Task-context-dependent Linear Representation of Multiple Visual Objects in Human Parietal Cortex

Abstract

A host of recent studies have reported robust representations of visual object information in the human parietal cortex, similar to those found in ventral visual cortex. In ventral visual cortex, both monkey neurophysiology and human fMRI studies showed that the neural representation of a pair of unrelated objects can be approximated by the averaged neural representation of the constituent objects shown in isolation. In this study, we examined whether such a linear relationship between objects exists for object representations in the human parietal cortex. Using fMRI and multivoxel pattern analysis, we examined object representations in human inferior and superior intraparietal sulcus, two parietal regions previously implicated in visual object selection and encoding, respectively. We also examined responses from the lateral occipital region, a ventral object processing area. We obtained fMRI response patterns to object pairs and their constituent objects shown in isolation while participants viewed these objects and performed a 1-back repetition detection task. By measuring fMRI response pattern correlations, we found that all three brain regions contained representations for both single object and object pairs. In the lateral occipital region, the representation for a pair of objects could be reliably approximated by the average representation of its constituent objects shown in isolation, replicating previous findings in ventral visual cortex. Such a simple linear relationship, however, was not observed in either parietal region examined. Nevertheless, when we equated the amount of task information present by examining responses from two pairs of objects, we found that representations for the average of two object pairs were indistinguishable in both parietal regions from the average of another two object pairs containing the same four component objects but with a different pairing of the objects (i.e., the average of AB and CD vs. that of AD and CB). Thus, when task information was held consistent, the same linear relationship may govern how multiple independent objects are represented in the human parietal cortex as it does in ventral visual cortex. These findings show that object and task representations coexist in the human parietal cortex and characterize one significant difference of how visual information may be represented in ventral visual and parietal regions.

A Tale of Two Visual Systems: Invariant and Adaptive Visual Information Representations in the Primate Brain

“…The sensitivity of PPC visual representation to task context was also found independently in another study examining how PPC representations of single objects may predict those of multiple objects ( Jeong & Xu 2017). In OTC, the neural representation of a pair of unrelated objects can be approximated by the averaged neural representation of the constituent objects shown in isolation, indicating the existence of a normalized response-averaging mechanism (Zoccolan et al 2005;MacEvoy & Epstein 2009Reddy et al 2009).…”

“…In the lateral occipital object processing region, fMRI response patterns for object pairs could be approximated by the averaged response patterns of the constitute objects shown alone. This simple linear relationship was not found in PPC ( Jeong & Xu 2017). However, attending to two streams of objects was more attentionally demanding and involved a different task context than attending to a single stream.…”

“…Attention and task also influence feature and object processing in OTC, with attended-to object features eliciting a stronger response amplitude or representation (as measured by fMRI adaptation or pattern decoding) than do unattended ones (O'Craven et al 1999, Murray & Wojciulik 2004, Reddy et al 2009. Nevertheless, when directly comparing OTC and PPC, the effects of attention and task on object representation are in general greater in PPC than in OTC (Bracci et al 2017, Jeong & Xu 2017, Vaziri-Pashkam & Xu 2017).…”

“…However, attending to two streams of objects was more attentionally demanding and involved a different task context than attending to a single stream. When we equated the amount of task demand and context by examining responses from two pairs of objects, we found that representations for the average of two object pairs was indistinguishable in PPC from the average of another two object pairs containing the same four component objects but with a different pairing of the objects (i.e., the average of AB and CD versus that of AD and CB) ( Jeong & Xu 2017). Thus, with the task demand and context held consistent, the same linear relationship between object pairs and single objects exists in both PPC and OTC.…”

Biased competition in semantic representation during natural visual search

“…Several neuroimaging studies provided evidence for competition among cortical representations of multiple objects across visual cortex in the absence of specific task demands (Kastner et al, 1998; MacEvoy and Epstein, 2009; Gentile and Jansma, 2010; Nagy et al, 2011; Baeck et al, 2013; Jeong and Xu, 2017). Gentile and Jansma (2010) measured average BOLD responses in fusiform face area (FFA) while subjects viewed a single face image or a pair of face images.…”