Browsing by Author "Fitzpatrick, David"

The visual system is thought to represent the trajectory of moving objects in the activity of large populations of cortical neurons that respond preferentially to the direction of stimulus motion. Here I employed in vivo voltage sensitive dye (VSD) imaging to explore how abrupt changes in the trajectory of a moving stimulus impact the population coding of motion direction in ferret primary visual cortex (V1). For motion in a constant direction, the peak of the cortical population response reliably signaled the stimulus trajectory; but for abrupt changes in motion direction, the peak of the population response departed significantly from the stimulus trajectory in a fashion that depended on the size of the direction deviation. For small direction deviation angles, the peak of the active population shifted from values consistent with the initial direction of motion to those consistent with the final direction of motion by progressing smoothly through intermediate directions not present in the stimulus. In contrast, for large direction deviation angles, peak values consistent with the initial motion direction were followed by: a small deviation away from the final motion direction, a rapid 180° jump, and a gradual shift to the final direction. These departures of the population response from the actual trajectory of the stimulus predict specific misperceptions of motion direction that were confirmed by human psychophysical experiments. I conclude that cortical dynamics and population coding mechanisms combine to place constraints on the accuracy with which abrupt changes in direction of motion can be represented by cortical circuits.

To solicit the attention or determine the intentions of another, we use our eyes. While many animals appear to use eyes as an important behavioral cue, for humans, these cues are especially critical. The power of the eyes to attract and direct attention shapes human behavior from an early age and likely serves as a foundation for social skill acquisition, ranging from simple, friendly eye contact to complex, spoken language, even to our almost mystical ability to empathize and "see the world through another's eyes". Humans have transformed our environment through our economic alliances and military competitions, and our individual successes and failures depend critically on social skills built on a foundation of shared attention. When these abilities break down, as in autism, pervasive social awkwardness can challenge the close relationship of individuals with their friends, family, and community. Nonetheless, we know almost nothing about the brain mechanisms that have evolved to process social cues and convert them into a rich experience of shared attention. To investigate this process, we explored the ability of human and nonhuman primates to follow the attention of other individuals. First, we characterized natural gaze-following behavior using a novel telemetric device in socially-interacting prosimian primates, and later in monkeys and humans responding to gaze cues in the lab. Finally, we examined the neuronal responses to gaze cues in a macaque posterior parietal area implicated in attention control--the lateral intraparietal area, LIP. Our findings suggest that gaze-following abilities may be widespread in social primates, relying on conserved, homologous brain pathways; and that they may not be informationally-encapsulated reflexes, but rather are densely interwoven with diverse social processes. Indeed, we found gaze cues influenced neurons in LIP, part of the dorsal frontoparietal attention network. Finally, we report that "mirror" neurons in parietal areas may thus play a role not only in representing perceived bodily actions, but also perceived mental states such as observed attention.

Negotiating the complex decisions that we encounter daily requires coordinated neu-

ronal activity. The enormous variety of decisions we make, the intrinsic complexity

of the situations we encounter, and the extraordinary flexibility of our behaviors

suggest the existence of intricate neural mechanisms for negotiating contexts and

making choices. Further evidence for this prediction comes from the behavioral al-

terations observed in illness and after injury. Both clinical and scientific evidence

suggest that decision signals are carried by electrical neuronal activity and influenced

by neuromodulatory chemicals. This dissertation addresses the function of two puta-

tive contributors to decision-making: neuronal activity in posterior cingulate cortex

and modulatory effects of serotonin. I found that posterior cingulate neurons respond

phasically to salient events (informative cues; intentional saccades; and reward deliv-

ery) across multiple contexts. In addition, these neurons signal heuristically guided

choices across contexts in a gambling task. These observations suggest that posterior

cingulate neurons contribute to the detection and integration of salient information

necessary to transform event detection to expressed decisions. I also found that

lowering levels of the neuromodulator serotonin increased the probability of making

risky decisions in both monkeys and mice, suggesting that this neurotransmitter con-

tributes to preference formation across species. These results suggest that posterior

cingulate cortex and serotonin each contribute to decision formation. In addition, the

unique serotonergic pro jections to posterior cingulate cortex, as well as the frequent

implication of altered serotonergic and posterior cingulate function in psychiatric dis-

orders, suggest that the confluence of cingulate and serotonergic activity may offer

key insights into normal and pathological mechanisms of decision making.