Processing of speech signals for physical and sensory disabilities.

Assistive technology involving voice communication is used primarily by people who are deaf, hard of hearing, or who have speech and/or language disabilities. It is also used to a lesser extent by people with visual or motor disabilities. A very wide range of devices has been developed for people with hearing loss. These devices can be categorized not only by the modality of stimulation [i.e., auditory, visual, tactile, or direct electrical stimulation of the auditory nerve (auditory-neural)] but also in terms of the degree of speech processing that is used. At least four such categories can be distinguished: assistive devices (a) that are not designed specifically for speech, (b) that take the average characteristics of speech into account, (c) that process articulatory or phonetic characteristics of speech, and (d) that embody some degree of automatic speech recognition. Assistive devices for people with speech and/or language disabilities typically involve some form of speech synthesis or symbol generation for severe forms of language disability. Speech synthesis is also used in text-to-speech systems for sightless persons. Other applications of assistive technology involving voice communication include voice control of wheelchairs and other devices for people with mobility disabilities.

Receptive field structure in the visual cortex: does selective stimulation induce plasticity?

Sensory areas of adult cerebral cortex can reorganize in response to long-term alterations in patterns of afferent signals. This long-term plasticity is thought to play a crucial role in recovery from injury and in some forms of learning. However, the degree to which sensory representations in primary cortical areas depend on short-term (i.e., minute to minute) stimulus variations remains unclear. A traditional view is that each neuron in the mature cortex has a fixed receptive field structure. An alternative view, with fundamentally different implications for understanding cortical function, is that each cell's receptive field is highly malleable, changing according to the recent history of the sensory environment. Consistent with the latter view, it has been reported that selective stimulation of regions surrounding the receptive field induces a dramatic short-term increase in receptive field size for neurons in the visual cortex [Pettet, M. W. & Gilbert, C. D. (1992) Proc. Natl. Acad. Sci. USA 89, 8366-8370]. In contrast, we report here that there is no change in either the size or the internal structure of the receptive field following several minutes of surround stimulation. However, for some cells, overall responsiveness increases. These results suggest that dynamic alterations of receptive field structure do not underlie short-term plasticity in the mature primary visual cortex. However, some degree of short-term adaptability could be mediated by changes in responsiveness.

Glutamate as a hippocampal neuron survival factor: an inherited defect in the trisomy 16 mouse.

The survival of cultured mouse hippocampal neurons was found to be greatly enhanced by micromolar concentrations of the excitatory neurotransmitter glutamate. Blockade of kainate/AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) glutamate receptors increased the rate of neuron death, suggesting that endogenous glutamate in the cultures promotes survival. Addition of glutamate (0.5-1 microM) further increased neuron survival, whereas glutamate in excess of 20 microM resulted in increased death. Thus, the survival vs. glutamate dose-response relation is bell-shaped with an optimal glutamate concentration near 1 microM. We found that hippocampal neurons from mice with the genetic defect trisomy 16 (Ts16) died 2-3 times faster than normal (euploid) neurons. Moreover, glutamate, at all concentrations tested, failed to increase survival of Ts16 neurons. In contrast, the neurotrophic polypeptide basic fibroblast growth factor did increase the survival of Ts16 and euploid neurons. Ts16 is a naturally occurring mouse genetic abnormality, the human analog of which (Down syndrome) leads to altered brain development and Alzheimer disease. These results demonstrate that the Ts16 genotype confers a defect in the glutamate-mediated survival response of hippocampal neurons and that this defect can contribute to their accelerated death.

Olfactory transduction is intrinsically noisy.

The sources of noise that limit olfactory signal detection were investigated in dissociated rat olfactory receptor cells. Near-threshold odorant-evoked currents exhibited large random fluctuation. However, similar fluctuations were observed in the absence of applied odorants when currents were induced by elevating the intracellular cyclic AMP concentration. This suggests that the fluctuations reflect noise intrinsic to the transduction mechanism, rather than the quantal nature of an odorant stimulus. For many odorants, this intrinsic noise may preclude the reliable detection of single odorant molecules.

Herpes simplex virus vectors overexpressing the glucose transporter gene protect against seizure-induced neuron loss.

