Ch. 13 A face in the crowd: which groups of neurons process face stimuli, and how do they interact? KARI L. HOFFMANN 2009/1/13 BI, Population Coding Seminar Ji Hoon Lee

Summary Where in the brain does face processing occur?  Temporal lobe Neural coding about faces is sparse and distributed The face cells are clustered The neurons relevant to face-processing are synchronized. (C) 2008, SNU Biointelligence Lab,

Introduction Neural responses to face stimuli may seem like an unwieldy subject for investigating population activity. The nature of perceiving and learning about facial signals, in terms of population dynamics, is very under-explored territory. The population interactions of face-processing systems across space (circuits), and time (oscillations) will be discussed. 2/11(C) 2008, SNU Biointelligence Lab,

The many facets of faces: what can be extracted from a face Information provided by the face includes the gender and identity of the monkey, and his or her associated position in the social hierarchy. Head and gaze direction indicate the intended target of facial expressions. Monkeys are sensitive to communicative gestures such as silent facial expressions Another’s gaze direction appears to be used as an indication of salient objects or locations in the environment. 3/11(C) 2008, SNU Biointelligence Lab,

Where in the brain does face processing occur? Human resources  Temporal lobe Monkey homologues Spatial clustering  Recording from temporal lobe neurons with similar responses where observed to cluster together  All neurons sampled were selective for faces as a class compared to various non-face stimuli as a class. 4/11(C) 2008, SNU Biointelligence Lab,

Face encoding I: single-unit information Coding is considered to be through firing rate in most studies Latency differences have provided information about stimulus type Subclass of face cells  Holistic, Head direction, Gaze, Expression( closed or open- mouth), Motion, identity Neural coding about faces is sparse and distributed  Sparse  A neuron will fire strongly to only a few stimuli  Sparsity leads to greater capacity  Distributed  A stimulus will elicit activity in many neurons  Generalization and pattern completion from incomplete or degraded stimuli, or from biological noise 5/11(C) 2008, SNU Biointelligence Lab,

Face encoding II: How is information integrated across single units? One of the ways that faces might be represented in a distributed code across neurons and even brain regions is through the synchronized activity of relevant neurons. Hebb postulated that the neural representation of an event could be formed by cell assemblies. The brain regions responding to actual face stimuli are also selectively active during imagined faces. Cell assembly offers a convenient mechanism allowing proper grouping of relevant distributed groups of neurons in time, and their elicitation as a whole from incomplete or even absent sensory inputs. 6/11(C) 2008, SNU Biointelligence Lab,

Multi-site synchronization and evidence for cell assemblies (in other systems) What evidence exists for cell assembly representation of faces, particularly in the necessary temporal lobe regions?  Through the synchronized grouping of cells (signs of cell assemblies) in other systems. Two brain regions, LIP and PFC showed synchronized, or coherent oscillations (Buschman and Miller, 2007) Inter-area synchronization during task performance was reported between areas MT and parietal cortex.(Saalmann et al., 2007) …… There is growing evidence that oscillations can coordinate activity among spatially distributed neurons, as in cell assemblies, and that this results in behavioral benefits (but, limited in the temporal lobe and for face processing) 7/11(C) 2008, SNU Biointelligence Lab,

Synchronization during face processing 8/11(C) 2008, SNU Biointelligence Lab, Rodriguez et al., 1999

Multi-site synchronization and evidence for cell assemblies 9/11(C) 2008, SNU Biointelligence Lab, Monkey temporal lobe obtained during slow-wave sleep Slow oscillations, indicated by vertical lines, can be seen as synchronized deflections. The temporal lobe appears to contain multi-site synchronized oscillations, but the spatiotemporal patterns and their behavioral significance are not yet understood.

Where to go from here Existing studies suggest that the temporal lobe is concerned with faces, as part of a network of face-processing regions. Stimulus coding in neurons appears to be selective for a small number of stimuli (sparse), but with populations of cells responding (distributed). The spatial distribution of face- selective cells in the temporal lobe tends to be clustered. There is very little to inform us about whether cell assemblies are created and used for face processing in the temporal lobe. The nature of coding in the temporal lobe for behaviorally relevant stimuli is an attainable goal. 10/11(C) 2008, SNU Biointelligence Lab,