A method for real-time visual stimulus selection in the study of cortical object perception

Abstract

The properties utilized by visual object perception in the mid- and high-level ventral visual pathway are poorly understood. To better establish and explore possible models of these properties, we adopt a data-driven approach in which we repeatedly interrogate neural units using functional Magnetic Resonance Imaging (fMRI) to establish each unit’s image selectivity. This approach to imaging necessitates a search through a broad space of stimulus properties using a limited number of samples. To more quickly identify the complex visual features underlying human cortical object perception, we implemented a new functional magnetic resonance imaging protocol in which visual stimuli are selected in real-time based on BOLD responses to recently shown images. Two variations of this protocol were developed, one relying on natural object stimuli and a second based on synthetic object stimuli, both embedded in feature spaces based on the complex visual properties of the objects. During fMRI scanning, we continuously controlled stimulus selection in the context of a real-time search through these image spaces in order to maximize neural responses across predetermined 1 cm3 brain regions. Elsewhere we have reported the patterns of cortical selectivity revealed by this approach (Leeds 2014). In contrast, here our objective is to present more detailed methods and explore the technical and biological factors influencing the behavior of our real-time stimulus search. We observe that: 1) Searches converged more reliably when exploring a more precisely parameterized space of synthetic objects; 2) Real-time estimation of cortical responses to stimuli are reasonably consistent; 3) Search behavior was acceptably robust to delays in stimulus displays and subject motion effects. Overall, our results indicate that real-time fMRI methods may provide a valuable platform for continuing study of localized neural selectivity, both for visual object representation and beyond.

Individual differences in rate of acquiring stable neural representations of tasks in fMRI

“…In this study, we sought to explore possible individual differences in task-related fMRI responses to a commonly applied fMRI task, the Stop Signal Task (SST), but at the level of the temporal development of stable neural responses to task-related processing demands. This individual brain-behavior response variable has been proposed in theory [ 3 ] and simulation [ 25 ] but has to our knowledge not been deployed in practice, either prospectively nor retrospectively.…”

“…There has also been growing interest in mapping individual differences in fMRI task behavior and functional brain responses, particularly to address the barriers posed by group-level brain-behavior mapping [ 1 ]. Recent fMRI studies have explored individual differences in neural population activation on a trial-by-trial basis–specifically, by exploiting the temporal development of neural activations associated with task processing demands in order to individually titrate task parameters (such as task type, task duration, or task stimulus type) [ 2 , 3 ].…”

“…First, the number of discrete task trials administered to a participant (i.e. paradigm duration) may be dynamically adjusted to optimally characterize the individual’s behavioral-neural response coupling, e.g., sampling until estimation of task-evoked neural activity reaches a steady state; an approach which strengthens functional neuroanatomical inferences while simultaneously minimizing participant scanner time, discomfort, and fatigue (all of which are confounds to task performance) [ 3 ]. Second, trial types may be dynamically selected based on a pre-defined neural response criterion, e.g., selecting the next trial’s parameters to maximize the novelty of its observed neural response [ 4 ], which may individually optimize the quality of derived brain-behavior relationships or behavioral performance parameters while simultaneously minimizing scan duration.…”

Computational approaches to fMRI analysis

“…In ‘adaptive’ designs, the experimenter determines not whether to present a trial—trials occur at regular intervals irrespective of brain activity—but rather the content of the next trial (for example, the stimulus or task). This has been done with the goal of characterizing the tuning properties of the visual system, by adjusting stimulus parameters until an area’s response is maximal 57 , but could also be used to examine a wide variety of systems (for example, those involved in attention or memory).…”