We have generated herpes simplex virus (HSV) vectors vIE1GT and v alpha 4GT bearing the GLUT-1 isoform of the rat brain glucose transporter (GT) under the control of the human cytomegalovirus ie1 and HSV alpha 4 promoters, respectively. We previously reported that such vectors enhance glucose uptake in hippocampal cultures and the hippocampus. In this study we demonstrate that such vectors can maintain neuronal metabolism and reduce the extent of neuron loss in cultures after a period of hypoglycemia. Microinfusion of GT vectors into the rat hippocampus also reduces kainic acid-induced seizure damage in the CA3 cell field. Furthermore, delivery of the vector even after onset of the seizure is protective, suggesting that HSV-mediated gene transfer for neuroprotection need not be carried out in anticipation of neurologic crises. Using the bicistronic vector v alpha 22 beta gal alpha 4GT, which coexpresses both GT and the Escherichia coli lacZ marker gene, we further demonstrate an inverse correlation between the extent of vector expression in the dentate and the amount of CA3 damage resulting from the simultaneous delivery of kainic acid.

bcl-2 transgene expression can protect neurons against developmental and induced cell death.

The bcl-2 protooncogene, which protects various cell types from apoptotic cell death, is expressed in the developing and adult nervous system. To explore its role in regulation of neuronal cell death, we generated transgenic mice expressing Bcl-2 under the control of the neuron-specific enolase promoter, which forced expression uniquely in neurons. Sensory neurons isolated from dorsal root ganglia of newborn mice normally require nerve growth factor for their survival in culture, but those from the bcl-2 transgenic mice showed enhanced survival in its absence. Furthermore, apoptotic death of motor neurons after axotomy of the sciatic nerve was inhibited in these mice. The number of neurons in two neuronal populations from the central and peripheral nervous system was increased by 30%, indicating that Bcl-2 expression can protect neurons from cell death during development. The generation of these transgenic mice suggests that Bcl-2 may play an important role in survival of neurons both during development and throughout adult life.

G-utrophin, the autosomal homologue of dystrophin Dp116, is expressed in sensory ganglia and brain.

The utrophin gene is closely related to the dystrophin gene in both sequence and genomic structure. The Duchenne muscular dystrophy (DMD) locus encodes three 14-kb dystrophin transcripts in addition to several smaller isoforms, one of which, Dp116, is specific to peripheral nerve. We describe here the corresponding 5.5-kb mRNA from the utrophin locus. This transcript, designated G-utrophin, is of particular interest because it is specifically expressed in the adult mouse brain and appears to be the predominant utrophin transcript in this tissue. G-utrophin is expressed in brain sites generally different from the regions expressing beta-dystroglycan. During mouse embryogenesis G-utrophin is also seen in the developing sensory ganglia. Our data confirm the close evolutionary relationships between the DMD and utrophin loci; however, the functions for the corresponding proteins probably differ.

The photoreceptor sensory rhodopsin I as a two-photon-driven proton pump.

Proton translocation experiments with intact cells of Halobacterium salinarium overproducing sensory rhodopsin I (SRI) revealed transport activity of SRI in a two-photon process. The vectoriality of proton translocation depends on pH, being outwardly directed above, and inwardly directed below, pH 5.7. Activation of the transport cycle requires excitation of the initial dark state of SRI, SRI590, to form the intermediate SRI380. Action spectra identify the photocycle intermediates SRI380 and SRI520 as the two photochemically reactive species in the outwardly directed transport process. As shown by flash photolysis experiments, SRI520 undergoes a so-far unknown photochemical reaction to SRI380 with a half-time of <200 micros. Mutation of SRI residue Asp-76, the residue which is equivalent to the proton acceptor Asp-85 in bacteriorhodopsin, to asparagine leads to inactivation of proton translocation. This demonstrates that the underlying mechanisms of proton transport in both retinal proteins share similar features. However, SRI is to our knowledge the first case where photochemical reactions between two thermally unstable photoproducts of a retinal protein constitute a catalytic ion transport cycle.

Electrical and synaptic properties of embryonic luteinizing hormone-releasing hormone neurons in explant cultures.

