Sensory sampling

Rubén Moreno-Bote, David C. Knill, Alexandre Pouget, Bayesian sampling in visual perception PNAS July 26, 2011 vol. 108 no. 30 12491-1249.

Abstract: It is well-established that some aspects of perception and action can be understood as probabilistic inferences over underlying probability distributions. In some situations, it would be advantageous for the nervous system to sample interpretations from a probability distribution rather than commit to a particular interpretation. In this study, we asked whether visual percepts correspond to samples from the probability distribution over image interpretations, a form of sampling that we refer to as Bayesian sampling. To test this idea, we manipulated pairs of sensory cues in a bistable display consisting of two superimposed moving drifting gratings, and we asked subjects to report their perceived changes in depth ordering. We report that the fractions of dominance of each percept follow the multiplicative rule predicted by Bayesian sampling. Furthermore, we show that attractor neural networks can sample probability distributions if input currents add linearly and encode probability distributions with probabilistic population codes [link].

Bayesian inference

Bayesian inference is a function of Bayesian probability. Bayesian probability is a measure of the likelihood of a desired outcome (H) (Colts winning the playoffs for instance) based on the conditional probabilities computed for a set of event-sequences (D) that would lead to the desired outcome ..adjusted for (divided-by) the conditional probabilities computed for the set of event-sequences leading to other possible outcomes (Hi ) (Dallas Giants or Eagles winning the playoffs).

Bayesian inference may be native to the way people make judgments. At the level of sensory processing, studies show that the nervous system perpetually distinguishes the most relevant signals, from incidental/peripheral signals, using likelihood estimates of a Bayesian sort. Signals that are the most likely outcome of ongoing activity, based on the contents of working memory, are given a boost. Signals considered less likely are held in abeyance and immediately suppressed if subsequent events do nothing to rehabilitate them.

In Bayesian terms, where H is the candidate signal and D is the current state of sensory memory, then the probability that H will be the winning candidate or P(D|H) ..is a function of sensory memory (D) mitigated by P(D|Hi) ..or the probability that the contents of sensory memory might favor other winning candidates (Hi ).

speech recognition

The way we process and interpret speech is largely dependent on the neuro-anatomy of the brain. Speech signals must travel from lower to higher regions before something resembling someone speaking can occur. Sound waves enter the ear canal where they are first broken down into their component-frequencies or ‘tones’. Individual tones are then converted to signals that get transmitted, over auditory pathways, to higher centers of the brain responsible for processing and synthesizing complex signals of speech such as phonemes, which are essentially complex bursts of multiple frequencies [link]. After completing sufficient cycles of phonemic synthesis - the phonic representation of a word is formed. Compound signals representing word-sounds are then passed to higher centers of the auditory cortex (Wernicke’s area) where word-meaning is retrieved from areas in the cerebral cortex where semantic processing is performed.

agents of expression

Most children learn to speak and understand what’s said effortlessly. It’s a spontaneous process that doesn’t require classroom training. The brain is innately tuned to extract the rules of spoken language. Observations show that parents rarely correct for rules of grammar during early childhood. However, they frequently correct for the rules of semantics ..making sure their children convey the proper idea [link]. That’s why it’s interesting for me to see that, while children may discover the correct rules of grammar on their own ..by adolescence they’re playing pretty loose with the rules of semantics they’d been taught. In other words, they frequently use well-formed sentences to fabricate and misrepresent what’s going on.