Location Of Auditory Stimuli Research Articles

Assessment of Vestigial Auriculomotor Activity to Acoustic Stimuli Using Electrodes In and Around the Ear.

Recently, it has been demonstrated that electromyographic (EMG) activity of auricular muscles in humans, especially the postauricular muscle (PAM), depends on the spatial location of auditory stimuli. This observation has only been shown using wet electrodes placed directly on auricular muscles. To move towards a more applied, out-of-the-laboratory setting, this study aims to investigate if similar results can be obtained using electrodes placed in custom-fitted earpieces. Furthermore, with the exception of the ground electrode, only dry-contact electrodes were used to record EMG signals, which require little to no skin preparation and can therefore be applied extremely fast. In two experiments, auditory stimuli were presented to ten participants from different spatial directions. In experiment 1, stimuli were rapid onset naturalistic stimuli presented in silence, and in experiment 2, the corresponding participant's first name, presented in a "cocktail party" environment. In both experiments, ipsilateral responses were significantly larger than contralateral responses. Furthermore, machine learning models objectively decoded the direction of stimuli significantly above chance level on a single trial basis (PAM: 80%, in-ear: 69%). There were no significant differences when participants repeated the experiments after several weeks. This study provides evidence that auricular muscle responses can be recorded reliably using an almost entirely dry-contact in-ear electrode system. The location of the PAM, and the fact that in-ear electrodes can record comparable signals, would make hearing aids interesting devices to record these auricular EMG signals and potentially utilize them as control signals in the future.

Causal inference regulates audiovisual spatial recalibration via its influence on audiovisual perception.

To obtain a coherent perception of the world, our senses need to be in alignment. When we encounter misaligned cues from two sensory modalities, the brain must infer which cue is faulty and recalibrate the corresponding sense. We examined whether and how the brain uses cue reliability to identify the miscalibrated sense by measuring the audiovisual ventriloquism aftereffect for stimuli of varying visual reliability. To adjust for modality-specific biases, visual stimulus locations were chosen based on perceived alignment with auditory stimulus locations for each participant. During an audiovisual recalibration phase, participants were presented with bimodal stimuli with a fixed perceptual spatial discrepancy; they localized one modality, cued after stimulus presentation. Unimodal auditory and visual localization was measured before and after the audiovisual recalibration phase. We compared participants’ behavior to the predictions of three models of recalibration: (a) Reliability-based: each modality is recalibrated based on its relative reliability—less reliable cues are recalibrated more; (b) Fixed-ratio: the degree of recalibration for each modality is fixed; (c) Causal-inference: recalibration is directly determined by the discrepancy between a cue and its estimate, which in turn depends on the reliability of both cues, and inference about how likely the two cues derive from a common source. Vision was hardly recalibrated by audition. Auditory recalibration by vision changed idiosyncratically as visual reliability decreased: the extent of auditory recalibration either decreased monotonically, peaked at medium visual reliability, or increased monotonically. The latter two patterns cannot be explained by either the reliability-based or fixed-ratio models. Only the causal-inference model of recalibration captures the idiosyncratic influences of cue reliability on recalibration. We conclude that cue reliability, causal inference, and modality-specific biases guide cross-modal recalibration indirectly by determining the perception of audiovisual stimuli.

The eardrums move when the eyes move: A multisensory effect on the mechanics of hearing

Interactions between sensory pathways such as the visual and auditory systems are known to occur in the brain, but where they first occur is uncertain. Here, we show a novel multimodal interaction evident at the eardrum. Ear canal microphone measurements in humans (n = 19 ears in 16 subjects) and monkeys (n = 5 ears in 3 subjects) performing a saccadic eye movement task to visual targets indicated that the eardrum moves in conjunction with the eye movement. The eardrum motion was oscillatory and began as early as 10 ms before saccade onset in humans or with saccade onset in monkeys. These eardrum movements, which we dub Eye Movement Related Eardrum Oscillations (EMREOs), occurred in the absence of a sound stimulus. The EMREOs’ amplitude and phase depended on the direction and horizontal amplitude of the saccade. They lasted throughout the saccade and well into subsequent periods of steady fixation. We discuss the possibility that the mechanisms underlying EMREOs create eye movement-related binaural cues that may aid the brain in evaluating the relationship between visual and auditory stimulus locations as the eyes move.

