The Neural Mechanisms Underlying Categorical Decision Making - PROJECT SUMMARY/ABSTRACT When we are confronted with unclear or vague information about our world, we rely heavily on memory to fill in missing data and ultimately guide behaviors. For example, if you encounter a massive four-legged animal alongside its cub while hiking in a forest, your subsequent actions rely not merely on identifying the animal, but reacting appropriately once you remember that bears are dangerous. What mechanisms in the brain are responsible for recognizing, identifying, and categorizing objects, all of which can occur within hundreds of milliseconds? This question is inherently linked to the semantic memory system, which acts as the interface between incoming sensory information and our preexisting knowledge of the meaning of words, images, concepts, and their associations. The primary goal of the proposed research is to better understand the temporal dynamics of how semantic categorization occurs in the human brain. One method to study semantic categorization is by framing it as a decision that occurs in the brain. Research on decision-making across several species (e.g., mice, monkeys, and humans) has shown that decision-making requires a specific computation where evidence is accumulated until a particular threshold is met, indicating a decision has been reached. This framework (drift-diffusion models, DDMs) has helped to uncover several decision-making signals across the brain depending on the task, leading researchers to believe DDMs could be a brain-general mechanism. For this to be true, DDMs should be able to explain activity in the human ventral visual stream during object recognition tasks. The first aim of this study is to identify where in the human ventral visual stream can best be described by decision-making signals. The main hypothesis is that during a semantic categorization task, decision-making signals should occur in the anterior temporal lobe (ATL), which has been implicated in recent decades as the brain’s semantic hub. The second aim of this study is to examine how the brain categorizes (e.g., blurred, occluded) visual stimuli. On a behavioral level, humans require more time to perform object recognition when images are less clear. One possible explanatory neural mechanism would involve the same neural processes as unambiguous visual recognition that just occurs more slowly. A contrasting hypothesis is that additional brain areas must be recruited to solve ambiguity. For instance, accessing stored memories via the medial temporal lobe (MTL) may be enlisted to call upon previous experiences. Another option relies on a greater degree of cognitive control from the prefrontal cortex (PFC). This research project will leverage the spatiotemporal precision offered by intracranial recordings in humans. This study can illuminate cognitive systems and brain networks that are often damaged in diseases affecting memory, including Alzheimer’s Disease and Semantic Dementia.
