Biology subjects

Sajid, N.

Publications and source records attributed to Sajid, N..

4 recordsLinked to original sources

bioRxiv2022

Degeneracy in the neurological model of auditory speech repetition

In the neurological model of language, repeating heard speech involves four left hemisphere regions: primary auditory cortex for processing sounds; Wernickes area for processing auditory images of speech; Brocas area for processing motor images of speech; and primary motor cortex for overt speech articulation. Previous functional-MRI (fMRI) studies confirm that auditory repetition activates these regions. Here, we used dynamic causal modelling (DCM) to test how the four regions interact with each other during single word and pseudoword auditory repetition. Contrary to expectation, we found that, for both word and pseudoword repetition, the effective connectivity between Wernickes and Brocas areas was predominantly bidirectional and inhibitory; activity in the motor cortex could be driven by either Wernickes area or Brocas area; and the latter effect varied both within and between individuals. Such variability speaks to degenerate functional architectures that support auditory repetition and may explain resilience to functional loss after brain damage.

neuroscience↗

bioRxiv2022

A mixed generative model of auditory word repetition

In this paper, we introduce a word repetition generative model (WORM), which--when combined with an appropriate belief updating scheme--is capable of inferring the word that should be spoken when presented with an auditory cue. Our generative model takes a deep temporal form, combining both discrete and continuous states. This allows a (synthetic) WORM agent to perform categorical inference on continuous acoustic signals, and--based on the same model--to repeat heard words at the appropriate time. From the perspective of word production, the model simulates how high-level beliefs about discrete lexical, prosodic and context attributes give rise to continuous acoustic signals at the sensory level. From the perspective of word recognition, it simulates how continuous acoustic signals are recognised as words and, how (and when) they should be repeated. We establish the face validity of our generative model by simulating a word repetition paradigm in which a synthetic agent or a human subject hears a target word and subsequently reproduces that word. The repeated word should be the target word but differs acoustically. The results of these simulations reveal how the generative model correctly infers what must be repeated, to the extent it can successfully interact with a human subject. This provides a formal process theory of auditory perception and production that can be deployed in health and disease. We conclude with a discussion of how the generative model could be scaled-up to include a larger phonetic and phonotactic repertoire, complex higher-level attributes (e.g., semantic, concepts, etc.), and produce more elaborate exchanges.

neuroscience↗

bioRxiv2021

Simulating lesion-dependent functional recovery mechanisms

Functional recovery after brain damage varies widely and depends on many factors, including lesion site and extent. When a neuronal system is damaged, recovery may occur by engaging residual (e.g., perilesional) components. When damage is extensive, recovery depends on the availability of other intact neural structures that can reproduce the same functional output (i.e., degeneracy). A systems response to damage may occur rapidly, require learning or both. Here, we simulate functional recovery from four different types of lesions, using a generative model of word repetition that comprised a default premorbid system and a less used alternative system. The synthetic lesions (i) completely disengaged the premorbid system, leaving the alternative system intact, (ii) partially damaged both premorbid and alternative systems, and (iii) limited the experience-dependent plasticity of both. The results, across 1000 trials, demonstrate that (i) a complete disconnection of the premorbid system naturally invoked the engagement of the other, (ii) incomplete damage to both systems had a much more devastating long-term effect on model performance and (iii) the effect of reducing learning capacity within each system. These findings contribute to formal frameworks for interpreting the effect of different types of lesions.

neuroscience↗

bioRxiv2020

Active Listening

This paper introduces active listening, as a unified framework for synthesising and recognising speech. The notion of active listening inherits from active inference, which considers perception and action under one universal imperative: to maximise the evidence for our (generative) models of the world. First, we describe a generative model of spoken words that simulates (i) how discrete lexical, prosodic, and speaker attributes give rise to continuous acoustic signals; and conversely (ii) how continuous acoustic signals are recognised as words. The active aspect involves (covertly) segmenting spoken sentences and borrows ideas from active vision. It casts speech segmentation as the selection of internal actions, corresponding to the placement of word boundaries. Practically, word boundaries are selected that maximise the evidence for an internal model of how individual words are generated. We establish face validity by simulating speech recognition and showing how the inferred content of a sentence depends on prior beliefs and background noise. Finally, we consider predictive validity by associating neuronal or physiological responses, such as the mismatch negativity and P300, with belief updating under active listening, which is greatest in the absence of accurate prior beliefs about what will be heard next.

neuroscience↗