We report a novel multisensory decision task designed to encourage subjects to combine information across both time and sensory modalities. We presented subjects, humans and rats, with multisensory event streams, consisting of a series of brief auditory and/or visual events. Subjects made judgments about whether the event rate of these streams was high or low. We have three main findings: First, we report that subjects can combine multisensory information over time to improve judgments about whether a fluctuating rate is high or low. Importantly, the improvement we observed was frequently close to, or better than, the statistically optimal prediction. Second, we found that subjects showed a clear multisensory enhancement both when the inputs in each modality were redundant and also when they provided independent evidence about the rate. This latter finding suggests a model where event rates are estimated separately for each modality and fused at a later stage. Finally, because a similar multisensory enhancement was observed in both humans and rats, we conclude that the ability to optimally exploit sequentially presented multisensory information is not restricted to a particular species.

Neurons in putative decision-making structures can reflect both sensory and decision signals, making their causal role in decisions unclear. Here, we tested whether rat posterior parietal cortex (PPC) is causal for processing visual sensory signals or instead for accumulating evidence for decision alternatives. We disrupted PPC activity optogenetically during decision making and compared effects on decisions guided by auditory versus visual evidence. Deficits were largely restricted to visual decisions. To further test for visual dominance in PPC, we evaluated electrophysiological responses after individual sensory events and observed much larger response modulation after visual stimuli than auditory stimuli. Finally, we measured trial-to-trial spike count variability during stimulus presentation and decision formation. Variability decreased sharply, suggesting that the network is stabilized by inputs, unlike what would be expected if sensory signals were locally accumulated. Our findings suggest that PPC plays a causal role in processing visual signals that are accumulated elsewhere. Defining the neural circuits that support decision making bridges a gap between our understanding of simple sensorimotor reflexes and our understanding of truly complex behavior. However, identifying brain areas that play a causal role in decision making has proved challenging. We tested the causal role of a candidate component of decision circuits, the rat posterior parietal cortex (PPC). Our interpretation of the data benefited from our use of animals trained to make decisions guided by either visual or auditory evidence. Our results suggest that PPC plays a causal role specifically in visual decision making and may support sensory aspects of the decision, such as interpreting the visual signals so that evidence for a decision can be accumulated elsewhere.

Objectives-Previous research has attributed older adult's difficulty with perceiving speech in noise to peripheral hearing loss. Recent studies have suggested a more complex picture, however, and implicate the central nervous system in sensation and sensory deficits. This study examines the relationship between the neuroanatomical structure of cognitive regions and the ability to perceive speech in noise in older adults. In particular, the neuroanatomical characteristics of the left ventral and dorsal prefrontal cortex are considered relative to standard measures of hearing in noise.Design-The participants were fifteen older and fourteen younger right-handed native speakers of American English who had no neurological deficits and scored better than normal on standardized cognitive tests. We measured the participants' peripheral hearing ability as well as their ability to perceive speech in noise using standardized tests. Anatomical magnetic resonance images were taken and analyzed to extract regional volumes and thicknesses of several key neuroanatomical structures.Results-The results showed that younger adults had better hearing sensitivity and better speech perception in noise ability than older adults. For the older adults only, the volume of the left pars triangularis and the cortical thickness of the left superior frontal gyrus were significant predictors of performance on the speech-in-noise test.Discussion-These findings suggest that, in addition to peripheral structures, the central nervous system also contributes to the ability to perceive speech in noise. In older adults, a decline in the volume and cortical thickness of the prefrontal cortex (PFC) during aging can therefore be a factor in a declining ability to perceive speech in a naturalistic environment. Our study shows a link between anatomy of PFC and speech perception in older adults. These findings are consistent with the decline-compensation hypothesis, which states that a decline in sensory processing due to cognitive aging can be accompanied by an increase in the recruitment of more general cognitive areas as a means of compensation. We found that a larger PFC volume may compensate for declining peripheral hearing. Clinically, recognizing the contribution of the cerebral cortex expands treatment possibilities for hearing loss in older adults beyond peripheral hearing aids to include strategies for improving cognitive function. We conclude by considering several mechanisms by which the PFC may facilitate speech perception in noise including inhibitory control, attention, cross-modal compensation, and phonological working memory, though no definitive conclusion can be drawn.