Registration of neural maps through value-dependent learning: modeling the alignment of auditory and visual maps in the barn owl's optic tectum

In the optic tectum (OT) of the barn owl, visual and auditory maps of space are found in close alignment with each other. Experiments in which such alignment has been disrupted have shown a considerable degree of plasticity in the auditory map. The external nucleus of the inferior colliculus (ICx), an auditory center that projects massively to the tectum, is the main site of plasticity; however, it is unclear by what mechanisms the alignment between the auditory map in the ICx and the visual map in the tectum is established and maintained. In this paper, we propose that such map alignment occurs through a process of value-dependent learning. According to this paradigm, value systems, identifiable with neuromodulatory systems having diffuse projections, respond to innate or acquired salient cues and modulate changes in synaptic efficacy in many brain regions. To test the self-consistency of this proposal, we have developed a computer model of the principal neural structures involved in the process of auditory localization in the barn owl. This is complemented by simulations of aspects of the barn owl phenotype and of the experimental environment. In the model, a value system is activated whenever the owl carries out a foveation toward an auditory stimulus. A term representing the diffuse release of a neuromodulator interacts with local pre- and postsynaptic events to determine synaptic changes in the ICx. Through large-scale simulations, we have replicated a number of experimental observations on the development of spatial alignment between the auditory and visual maps during normal visual experience, after the retinal image is shifted through prismatic goggles, and after the reestablishment of normal visual input. The results suggest that value-dependent learning is sufficient to account for the registration of auditory and visual maps of space in the OT of the barn owl, and they lead to a number of experimental predictions.     

Inadequate cortical feature maps: a neural circuit theory of autism

The autistic syndromes are caused by neurological dysfunctions. The capacity of autistic individuals to form representations of previous sensory impressions, useful for the processing of present information, is impaired. Self-organizing feature maps are mathematical models of cortical feature maps and may be used to simulate cortical processing. Dysfunctional self-organization, resulting in disability to extract features from stimuli, is proposed as a neural circuit theory of autism. The nature and a possible cause of dysfunction self-organization are examined. It is shown that impaired feature detection is valid for explaining the memory function in autism, the lack of drive for central coherence according to Frith's theory of autism, and a number of impairments from the diagnostic criteria. Unequal levels of impairment of different cortical feature maps can account for the typically uneven intelligence profile of autistic individuals. Excessive inhibitory lateral feedback synaptic connection strengths are presented as one factor impairing the development of feature maps. Strong or excessive inhibitory lateral feedback synaptic connection strengths also cause high sensory discrimination and abnormal sensory responses, both documented in autism. A neural circuit theory for autism has been presented. For a proof of this neural circuit theory neurological investigations are required.     

Plasticity of tonotopic maps in auditory midbrain following partial cochlear damage in the developing chinchilla

In order to analyze the immunopathologic mechanisms of Beh?et's disease, the gene (bes-1) encoding a streptococcal antigen correlated with the disease was cloned and sequenced, and protein produced by this clone was identified by Western immunoblotting using serum antibody from the patient. Cellular DNA of Streptococcus (S.) sanguis serotype KTH-1 (uncommon serotype 1, strain 113-20) from the patient was extracted and digested with EcoRI. The digested fragments were cloned into the cloning vector lambda gt11, and then the resulting DNA library was immunoscreened using the patient's serum antibody to serotype KTH-1. The immunopositive clone of the 1.5 kbp fragment was subcloned into pUC 118 plasmid (pU8BeS1-1) and sequenced. The sequence showed that the 3'-terminal half side region of this insert contained 962bp of open-reading frame (ORF) discontinued at the EcoRI restriction site, and the stop codon was not found. The nucleotide sequence of the remaining additional 3'-terminal region of this gene encoding whole BES-1 was determined by genome walking. The whole ORF of bes-1 consisted of 849 amino acid residues with a calculated molecular mass of 95 kDa. The residues in a portion of the amino acid sequence showed a 60% correspondence to those of the human intraocular peptide Brn-3b.     

