Responses in human early visual cortex are more sensitive to task difficulty than object category.

Abstract

Early visual cortex (EVC) exhibits widespread hemodynamic responses in the absence of visual stimulation, which are entrained to the timing of a task and not predicted by local spiking or local field potentials. These task-related responses (TRRs) are associated with aspects of cognitive and perceptual tasks, including reward magnitude and task difficulty, and correlate with markers of arousal, such as pupil size. Here, we tested the role of TRRs in an object discrimination task that has been used previously to study feedback signals in human V1. We measured brain activity (7T BOLD fMRI) and pupil size as subjects performed a two-alternative forced-choice shape discrimination task on peripherally-presented stimuli. On each trial, two stimuli from the Validated Circular Shape Space (Li et al., 2020) appeared simultaneously for 100 ms in diagonally opposite quadrants of the screen at 7 deg eccentricity. Subjects reported whether the stimuli were the same or different. In Expt 1, we partitioned the circular space into 3 equidistant sectors to create object 'categories,' with different categories presented in separate blocks of trials. In Expt 2, we modulated task difficulty by selecting stimuli from either proximate (for difficult trials) or diametrically opposed points (for easy trials) on the circle. We employed multi-variate pattern analysis to identify brain regions sensitive to object category and task difficulty. We were able to decode object category from the lateral occipital complex (LOC), consistent with its established role in object processing. But we were unable to decode category from EVC, inconsistent with the notion that LOC sends object-selective feedback to EVC. However, we could decode task difficulty, both from fMRI responses in EVC, and from trial-locked pupil responses. Our results demonstrate that TRRs are sensitive to important covariates of task performance, and underscores the significance of TRRs in fMRI studies of core visual functions.