Voltage- and ligand-activated channels in embryonic neurons containing luteinizing hormone-releasing hormone (LHRH) were studied by patch-pipette, whole-cell current and voltage clamp techniques. LHRH neurons were maintained in explant cultures derived from olfactory pit regions of embryonic mice. Cells were marked intracellularly with Lucifer yellow following recording. Sixty-two cells were unequivocally identified as LHRH neurons by Lucifer yellow and LHRH immunocytochemistry. The cultured LHRH neurons had resting potentials around -50 mV, exhibited spontaneous discharges generated by intrinsic and/or synaptic activities and contained a time-dependent inward rectifier (Iir). Voltage clamp analysis of ionic currents in the LHRH neuron soma revealed a tetrodotoxin-sensitive Na+ current (INa) and two major types of K+ currents, a transient current (IA), a delayed rectifier current (IK) and low- and high-voltage-activated Ca2+ currents. Spontaneous depolarizing synaptic potentials and depolarizations induced by direct application of gamma-aminobutyrate were both inhibited by picrotoxin or bicuculline, demonstrating the presence of functional gamma-aminobutyrate type A synapses on these neurons. Responses to glutamate were found in LHRH neurons in older cultures. Thus, embryonic LHRH neurons not yet positioned in their postnatal environment in the forebrain contained a highly differentiated repertoire of voltage- and ligand-gated channels.

Induction of cell proliferation in mammalian inner-ear sensory epithelia by transforming growth factor alpha and epidermal growth factor.

Regenerative proliferation occurs in the inner-ear sensory epithelial of warm-blooded vertebrates after insult. To determine how this proliferation is controlled in the mature mammalian inner ear, several growth factors were tested for effects on progenitor-cell division in cultured mouse vestibular sensory epithelia. Cell proliferation was induced in the sensory epithelium by transforming growth factor alpha (TGF-alpha) in a dose-dependent manner. Proliferation was also induced by epidermal growth factor (EGF) when supplemented with insulin, but not EGF alone. These observations suggest that stimulation of the EGF receptors by TGF-alpha binding, or EGF (plus insulin) binding, stimulates cell proliferation in the mature mammalian vestibular sensory epithelium.

Residue replacements of buried aspartyl and related residues in sensory rhodopsin I: D201N produces inverted phototaxis signals.

Residue replacements were made at five positions (Arg-73, Asp-76, Tyr-87, Asp-106, and Asp-201) in the Halobacterium salinarium phototaxis receptor sensory rhodopsin I (SR-I) by site-specific mutagenesis. The sites were chosen for their correspondence in position to residues of functional importance in the homologous light-driven proton pump bacteriorhodopsin found in the same organism. This work identifies a residue in SR-I shown to be of vital importance to its attractant signaling function: Asp-201. The effect of the substitution with the isosteric asparagine is to convert the normally attractant signal of orange light stimulation to a repellent signal. In contrast, similar neutral substitution of the four other ionizable residues near the photoactive site allows essentially normal attractant and repellent phototaxis signaling. Wild-type two-photon repellent signaling by the receptor is intact in the Asp-201 mutant, genetically separating the wild-type attractant and repellent signal generation processes. A possible explanation and implications of the inverted signaling are discussed. Results of neutral residue substitution for Asp-76 confirm our previous evidence that proton transfer reactions involving this residue are not important to phototaxis but that Asp-76 functions as the Schiff base proton acceptor in proton translocation by transducer-free SR-I.

Refinement of odor molecule tuning by dendrodendritic synaptic inhibition in the olfactory bulb.

Mitral/tufted cells (M/T cells) and granule cells form reciprocal dendrodendritic synapses in the main olfactory bulb; the granule cell is excited by glutamate from the M/T cell and in turn inhibits M/T cells by gamma-aminobutyrate. The trans-synaptically excited granule cell is thought to induce lateral inhibition in neighboring M/T cells and to refine olfactory information. It remains, however, elusive how significantly and specifically this synaptic regulation contributes to the discrimination of different olfactory stimuli. This investigation concerns the mechanism of olfactory discrimination by single unit recordings of responses to a series of normal aliphatic aldehydes from individual rabbit M/T cells. This analysis revealed that inhibitory responses are evoked in a M/T cell by a defined subset of odor molecules with structures closely related to the excitatory odor molecules. Furthermore, blockade of the reciprocal synaptic transmission by the glutamate receptor antagonist or the gamma-aminobutyrate receptor antagonist markedly suppressed the odor-evoked inhibition, indicating that the inhibitory responses are evoked by lateral inhibition via the reciprocal synaptic transmission. The synaptic regulation in the olfactory bulb thus greatly enhances the tuning specificity of odor responses and would contribute to discrimination of olfactory information.