The eardrums move when the eyes move: A multisensory effect on the mechanics of hearing

Interactions between sensory pathways such as the visual and auditory systems are known to occur in the brain, but where they first occur is uncertain. Here, we show a multimodal interaction evident at the eardrum. Ear canal microphone measurements in humans (n = 19 ears in 16 subjects) and monkeys (n = 5 ears in three subjects) performing a saccadic eye movement task to visual targets indicated that the eardrum moves in conjunction with the eye movement. The eardrum motion was oscillatory and began as early as 10 ms before saccade onset in humans or with saccade onset in monkeys. These eardrum movements, which we dub eye movement-related eardrum oscillations (EMREOs), occurred in the absence of a sound stimulus. The amplitude and phase of the EMREOs depended on the direction and horizontal amplitude of the saccade. They lasted throughout the saccade and well into subsequent periods of steady fixation. We discuss the possibility that the mechanisms underlying EMREOs create eye movement-related binaural cues that may aid the brain in evaluating the relationship between visual and auditory stimulus locations as the eyes move.

Visually-guided auditory adaptation and reference frame of the ventriloquism after effect

Ventriloquism aftereffect (VA) is observed as a shift in the perceived locations of auditory stimuli, induced by repeated presentation of audiovisual signals with incongruent locations of auditory and visual components. Since the two modalities use a different reference frame (RF), audition is head-centered (HC) while vision is eye-centered (EC), the representations have to be aligned. A previous study examining RF of VA found inconsistent results: the RF was a mixture of HC and EC for VA induced in the center of the audiovisual field, while it was predominantly HC for VA induced in the periphery [Lin et al., JASA 121, 3095, 2007]. In addition, the study found an adaptation in the auditory space representation even for congruent AV stimuli in the periphery. Here, a computational model examines the origins of these effects. The model assumes that multiple stages of processing interact: (1) the stage of auditory spatial representation (HC), (2) the stage of saccadic eye responses (EC), and (3) some stage at which the representation is mixed (HC + EC). Observed results are most consistent with a suggestion that the neural representation underlying spatial auditory plasticity incorporates both HC and EC auditory information, possibly at different processing stages. [Work supported by VEGA-1/1011/16.]

Touch the sound: The role of audio-tactile and audio-proprioceptive interaction on the spatial orientation in virtual scenes

Being able to localize objects in the space close to the body is an important prerequisite for precise object interaction. It is also very important for the spatial orientation in virtual scenes. Since sound is usually produced by the vibrations of a body, sound emitting objects, such as shaver or hair dryer, provide both auditory and haptic information. This study focuses on auditory-haptic localization in the spatial domain. We carried out two experiments to investigate the interaction effects. In the first experiment, the influence of tactile signals on auditory localization task was investigated. Similar to the ventriloquist effect from auditory-visual interaction, the results of the first experiment show that the perceived location of auditory stimuli is influenced by tactile stimulation. The results also indicate some hints that there may be an audiotactile precedence effect. In the second experiment, the influence of auditory signals on proprioception was investigated. The results show that the auditory and proprioceptive information can be combined in such a way that the localization errors in a virtual scene are minimized.

Light Location Influences the Perceived Locations of Internal Sounds

Numerous studies have shown that visual stimuli can bias the perceived location of auditory stimuli. Here, we sought to determine if a visual stimulus can also bias the perceived location of multiple internal auditory stimuli. Fifty subjects were presented with a train of eight binaural click-pairs over headphones while a single flash of light was presented either to the left or to the right side of a central fixation point on an otherwise black CRT screen. A no-flash baseline was also implemented, as was a no-fixation control. The subjects used a rating scale to indicate the perceived location of each of the eight click-pairs within their heads. The results showed that the flash of light significantly influenced the perceived location of the click-pairs, biasing them in the same direction as the flash of light. This suggests that, even without perceptual correspondence, cross-modal interactions can occur.

Visual–auditory interaction in saccadic reaction time: Effects of auditory masker level

Saccadic reaction time (SRT) to a visual target tends to be shorter when auditory stimuli are presented in close temporal and spatial proximity, even when subjects are instructed to ignore the auditory non-target (focused attention paradigm). Observed SRT reductions typically range between 10 and 50 ms and decrease as spatial disparity between the stimuli increases. Previous studies using pairs of visual and auditory stimuli differing in both azimuth and vertical position suggest that the amount of SRT facilitation decreases not with the physical but with the perceivable distance between visual target and auditory accessory. Here we probe this hypothesis by presenting an additional white-noise masker background of 3 s duration. Increasing the masker level had a diametrical effect on SRTs in spatially coincident vs. disparate stimulus configurations: saccadic responses to coincident visual–auditory stimuli are slowed down, whereas saccadic responses to disparate stimuli are speeded up. As verified in a separate auditory localization task, localizability of the auditory accessory decreases with masker level. The SRT results are accounted for by a conceptual model positing that increasing masker level enlarges the area of possible auditory stimulus locations: it implies that perceivable distances decrease for disparate stimulus configurations and increase for coincident stimulus pairs.

