Duration encoding of brief temporal intervals by the visual system

  • Lee Beattie

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

The classical model of timing describes a global, cross-modal, centralised clock that processes durations across all timescales. Such a model is viable when processing durations in the range of seconds but not milliseconds, which seem to be encoded by modality-specific timing mechanisms. Studies using stimuli that selectively activate discrete areas of the visual pathway have provided evidence of these mechanisms in pre-cortical and cortical areas, though precise localisation has remained elusive. A productive area of investigation has been to study duration compression, an aftereffect caused by a dynamic visual adaptor that results in the duration of a stimulus presented in the adapted location to be underestimated. The experiments described in the present thesis used stimuli with varied local and global motion parameters to try to isolate their influence on duration perception and, from this, infer likely cortical locations for temporal processes. The experiments in Chapter 3 established that the duration compression effect is tuned for the speed of the adaptor when using unidirectional random dot kinematograms (RDKs). This information was then used to generate RDKs that presented multiple dot speeds simultaneously that could isolate local motion effects from those of the global pattern speed of the stimuli. Coherence RDKs, where local and global speeds differed through dots changing direction multiple times throughout their presentation, were used in Chapter 4’s experiments. Those in Chapter 5 tested the effects of adaptation to coherent drifting plaid stimuli, which have superimposed component gratings that generate the percept of a single global pattern that moves with a speed and direction different to that of the underlying local motion. The key finding of these experiments is that duration compression magnitude is tuned for the global pattern speed of RDK and coherent bikinetic plaid adaptors. That global effects were found in all three chapters indicates the results are due to mechanisms found late in the visual pathway; MT is the earliest area of the processing hierarchy where local motion signals are pooled to calculate global pattern information. Duration compression was also found in the pattern direction of a coherent unikinetic plaid adaptor, which has one stationary and one moving component grating. The global motion of these stimuli is thought to be processed in area MST, showing timing mechanisms extend beyond area MT. Taken as a whole, the experiments described in the present thesis clearly indicate the existence of subsecond duration encoding in the visual cortex. When combined with the results of previous studies, the present findings strengthen the argument that temporal mechanisms exist at multiple levels throughout the visual processing hierarchy.
Date of AwardJul 2019
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorWilliam Curran (Supervisor)

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