The project entitled ?Neurophysiology of Visual Perception? investigates the mechanisms in the brain that link basic sensory visual processing with higher level aspects of our visual experience. We are presently setting up a neurophysiology laboratory, as well as a functional imaging laboratory, both of which are dedicated to the study of brain function in monkeys. Our groups main focus is on the neural mechanisms underlying how we perceive visual patterns. In the last months, the main construction and population of the neurophysiology laboratory has been largely completed, and a large portion of the experimental equipment is soon due to arrive. The imaging laboratory, a core facility called the neurophysiology imaging facility (NIF), is specialized in functional magnetic resonance imaging (fMRI) in the nonhuman primate, is already partially functiononal. This facility is available to scientists from several institutes for both the anatomical and functional evaluation of brain function in monkeys. In our research program, we exploit the combinatino of both techniques, which afford complementary views of neural function. The imaging experiments in awake monkeys pose a number of technical challenges that are currently being addressed. We have, for example, designed and built a specialized chair in which a monkey can sit comfortably, observe stimuli, and give manual responses while scans of his brain are obtained. Of course, each aspect of this constraint, as well as behavioral control and measurement, must be achieved in the context of the main magnetic field, and therefore be constructed of special materials.
The aim, which sets aside the NIF as a unique facility in the world at present, will be the testing of the same trained monkeys, in the same chair, with both electrophysiological and fMRI. Each animal will perform the same set of tasks in both environments. In the first phase of the setup, the electrophysiology, i.e. microelectrode recordings, will be performed outside the magnet, in a traditional neurophysiology setting. Microelectrode measurements permit the measurement of both single neuron activation, as well as more global electrical fields (similar to the electroencephalogram, or EEG, in humans), which together tap into several aspect of neural function. These measures, when combined with the large-scale activity patterns available with fMRI, will provide excellent complementary views of brain function. Finally, as trained experts, each monkey is extremely valuable, and therefore great care is given to ensure that they remain in good health. For this reason we are also developing a new style of implants aimed to be both biocompatible and cause minimal magnetic field distortion during imaging. Each of these projects and modifications has been undertaken in order to ask questions in the brain related to visual perception, some of which will be described now. In the process of seeing, the brain is faced at each moment in time with the task of interpreting its sensory environment. What we """"""""see"""""""" is a product of the automatic processing of a pattern of stimulation on our retina, as well as active, inquisitive processes that seek to make sense of this processing. This is perhaps best conveyed when one considers that an image cast on the retina, after passing through the pupil and lens, is inherently two-dimensional, and must be interpreted as a 3-D world within which we must navigate, interact, etc. The neural mechanisms by which objects and their spatial relationships are extracted from this primary input are poorly understood, and are of great interest to many students of vision. Our laboratory has previously approached active elements of visual perception using several tools derived from human psychophysics. For example, in several studies, monkeys were required to make judgments about their perception of stimuli that were either barely recoognizable or completely ambiguous. Microelectrode recordings of neurons were then compared to the animals? subjective perception. In the present project, we aim to extend this work on ambiguous perception, in the more general context of perceptual organization. This latter term roughly refers more generally to the set of processes that shape the sensory input into structurally meaningful shapes and objects. It relates to how we perceive depth, color, motion, in a scene, as well as how we recognize objects such as the identity of a face. As before, the monkey?s subjective perception will be a crucial element in our experiments, since relating a neuron?s firing to a """"""""percept"""""""" is distinctly different, and often more informative, than describing it as a transformation of a sensory input. While this component of the research places behavioral demands on the monkeys, and thereby adds time and effort in training and testing, it permits insight into fascinating questions about the brain?s production of perception, for which some neural mechanism may only be addressable using such a combined approach as described here. We are hopeful that both the microelectrode recordings and neuroimaging experiments will be running by early part of 2005.
|Müller, K-M; Wilke, M; Leopold, D A (2009) Visual adaptation to convexity in macaque area V4. Neuroscience 161:655-62|
|Shmuel, Amir; Leopold, David A (2008) Neuronal correlates of spontaneous fluctuations in fMRI signals in monkey visual cortex: Implications for functional connectivity at rest. Hum Brain Mapp 29:751-61|
|Maier, Alexander; Logothetis, Nikos K; Leopold, David A (2007) Context-dependent perceptual modulation of single neurons in primate visual cortex. Proc Natl Acad Sci U S A 104:5620-5|
|Wang, Zhisong; Maier, Alexander; Leopold, David A et al. (2007) Single-trial evoked potential estimation using wavelets. Comput Biol Med 37:463-73|
|Wilke, Melanie; Logothetis, Nikos K; Leopold, David A (2006) Local field potential reflects perceptual suppression in monkey visual cortex. Proc Natl Acad Sci U S A 103:17507-12|
|Wang, Zhisong; Maier, Alexander; Leopold, David A et al. (2006) Relaxation-based multichannel signal combination (RELAX-MUSIC) for ROC analysis of percept-related neuronal activity. IEEE Trans Biomed Eng 53:2615-8|
|Leopold, David A; Bondar, Igor V; Giese, Martin A (2006) Norm-based face encoding by single neurons in the monkey inferotemporal cortex. Nature 442:572-5|
|Fukunaga, Masaki; Horovitz, Silvina G; van Gelderen, Peter et al. (2006) Large-amplitude, spatially correlated fluctuations in BOLD fMRI signals during extended rest and early sleep stages. Magn Reson Imaging 24:979-92|
|Leopold, David A; Maier, Alexander (2006) Neuroimaging: perception at the brain's core. Curr Biol 16:R95-8|
|Leopold, David A; Rhodes, Gillian; Muller, Kai-Markus et al. (2005) The dynamics of visual adaptation to faces. Proc Biol Sci 272:897-904|
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