Intensity coding of auditory stimuli: an fMRI study

The effect of stimulus intensity (sound pressure level, SPL) of auditory stimuli on the BOLD response in the auditory cortex was investigated in 14 young and healthy subjects, with no hearing abnormalities, using echo-planar, functional magnetic resonance imaging (fMRI) during a verbal and a non-verbal auditory discrimination task. The stimuli were presented block-wise at three different intensities: 95, 85 and 75 dB (SPL). All subjects showed fMRI signal increases in superior temporal gyrus (STG) covering primary and secondary auditory cortex. Most importantly, the spatial extent of the fMRI response in STG increased with increasing stimulus intensity. It is hypothesized that spreading of excitation is associated with the encoding of increasing stimulus intensity levels. In addition, we found bifrontal activation supposedly evoked by the auditory-articulary loop of working memory. The results presented here should assist in the design of optimal activation strategies for studying the auditory cortex with fMRI paradigms and may help in understanding intensity coding of auditory stimuli.

Intensity discrimination and auditory brainstem responses in cochlear implant and normal-hearing listeners

Intensity-discrimination limens (IDLs) and auditory brainstem responses (ABRs) were measured as a function of stimulus intensity in 6 cochlear implant (CI) and 8 normal-hearing (NH) listeners. Pulse-train stimuli were delivered electrically to the auditory nerve in CI listeners and acoustically in NH listeners. In CI listeners, the IDLs expressed as Weber fractions decreased monotonically with increasing intensity. In NH listeners, a nonmonotonic IDL function showing a peak a midintensities was observed. ABR wave amplitudes increased regularly with intensity only in CI listeners. Results support the notion that the slight decrease in Weber's fractions with increasing sound intensity--generally referred to as "the near-miss to Weber's law"--is subtended by retrocochlear processes, whereas the increase in Weber's fractions at midlevels--known as "the severe departure from Weber's law"--originates in cochlear mechanisms.

Contributions of the hippocampus, amygdala, and dorsal striatum to the response elicited by reward reduction

Rats were trained to run down a runway for either 1 or 10 food pellets. After training, those receiving 10 pellets were shifted to 1 pellet. Such shifts typically elicit a temporary decrease in running speed. Groups of normal rats and rats with bilateral lesions of the fimbria-fornix, lateral-basolateral complex of the amygdala, or dorsal striatum were tested with the shifted and unshifted procedures. Separate experiments, identical except for the intertrial intervals (ITIs; 3 min vs. 30 s), were carried out. The data are consistent with the view that an integrated action of multiple neural systems is required to observe the typical response to reward reduction in unlesioned rats. One system that includes the dorsal striatum promotes a reinforced approach response to the goal box. A neural system that includes fimbria-fornix is required to retain information about reduced reward over the 3-min ITI. A system that includes the amygdala may acquire a conditioned aversive response to the goal box after the shift is detected, leading to reduced speeds over testing.

Cue control and head direction cells

Previous research has shown that head direction (HD) cells in both the anterior dorsal thalamus (ADN) and the postsubiculum (PoS) in rats discharge in relation to familiar, visual landmarks in the environment. This study assessed whether PoS and ADN HD cells would be similarly responsive to nonvisual or unfamiliar environmental cues. After visual input was eliminated by blindfolding the rats, HD cells maintained direction-specific discharge, but their preferred firing directions became less stable. In addition, rotations of the behavioral apparatus indicated that some nonvisual cues (presumably tactile, olfactory, or both) exerted above chance stimulus control over a cell's preferred firing direction. However, a prominent auditory cue was not effective in exerting stimulus control over a cell's preferred direction. HD cell activity also was assessed after rotation of a novel visual cue exposed to the rat for 1, 3, or 8 min. An 8-min exposure was enough time for a novel visual cue to gain control over a cell's preferred direction, whereas an exposure of 1 or 3 min led to control in only about half the sessions. These latter results indicate that HD cells rely on a rapid learning mechanism to develop associations with landmark cues.

Taste responses in the greater superficial petrosal nerve: substantial sodium salt and amiloride sensitivities demonstrated in two rat strains

A great quantity of research has focused on neural responses of the chorda tympani nerve (CT) to taste stimuli. This report examined salt and sugar sensitivity of the greater superficial petrosal nerve (GSP) and the effect of amiloride on these neural responses. In addition to Sprague-Dawley (SD) rats that have CT responses typical of most rat strains, we included Fischer 344 (F344) rats whose CT responses to sodium chloride (NaCl) are higher than those of other strains. After a stimulation series in which water served as the rinse, a series of stimuli was presented in 100 microM amiloride. The GSP was highly responsive to NaCl, sodium acetate (NaAc), ammonium chloride, and sucrose; NaCl and NaAc responses were strongly suppressed by amiloride. Relative responses to NaCl were significantly higher in F344 than in SD rats. In summary, the GSP is highly sensitive to salt and sugar stimulation, and palatal taste receptors have a considerable degree of amiloride sensitivity.

