Barbara E. Norris

Single Auditory Units in the Superior Olivary Complex: II: Locations of Unit Categories and Tonotopic Organization

The locations of various categories of units are described and correlations are found between unit properties and unit locations. Units on the edges of the medial superior olive (MSO) and the lateral superior olive (LSO) appear to have properties different from units inside these nuclei. Many units in the dorsomedial periolivary nucleus had wide tuning curves and some had very long latencies. Almost all units in the medial nucleus of the trapezoid body (MNTB) had irregular firing patterns, but most units in the LSO and the dorsolateral periolivary nucleus had regular firing patterns. Class 2 (off) units may correspond to ‘stellate’ cells of the MNTB.With few exceptions, units dorsolateral to the MSO were excited by ipsilateral sounds, and units ventromedial to the MSO, or in the ventral nucleus of the lateral lemniscus (VNLL), were excited by contralateral sounds.Best frequencies were arranged from high to low (1) from ventromedial to dorsolateral in the MSO, (2) from the medial limb to the lateral limb i...

Single Auditory Units in the Superior Olivary Complex: I: Responses to Sounds and Classifications Based on Physiological Properties

Response properties to sounds were examined in single auditory units in the superior olivary complex and the ventral nucleus of the lateral lemniscus. Units were divided into ten ‘classes’; four classes were defined by single properties (spike waveshape or the timing of responses to sound bursts) and the remaining six classes were defined by which ear excited or inhibited the units. Units were also divided according to the types of poststimulus-time histograms they produced in response to sound bursts, and they were separately divided according to the shape of their inter-spike interval histograms. These latter two groupings were highly correlated with each other and with unit class, and were combined into a single classification scheme called ‘firing-pattern type’.Units were divided into six groups based on the shapes of their tuning curves; data on the bandwidths of tuning curves are also presented. Data are presented on spontaneous firing rate, thresholds to best-frequency tones, and latency of respons...

Generators of the brainstem auditory evoked potential in cat. I. An experimental approach to their identification

This paper is the first in a series aimed at identifying the cellular generators of the brainstem auditory evoked potential (BAEP) in cats. The approach involves (1) developing experimental procedures for making small selective lesions and determining the corresponding changes in BAEP waveforms, (2) identifying brainstem regions involved in BAEP generation by examining the effects of lesions on the BAEP and (3) identifying specific cell populations involved by combining the lesion results with electrophysiological and anatomical information from other kinds of studies. We created lesions in the lower brainstem by injecting kainic acid which is generally toxic for neuronal cell bodies but not for axons and terminals. This first paper describes the justifications for using kainic acid, explains the associated problems, and develops a methodology that addresses the main difficulties. The issues and aspects of the specific methods are generally applicable to physiological and anatomical studies using any neurotoxin, as well as to the present BAEP study. The methods chosen involved (1) measuring the BAEP at regular intervals until it reached a post-injection steady state and perfusing the animals with fixative shortly after the last BAEP recordings were made, (2) using objective criteria to distinguish injection-related BAEP changes from unrelated ones, (3) making control injections to identify effects not due to kainic acid toxicity, (4) verifying the anatomical and functional integrity of axons in lesioned regions, and (5) examining injected brainstems microscopically for cell loss and cellular abnormalities indicating dysfunction. This combination of methods enabled us to identify BAEP changes which are clearly correlated with lesion locations.

Paucity of Unit Responses in the Accessory Superior Olive of Barbiturate‐Anesthetized Cats

Platinum‐tipped or fluid‐filled electrodes (tip diameter 2.5–12 μ) were used to record extracellular unit responses to acoustic stimuli (e.g., tone bursts, noise bursts, or clicks, presented monauraly or binaurally). In data from approximately 40 electrode passes through the accessory superior olive (ASO), histology indicated that only 14 units (4 suitable for study) were encountered that were located within the ASO. Throughout the whole superior olivary region sampled by these passes, a total of 214 units (114 suitable for study) were encountered and histologically localized. The number of units found in the ASO seems disproportionately low considering the size of the ASO relative to the size of the region sampled. This paucity of unit responses suggests that previous data localizing units within the ASO might be profitably reexamined. [Supported in part by the Joint Services Electronics Program of the National Science Foundation, the National Institutes of Health, U. S. Department of Health, Education, and Welfare, and National Aeronautics and Space Administration.]

Single Auditory Units Recorded in the Medial Nucleus of the Trapezoid Body (MNTB) of Anesthetized Cats

We classified 392 Superior Olivary (SOC) units using physiological criteria and independently determined their locations. Of 169 MNTP units, 120 were in a single class characterized by a distinctive spike waveform (Pfeiffer, Science 154, 667). All 127 units in this class (except one) were monaural (contralateral excitatory), had firing patterns similar to auditory‐nerve fibers, had short click latencies relative to (other SOC units, were located near calyces of Held, and were tonotopically arranged with higher best frequencies more ventromedial. Sixteen MNTB units responded like the above units but lacked the characteristic spike waveform and were placed in a different class. Twenty‐three MNTB units were in a class characterized by an “OFF response” to contralateral stimuli. None of the 35 units in this class showed spontaneous activity or was located far from the MNTB. The 10 remaining MNTB units were distributed throughout five other classes. [Supported by NIH grants and the Joint Services Electronics Program.]We classified 392 Superior Olivary (SOC) units using physiological criteria and independently determined their locations. Of 169 MNTP units, 120 were in a single class characterized by a distinctive spike waveform (Pfeiffer, Science 154, 667). All 127 units in this class (except one) were monaural (contralateral excitatory), had firing patterns similar to auditory‐nerve fibers, had short click latencies relative to (other SOC units, were located near calyces of Held, and were tonotopically arranged with higher best frequencies more ventromedial. Sixteen MNTB units responded like the above units but lacked the characteristic spike waveform and were placed in a different class. Twenty‐three MNTB units were in a class characterized by an “OFF response” to contralateral stimuli. None of the 35 units in this class showed spontaneous activity or was located far from the MNTB. The 10 remaining MNTB units were distributed throughout five other classes. [Supported by NIH grants and the Joint Services Electronics P...

Motoneuron axon distribution in the cat stapedius muscle.

Stapedius-motoneuron cell bodies in the brainstem are spatially organized according to their acoustic response laterality, as demonstrated by intracellular labeling of physiologically identified motoneurons [Vacher et al., 1989. J. Comp. Neurol. 289, 401-415]. To determine whether a similar functional spatial segregation is present in the muscle, we traced physiologically identified, labeled axons into the stapedius muscle. Ten labeled axons were visible in the facial nerve and five could be traced to endplates within the muscle. These five axons had 39 observed branches (others may have been missed). This indicates an average innervation ratio (> or = 7.8) which is much higher than that obtained from previous estimates of the numbers of stapedius motoneurons and muscle fibers in the cat. One well-labeled stapedius motor axon innervated only a single muscle fiber. In contrast, two labeled axons had over 10 endings and innervated muscle fibers spread over wide areas in the muscle. Two of the axons branched and coursed through two primary stapedius fascicles, indicating that the muscle zones innervated by different primary fascicles are not functionally segregated. In another series of experiments, retrograde tracers were deposited in individual primary nerve fascicles. In every case, labeled stapedius-motoneuron cell bodies were found in each of the physiologically identified stapedius-motoneuron regions in the brainstem. These observations suggest there is little, if any, functional spatial segregation based on separate muscle compartments in the stapedius muscle, despite there being functional spatial segregation in the stapedius-motoneuron pool centrally.