Effect of a central fixation light on auditory spatial responses in area LIP.

A recent report demonstrated that, while fixating a central light, lateral intraparietal area (LIP) neurons are not modulated by the location of auditory stimuli until monkeys learn to saccade to the location of an auditory stimulus. This finding suggests that auditory spatial responses in area LIP are dependent on auditory-saccadic training. We found that, in monkeys that had not been trained to make behavioral responses to auditory stimuli, LIP neurons are modulated by auditory-stimulus location when a central light is not present in the environment. These results indicate that LIP auditory responses are not wholly dependent on behavioral training with auditory stimuli.

Spatial Processing in the Primate Auditory Cortex

Spatial localization of auditory stimuli is dependent on the cerebral cortex, yet it remains unclear how cortical activity gives rise to spatial percepts. It has recently been proposed that spatial information is processed serially within the primate auditory cortex, initially in the primary auditory cortex (AI) through the auditory areas caudal to AI, particularly the caudomedial (CM) and caudolateral fields, and onward to the parietal lobe. The activity of single neurons in AI and CM supports this hypothesis, where a greater percentage of CM neurons are sensitive to the spatial location of acoustic stimuli than AI neurons, and the spatial sensitivity of CM neurons extends across a broader range of the stimulus spectrum compared to AI neurons. Further, populations of CM neurons are better able to predict sound localization ability than are populations of AI neurons. We have recently explored the effects of stimulus intensity on both sound localization performance and the spatial sensitivity of auditory cortical neurons. The preliminary results of these experiments again indicate that spatial information is serially processed between AI and the caudal fields. The effects of visual stimulation on auditory localization have also been investigated. Under the appropriate circumstances, visual stimuli can ‘capture’ the spatial location of auditory stimuli in both humans and monkeys. This perceptual illusion suggests that there is a plastic shift in auditory spatial perception. Where the representation of this shift resides is unknown, although two likely candidates are the multimodal regions of the parietal lobe and the superior temporal sulcus.

Electrophysiological evidence for an early(pre-attentive) information processing deficit in patients with right hemisphere damage and unilateral neglect.

Patients with right hemisphere damage and contralesional neglect are often unaware of visual, auditory or tactile stimuli occurring on their left side. In an effort to understand the contribution of pre-attentive processes to this phenomenon, we examined the processing of the pitch, duration and spatial location of auditory stimuli using an electrophysiological probe, the mismatch negativity (MMN). This event-related brain potential indexes the integrity of cerebral processes that respond automatically to deviations from regularity in the acoustic environment. We compared the MMN elicited by right- and left-sided deviant stimuli in 10 patients with left unilateral neglect and 10 age-matched healthy volunteers, exploring an anticipated dissociation between the processing of spatial localization of sounds and the processing of the other auditory dimensions. Across dimensions, the MMN elicited by deviance occurring to the left of the patients was reduced relative to that elicited by deviance occurring to the right. This effect was robust for spatial location, and less so for pitch, whereas the processing of stimulus duration was not significantly affected by the side of stimulation. In healthy subjects, deviance in either side elicited similar MMN. We suggest that an early deficit in detecting changes in the environment hampers the involuntary triggering of attention in those patients and discuss the specific role of encoding spatial location in the establishment of conscious awareness.

Early Visual Experience Shapes the Representation of Auditory Space in the Forebrain Gaze Fields of the Barn Owl

Auditory spatial information is processed in parallel forebrain and midbrain pathways. Sensory experience early in life has been shown to exert a powerful influence on the representation of auditory space in the midbrain space-processing pathway. The goal of this study was to determine whether early experience also shapes the representation of auditory space in the forebrain. Owls were raised wearing prismatic spectacles that shifted the visual field in the horizontal plane. This manipulation altered the relationship between interaural time differences (ITDs), the principal cue used for azimuthal localization, and locations of auditory stimuli in the visual field. Extracellular recordings were used to characterize ITD tuning in the auditory archistriatum (AAr), a subdivision of the forebrain gaze fields, in normal and prism-reared owls. Prism rearing altered the representation of ITD in the AAr. In prism-reared owls, unit tuning for ITD was shifted in the adaptive direction, according to the direction of the optical displacement imposed by the spectacles. Changes in ITD tuning involved the acquisition of unit responses to adaptive ITD values and, to a lesser extent, the elimination of responses to nonadaptive (previously normal) ITD values. Shifts in ITD tuning in the AAr were similar to shifts in ITD tuning observed in the optic tectum of the same owls. This experience-based adjustment of binaural tuning in the AAr helps to maintain mutual registry between the forebrain and midbrain representations of auditory space and may help to ensure consistent behavioral responses to auditory stimuli.