Visual hemifield mapping using transcranial magnetic stimulation coregistered with cortical surfaces derived from magnetic resonance images

The perception of a visual stimulus can be inhibited by occipital transcranial magnetic stimulation. This visual suppression effect has been attributed to disruption in the cortical gray matter of primary visual cortex or in the fiber tracts leading to V1 from the thalamus. However, others have suggested that the visual suppression effect is caused by disruption in secondary visual cortex. Here the authors used a figure-eight coil, which produces a focal magnetic field, and a Quadropulse stimulator to produce visual suppression contralateral to the stimulated hemisphere in five normal volunteer subjects. The authors coregistered the stimulation sites with magnetic resonance images in these same subjects using optical digitization. The stimulation sites were mapped onto the surface of the occipital lobes in three-dimensional reconstructions of the cortical surface to show the distribution of the visual suppression effect. The results were consistent with disruption of secondary visual cortical areas.

Color selection and location selection in ERPs: differences, similarities and 'neural specificity'

It was hypothesized that color selection consists of two stages. The first stage represents a feature specific selection in neural populations specialized in processing color. The second stage constitutes feature non-specific selections, related to executive attentional processes and/or motor processes. This hypothesis was tested by investigating the effects of selectively attending to a specific color, location, or conjunction of location and color on the ERPs elicited by briefly flashed gratings. The gratings differed on three dimensions: color (red or blue), location in the visual field (4.4 degrees to the left or right of fixation) and form (target or non-target). Subjects had to respond to the presentation of target gratings in the attended category. Color selection was reflected in an enhanced parietal positivity in the 150-190 ms interval. Source analyses suggested that this color selection positivity might be generated in the basal occipital cortex, possibly human V4, an area of the brain specialized in color processing. The effect was separated from the P1 spatial attention effect both in topography and sources. Color selection was also reflected in a contralateral occipitotemporal negativity, which resembled the N1 spatial attention effect both in timing and topography. And finally, color selection was reflected in an N2b component. This N2b was similar in timing, topography and sources to the N2b's elicited by location selection and conjunction selection. We suggested that the N2b reflects feature non-specific selection processes, elicited by a range of attended stimuli, and possibly reflects activity in the anterior cingulate cortex. The NP80 was unaffected by attention to color and/or location and localized in striate cortex.

Comparison of ventral subicular and hippocampal neuron spatial firing patterns in complex and simplified environments

Activity from ventral subicular and hippocampal CA1 neurons was recorded in rats exploring a 4-arm radial maze in which the local and distal cues could be manipulated. Cells from both regions exhibited place fields, although ventral subicular neurons had larger fields than hippocampal cells. Rotation of the local and distal cues in opposite directions produced movement of the place fields in either direction or a complete change in firing pattern. Simplifying the environment also produced changes in place field location. Despite similarities between regions, subiculum fields decreased in size whereas hippocampal fields increased in the simple environment. These findings suggest that subicular cells may receive converging input from several hippocampal neurons and code more complex configurations of the cues.

[Contrast sensitivity in amblyopia, abiotrophy and optic nerve atrophy in children]

The threshold spatial contrast sensitivity was assessed in children with amblyopia, tapetoretinal abiotrophy, and partial atrophy of the optic nerve of different origin. The measurements were carried out at spatial frequencies of 0.5-22 cycle/grad. Tests with white, red, green, and blue arrays against a black background realized in the Zebra software for IBM computers with VGA or SVGA displays were carried out. Classification of curves representing the contrast sensitivity for the above conditions is proposed. Characteristics of the groups with the above diseases are described, based on the height, shape, and mutual position of the curves.

[The electrophysiological indices of the state of the different sections of the auditory analyzer in persons with a normal voice and with functional voice disorders]

Individuals with normal voice and patients with voice functional impairments undergone electrophysiological investigation of various parts of the hearing system, using tone audiometry, including the extended frequency band (10, 12, 14 and 16 kHz), as well as short- and long-latency acoustic evoked potentials (SLAEP and LLAEP). It was found out, that individuals with voice functional impairments had all of their hearing system's parts impaired to various extent, with more marked impairments in the central, rather than in the peripheral part of the hearing system. It was shown, that hearing at 4-8 kHz, as well as with the extended frequency band, especially at 14-16 kHz, time patterns of acoustic evoked potentials (latencies of waves III and V of SLAEP, the interpeak interval I-V, as well as the latency periods of the LLAEP components P2 and N2) could be useful in professional selection of individuals of voice and speech professions and for solving labor expertise matters. Of those individuals with normal voice but systematic vocal stress, 17.5% had impaired hearing at 14 and 16 kHz, as well as significant latency prolongation of the LLAEP wave N2 with tone stimulation at 1 and 4 kHz. Apparently, individuals of voice and speech professions should be referred to as the "risk" group. It may well be, that extended band audiometry and acoustic evoked potentials time patterns could be useful in determining the thresholds between the normality and pathology in voice dysfunctions.

