Omer Bar-Yosef

Reduction of information redundancy in the ascending auditory pathway.

Information processing by a sensory system is reflected in the changes in stimulus representation along its successive processing stages. We measured information content and stimulus-induced redundancy in the neural responses to a set of natural sounds in three successive stations of the auditory pathway-inferior colliculus (IC), auditory thalamus (MGB), and primary auditory cortex (A1). Information about stimulus identity was somewhat reduced in single A1 and MGB neurons relative to single IC neurons, when information is measured using spike counts, latency, or temporal spiking patterns. However, most of this difference was due to differences in firing rates. On the other hand, IC neurons were substantially more redundant than A1 and MGB neurons. IC redundancy was largely related to frequency selectivity. Redundancy reduction may be a generic organization principle of neural systems, allowing for easier readout of the identity of complex stimuli in A1 relative to IC.

Responses of Neurons in Cat Primary Auditory Cortex to Bird Chirps: Effects of Temporal and Spectral Context

The responses of neurons to natural sounds and simplified natural sounds were recorded in the primary auditory cortex (AI) of halothane-anesthetized cats. Bird chirps were used as the base natural stimuli. They were first presented within the original acoustic context (at least 250 msec of sounds before and after each chirp). The first simplification step consisted of extracting a short segment containing just the chirp from the longer segment. For the second step, the chirp was cleaned of its accompanying background noise. Finally, each chirp was replaced by an artificial version that had approximately the same frequency trajectory but with constant amplitude. Neurons had a wide range of different response patterns to these stimuli, and many neurons had late response components in addition, or instead of, their onset responses. In general, every simplification step had a substantial influence on the responses. Neither the extracted chirp nor the clean chirp evoked a similar response to the chirp presented within its acoustic context. The extracted chirp evoked different responses than its clean version. The artificial chirps evoked stronger responses with a shorter latency than the corresponding clean chirp because of envelope differences. These results illustrate the sensitivity of neurons in AI to small perturbations of their acoustic input. In particular, they pose a challenge to models based on linear summation of energy within a spectrotemporal receptive field.

Neurons and objects: the case of auditory cortex

Sounds are encoded into electrical activity in the inner ear, where they are represented (roughly) as patterns of energy in narrow frequency bands. However, sounds are perceived in terms of their high-order properties. It is generally believed that this transformation is performed along the auditory hierarchy, with low-level physical cues computed at early stages of the auditory system and high-level abstract qualities at high-order cortical areas. The functional position of primary auditory cortex (A1) in this scheme is unclear – is it ‘early’, encoding physical cues, or is it ‘late’, already encoding abstract qualities? Here we argue that neurons in cat A1 show sensitivity to high-level features of sounds. In particular, these neurons may already show sensitivity to ‘auditory objects’. The evidence for this claim comes from studies in which individual sounds are presented singly and in mixtures. Many neurons in cat A1 respond to mixtures in the same way they respond to one of the individual components of the mixture, and in many cases neurons may respond to a low-level component of the mixture rather than to the acoustically dominant one, even though the same neurons respond to the acoustically-dominant component when presented alone.

The effects of background noise on the neural responses to natural sounds in cat primary auditory cortex

Animal vocalizations in natural settings are invariably accompanied by an acoustic background with a complex statistical structure. We have previously demonstrated that neuronal responses in primary auditory cortex of halothane-anesthetized cats depend strongly on the natural background. Here, we study in detail the neuronal responses to the background sounds and their relationships to the responses to the foreground sounds. Natural bird chirps as well as modifications of these chirps were used. The chirps were decomposed into three components: the clean chirps, their echoes, and the background noise. The last two were weaker than the clean chirp by 13 and 29 dB on average respectively. The test stimuli consisted of the full natural stimulus, the three basic components, and their three pairwise combinations. When the level of the background components (echoes and background noise) presented alone was sufficiently loud to evoke neuronal activity, these background components had an unexpectedly strong effect on the responses of the neurons to the main bird chirp. In particular, the responses to the original chirps were more similar on average to the responses evoked by the two background components than to the responses evoked by the clean chirp, both in terms of the evoked spike count and in terms of the temporal pattern of the responses. These results suggest that some of the neurons responded specifically to the acoustic background even when presented together with the substantially louder main chirp, and may imply that neurons in A1 already participate in auditory source segregation.

