Monaural and binaural adaptation in sequence: SDLB analogue?

Magnitude estimates (M.E.) of loudness of a 1-kc/s tone in the R ear at 60 db SPL were required over three 3-phase sessions from 15 normal-hearing young adults. The initial tone in the session was arbitrarily assigned a value of "60 units" in instructions to the S. The session was arranged to be analogous to that commonly used in Simultaneous Dichotic Loudness Balancing (SDLB). In a baseline Phase I in any of 3 sessions both ears received identical, simultaneous, intermittent stimulation. In each session, one of 3 duty cycles (randomized) were used (all on-times of .5 sec, off-times of .5, 4.5, or 24.5 sec). An M.E. judgment was requested every 10 sec for just over 1 min. Then immediately in Phase II the L ear received nothing while the R ear received continuous stimulation at 60 db SPL for 7 min, with M.E. Judgements requested every 10 sec during Min 1 and every min thereafter. Then in Phase III the R ear continued to receive continuous stimulation at 60 db SPL for 1 min while in the contralateral ear the condition of Phase I was re-introduced. M.E. loudness judgements for the R ear were requested as in Phase I. During Phase I there was, as expected, no loudness adaptation. During the adapting Phase II, M.E. group values declined progressively and significantly, but without significant differences among duty cycles. M.E. values in Phase III continued to decline significantly even though the contralateral ear was being given intermittent stimulation (irrespective of duty cycle). Previous studies had found that, for most listeners, M.E. loudness adaptation does not occur when the continuous stimulation is presented by itself. As far as is known, the present study is the first M.E. loudness adaptation study to present binaural intermittent stimulation before the continuous monaural stimulus. It is suggested that the redundancy of the continuous monaural stimulation, in comparison with the similar intermittent stimulus, is associated with the diminution of loudness.