When human subjects hear a sequence of two alternating pure tones, they often perceive it in one of two ways: as one integrated sequence (a single "stream" consisting of the two tones), or as two segregated sequences, one sequence of low tones perceived separately from another sequence of high tones (two "streams"). Perception of this stimulus is thus bistable. Moreover, subjects report on-going switching between the two percepts: unless the frequency separation is large, initial perception tends to be of integration, followed by toggling between integration and segregation phases. The process of stream formation is loosely named “auditory streaming”. Auditory streaming is believed to be a manifestation of human ability to analyze an auditory scene, i.e. to attribute portions of the incoming sound sequence to distinct sound generating entities. Previous studies suggested that the durations of the successive integration and segregation phases are statistically independent. This independence plays an important role in current models of bistability. Contrary to this, we show here, by analyzing a large set of data, that subsequent phase durations are positively correlated. To account together for bistability and positive correlation between subsequent durations, we suggest that streaming is a consequence of an evidence accumulation process. Evidence for segregation is accumulated during the integration phase and vice versa; a switch to the opposite percept occurs stochastically based on this evidence. During a long phase, a large amount of evidence for the opposite percept is accumulated, resulting in a long subsequent phase. In contrast, a short phase is followed by another short phase. We implement these concepts using a probabilistic model that shows both bistability and correlations similar to those observed experimentally.
As the animal moves in its environment, the brain detects and learns the structure of the surrounding stimuli, independently of the immediate relevance this has for the animal. This experience influences subsequent learning in a manner quantified using paradigms such as latent inhibition or stimulus preconditioning, which measure the effect that unsupervised (not-reinforced) learning has on subsequent reinforced learning. Despite our understanding of the behavioural consequences of prior neutral experience, there is little understanding about the influence of this previous experience on neuronal plasticity. Using latent inhibition, we have shown in mice that learning a two tone discrimination is slower in mice that have had previous neutral exposure to the same or similar tones (<2/3 octave away). Neutral exposure thus elicits profound changes in the brain that influence subsequent learning. To study how previous experience influences experience-dependent plasticity, and better understand the interactions between experience, learning, and plasticity, we recorded sound evoked responses in the auditory cortex of exposed and trained mice. We studied both changes in response magnitude and changes in sensory dynamics, and related both to the differential behavioral effects of different pre-exposure conditions. Here we describe the neuronal changes that paralleled the behavioral findings. We found that discrimination learning led to stronger initial sound-evoked responses and a long-lasting increase in response adaptation and an increase. The first effect was delayed in animals that showed latent inhibition, paralleling behavioural learning. Overall our data reveal that slow changes in behaviour that accompanied learning, paralleled the slow dynamics of experience-dependent plasticity in auditory cortex.
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