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Anthony Stiliani
Graduate Student

Temporal Processing Capacity in the Human Visual System

The world around us is constantly changing. While some variations occur on timescales that are too fast to detect with the naked eye (e.g., the 120 Hz flicker of fluorescent lights), others occur on timescales that are much too slow (e.g., gradual changes in natural lighting). Despite tremendous variability in the dynamics of our physical surroundings, the visual system is able to perceive a critical range of temporal frequencies that enables us to distinguish relatively stable aspects of the environment from changing ones. This visible range of timescales may be further optimized for regularities in the temporal statistics of different object categories and the nature of our interactions with them. Consistent with this hypothesis, our previous research indicates that putative networks of domain-specific regions in the human visual system have differential temporal processing capacities - defined as the amount of information (i.e., number of stimuli) that a brain region can process in a unit of time (Stigliani et al., 2015).

Temporal processing capacity in high-level visual cortex is domain specific

Anthony Stigliani, Kevin Weiner, & Kalanit Grill-Spector

Visual stimuli bombard us at different rates every day. For example, words and scenes are typically stationary and vary at slow rates. In contrast, bodies are dynamic and typically change at faster rates. Using a novel fMRI paradigm, we measured temporal processing capacities of functional regions in human high-level visual cortex. Contrary to prevailing theories, we find that different regions have different processing capacities, which have behavioral implications. In general, character-selective regions have the lowest capacity, face- and place-selective regions have an intermediate capacity, and body-selective regions have the highest capacity. These results suggest that temporal processing capacity is a characteristic of domain-specific networks in high-level visual cortex and contributes to the segregation of cortical regions.

 

Motivated by these findings, we speculate that domain-specific temporal processing capacities in high-level visual cortex emerge through differential input from transient and sustained temporal channels. To examine this possibility, we next develop a method for modeling the relative contributions of these channels in early visual cortex that we hope to extend to high-level regions in future work.

Independent responses to transient and sustained visual stimulation across the cortical processing hierarchy

Anthony Stigliani, Brianna Jeska, & Kalanit Grill-Spector

Early stages of the human visual system contain separate temporal channels for processing static and dynamic features of the environment (Tolhurst, 1975; Horiguchi et al., 2009; Nassi & Callaway, 2009), but the relative contributions of these channels at later stages are unknown. By separately measuring fMRI responses to sustained and transient visual stimuli across a hierarchy of retinotopic regions, we find that contributions from these channels are functionally independent across visual cortex and sum to predict responses to novel combinations of transient and sustained stimuli. Ascending the hierarchy from primary visual cortex (V1), we also find that the magnitude of response to sustained stimuli decreases more than response to transient stimuli, especially in lateral visual regions. These findings reveal the functional independence of transient and sustained channels in extrastriate cortex, and a hierarchical clustering of visual areas according to contributions from these channels.