s known about a morphologically similar complex (the off-response) tha t can be recorded under suitable stimulus conditions. The interaction between the two responses indicated that the responses were not physio logically independent. The present experiment employed neuromagnetic m ethods to determine the cortical sources of N1 and P2 on- and off-resp onses and their relation to other events observed in the auditory evok ed field. Design: Five female and five male subjects with no history o f otologic or neurological disorders and with normal audiological stat us participated in this study. Tone bursts of 2 sec duration (10 msec rise and decay time, cosine function), carrier frequency of 1 kHz, and intensity of 60 dB nHL (normative hearing level) were presented 512 t imes to the subject's right ear (contralateral to the investigated hem isphere) with an interstimulus interval randomized between 5 and 7 sec . Results: The present study is unique in that several components of t he complex auditory evoked response (P1, N1on, P2on, sustained-field, N1off, P2off) were recorded and localized in the same subjects and in the same experiment. The source coordinates obtained for N1 and P2 on- and off-responses indicated that the two responses are generated by o verlapping cortical regions. Sources for the P2 components were situat ed anterior and medial to sources for the N1 components and were indis tinguishable from sources for the auditory sustained-field. An early P ion event preceded the N1on (but not the N1off) response and was spati ally indistinguishable from the N1on response. The equivalent source s trength was greater for N1on and P2on sources compared with N1off and P2off sources. Conclusions: The recoding process signaled by on- and o ff-responses may be a dynamic form of plasticity in the auditory corte x with a time constant on the order of hundreds of milliseconds, corre sponding to the duration of sustained-responses released by acoustic c hanges and to the duration of the acoustic foreperiod that is necessar y before on- and off-responses to acoustic changes can be observed.