Systems Neuroscience Institute Seminar

Title: Ventral premotor-motor cortex interactions in the macaque monkey during grasp . . . and mirror neurons
Presenter: Alexander Kraskov
Location: 4075 BST3
 

Dissertation

Title: The Temporal Dynamics of Word Comprehension and Response Selection: Computational and Behavioral Studies

Presenter: Blair Armstrong

Location: Steinberg Auditorium, BA53 [Baker Hall]

Abstract:
Most words are semantically ambiguous in that their meaning depends on the context in which they occur (e.g., <river> vs. <money> BANK).  Developing a theory of how the meanings of semantically ambiguous words are comprehended has proven difficult because of discrepancies in the effects of relatedness of meaning observed across tasks.  Further, existing accounts are underspecified, narrow in the scope of issues they address, and mutually inconsistent.

The current work proposes a theory of semantic ambiguity resolution in which the discrepant effects are explained by the temporal settling dynamics in semantics and how these dynamics interact with the semantic representations of ambiguous words.  This account was instantiated using a distributed connectionist network that incorporated biologically processing constraints.  The network shows that the semantic activity evoked at different points in time is consistent with the effects observed in different tasks.  The account is further supported by behavioral studies of lexical decision, to evaluate whether differences in processing time, as opposed to qualitative task differences, are responsible for the different ambiguity effects observed across tasks.   In these experiments, task difficulty---and the presumed amount of semantic processing---was manipulated both by altering the wordlikeness of the nonword foils and by altering the visual contrast of the stimuli.  The selection of optimized word stimuli was enhanced by the development of an automatic stimulus selection algorithm which allowed for a large number of confounding variables to be controlled for, including a measure of the relative frequency of an ambiguous word's meanings that was collected using a new norming method.  The results of the lexical decision experiment show that the contrast manipulation caused large increases in overall latencies and produced semantic ambiguity effects consistent with later semantic processing.  This coordinated computational and behavioral work suggests that properties of settling dynamics within a distributed network explain the discrepancies observed across tasks, and generate predictions that can guide future research.

Furthermore, this work points to the importance of understanding how the semantic, orthographic, and phonological representations interact with the response selection system to generate the patterns of effects observed in different tasks.  The second portion of the dissertation develops a model of response selection that employs a similar set of domain-general learning, processing and representation principles to those that were used to model semantic ambiguity effects.  This work was challenged by previous computational and behavioral investigations using a numerosity judgement task that revealed numerous disagreements between the connectionist models and the behavioral data.  New behavioral data collected in an extension of the original numerosity judgment paradigm show that some of these findings do not replicate and were likely due to several idiosyncratic aspects of the original experiment.  Connectionist simulations of this extension of the original task succeed in capturing key elements of these new data, including some that are not captured by other models.  This work provides the foundation for developing models that integrate the word comprehension system and the response selection system to understand and predict the effects of ambiguity in different tasks, and beyond.

Dissertation Defense

Title: An auditory perspective on verbal short-term memory

Presenter: Jingyuan Huang

Location: College Conference Room, BA154r [Baker Hall]

Abstract:
It has long been hypothesized that short-term memory of verbal information
is represented phonologically and all other acoustic details are lost
during the perceptual process. The abstract, phonological nature of verbal
short-term memory is believed supported by the phonological similarity
effect: immediate serial recall of phonologically similar items (e.g., b,
d, g, t, c) is poorer than that of dissimilar items (e.g., f, q, r, h, y;
Conrad, 1964). Further suggesting a phonological code, visual orthographic
speech elicits a similar error pattern to that produced by auditory speech
(Baddeley, 1968). However, emerging empirical data suggest the persistence
of auditory details in short-term memory representations (e.g. Jones,
Macken & Nicholls, 2004; Frankish, 2007; Schweppe, Grice & Rummer, 2011)
and put the traditional phonological account into question.

The current research explicitly examines whether graded acoustical
information is preserved in verbal short-term memory. The first series of
experiments examines the phonological similarity effect, which serves as a
significant benchmark of the phonological nature of short-term memory
representations. By independently manipulating the phonological and
acoustical similarity of serial recall list items, the current experiments
found that when phonological similarity is held constant, serial recall
accuracy was poorer for acoustically similar list items than for
acoustically dissimilar items. The second series of experiments examined
the irrelevant speech effect, another influential effect putatively
supporting the phonological nature of representation in verbal short-term
memory. Concurrent background sounds significantly impair serial recall
performance and this influence may be modulated by similarity.
Manipulating the acoustics of nonspeech background sounds, the current
experiments found that the acoustic similarity between speech target and
nonspeech background items influences serial recall performance, even when
list items are presented orthographically.

Overall, the current data are consistent with the hypothesis that more
graded acoustical information persists through the sensory registration
and is encoded in short-term memory representations. Further, nonspeech
signals enter the same short-term store and interact with the speech
representations. These findings challenge the prevailing short-term memory
theories and models that posit abstract, phonological representations, and
open up a new perspective in understanding the mechanisms of both
perceptual speech processing and short-term memory.

Seminar

Title: "Neural Basis of Adaptive Decision-Making in the Macaque"
Presenter: Tobias Teichert, PhD
Location: BST 1695

Seminar
Title:  "How Do We Stop Ourselves?"
Presenter: Weidong Cai, Ph.D.
Location: Mellon Institute, Room 130
Abstract:
The ability to stop or prepare to stop inappropriate responses is important in everyday life. Many neurological and psychiatric disorders are associated with stopping deficit. However, the exact mechanism underlying stopping remains unclear. My research aims to understand how the behavioral stopping is achieved and what the neural mechanism underlying stopping is. To answer these questions, I have studied behavioral stopping using various modified stop-signal tasks. In this talk, I will present a series of TMS and fMRI studies in which I decomposed different cognitive processes in stopping. In two TMS studies, I have demonstrated a proactive and selective suppression mechanism and a global suppression mechanism. I will argue that behavioral stopping can be implemented by an active suppression mechanism and response tendencies can be proactively and selectively suppressed in advance. In one fMRI and another TMS studies, I have examined the role of the inferior frontal gyrus and the pre-supplementary motor area in stopping. Our results suggest that the inferior frontal gyrus is a functional heterogeneous structure and its subdivisions may play very different roles in stopping, and the pre-supplementary motor area or the network that it connects to is critical for implementing stopping

Seminar Title: Data Blitz – Spinal Cord Injury

Presenter: Speakers from the following labs: de Groat, Weber, Oudega, Wang, Gaunt, Perez

Special Guest: Susan Harkema

Location: 4075 BST3