Towards large-scale, high resolution maps of object selectivity in inferior temporal cortex

TitleTowards large-scale, high resolution maps of object selectivity in inferior temporal cortex
Publication TypeConference Proceedings
Year of Conference2010
AuthorsIssa EB, Papanastassiou AM, Andken BB, DiCarlo JJ
Conference NameCosyne 2010
Date Published03/2010
Conference LocationSalt Lake City, UT
Full Text

Inferior temporal cortex (IT) has been shown to have large-scale (mm to cm) maps of object category selectivity as well as small-scale (sub-millimeter) organization for object features.  These two scales of spatial organization have yet to be linked because they were measured using different techniques (fMRI, optical imaging, and local electrophysiology), each with their own limitations.  For example, fMRI has poor spatial resolution while optical imaging has higher resolution at the expense of a narrow field of view of only surface-accessible cortex.  Given that much of IT lies inside a major cortical sulcus or at the skull base, what is needed is a method that can access the whole of IT with high resolution.  Microelectrode based mapping has such potential: electrodes can reach almost anywhere in IT (high spatial coverage) and record from single cells (high spatial resolution). This potential has not yet been realized because of the difficulty of precisely localizing and co-registering many electrode recordings in vivo.  Methods such as histological reconstruction of lesion sites or MRI visualization of electrodes are post-hoc and can introduce additional spatial errors.  Here, we have adopted a microfocal stereo x-ray system for localizing electrodes that can be used at an unlimited number of sites and operates virtually in real-time (Cox et al., J. Neurophys. 2008).  We have used this system to construct broad scale maps of object category selectivity in IT for comparison to fMRI-based maps.  We found a weak but significant correspondence between physiology and fMRI maps collected in the same animal, and this correspondence improved substantially when MUA and LFP signals were smoothed (~3-5 mm) to broader scales suggesting the spatial low pass nature of fMRI.  Transformations other than spatial smoothing such as dividing the LFP into power in different frequency bands did not produce noticeable improvement in map correspondence.
    Currently, we are extending our approach to address fine-scale organization in IT at spatial scales more similar to those obtained in optical imaging studies.  Although the ex vivo, skull-based accuracy of our system is 50 microns, in vivo resolution may be limited by tissue movement within the skull. For example, the brain may not be in exactly the same position today as yesterday, and cortex may deform locally during electrode recording. To address these issues, we tracked the movements of implanted internal markers within and across sessions.  We also measured the neural ‘fingerprint’ (selectivity profile across a battery of images) of nearby sites recorded on separate days as an empirical test of how reproducible serial samples are.  Finally, we estimated local non-rigid deformations of the brain around the electrode.  These measurements will determine the current in vivo, tissue-based accuracy of the serial mapping approach and guide mechanical modeling of brain tissue to compensate position shifts.  Going forward, linking broad scale and fine scale maps of neural organization in IT will help reveal structure-function relationships and yield insights into the organization of computation in IT.

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