Relating cluster and population responses to natural sounds and tonal stimuli in cat primary auditory cortex

Most information about neuronal properties in primary auditory cortex (AI) has been gathered using simple artificial sounds such as pure tones and broad-band noise. These sounds are very different from the natural sounds that are processed by the auditory system in real world situations. In an attempt to bridge this gap, simple tonal stimuli and a standard set of six natural sounds were used to create models relating the responses of neuronal clusters in AI of barbiturate-anesthetized cats to the two classes of stimuli. A significant correlation was often found between the response to the separate frequency components of the natural sounds and the response to the natural sound itself. At the population level, this correlation resulted in a rate profile that represented robustly the spectral profiles of the natural sounds. There was however a significant scatter in the responses to the natural sound around the predictions based on the responses to tonal stimuli. Going the other way, in order to understand better the non-linearities in the responses to natural sounds, responses of neuronal clusters were characterized using second order Volterra kernel analysis of their responses to natural sounds. This characterization predicted reasonably well the amplitude of the response to other natural sounds, but could not reproduce the responses to tonal stimuli. Thus, second order non-linear characterizations, at least those using the Volterra kernel model, do not interpolate well between responses to tones and to natural sounds in auditory cortex.

Transformation of stimulus representations in the ascending auditory system

1 Dept. Of Physiology, Hebrew University – Hadassah Medical School, Jerusalem, Israel, {Israel, nachumu, lioraa, omerbary}@md.huji.ac.il 2 Interdisciplinary Center for Neural Computation, Hebrew University, Jerusalem, Israel 3 Institute for Computer Science, Hebrew University, Israel, {ggal, tishby}@cs.huji.ac.il 4 Dept. of Biomedical engineering, Johns Hopkins University, Baltimore MD, USA, {anderson, eyoung}@bme.jhu.edu

Prenatal abnormal width of the cavum septum pellucidum - MRI features and neurodevelopmental outcome.

Abstract Purpose: The purpose of this study is to describe the morphological characteristics of fetal MRIs and outcome of prenatally-detected abnormal width of the CSP (cavum septum pellucidum). Material and methods: A retrospective study of cases with abnormal width of the CSP demonstrated prenatally on MRI. Data collected included: prenatal history, MRI features, sonographic follow up, and neurodevelopmental outcome. Results: Most of the cases referred (34/39) had an abnormal CSP on MRI. Thirty cases had an abnormal width: 23 had narrow CSP and seven had wide CSP. Only three out of 12 cases that were referred with US diagnosis of absent CSP were confirmed by MRI, the rest had narrow CSP. Follow up was performed in 24 out of 30 cases with an abnormal CSP width; all had normal neurodevelopment. Conclusion: To the best of our knowledge, this is the first study to examine the outcome in narrow and wide CSPs detected prenatally. An abnormal width of the CSP prenatally, without an associated fetal abnormality such as aneuploidy, appears to have normal outcome. MRI should be offered when an absence of the CSP is suspected on US to rule out narrow CSP, which seems to be a benign finding.

Natural history of fetal isolated ventriculomegaly: Comparison between pre- and post-natal imaging

Abstract Purpose: The aim of our study was to assess the agreement between pre- and post-natal measurements in prenatal isolated ventriculomegaly. Methods: Ninety-two women were referred for lateral ventricular abnormality and followed prospectively. Cases with at least one dilated lateral ventricle ≥10 mm and a normal work up (serological tests, detailed anatomical scan, fetal brain MR imaging, genetic counseling, and amniocentesis) were considered idiopathic ventriculomegaly and comprised the study group for post-natal follow up. Prenatal measurements were performed by ultrasound and MR imaging. Post-natal measurements were performed by cranial sonography at age of 1–3 months. Measurements were performed in the customary plane for each modality. Paired Student’s t test was used to assess the mean difference between pre- and postnatal measurements. Results: Forty three cases comprised the study group for post-natal measurements. A statistically significant decrease in ventricular width (p < .001) was observed between pre- and post-natal measurements. On clinical follow up for 24 months, all cases were normal except three who demonstrated very mild neurological deficits. Conclusions: Our study indicates statistically significant regression of prenatal isolated ventriculomegaly in the post-natal period.