Representation of frequency in the primary auditory field of the barn owl forebrain.

1. The primary auditory field (PAF) constitutes the first telencephalic stage of auditory information processing in the classical auditory pathway. In this study we investigated the frequency representation in the PAF of the barn owl, a species with a broad frequency range of hearing and a highly advanced auditory system. 2. Single- and multiunit sites were recorded extracellularly in ketamine-anesthetized owls. The frequency response properties of PAF sites were assessed with the use of digitally synthesized dichotic stimuli. PAF sites (n = 442) either were unresponsive to tonal stimulation (but responsive to noise stimuli), were tuned for frequency, or had multipeaked frequency response profiles. Tuned sites responded best to frequencies between 0.2 and 8.8 kHz, a range that encompasses nearly the entire hearing range of the barn owl. Most sites responding best to frequencies < 4 kHz had relatively broad frequency tuning, whereas sites responding best to higher frequencies had either broad or narrow frequency tuning. Sites with multipeaked frequency response profiles typically had two response peaks. The first peak was usually between 1 and 3 kHz and the second was usually between 5 and 8 kHz; there was no systematic relationship between the two peak frequencies. 3. In dorsoventral electrode penetrations that contained sites with tuned and/or multipeaked response profiles, a "common frequency" was identified that elicited a maximal response from all of the sites in the penetration. 4. The PAF contains a single tonotopic field. Units tuned to low frequencies are located caudomedially, whereas units tuned to high frequencies are located rostrolaterally. Compared with the frequency representation along the basilar papilla and in other auditory structures, the PAF overrepresents low frequencies (< 4 kHz) that are important for barn owl vocalizations. Conversely, high frequencies (> or = 4 kHz), which are necessary for precise sound localization, are underrepresented relative to these more peripheral auditory structures. 5. There was considerable interindividual variability both in the relative magnification of different frequency ranges and in the orientation of the tonotopic map in the brain. 6. These results suggest that the barn owl PAF, like the mammalian primary auditory cortex, is a general processor of auditory information that is involved in the analysis of both the meaning (such as species-specific vocalizations) and the location of auditory stimuli. In addition, the high degree of interindividual variability in the representation of frequency information suggests that the barn owl PAF, like the mammalian auditory cortex, is subject to modification by sensory experience.

Disruption of auditory spatial working memory by inactivation of the forebrain archistriatum in barn owls.

Barn owls not only localize auditory stimuli with great accuracy, they also remember the locations of auditory stimuli and can use this remembered spatial information to guide their flight and strike. Although the mechanisms of sound localization have been studied extensively, the neurobiological basis of auditory spatial memory has not. Here we show that the ability of barn owls to orient their gaze towards and fly to the remembered location of auditory targets is lost during pharmacological inactivation of a small region in the forebrain, the anterior archistriatum. In contrast, archistriatal inactivation has no effect on stimulus-guided responses to auditory targets. The memory-dependent deficit is evident only for acoustic events that occur in the hemifield contralateral to the side that is inactivated. The data demonstrate that in the avian archistriatum, as in the mammalian frontal cortex, there exists a region that is essential for the expression of spatial working memory and that, in the barn owl, this region encodes auditory spatial memory.

Localization and recall of the spatial position of sounds

Memory for the location of auditory stimuli was tested after delays of 1 min, 30 min, or 24 h. Eleven environmental sounds were presented to 22 subjects in each of 11 different free-field positions, spaced evenly in a semicircle around the subject in the horizontal plane. Initial localization acuity was recorded upon presentation of each stimulus. To test memory for location, target stimuli were presented over headphones, and subjects attempted to recall the original position. Memory for location was significantly less accurate than initial localization, even after 1 min. Male subjects’ accuracy decreased over time, but there was no effect of delay for female subjects. A large effect of stimulus position was also noted, with stimuli coming from central locations remembered better than those coming from the extremes, an effect not accounted for by azimuthal differences in localization accuracy. Analysis of the error distributions indicated that this pattern was largely due to the presence of right–left reversals. These reversals suggest the existence of two separate components to auditory spatial memory: side and azimuth, and that these two parts can be dissociated.

