Kenneth O. Johnson - US grants

DESCRIPTION: (Adapted From The Applicant's Abstract): Most of what is known about the neural basis of perception comes from detailed correlation and hypothesis testing of putative neural mechanisms against detailed psychophysical outcomes. Although most of those studies have employed peripheral neural data the hypothesis testing methods used in those studies applies equally to the cortical mechanisms underlying perception. The first ai is to identify which cortical neuronal populations and neural coding mechanism can and which cannot account for the primate capacity for tactile form and texture perception. This will be attacked in three ways. The first employs exactly the same stimuli in cortical neurophysiological experiments as in psychophysical experiments. The question in areas 3b, 1 and 2 and the areas in and around Sll cortex will be whether the cortical neural responses to these stimuli can account for the associated psychophysics. This correlative strateg depends on the cross-species assumption that psychophysical capacity and neura mechanisms in man and monkey are identical. To test this assumption, new psychophysical studies will be undertaken in both man and monkey involving the discrimination of sinusoidal gratings. The fourth approach is different and is based on the substantial evidence that SAI afferents are responsible for form and texture perception while RA afferents are responsible for the detection of motion. By selective activation of SAI and RA afferents, we hope to obtain additional clues to the central pathways subserving these functions. Identifying the neurons responsible for form and texture processing is only part of the story, the second part is how they do it. Two developments during the current funding period allow us to attack this question more effectively than in the past. One is a new tactile stimulator with 400 independently controllable probes. The second is the development of new stimulus methods and analytical techniques for investigating the spatial distribution of excitation and inhibition that determines a neuron's response properties. These have allowed us to determine the receptive field structure of 247 neurons in area 3 of the alert monkey. For the first time, we have a picture of the receptive field organization in area 3b. The same methods, made more powerful with the new stimulator, will be used to study neuronal responses in the remainder of S and in Sll cortex.

The project will develop two high resolution tactual displays (also called dense arrays). Each device will contain 400 elements, arranged in a 20 by 20 array. The frequency, 0 to 400 Hz, and amplitudes, up to 3.0 mm peak-to-peak, of the mechanical vibration of each element in the array will be individually controlled. The center-to-center spacing of the stimulator tips in contact with the skin will be 0.4 mm when contacting the most sensitive skin surfaces. For less sensitive areas the spacing will be increased. The frequency, amplitude, and spatial separation of the stimulator tips permit the dense array to match or exceed the sensory capabilities of the skin. Initially the dense stimulators will be used to investigate tactual spatial pattern processing. Psychophysical studies with human subjects will be conducted in conjunction with neurophysiological studies with nonhuman primates. Neurophysiological recordings will be made of first-order afferents as well as cells in primary somatosensory cortex. By appropriate selection of frequency and amplitude of stimulation, spatial patterns will be generated that stimulate one or another of the primary afferent systems selectively. Spatial sensitivity of and interactions between the afferent systems will be examined both psychophysically and neurophysiologically. The dense array will be used to evaluate how prolonged exposure to distorted Spatial patterns affects the spatial organization of limited areas of skin. The arrays will be used to simulate realistic tactual stimuli and to evaluate current theories of the perception of roughness and slip. The arrays will also be used to examine biomechanical responses of the skin.

Three-dimensional (3D) form is a fundamental perceptual category that transcends individual sensory systems. A vivid percept of 3D form can be obtained from touch, from vision, or from both. The neural representations that underlie 3D for perception may be supramodal (independent or the individual sensory systems), multimodal (based on strong links between representations in the individual systems, or a mixture of the two. There are two compelling reasons to undertake the study of 3D form perception. The first is that 3D form perception is a paradigm of sensory integration in the brain. The second is that 3D form perception is a fundamental cognitive function whose neural mechanisms are almost completely unknown. Four individual projects are proposed, two involving somatosensory neurophysiology and two involving visual neurophysiology. Tactual perception of 3D form simultaneously on the conformation on the hand and on the integration of information from the multiple points of contact with the fingertips. Project 1 (Johnson) deals with how neurons respond to 3D stimuli (edges, corners, curvature) at multiple points of contact. Project 3 (Hsiao) concentrates on how responses depend on the conformation of the hand. Visual perception of 3D form is derived from 2D images; this process depends on the integration of visual information with implicit knowledge about optics and the 3D world. Project 2 (Conner) addresses how neurons in visual cortex represent the basic elements of 3D share (3D edges, corners, surface curvature, and simple volumes like blocks and cylinders). Project 4 (von der Hyedt) concentrations on the integration of binocular and monocular and cue conflict. The core project concerns shared services, administration, and project coordination.

