Insub Kim

All Publications

• Rethinking simultaneous suppression in visual cortex via compressive spatiotemporal population receptive fields. Nature communications Kupers, E. R., Kim, I., Grill-Spector, K. 2024; 15 (1): 6885

  Abstract

  When multiple visual stimuli are presented simultaneously in the receptive field, the neural response is suppressed compared to presenting the same stimuli sequentially. The prevailing hypothesis suggests that this suppression is due to competition among multiple stimuli for limited resources within receptive fields, governed by task demands. However, it is unknown how stimulus-driven computations may give rise to simultaneous suppression. Using fMRI, we find simultaneous suppression in single voxels, which varies with both stimulus size and timing, and progressively increases up the visual hierarchy. Using population receptive field (pRF) models, we find that compressive spatiotemporal summation rather than compressive spatial summation predicts simultaneous suppression, and that increased simultaneous suppression is linked to larger pRF sizes and stronger compressive nonlinearities. These results necessitate a rethinking of simultaneous suppression as the outcome of stimulus-driven compressive spatiotemporal computations within pRFs, and open new opportunities to study visual processing capacity across space and time.

  View details for DOI 10.1038/s41467-024-51243-7

  View details for PubMedID 39128923

  View details for PubMedCentralID PMC11317513

• Characterizing spatiotemporal population receptive fields in human visual cortex with fMRI. The Journal of neuroscience : the official journal of the Society for Neuroscience Kim, I., Kupers, E. R., Lerma-Usabiaga, G., Grill-Spector, K. 2023

  Abstract

  The use of fMRI and computational modeling has advanced understanding of spatial characteristics of population receptive fields (pRFs) in human visual cortex. However, we know relatively little about the spatiotemporal characteristics of pRFs because neurons' temporal properties are one to two orders of magnitude faster than fMRI BOLD responses. Here, we developed an image-computable framework to estimate spatiotemporal pRFs from fMRI data. First, we developed a simulation software that predicts fMRI responses to a time-varying visual input given a spatiotemporal pRF model and solves the model parameters. The simulator revealed that ground-truth spatiotemporal parameters can be accurately recovered at the millisecond resolution from synthesized fMRI responses. Then, using fMRI and a novel stimulus paradigm, we mapped spatiotemporal pRFs in individual voxels across human visual cortex in 10 participants (both females and males). We find that a compressive spatiotemporal (CST) pRF model better explains fMRI responses than a conventional spatial pRF model across visual areas spanning the dorsal, lateral, and ventral streams. Further, we find three organizational principles of spatiotemporal pRFs: (i) from early to later areas within a visual stream, spatial and temporal windows of pRFs progressively increase in size and show greater compressive nonlinearities, (ii) later visual areas show diverging spatial and temporal windows across streams, and (iii) within early visual areas (V1-V3), both spatial and temporal windows systematically increase with eccentricity. Together, this computational framework and empirical results open exciting new possibilities for modeling and measuring fine-grained spatiotemporal dynamics of neural responses using fMRI.Significance Statement We developed a computational framework for estimating spatiotemporal receptive fields of neural populations using fMRI. This framework pushes the boundary of fMRI measurements, enabling quantitative evaluation of neural spatial and temporal processing windows at the resolution of visual degrees and milliseconds, which was thought to be unattainable with fMRI. We not only replicate well-established visual field and pRF size maps, but also estimates of temporal windows from electrophysiology and electrocorticography. Notably, we find that spatial and temporal windows as well as compressive nonlinearities progressively increase from early to later visual areas in multiple visual processing streams. Together, this framework opens exciting new possibilities for modeling and measuring fine-grained spatiotemporal dynamics of neural responses in the human brain using fMRI.

  View details for DOI 10.1523/JNEUROSCI.0803-23.2023

  View details for PubMedID 37963768

• Rethinking simultaneous suppression in visual cortex via compressive spatiotemporal population receptive fields. bioRxiv : the preprint server for biology Kupers, E. R., Kim, I., Grill-Spector, K. 2023

  Abstract

  When multiple visual stimuli are presented simultaneously in the receptive field, the neural response is suppressed compared to presenting the same stimuli sequentially. The prevailing hypothesis suggests that this suppression is due to competition among multiple stimuli for limited resources within receptive fields, governed by task demands. However, it is unknown how stimulus-driven computations may give rise to simultaneous suppression. Using fMRI, we find simultaneous suppression in single voxels, which varies with both stimulus size and timing, and progressively increases up the visual hierarchy. Using population receptive field (pRF) models, we find that compressive spatiotemporal summation rather than compressive spatial summation predicts simultaneous suppression, and that increased simultaneous suppression is linked to larger pRF sizes and stronger compressive nonlinearities. These results necessitate a rethinking of simultaneous suppression as the outcome of stimulus-driven compressive spatiotemporal computations within pRFs, and open new opportunities to study visual processing capacity across space and time.

  View details for DOI 10.1101/2023.06.24.546388

  View details for PubMedID 37461470