This study aims to investigate the evoked and the induced activity in 40 Hz auditory responses. The 40 Hz activity, also called Pb or P50 or P1 component, has a latency of 50ms and belongs to the category of MLRs (Middle latency responses), which occur right after Auditory Brainstem Responses (ABRs) between 15 and 80ms. Its importance is related to possible clinical applications such as anesthesia, schizophrenia and auditory development. In addition to this, evoked and induced activities at 40 Hz might play an important role in cognitive processing. Trains of right ear rarefaction clicks at the mean rate of 39.1 Hz, intensity of 50dB and duration of 100 were used to elicit the above mentioned activities. Three different sequences have been used: steady state, low jittered and medium jittered. Low jittered sequence has been the main sequence used to study the 40 Hz activity. The advantage of using this sequence is the fact that a deconvolution analysis can be performed and also the fact that it does not differ too much from the standard 40Hz steady-state sequence and this means that a resonance at 40 Hz can still be obtained. Ten healthy subjects (8 males and 2 females; ages ranging from 25 to 47), with no history of audiological or neurological hearing impairment were recorded. Informed consent was signed according to approved IRB protocols. All recordings were done in a sound-proof chamber (Acoustic Systems, Inc.) with subjects lying on a bed comfortably. The subjects were not asked to perform any tasks, but just to passively listen to the acoustical stimuli. Evoked and induced activities were recorded in response to the above mentioned acoustic stimuli. The deconvolution analysis showed that the peak of activity occurs around 152ms. Wavelets analysis has confirmed this observation and has also unveiled and induced activity in the low beta range. This induced activity seems to be strictly related to the evoked activity, as it seems to occur around 390ms, which corresponds to the situation where the 40 Hz evoked activity enters a steady state condition, which lasts until the last acoustic stimulus has been applied. The latter observation is again in accordance with the literature, where it is reported that the 40Hz evoked activity could reflect the initial coactivation of neural assemblies representing specific stimulus features. A change in such stimulus features could be reflected as induced oscillations occurring in the middle beta range (16-22 Hz).

The purpose of these experiments was to determine the effects of cholinergic depletion on spontaneous and evoked activity of neurons in the different layers of the posteromedial barrel subfield (PMBSF) of the rat somatosensory cortex. Acetylcholine neurons in nucleus basalis of Meynert (NBM) were selectively lesioned with an immunotoxin (IT), 192 IgG-saporin. Spontaneous activity was significantly lower in layers II-III, Va, and VI in IT-injected animals compared to control animals. Evoked activity was significantly lower in layers II-III, IV, Vb, and VI of IT-injected animals compared to control animals. The largest difference was observed in layer Vb. Thus, cholinergic depletion causes significant changes in the magnitude of spontaneous and evoked activity but these differences are not completely in register with one another.