The perception of 3D form depends on hand conformation and the integration of stimuli of stimuli from the multiple points of contact with the hand. To understand the neural mechanisms underlying stereognosis we need to understand three things: 1) how neurons respond to surface structure at the individual points of contact, 2) how a stimuli at separate regions of skin are integrated, and 3) how this integration depends on hand conformation. The experiments proposed here are aimed at taking a first step towards understanding these three facets of 3D form perception. Aim 1 is to determine how neurons in SI and SII cortex respond to 3D surface structure contacting a single finger. This study will be limited to surface curvature and edges of the kind that occurs when a finger contacts a rigid object. 3D surface structure varies in the plane parallel to and normal to the skin surface. The experiments related to aim 1 will study neural responses to edges and corners whose orientation and curvature vary in the plane parallel to the skin surface and neural responses to surface curvature normal to the skin surface. Aim 2 is to determine how neurons in SI and SII cortex respond to stimuli presented to two fingers. Surfaces indenting tow finger tips signal different things about an object depending on the surfaces' relative motions, orientations, and curvatures. Experiments related to aim 2 will study 1) the effects of stimulus direction and velocity; b) the effects of edge orientation, and c) the effects of surface curvature and slope on adjacent fingers. Aim 3 is to determine the neural mechanisms of objective constancy in the face of varying finger position. When we scan our fingers over an object the primary information from two or more fingers varies with finger spread but the percept is unaffected. The experimented related to Aim 3 concentrate on the convergent mechanisms (between proprioceptive and cutaneous information) that must operate to compensate for finger position when abduction varies.

The purpose of this core is to provide common services to all of the projects, to administer the program, and to provide a mechanism for scientific interaction between the project. The service component consists of machining, electronics, histological, and electrode manufacturing services provided by facilities in the Mind/Brain Institute. The administrative component consists of budget administration, administration of the service facilities, scheduling of advisory meetings and weekly group discussions, and communications with NIH. Scientific integration will be effected through the organization of weekly, formal interactions between the members of the program project and through visits by internal and external advisors to review our progress and aims.

[unreadable] DESCRIPTION (provided by applicant): The broad, long-term aim of this study is to understand the neural mechanisms of tactile spatial perception in the human hand. Impaired tactile acuity leads to the inability to perform simple tasks such as buttoning and unbuttoning a button. Tactile spatial acuity is impaired in advanced age and in many neurological conditions. Understanding the underlying mechanisms is an essential first step in any treatment. The long-term aim of the study proposed here is to understand the neural mechanisms of tactile spatial perception in primary somatosensory (SI) cortex. A basic problem is that the functional organization of SI cortex is not well understood and this makes it difficult to formulate and test specific hypotheses. Combined psychophysical and neurophysiological studies over the last forty years have lead us to understand that the human hand is innervated by four types of mechanoreceptors and that each is responsible for a distinctly different aspect of tactile perception. Despite the distinct functional division evident in the peripheral nerve and despite the widespread, and justified belief that the information conveyed by the four afferent groups remains segregated within the central nervous system, the division of function within SI cortex is understood only in very broad terms. One reason is the lack of flexible, controlled stimuli with which to study the functions of neurons in St cortex, which we believe we have overcome with the development of a tactile stimulator with 400 independently controlled probes. Experiments proposed in Aim 1 examine the response properties of neurons in areas 3a, 3b, 1, and 2 of macaque SI cortex with temporal and spatial stimuli selected to span a wide range of tactile function. Experiments proposed in Aim 2 examine differences in the mechanisms underlying neuronal response properties in these areas using random stimuli and regression analyses. Experiments proposed in Aim 3 examine the neural mechanisms of tactile spatial perception with stimuli that have been particularly effective in previous studies (square-wave gratings) and with a new class of stimuli that combine the spatial and temporal properties of tactile function in a single framework (spatiotemporal sinusoids). [unreadable] [unreadable]