Biases of spatial attention in vision and audition.

Neurologically normal observers misperceive the midpoint of horizontal lines as systematically leftward of veridical center, a phenomenon known as pseudoneglect. Pseudoneglect is attributed to a tonic asymmetry of visuospatial attention favoring left hemispace. Whereas visuospatial attention is biased toward left hemispace, some evidence suggests that audiospatial attention may possess a right hemispatial bias. If spatial attention is supramodal, then the leftward bias observed in visual line bisection should also be expressed in auditory bisection tasks. If spatial attention is modality specific then bisection errors in visual and auditory spatial judgments are potentially dissociable. Subjects performed a bisection task for spatial intervals defined by auditory stimuli, as well as a tachistoscopic visual line bisection task. Subjects showed a significant leftward bias in the visual line bisection task and a significant rightward bias in the auditory interval bisection task. Performance across both tasks was, however, significantly positively correlated. These results imply the existence of both modality specific and supramodal attentional mechanisms where visuospatial attention has a prepotent leftward vector and audiospatial attention has a prepotent rightward vector of attention. In addition, the biases of both visuospatial and audiospatial attention are correlated.

Neural basis of auditory-induced shifts in visual time-order perception.

Attended objects are perceived to occur before unattended objects even when the two objects are presented simultaneously. This finding has led to the widespread view that attention modulates the speed of neural transmission in the various perceptual pathways. We recorded event-related potentials during a time-order judgment task to determine whether a reflexive shift of attention to a sudden sound modulates the speed of sensory processing in the human visual system. Attentional cueing influenced the perceived order of lateralized visual events but not the timing of event-related potentials in visual cortex. Attentional cueing did, however, enhance the amplitude of neural activity in visual cortex, which shows that attention-induced shifts in visual time-order perception can arise from modulations of signal strength rather than processing speed in the early visual-cortical pathways.

Auditory spatial localization and attention deficits in autistic adults.

The objective of this study was to compare autistic adults and matched control subjects in their ability to focus attention selectively on a sound source in a noisy environment. Event-related brain potentials (ERPs) were recorded while subjects attended to a fast paced sequence of brief noise bursts presented in free-field at a central or peripheral location. Competing sequences of noise bursts at adjacent locations were to be ignored. Both behavioral measures of target detection and auditory ERP amplitudes indicated that control subjects were able to focus their attention more sharply on the relevant sound source than autistic subjects. These findings point to a fundamental deficit in the spatial focusing of auditory attention in autism, which may be a factor that impedes social interactions and sensory-guided behavior, particularly in noisy environments.

Human auditory cortex tracks task-irrelevant sound sources.

The brain organizes sound into coherent sequences, termed auditory streams. We asked whether task-irrelevant sounds would be detected as separate auditory streams in a natural listening environment that included three simultaneously active sound sources. Participants watched a movie with sound while street-noise and sequences of naturally varying footstep sounds were presented in the background. Occasional deviations in the footstep sequences elicited the mismatch negativity (MMN) event-related potential. The elicitation of MMN showed that the regular features of the footstep sequences had been registered and their violations detected, which could only occur if the footstep sequence had been detected as a separate auditory stream. Our results demonstrate that sounds are organized into auditory streams irrespective of their relevance to ongoing behavior.

Psychophysiology and imaging of visual cortical functions in the blind: a review.

Imaging, transcranial magnetic stimulation, and psychophysiological recordings of the congenitally blind have confirmed functional activation of the visual cortex but have not extensively explained the functional significance of these activation patterns in detail. This review systematically examines research on the role of the visual cortex in processing spatial and non-visual information, highlighting research on individuals with early and late onset blindness. Here, we concentrate on the methods utilized in studying visual cortical activation in early blind participants, including positron emissions tomography (PET), functional magnetic resonance imaging (fMRI), transcranial magnetic stimulation (TMS), and electrophysiological data, specifically event-related potentials (ERPs). This paper summarizes and discusses findings of these studies. We hypothesize how mechanisms of cortical plasticity are expressed in congenitally in comparison to adventitiously blind and short-term visually deprived sighted participants and discuss potential approaches for further investigation of these mechanisms in future research.

The temporal window of integration in elderly and young adults.

Integration of information across time is an essential part of auditory processing. Evidence from a variety of experiments support the notion of an approximately 200-ms long time window following the onset of a sound, during which a unitary sound representation is formed (the temporal window of integration, TWI). The temporal resolution in the auditory system is assumed to decrease with aging suggesting that the duration of the TWI may be longer in elderly than young adults. The TWI duration was assessed in young and elderly adults using the oddball paradigm in which a regular auditory event (standard) is occasionally exchanged for a different event (deviant). Previous studies showed that when the stimulus onset asynchrony (SOA) exceeds the duration of the TWI, two successive deviations occurring infrequently in a repetitive sound sequence elicit two separate mismatch negativity (MMN) event-related brain potentials. However, only one MMN is elicited when the SOA is shorter than the TWI. Experiment 1 tested MMN elicitation for the second of two successive deviant sounds as a function of the SOA. Experiment 2 used the sound omission paradigm, in which MMN is only elicited by omissions when the SOA is shorter than the TWI. Again, MMN elicitation was tested by infrequent tone omissions as a function of the SOA. Results showed no significant differences between elderly and younger participants as a function of SOA. This suggests that the duration of the TWI is approximately between 200 and 250 ms in both groups of subjects. On the other hand, the lower MMN amplitudes elicited by frequency deviation in the elderly compared with the younger participants suggest that the specificity of frequency representation deteriorates with aging.

Control mechanisms mediating shifts of attention in auditory and visual space: a spatio-temporal ERP analysis.

The neural systems that mediate voluntary shifts of attention to visual and auditory stimuli were investigated by examining the patterns of human brain electricity elicited by attention-directing cues in auditory and visual tasks. Several lateralized event-related potential (ERP) components were observed when participants shifted attention in expectation of visual targets (experiment 1). One component was focused over frontal cortex and a second was focused primarily over the occipital-temporal cortex but also spread to parietal regions of the scalp. Previous work has indicated that the frontal component reflects supramodal processes involved in the executive control of attention and that the posterior component reflects either spatial attentional control processes in the posterior parietal lobe or modulation of processes in visual cortex. Here, the posterior component was observed when participants shifted attention in expectation of auditory targets (experiments 2-4), but the frontal component was found only in the visual task. The posterior component seemed to be generated in parietal and occipital areas even when there was no visual information about the to-be-attended locations. These results are consistent with the view that voluntary shifts of attention are mediated by supramodal processes in the parietal lobe and that the spatial coordinates of the to-be-attended location are based on visual representations of space.

Neural substrates of perceptual enhancement by cross-modal spatial attention.

Orienting attention involuntarily to the location of a sudden sound improves perception of subsequent visual stimuli that appear nearby. The neural substrates of this cross-modal attention effect were investigated by recording event-related potentials to the visual stimuli using a dense electrode array and localizing their brain sources through inverse dipole modeling. A spatially nonpredictive auditory precue modulated visual-evoked neural activity first in the superior temporal cortex at 120-140 msec and then in the ventral occipital cortex of the fusiform gyrus 15-25 msec later. This spatio-temporal sequence of brain activity suggests that enhanced visual perception produced by the cross-modal orienting of spatial attention results from neural feedback from the multimodal superior temporal cortex to the visual cortex of the ventral processing stream.