Spatial Representations and Sensorimotor Transformations

Each year, the Centre de recherche en sciences neurologiques of the Université de Montréal organizes a symposium on a topic in the neurosciences. For the IXth International Symposium, the theme chosen was "Spatial Representations and Sensorimotor Transformations."Many of the diverse functions performed by the central nervous system have an important spatial component in common. For instance, there are neural mechanisms for the analysis and perception of the three-dimensional structure of visual space, such as the location, form, and movement of objects in the visual environment. There exist processes to determine the spatial location of auditory stimuli. One can also regard the body as an "internal" space for which mechanisms have evolved for the kinesthetic perception of the position and movement of body parts relative to one another, and for the position and orientation of the body within its immediate external environment. Motor control also requires spatial information, since many movements of the eyes, head, and limbs follow specific paths or are aimed at the specific spatial location of an object as signalled by sensory processes.One can argue, therefore, that a major aspect of the functioning of the brain involves the generation of many different spatial representations, and the exchange of information among them. Each of these neural representations provides a spatial coordinate framework whose coordinate axes are based on certain types of information. For instance, movement of the limb toward an object can be described equally well in several different coordinate systems, such as those based on its spatial path, its dynamics (the direction and level of forces, torques, and external loads), or the muscle activity by which it is achieved. A better understanding of the coordinates in which the CNS codes these various types of information will provide a better appreciation of the neural mechanisms generating the spatial representations. It will also provide a clearer understanding of the transformations that must occur to relay information between sensory and motor representations, which permit an animal to interact successfully with its environment.The participants at this Symposium were invited to examine some of these issues as they pertain to the somatic and visual systems.

Auditory cortex lesions and discrimination of spatial location by the rat

Five normal rats and four rats with bilateral lesions of auditory cortex were tested by the conditioned suppression procedure to determine their abilities to discriminate between spatially separated sound sources. The discrimination involved detection of a change in location of a train of clicks from a speaker on the animals' left to a speaker on the right. The separation between speakers was varied from 180° to 90°, 45°, 22°, 12°, 6°, and psychophysical functions were obtained using a method of descending limits. Both normal and brain-damaged animals were capable of discriminating left from right clicks and psychophysical curves were similar for the two groups. Histological analysis indicated that the lesions in each of the four brain-damaged rats destroyed primary auditory cortex as well as surrounding belt areas. Therefore, for the rat, auditory cortex was not found to be essential for discrimination of the spatial locations of auditory stimuli. The results are discussed in light of impairments in sound localization following lesions of auditory cortex in other mammalian species.

Control of responding by location of auditory stimuli: role of differential and non-differential reinforcement.

Sound was presented to monkeys through one of two loudspeakers, each adjacent to a response key. A response on the key adjacent to the sound source was reinforced (correct response). A response on the other key produced a timeout (incorrect response). Under these conditions, over 90% of responses were correct within one or two sessions. When the procedure was changed so that a response on either key was reinforced independently of which speaker was sounding, similar control by location developed within one or two sessions. When conditions were modified by moving the keys away from the immediate vicinity of the speakers, the animals required about 20 sessions to reach a stable level of greater than 90% correct responses under differential reinforcement conditions. No control by location developed under nondifferential reinforcement conditions.

Control of responding by location of auditory stimuli: rapid acquisition in monkey and rat.

Monkeys require a considerably larger number of trials to bring responding under the control of the location of an auditory stimulus than cats, rats, and bats with the same experimental procedures. The present experiment sought to determine the conditions necessary for rapid acquisition of control of responding by location of noise and tone bursts in the monkey. Monkeys were run in an enclosure that contained four loudspeakers and four manipulanda. Two conditions were used in training. In the adjacent condition, a stimulus (noise or tone burst) was presented through one or other of two speakers and a response on the manipulandum adjacent to the speaker was reinforced with food. In the nonadjacent condition, a stimulus was presented through one of two speakers and a response on a manipulandum remote from the speaker was reinforced with food. Acquisition of control was measured by change in the percentage of reinforced responses during training. In the adjacent condition, responding came under control of location within zero to three sessions. In nonadjacent conditions, the animals required 14 to 20 sessions to come under control of location. These latter numbers are comparable to those reported in the literature for localization discrimination in monkeys.