Tonotopic organization of auditory cortical fields delineated by parvalbumin immunoreactivity in macaque monkeys

Tonotopic maps, obtained from single and multi-unit recordings in the primary and surrounding areas of the auditory cortex, were related to chemoarchitecture of the supratemporal plane, as delineated by immunoreactivity for parvalbumin. Neurons in the central core were sharply tuned and formed two complete tonotopic representations corresponding to the primary auditory area (AI) and the rostral (R) area. High frequencies were represented posteriorly in AI and anteriorly in R, the representation reversing in the anterior part of the core. Neurons in regions of less dense immunostaining previously described as lateral (L) and posteromedial (P-m) fields, showed broader frequency tuning. Two tonotopic representations were found in L: in an anterolateral (AL) field, corresponding to a field previously reported by others, high frequencies were represented anteriorly and low frequencies posteriorly; in a posterolateral field (PL) the trend reversed. There was a further reversal on entering P-m from the high frequency representation in PL and progressively lower frequencies tended to be represented more medially in P-m, but P-m may contain two representations reported by others. Neurons in the previously described anteromedial (A-m) and medial (M) fields of weaker immunostaining, were even more broadly tuned. A tonotopic progression from low frequency representation posteriorly to high frequency representation anteriorly was observed in the medial field. Frequency representation in A-m remains uncertain. No tonotopic representation could be demonstrated with the stimuli used in the zones of very weak parvalbumin immunostaining outside AL, PL, P-m, A-m, and M. The properties of neurons in the core and surrounding zones are likely to reflect inputs from the ventral and dorsal medial geniculate nuclei, respectively. The fields outside the core seem to be the starting points for separate streams of auditory corticocortical connections passing into association cortex.     

Fear conditioning to discontinuous auditory cues requires perirhinal cortical function.

Pretraining lesions of rat perirhinal (PR) cortex impair fear conditioning to ultrasonic vocalizations (USVs) but have no effect on conditioning to continuous tones. This study attempted to deconstruct USVs into simpler stimulus features that cause fear conditioning to be PR-dependent. Rats were conditioned to one of three cues: a multicall 19-kHz USV, a 19-kHz discontinuous tone, and a 19-kHz continuous tone. The discontinuous tone duplicated the on/off pattern of the individual calls in the USV, but it lacked the characteristic frequency modulations. Well-localized neurotoxic PR lesions impaired conditioning to the USV, the discontinuous tone, and the training context. However, PR lesions had no effect on conditioning to the continuous tone. The authors suggest that the lesion effects on fear conditioning to both cues and contexts reflect the essential role of PR in binding stimulus elements together into unitary representations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Delayed recovery of auditory cortical evoked potentials is correlated with cortical neuronal death after transient cerebral ischemia in awake gerbils

1. Spermatozoa in semen samples from 8 individual male domestic fowls were shown to have a differential and characteristic ability to hydrolyse holes in the inner perivitelline layer from laid eggs in an in vitro assay. 2. The number of holes produced by samples of spermatozoa per unit area of inner perivitelline layer in vitro was linearly correlated with sperm ATP content (r = 0.85) and motility (r = 0.76). 3. The number of holes formed in the inner perivitelline layer in vitro was also linearly correlated with the numbers of holes formed in the inner perivitelline layer of eggs fertilised in vivo, in inseminated hens (r = 0.90); and was correlated logarithmically with the proportion of fertile eggs laid by these hens.     

Spatial receptive fields of primary auditory cortical neurons in quiet and in the presence of continuous background noise

