Response properties of single units in the dorsal nucleus of the lateral lemniscus of decerebrate cats

Information Processing by Onset Neurons in the Cat Auditory Brainstem

Octopus cells in the ventral cochlear nucleus (VCN) have been difficult to study because of the very features that distinguish them from other VCN neurons. We performed in vivo recordings in cats on well-isolated units, some of which were intracellularly labeled and histologically reconstructed. We found that responses to low-frequency tones with frequencies < 1 kHz reveal higher levels of neural synchrony and entrainment to the stimulus than the auditory nerve. In responses to higher frequency tones, the neural discharges occur mostly near the stimulus onset. These neurons also respond in a unique way to 100 % amplitude-modulated (AM) tones with discharges exhibiting a bandpass tuning. Responses to frequency-modulated sounds (FM) are unusual: Octopus cells react more vigorously during the ascending than the descending parts of the FM stimulus. We examined responses of neurons in the ventral nucleus of the lateral lemniscus (VNLL) whose discharges to tones and AM sounds are similar to octopus cells. Repeated stimulation with short tone pips of VCN and VNLL onset neurons evokes trains of action potentials with gradual shifts toward later times in their first spike latency. This behavior parallels short-term post-synaptic depression observed by other authors in in vitro VCN recordings of octopus cells. VCN and VNLL onset units in cats respond to frozen noise stimuli with gaps as narrow as 1 ms with a robust discharge near the stimulus onset following the gap. This finding suggests that VCN and VNLL onset cells play a role in gap detection, which is of great importance to speech perception.

All the way from the cortex: a review of auditory corticosubcollicular pathways

Enrico Mugnaini has devoted part of his long and fruitful neuroscientific career to investigating the structural similarities between the cerebellar cortex and one of the first relay stations of the mammalian auditory pathway: the dorsal cochlear nucleus. The hypothesis of the cerebellar-like nature of the superficial layers of the dorsal cochlear nucleus received definitive support with the discovery and extensive characterization in his laboratory of unipolar brush cells, a neuron type unique to certain regions of the cerebellar cortex and to the granule cell domains of the cochlear nuclei. Paradoxically, a different line of research carried out in his laboratory revealed that, unlike the mammalian cerebellar cortex, the dorsal cochlear nucleus receives direct projections from the cerebral cortex, a fact that constitutes one of the main differences between the cerebellum and the dorsal cochlear nucleus. In an article published in 1995, Mugnaini's group described in detail the novel direct projections from the rat auditory neocortex to various subcollicular auditory centers, including the nucleus sagulum, the paralemniscal regions, the superior olivary complex, and the cochlear nuclei (Feliciano et al., Auditory Neuroscience 1995; 1:287-308). This review gives Enrico Mugnaini credit for his seminal contribution to the knowledge of auditory corticosubcollicular projections and summarizes how this growing field has evolved in the last 20 years.

Temporal properties of responses to sound in the ventral nucleus of the lateral lemniscus

Besides the rapid fluctuations in pressure that constitute the "fine structure" of a sound stimulus, slower fluctuations in the sound's envelope represent an important temporal feature. At various stages in the auditory system, neurons exhibit tuning to envelope frequency and have been described as modulation filters. We examine such tuning in the ventral nucleus of the lateral lemniscus (VNLL) of the pentobarbital-anesthetized cat. The VNLL is a large but poorly accessible auditory structure that provides a massive inhibitory input to the inferior colliculus. We test whether envelope filtering effectively applies to the envelope spectrum when multiple envelope components are simultaneously present. We find two broad classes of response with often complementary properties. The firing rate of onset neurons is tuned to a band of modulation frequencies, over which they also synchronize strongly to the envelope waveform. Although most sustained neurons show little firing rate dependence on modulation frequency, some of them are weakly tuned. The latter neurons are usually band-pass or low-pass tuned in synchronization, and a reverse-correlation approach demonstrates that their modulation tuning is preserved to nonperiodic, noisy envelope modulations of a tonal carrier. Modulation tuning to this type of stimulus is weaker for onset neurons. In response to broadband noise, sustained and onset neurons tend to filter out envelope components over a frequency range consistent with their modulation tuning to periodically modulated tones. The results support a role for VNLL in providing temporal reference signals to the auditory midbrain.

Fine frequency tuning in monkey auditory cortex and thalamus

The frequency resolution of neurons throughout the ascending auditory pathway is important for understanding how sounds are processed. In many animal studies, the frequency tuning widths are about 1/5th octave wide in auditory nerve fibers and much wider in auditory cortex neurons. Psychophysical studies show that humans are capable of discriminating far finer frequency differences. A recent study suggested that this is perhaps attributable to fine frequency tuning of neurons in human auditory cortex (Bitterman Y, Mukamel R, Malach R, Fried I, Nelken I. Nature 451: 197-201, 2008). We investigated whether such fine frequency tuning was restricted to human auditory cortex by examining the frequency tuning width in the awake common marmoset monkey. We show that 27% of neurons in the primary auditory cortex exhibit frequency tuning that is finer than the typical frequency tuning of the auditory nerve and substantially finer than previously reported cortical data obtained from anesthetized animals. Fine frequency tuning is also present in 76% of neurons of the auditory thalamus in awake marmosets. Frequency tuning was narrower during the sustained response compared to the onset response in auditory cortex neurons but not in thalamic neurons, suggesting that thalamocortical or intracortical dynamics shape time-dependent frequency tuning in cortex. These findings challenge the notion that the fine frequency tuning of auditory cortex is unique to human auditory cortex and that it is a de novo cortical property, suggesting that the broader tuning observed in previous animal studies may arise from the use of anesthesia during physiological recordings or from species differences.

Monaural spectral processing differs between the lateral superior olive and the inferior colliculus: physiological evidence for an acoustic chiasm

Evidence suggests that the lateral superior olive (LSO) initiates an excitatory pathway specialized to process interaural level differences (ILDs), the primary cues used by mammals to localize high-frequency sounds in the horizontal plane. Type I units in the central nucleus of the inferior colliculus (ICC) of decerebrate cats exhibit monaural and binaural response properties qualitatively similar to those of LSO units, and are thus supposed to be the midbrain component of the ILD pathway. Studies have shown, however, that the responses of ICC cells do not often reflect simply the output of any single source of excitatory inputs. The goal of this study was to compare directly the monaural, spectral response properties of LSO and type I units measured in unanesthetized decerebrate cats. Compared to LSO units, type I units have narrower V-shaped excitatory tuning curves, higher spontaneous rates, lower maximum stimulus-evoked firing rates and more nonmonotonic rate-level curves for tones and noise. In addition, low-frequency type I units have lower thresholds to tones than corresponding LSO units. Taken together, these results suggest that the excitatory ILD pathway from LSO to ICC is mostly a high-frequency channel, and that additional inputs transform LSO influences in the ICC.

Timing is everything: temporal processing deficits in the aged auditory brainstem

This summary article reviews the literature on neural correlates of age-related changes in temporal processing in the auditory brainstem. Two types of temporal processing dimensions are considered, (i) static, which can be measured using a gap detection or forward masking paradigms, and (ii) dynamic, which can be measured using amplitude and frequency modulation. Corresponding data from physiological studies comparing neural responses from young and old animals using acoustic stimuli as silent gaps-in-noise, amplitude modulation, and frequency modulation are considered in relation to speech perception. Evidence from numerous investigations indicates an age-related decline in encoding of temporal sound features which may be a contributing factor to the deficits observed in speech recognition in many elderly listeners.