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Digital archive of theses discussed at the University of Pisa

 

Thesis etd-01042010-134127


Thesis type
Tesi di dottorato di ricerca
Author
COSTAGLI, MAURO
URN
etd-01042010-134127
Thesis title
High-field functional magnetic resonance imaging reveals different patterns of responses elicited by transient changes in visual motion coherency
Academic discipline
BIO/12
Course of study
ESPLORAZIONE MOLECOLARE, METABOLICA E FUNZIONALE DEL SISTEMA NERVOSO E DEGLI ORGANI DI SENSO
Supervisors
tutor Prof. Pietrini, Pietro
Keywords
  • coherency
  • fMRI
  • hMT+
  • hV4
  • visual cortex
  • visual motion
Graduation session start date
14/01/2010
Availability
Withheld
Release date
14/01/2050
Summary
Previous work on human visual cortex demonstrated that when contrast is transiently increased or decreased from an intermediate, baseline level, neuronal activity in areas V1 through V3 follows the sign and the magnitude of the change. However, human area V4 (hV4) was reported to behave in a fundamentally different way, by responding always positively to both increments and decrements in contrast. This interesting behavior has been suggested to be part of the mechanisms responsible for signaling less salient changes in the visual stimulus – a function that cannot otherwise be performed by earlier areas, whose response to lesser stimuli is weak. However, despite the interestingness of this property of hV4 and its importance to advance knowledge about important aspects of early visual processing, this behavior has not been thoroughly investigated further and is not well documented in the literature. In particular, two main questions are of special interest: does hV4 respond positively to lesser stimuli in the presence of decrements of other stimulus attributes, besides image contrast? And do other cortical areas in the visual cortex behave in a similar way? To answer to these questions, we investigated the behavior of the visual cortex in response to transient increments and decrements in visual motion coherency. Two main response patterns were observed in the extrastriate visual cortex. In particular, the human MT complex, the parieto-occipital area V6, and V3A share the same kind of response pattern, in the sense that they are characterized by hemodynamic responses whose sign follows that of the change in motion coherency. On the other hand, areas in ventral occipital areas, including hV4, and in the vicinity of the intra-parietal sulcus exhibit a different kind of response, since they respond always positively to both increments and decrements in the level of motion coherency. Thus, we are able to provide evidence that hV4 signals transient decrements with positive responses, not only in the case of contrast, but also in the case of motion coherency – and, importantly, such a property is not restricted to hV4, but it is observed elsewhere as well, in other visual areas that are hierarchically located beyond V3. This previously unknown diversity in response patterns constitutes a new important finding for further understanding the mechanisms involved in visual processing in the human brain.
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