Spatial receptive fields of primary auditory (AI) neurons were studied by delivering, binaurally, synthesized virtual-space signals via earphones to cats under barbiturate anesthesia. Signals were broadband or narrowband transients presented in quiet anechoic space or in acoustic space filled with uncorrelated continuous broadband noise. In the absence of background noise, AI virtual space receptive fields (VSRFs) are typically large, representing a quadrant or more of acoustic space. Within the receptive field, onset latency and firing strength form functional gradients. We hypothesized earlier that functional gradients in the receptive field provide information about sound-source direction. Previous studies indicated that spatial gradients could remain relatively constant across changes in signal intensity. In the current experiments we tested the hypothesis that directional sensitivity to a transient signal, as reflected in the gradient structure of VSRFs of AI neurons, is also retained in the presence of a continuous background noise. When background noise was introduced three major affects on VSRFs were observed. 1) The size of the VSRF was reduced, accompanied by a reduction of firing strength and lengthening of response latency for signals at an acoustic axis and on-lines of constant azimuth and elevation passing through the acoustic axis. These effects were monotonically related to the intensity of the background noise over a noise intensity range of approximately 30 dB. 2) The noise intensity-dependent changes in VSRFs were mirrored by the changes that occurred when the signal intensity was changed in signal-alone conditions. Thus adding background noise was equivalent to a shift in the threshold of a directional signal, and this shift was seen across the spatial receptive field. 3) The spatial gradients of response strength and latency remained evident over the range of background noise intensity that reduced spike count and lengthened onset latency. Those gradients along the azimuth that spanned the frontal midline tended to remain constant in slope and position in the face of increasing intensity of background noise. These findings are consistent with our hypothesis that, under background noise conditions, information that underlies directional acuity and accuracy is retained within the spatial receptive fields of an ensemble of AI neurons.     

Neuronal responses across cortical field A1 in plasticity induced by peripheral auditory organ damage

The adult auditory cortex is capable of a plastic reorganization of its tonotopic map after damage to restricted parts of the cochlear sensory epithelium. We examine the precise conditions of cochlear damage required to demonstrate such plasticity in the primary auditory cortex (A1) of the cat and the changes observed in neuronal responses in the A1 which has reorganized in plasticity of the tonotopic map. From these data we attempt to predict the conditions required for similar plasticity to occur in humans after cochlear damage.     

Sex differences in prefrontal cortical brain activity during fMRI of auditory verbal working memory.

Functional imaging studies of sex effects in working memory (WMEM) are few, despite significant normal sex differences in brain regions implicated in WMEM. This functional MRI (fMRI) study tested for sex effects in an auditory verbal WMEM task in prefrontal, parietal, cingulate, and insula regions. Fourteen healthy, right-handed community subjects were comparable between the sexes, including on WMEM performance. Per statistical parametric mapping, women exhibited greater signal intensity changes in middle, inferior, and orbital prefrontal cortices than men (corrected for multiple comparisons). A test of mixed-sex groups, comparable on performance, showed no significant differences in the hypothesized regions, providing evidence for discriminant validity for significant sex differences. The findings suggest that combining men and women in fMRI studies of cognition may obscure or bias results. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of auditory cortical lesions on pure-tone sound localization by the albino rat.

Used 2-choice and 3-choice tests to evaluate the effects of bilateral auditory cortical lesions on pure-tone sound localization by 10 male albino rats. Both tests required that Ss approach a distant sound source to obtain water reinforcement. Stimuli were single noise and tone bursts, 65 msec in duration including 20-msec rise and fall times. Tone frequencies were 2, 4, 8, 16, and 32 kHz adjusted to 40 dB (sound pressure level) above the S's absolute threshold. Five Ss were tested in the 2-choice situation following bilateral ablation of auditory cortex. Some reduction in performance was observed relative to normals, but impairments were not severe. Similar results were obtained for 2 brain-damaged Ss tested in the 3-choice situation. Thus, the ability to localize sounds in space remained intact after complete destruction of auditory cortex, and there was no indication of a frequency-dependent deficit. Findings are considered in relation to the more severe deficits observed in other mammals after lesions of the auditory cortex. (30 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of bilateral auditory cortical lesions on gap-detection thresholds in the ferret (Mustela putorius).

Ferrets were tested for their ability to detect temporal gaps in noise before and after bilateral lesions of the primary auditory cortex. Thresholds for gap detection were determined first for normal animals with band-pass noises at various center frequencies (0.5 to 32 kHz) and at 8 kHz with various sound pressure levels (-10 to 70 dB). Gap-detection ability improved steadily as sound pressure increased up to 70 dB. No systematic relation was found between threshold and center frequency. To determine the effects of brain damage, ferrets were tested with 8-kHz band-pass noise at 70 dBSPL. After bilateral lesions of auditory cortex, ferrets were still capable of detecting gaps, but the mean threshold was elevated from 10.1 to 20.1 ms. The data demonstrate that auditory cortex is important for perceptual tasks requiring fine temporal resolution. (PsycINFO Database Record (c) 2010 APA, all rights reserved)