A method for achieving an order-of-magnitude increase in the temporal resolution of a standard CRT computer monitor

Inferior parietal lobule encodes visual temporal resolution processes contributing to the critical flicker frequency threshold in humans

The measurement of the Critical Flicker Frequency threshold is used to study the visual temporal resolution in healthy subjects and in pathological conditions. To better understand the role played by different cortical areas in the Critical Flicker Frequency threshold perception we used continuous Theta Burst Stimulation (cTBS), an inhibitory plasticity-inducing protocol based on repetitive transcranial magnetic stimulation. The Critical Flicker Frequency threshold was measured in twelve healthy subjects before and after cTBS applied over different cortical areas in separate sessions. cTBS over the left inferior parietal lobule altered the Critical Flicker Frequency threshold, whereas cTBS over the left mediotemporal cortex, primary visual cortex and right inferior parietal lobule left the Critical Flicker Frequency threshold unchanged. No statistical difference was found when the red or blue lights were used. Our findings show that left inferior parietal lobule is causally involved in the conscious perception of Critical Flicker Frequency and that Critical Flicker Frequency threshold can be modulated by plasticity-inducing protocols.

Lengthened temporal integration in schizophrenia

Research in schizophrenia has tended to emphasize deficits in higher cognitive abilities, such as attention, memory, and executive function. Here we provide evidence for dysfunction at a more fundamental level of perceptual processing, temporal integration. On a measure of flicker fusion, patients with schizophrenia exhibited significantly lower thresholds than age and education matched healthy controls. We reasoned that this finding could result from a longer window of temporal integration or could reflect diminished repetition suppression: if every frame of the repeating stimulus were represented as novel, its perceived duration would be accordingly longer. To tease apart these non-exclusive hypotheses, we asked patients to report the number of stimuli perceived on the screen at once (numerosity) as they watched rapidly flashing stimuli that were either repeated or novel. Patients reported significantly higher numerosity than controls in all conditions, again indicating a longer window of temporal integration in schizophrenia. Further, patients showed the largest difference from controls in the repeated condition, suggesting a possible effect of weaker repetition suppression. Finally, we establish that our findings generalize to several different classes of stimuli (letters, pictures, faces, words, and pseudo-words), demonstrating a non-specific effect of a lengthened window of integration. We conclude that the visual system in schizophrenics integrates input over longer periods of time, and that repetition suppression may also be deficient. We suggest that these abnormalities in the processing of temporal information may underlie higher-level deficits in schizophrenia and account for the disturbed sense of continuity and fragmentation of events in time reported by patients.

A low-cost, portable, micro-controlled device for multi-channel LED visual stimulation

Light emitting diodes (LEDs) are extensively used as light sources to investigate visual and visually related function and dysfunction. Here, we describe the design of a compact, low-cost, stand-alone LED-based system that enables the configuration, storage and presentation of elaborate visual stimulation paradigms. The core functionality of this system is provided by a microcontroller whose ultra-low power consumption makes it well suited for long lasting battery applications. The effective use of hardware resources is managed by multi-layered architecture software that provides an intuitive and user-friendly interface. In the configuration mode, different stimulation sequences can be created and memorized for ten channels, independently. LED-driving current output can be set either as continuous or pulse modulated, up to 500 Hz, by duty cycle adjustments. In run mode, multiple-channel stimulus sequences are automatically applied according to the pre-programmed protocol. Steady state visual evoked potentials were successfully recorded in five subjects with no visible electromagnetic interferences from the stimulator, demonstrating the efficacy of combining our prototyped equipment with electrophysiological techniques. Finally, we discuss a number of possible improvements for future development of our project.

Multiple serial picture presentation with millisecond resolution using a three-way LC-shutter-tachistoscope

Throughout recent years there has been an increasing interest in studying unconscious visual processes. Such conditions of unawareness are typically achieved by either a sufficient reduction of the stimulus presentation time or visual masking. However, there are growing concerns about the reliability of the presentation devices used. As all these devices show great variability in presentation parameters, the processing of visual stimuli becomes dependent on the display-device, e.g. minimal changes in the physical stimulus properties may have an enormous impact on stimulus processing by the sensory system and on the actual experience of the stimulus. Here we present a custom-built three-way LC-shutter-tachistoscope which allows experimental setups with both, precise and reliable stimulus delivery, and millisecond resolution. This tachistoscope consists of three LCD-projectors equipped with zoom lenses to enable stimulus presentation via a built-in mirror-system onto a back projection screen from an adjacent room. Two high-speed liquid crystal shutters are mounted serially in front of each projector to control the stimulus duration. To verify the intended properties empirically, different sequences of presentation times were performed while changes in optical power were measured using a photoreceiver. The obtained results demonstrate that interfering variabilities in stimulus parameters and stimulus rendering are markedly reduced. Together with the possibility to collect external signals and to send trigger-signals to other devices, this tachistoscope represents a highly flexible and easy to set up research tool not only for the study of unconscious processing in the brain but for vision research in general.