Short Temporal Delays Research Articles

325 DISTINCT INTEGRATION OF RAPIDLY PRESENTED AUDIO-VISUAL EVENTS IN OLDER ADULT RECURRENT FALLERS

Abstract Background The ability to precisely integrate sensory information is critical to everyday functions, including balance/postural control and navigation. Older adults typically integrate sensory information over longer time delays than younger adults, which can reduce the precision of their perceptual judgments. This is particularly evident in older adults with a recent history of falls. However, the extent to which longitudinal patterns of falling or fall risk are associated with the precision of multisensory integration in older adults remains uncertain. Methods A sample of 2,321 older adults were grouped into longitudinal trajectories via self-report fall number (decreasing, stable, increasing) and, separately, sensorimotor fall risk via Timed-Up-and-Go (TUG) performance (stable, moderately declining, severely declining). Multisensory integration was measured once with the Sound Induced Flash Illusion, which examines how sound influences vision. A single ‘flash’ and two ‘beeps’ were presented over short (70ms) and longer (150ms and 230ms) temporal delays and older adults were asked to report only the number of ‘flashes’ they perceived. Accuracy on this task was the main outcome measure. Results Our results showed that the likelihood of incorrectly perceiving two ‘flashes’ at longer temporal delays increased with age and this was particularly evident for those with an increasing fall number trajectory. Furthermore, the likelihood of incorrectly perceiving two ‘flashes’ at the short temporal delay decreased with age but only for those same recurrent fallers. There was no association between longitudinal TUG trajectories and task performance. Conclusion Experiencing increasing incidents of falling over ten years is associated with distinct patterns of audio-visual integration in ageing. In contrast, long-term functional mobility is not. This may reflect links between multisensory integration and perturbed balance function and/or compromised cognitive functions (e.g. inhibitory control). Our findings contribute to our understanding of the mechanisms underpinning brain health in older age.

Transient perceptual enhancements resulting from selective shifts of exogenous attention in the central fovea

Exogenous attention, a powerful adaptive tool that quickly and involuntarily orients processing resources to salient stimuli, has traditionally been studied in the lower-resolution parafoveal and peripheral visual field.1-4 It is not known whether and how it operates across the 1° central fovea where visual resolution peaks.5,6 Here we investigated the dynamics of exogenous attention in the foveola. To circumvent the challenges posed by fixational eye movements at this scale, we used high-precision eye-tracking and gaze-contingent display control for retinal stabilization.7 High-acuity stimuli were briefly presented foveally at varying delays following an exogenous cue. Attended and unattended locations were just a few arcminutes away from the preferred locus of fixation. Our results show that for short temporal delays, observers' ability to discriminate fine detail is enhanced at the cued location. This enhancement is highly localized and does not extend to the nearby locations only 16' away. On a longer timescale, instead, we report an inverse effect: paradoxically, acuity is sharper at the unattended locations, resembling the phenomenon of inhibition of return at much larger eccentricities.8-10 Although exogenous attention represents a mechanism for low-cost monitoring of the environment in the extrafoveal space, these findings show that, in the foveola, it transiently modulates vision of detail with a high degree of resolution. Together with inhibition of return, it may aid visual exploration of complex foveal stimuli.11.

Contextual experience modifies functional connectome indices of topological strength and efficiency

Stimuli presented at short temporal delays before functional magnetic resonance imaging (fMRI) can have a robust impact on the organization of synchronous activity in resting state networks. This presents an opportunity to investigate how sensory, affective and cognitive stimuli alter functional connectivity in rodent models. In the present study we assessed the effect on functional connectivity of a familiar contextual stimulus presented 10 min prior to sedation for imaging. A subset of animals were co-presented with an unfamiliar social stimulus in the same environment to further investigate the effect of familiarity on network topology. Rats were imaged at 11.1 T and graph theory analysis was applied to matrices generated from seed-based functional connectivity data sets with 144 brain regions (nodes) and 10,152 pairwise correlations (after excluding 144 diagonal edges). Our results show substantial changes in network topology in response to the familiar (context). Presentation of the familiar context, both in the absence and presence of the social stimulus, strongly reduced network strength, global efficiency, and altered the location of the highest eigenvector centrality nodes from cortex to the hypothalamus. We did not observe changes in modular organization, nodal cartographic assignments, assortative mixing, rich club organization, and network resilience. We propose that experiential factors, perhaps involving associative or episodic memory, can exert a dramatic effect on functional network strength and efficiency when presented at a short temporal delay before imaging.

Modelling detection thresholds for sounds repeated at different delays

Detection thresholds for pairs or multiple copies of sounds are better than those for a single sound, an observation commonly interpreted as indicating temporal integration by the auditory system. Detection thresholds for pairs of brief tones depend on the delay between the tones (if short) and on frequency, suggesting frequency-dependent temporal overlap of auditory-filter responses elicited by the two successive stimuli (Krumbholz and Wiegrebe, 1998). The model presented by Krumbholz and Wiegrebe did not account for all aspects of their data, despite its complexity. This study shows that a simple probabilistic model based on Neubauer and Heil (2008) predicts the increase in threshold for short temporal delays as well as the asymptotic behaviour towards longer delays. The model entails (i) a 4th-order gammatone filter with a brief impulse response and thus broad bandwidth (shorter and broader than those of a filter normally assumed), (ii) the formation of stochastic ‘spikes’ or ‘events’ whose probability of occurrence is proportional to the filter output (half-wave rectified fine-structure or amplitude envelope), raised to a power of 3, and (iii) probability summation. The same model with the same front-end filter also predicts thresholds for pairs of clicks presented in band-reject noise, measured by Hall and Lummis (1973). The model accurately predicts the magnitudes and the decay of the alternating increase and decrease of thresholds as the delay between the click varies, the small effects of click polarity, and the dependence of thresholds for pairs of clicks with unequal intensities on their temporal order. Finally, we show that this model also correctly predicts the decrease in threshold with increasing number of temporally separated brief sounds, reported in several studies. While the latter data do not constrain the characteristics of the front-end filter, they do confirm the exponent of 3 in the model. Our paper stresses the viability of the model and raises the possibility that the bandwidths of filters estimated with psychophysical techniques may depend more strongly on the experimental paradigms and stimuli than hitherto thought.

Effects of fundamental frequency contrast on discrimination and identification of dichotic CV syllables at various temporal delays

Dichotic CV syllables (identical and nonidentical pairs) were presented at nine temporal offsets between 0 and 500 msec. One task consisted in judging quickly whether the syllables in a pair were phonetically the same or different; the other task was to identify both syllables. The fundamental frequency (pitch) of the synthetic stimuli was either the same or different, and either predictable or unpredictable. The pitch variable had surprisingly little effect on the latencies of "same"-"different" judgments, and the expected "preparation" effect of pitch predictability was ba]rely present. Instead, there were strong effects on the frequencies of errors at short temporal delays, which suggests shifts or biases in the phonetic "same"-"different" criterion with context. A comparison with analogous errors in the identification task revealed identical patterns. Further analysis of identification errors showed no overall "feature sharing advantage": The direction of this effect depends on the kind of error committed. Also, a lag effect was found only in nonidentical pairs that received two identical responses. The results are discussed in the framework of a two-stage information-processing model. Effects of pitch are tentatively explained as biases from implicit (pitch) decisions at the auditory level on phonetic decisions in the presence of uncertainty. Four sources of errors are identified: fusion at the auditory level; "integration," confusions, and transpositions at the phonetic level.