Auditory Information Processing Of Neurons In Rat's Inferior Colliculus

Frequency modulation, the basic elements of the human speech and vocalizations of many species, convey the temporal-spectral cues for the recognizing of auditory information. The processing of the frequency modulated information is one most important characteristic of the central auditory nervous system, and the studies of the mechanisms of which will further the investigation the neural mechanisms for the auditory information processing.There are frequency modulated information in the vocalizations of rats, behavioral results had shown that rats can discriminate the modulation rate and direction of the frequency modulation sweeps and the right auditory cortex of rat's brain was dominance in the discrimination of the direction of frequency modulation sweeps. These data suggested that the frequency modulation information processing mechanisms exist in the central auditory system. Some experiments about the frequency modulation information processing have been conducted on rats' inferior colliculus (IC) and auditory cortex (AC), while the progress on IC was not fast as that in AC. The experiments about the FM information processing on rat's IC had always been done on the relative sound intensity, such as 30 dB above the neurons mini-thresholds (MT), while more recently, data from other species suggested that sound intensity will affect the preference of neurons to the FM sweeps, whether the preferred characteristics to the FM sweeps of neurons in rat's IC are affected by the sound intensity or not is still a question, and the neural mechanisms for the processing of FM information on neurons in rat's IC has been ignored for a long time. Based on this situation, we designed some experiments to investigate the neural mechanisms for the FM information processing of neurons in rat's IC. For example, the response characteristics of rat inferior colliculus (IC) neurons to frequency modulated (FM) sounds were studied in a wide range of sound levels to investigate the effects of sound intensity on the preference to FM sweeps, the relationships between preference to duration of pure tone and the preference to modulation rate of FM sweep were studied to investigate the possible sideband inhibition mechanism and duration tuning mechanism for the modulation rate sensitivity. Electric stimulation method was used to change the exciting balance between the neurons of both side of IC to investigate the modulation between them. Modified morris water maze was used to study the orientation behavior induced by the auditory cue alone or audio-visual bimodal cue with spatial disparities to investigate the role of auditory cues to the space navigation.Total 88 neurons, which responded to both pure tone and FM sweeps, were isolated in 28 IC of rats, Nearly 90% of IC neurons exhibited preference to sweep direction and frequency modulation rate (FMR) of FM sweep; the proportions of FM sweep directional selective and FMR preferred neurons varied with sound level. The directional selectivity of nearly 60% of neurons and the FMR preference of 84.2% of neurons determined at 30dB above minimal threshold to FM sound were not consistent with that determined at lower sound levels. The results demonstrate that the intensity of FM sweeps affects the FM response characteristics of most IC neurons. The proportions of neurons exhibited FM preference at sound intensity relative to minimal threshold (MT) to FM were different from that at the intensity relative to the stander basic level-20 uPa, it was significant that the proportion at 0 dB relative to MT was higher than that at 30 dB relative to MT, while this significance disappeared at intensity relative to the stander basic level-20 uPa, this suggested that there maybe one mechanism to code the FM and intensity information at the same time.Twenty rats were used to study the relations between preference to duration of pure tone and the preference to modulation rate of FM sweep, pure tone and narrow band (located in the excited frequency area) FM sweep, broad band (overflow the excited frequency area) were used as sound stimulation to investigate the possible sideband inhibition mechanism and duration tuning mechanism for the modulation rate sensitivity. The side band inhibition was always located beside the excited frequency area, if it is existing and contributed to the modulation rate preference, the preference to modulation rate will be eliminated with the changing of the band range of FM sweep to in the excited frequency area. Total 80 neurons were isolated and the response characteristics to pure tone, narrow band FM sweep and broad band FM sweep of 54 neurons were recorded detailly. Among the 26 neurons, which were not sensitive to the duration of pure tone, 5 neurons keep this no-sensitive characteristic when stimulated with narrow band FM sweeps but show preference to the broad band FM sweep, this suggested that the inhibition side band may contributed to the modulation rate preference to neurons in IC of rats. Total 28 neurons show duration tuning to pure tone, only 16 of them have the same duration response functions under the stimulation of pure tone and FM sweeps, the modulation rate preferences of 12 neurons were not affected by the bandwidth of FM sweeps. The duration 0.5, 0.8 cut-off points of these 12 neurons can predicate the modulation rate 0.5, 0.8 cut-off points perfectly, this suggested that the preference to modulation rate may be created by the mechanism which contributed to the duration tuning of neurons.Total 20 rats were used to study the modulation between the neurons in the bilateral IC. the rate-intensity functions and preference to FM sweeps of neurons in con-sound source IC were recorded before, during and after the electric stimulation of the neurons in the ipsi-sound source IC. Results show that the response of most sampled neurons were changed, and the response pulses of most decreased and little increased, this was similar to the previous report which showed that when the activity of neurons in ipsi-sound source IC decreased the activity of con-IC neurons increased. The 30-70% dynamic ranges of rate-intensity functions of 27.8% (5/18) of neurons were extended and 33.3% were contracted, 11.1% of neurons were not changed but the location of 30% and 70% cut-off point changed. Electric stimulation changed the preference to modulation rate at 92.8% (26/28) of sampled neurons and preference to directions of FM sweeps at 46.4% of sampled neurons. These data suggested that there is modulation between the neurons in bilateral IC of rats.The auditory cue alone and audio-visual cue were used to induce the orientation behavior of rats, respectively. The results showed that rat will orientated better when auditory cue were used alone than random, this suggested that alone auditory beacon can be used to induced the navigation of rats. Under the inducing of the auditory-visual combined cue presented with the spatial and temporal coincidence, the reaction time decreased and the success rate increased than the visual cue induced alone. This suggested that the rat can integrate the information from different modal and have the multi-modal integration ability, and this kind integration will follow the rules generally.