[Objective assessment of the olfactory analyzer function based on recording of olfacto-vegetative and olfacto-vestibular reactions]

To register objectively olfactory defects and to assess them qualitatively and quantitatively, the authors propose to use olfactometric techniques reflecting olfactovegetative and olfactovestibular manifestations. Automatic pupillographic complex APK-OI provides speed, time and amplitude parameters of the pupillary and nyctating reflexes. 600 pupillograms and records of the nictation reflex proved high diagnostic significance of these methods. 104 stabilographic examinations (ST-02 unit) illustrated the changes in 12 vestibular parameters prior to and after olfactory impact. The results give grounds for application of olfactovestibular reactions registration as an objective olfactometric test.

Functional streams in occipito-frontal connections in the monkey

It is known that the prestriate cortical regions that project to area LIP in parietal cortex and to areas TEO and TE in temporal cortex are mostly separated. Two separate streams of information transfer from occipital cortex can this be distinguished. We wished to determine whether the parietal and temporal streams remain segregated in their projections to frontal cortex. Paired injections of retrograde fluorescent tracers were placed in parietal and temporal cortex, or in the lateral and medial parts of the frontal eye field (FEF). The cortical regions containing retrogradely labeled cells were reconstructed in two-dimensional maps. The results show that temporal cortex mainly projects to lateral FEF (area 45). Parietal cortex sends projections to medial FEF (area 8a) and to lateral FEF, as well as to area 46. Thus, the parietal and temporal streams converge in lateral FEF. Most of the occipital regions projecting to medial FEF are the same as those projecting to parietal cortex, whereas lateral FEF receives afferents from the same occipital regions as those sending projections to temporal cortex. Thus, one can distinguish two interconnected networks. One is associated with the inferotemporal cortex and includes areas of the ventral bank and fundus of the superior temporal sulcus (STS), lateral FEF and ventral prestriate cortex. This network emphasizes central vision, small accades and form recognition. The other network is linked to cortex of the intraparietal sulcus. It consists of areas of the upper bank and fundus of STS, medial FEF and dorsal prestriate cortex. These areas encode peripheral visual field and are active during large saccades.

On the specificity of neurons and visual areas

The dominant view during the past 40 years has been that the visual system analyzes the visual scene by breaking it down into basic attributes such as color, form, motion, depth and texture. Individual dedicated neurons and specific visual areas were believed to be devoted to the analysis of each of these attributes. Current research has challenged these views by emphasizing that neurons, especially in the cortex, have multifunctional properties and therefore serve as general-purpose analyzers rather than feature detectors. Consequently, it appears that most extrastriate visual areas, rather than each being devoted to the analysis of a specific basic visual attribute, perform several different tasks and thereby engage in more advanced and complex analyses than had been realized.

Cortical mechanisms for visual perception of object motion and position in space

The present review is aimed at analyzing and discussing some of the cortical mechanisms possibly involved in the perception of object motion and object localization in the visual field. A comprehensive approach to these topics would be beyond the scope of this work. The highest priority, therefore, will be given to the cortical machinery involved in these processes, while very little (or nothing at all) will be said on the possible role played by subcortical structures such as the lateral geniculate nucleus and the superior colliculus which, albeit not directly involved in perception, might contribute to it.

[A model for the study of the ototoxic effects of drugs in an experiment]

The authors compared sensitivity of the acoustic analyzer elements to ototoxic drugs in cat experiments. The model proposed assesses primary responses of the brain cortical acoustic zone (BCAZ) to sound stimulation. The drugs effect on the analyzer's periphery was determined from changes in bioelectric responses of the cochlea to sound stimulation (microphone potential, potential of the acoustic nerve action). The arguments are presented for feasibility of using primary BCAZ responses evoked by electric stimulation of thalamocortical radiation fibers coming from the medial geniculate body for examination of the drug effects on the analyzer's cortex. Parallel registration of the above bioelectrical reactions in the same animal for a long time makes it possible to compare sensitivity of different elements of the acoustic analyzer to ototoxic medication.

[The epiphysis in the visual analyzer system]

The brain gland epiphysis, participated in the regulation of the endocrine functions of mammals, is rudimentary third eye of lower vertebrates. In the process of evolution the gland has lost it own photoreceptor functions, but saved very close contacts with visual system. By means of it biologically active combinations and, first of all, melatonin hormone, the epiphysis support control over transmission of visual information and participate in realization of biological effects of light on organism.