Besle, J. | Search bioRxiv

bioRxiv2022

Combining ultra-high-field fMRI adaptation and computational modelling to estimate neuronal frequency selectivity in human auditory cortex

Measuring neuronal frequency selectivity in human auditory cortex may be crucial for understanding common auditory deficits such as speech-in-noise difficulty. Non-invasive methods measure aggregate responses of large populations of neurons and therefore overestimate single-neuron tuning selectivity. Here we explore whether cortical frequency selectivity can be estimated through fMRI adaptation. Using ultra-high-field (7T) BOLD-fMRI and individualized functional parcellation of auditory cortex, we measured the suppression (or adaptation) of primary and non-primary cortical responses to a high-frequency (3.8 kHz) probe sound as a function of the frequency of a preceding adaptor sound (ranging from 0.5 to 3.8 kHz). The degree of frequency tuning of the adaptation effect strongly depended on the temporal structure of the adaptor. Suppression by a single 200-ms adaptor showed little or no tuning, despite clear frequency tuning of the responses to the different adaptors. In contrast, suppression by multiple (four) 50-ms adaptor bursts was clearly tuned, with greater frequency selectivity than the adaptor response tuning, suggesting that fMRI adaption to multiple adaptors may reflect the frequency tuning of the underlying neuronal response. Importantly, adaptation tuning differed between primary and non-primary regions, suggesting a local suppression effect, rather than inheritance from upstream subcortical structures. Using a computational model of fMRI adaptation in a tonotopically-organized neuronal array, we identify key factors determining the relationship between observed fMRI adaptation tuning and the frequency selectivity of the underlying neuronal response. Using this model, we derive a plausible range for the frequency selectivity of individual neurons in each region of auditory cortex.

neuroscience↗

bioRxiv2022

Fast event-related mapping of fingertip population receptive fields in human somatosensory and motor cortex

fMRI studies that investigate somatotopic tactile representations in the human cortex typically use either block or phase-encoded stimulation designs. Event-related (ER) designs allow for more flexible and unpredictable stimulation sequences than the other methods, but they are less efficient. Here we compared an efficiency-optimized fast ER design (2.8s average intertrial interval, ITI) to a conventional slow ER design (8s average ITI) for mapping voxelwise fingertip tactile tuning properties in the sensorimotor cortex of 6 participants at 7 Tesla. The fast ER design yielded more reliable responses compared to the slow ER design, but with otherwise similar tuning properties. Concatenating the fast and slow ER data, we demonstrate in each individual brain the existence of two separate somatotopically-organized tactile representations of the fingertips, one in the primary somatosensory cortex (S1) on the post-central gyrus, and the other shared across the motor and pre-motor cortices on the pre-central gyrus. In both S1 and motor representations, fingertip selectivity decreased progressively, from narrowly-tuned Brodmann areas 3b and 4a respectively, towards associative parietal and frontal regions that responded equally to all fingertips, suggesting increasing information integration along these two pathways. In addition, fingertip selectivity in S1 decreased from the cortical representation of the thumb to that of the pinky. Significance StatementSensory and motor cortices in the human brain contain map-like representations of the body in which adjacent brain regions respond to adjacent body parts. The properties of these somatotopic maps provide important insight into how tactile and motor information is processed by the brain. Here, we describe an efficient mapping method using functional MRI to measure somatotopic maps and their tuning properties. We used a fast event-related sequence to map the five fingers of the left hand in six human participants, and show that this method is more efficient than a conventional, slower event-related design. Furthermore, we confirm previously-identified tuning properties of fingertip representations in somatosensory cortex, and reveal a hitherto unknown tactile fingertip map in the motor cortex.

neuroscience↗