Flicker induced subjective colour and form

Our understanding of human visual perception generally rests on the assumption that conscious visual states represent the interaction of spatial structures in the environment and our nervous system. This assumption is questioned by circumstances where conscious visual states can be triggered by external stimulation which is not primarily spatially defined. In my PhD project I designed psychophysical experiments to investigate flicker induced colour and form. I constructed and programmed an experimental apparatus which is capable of the presentation of spatially uniform flicker with a precise temporal resolution. The statistical analysis, including analysis of density estimations, circular statistics and multidimensional scaling, revealed that subjective experiences do not only depend on stimulation frequency, but also on stimulation phase. What's more, the occurrence of one subjective experience appears to be associated with the occurrence of others. While these data indicate that conscious visual experience may be evoked directly by particular variations in the flow of spatially unstructured light over time it must be assumed that the systems responsible are essentially temporal in character and capable of representing a variety of visual forms and colors, coded in different frequencies or at different phases of the same processing rhythm.

Becker, C. & Elliott, M.A. (2006). Flicker induced color and form: Interdependencies and relation to stimulation frequency and phase. Consciousness and Cognition, 15 (1), 175-196.

Visuo-perceptual organization and temporal processes in developmental dyslexics

In the course of my diploma research project I tested the hypothesis, developed by myself, that temporal binding mechanisms might also play a role in dyslexia. I used psychophysical methods (the temporal priming paradigm) to investigate deficits in temporal processing in dyslexics. The hypothesis related to clinical variations of reading ability in the population required the careful selection and prior clinical testing of the participant sample. Planning of the study, the performance of the reading tests and experiments as well as the statistical analysis were performed by myself independently. An analysis of normal and dyslexic readers' reaction-time (RT) performance revealed a tendency for significantly longer search and detection RTs for dyslexic relative to the performance of normal readers. Consistent with previous studies, the RTs of normal readers and fast dyslexic responders exhibited target-specific priming effects. In contrast, a set of slower dyslexic responders showed strong negative priming on target-absent trials. The enhanced positive and the negative priming effects are both interpreted in the context of the possible deployment of attentional mechanisms to the priming stimulus. The extent to which this strategy is characteristic of dyslexic performance as a whole may relate to the degree to which the dyslexic responder concerned experiences some general temporal processing impairment: Attentional deployment in this instance serving to compensate a lack of the requisite temporal resolution required for coding the spatio-temporal structure of the prime.

Becker, C., Lachmann, T., & Elliott, M.A. (2005). Evidence for impaired visuo-perceptual organization in developmental dyslexics and its relation to temporal processes. Cognitive Neuropsychology, 22 (5), 499-522.

GABAergic effects on synchrony coding in human perception

The aim of my maitrise research project, developed in collaboration with Dr. M. A. Elliott and Dr. M. Boucart and linking pharmacology with psychophysical experimentation, was the investigation of the effects of a single dose of benzodiazepines on temporal priming. The benzodiazepine lorazepam enhances the efficiency of local, inhibitory GABAA synapses in the cortex, which stabilize postsynaptic, excitatory activity by synchronizing their own discharges at around 40 Hz. Treatment with the lorazepam has also been shown to adversely influence detection performance in perceptual tasks, suggesting a role for GABAA-mediated synchronization during visuo-perceptual organization. Consistent with these findings we found that effects of priming were amplified under the administration of the benzodiazepine lorazepam relative to baseline (no drug) and control (diazepam) conditions. We concluded that enhanced GABAA-induced inhibition enhances stimulus-evoked synchronization with differential effects upon mechanisms of perceptual segmentation and grouping.

Elliott, M. A., Becker, C., Boucart, M., & Müller, H. J. (2000). Enhanced GABAA inhibition enhances synchrony coding in human perception. Neuroreport, 11 (15), 3403-3407.