Progress in Sensory Physiology PDF
Preview Progress in Sensory Physiology
Foreword I fancy that many of you, like myself, have woken up in the night with a "sleeping" arm or leg. It is a very peculiar feeling to have that arm or leg, cold and lifeless, hanging there at your side as if it were something which does not belong to you. In such situations you recover some of the motor functions before the sensory functions, which en ables you to move the limb like a pendulum. For a few sec onds the arm functions as an artificial limb - a prosthesis without sensors. In general we are not aware of the importance of our sensory organs until we lose them. You do not feel the pressure of your clothes on the skin or the ring on your finger. In the nineteenth century such phenomena generally named adaptation, were studied to a great extent, partic ularly in vision, as well as in the so-called lower senses. The question whether sensory adaptation was due to changes in the peripheral sensory receptors or in the central nervous structure remained in general open until the 1920s. Then the development of the electronic arsenal gave us the means to attack the problem by direct observations of the electrical events in the peripheral as well as the central nervous system. But even today there are still some blank areas in our knowledge of adaptation. More remarkable, however, is in my'opinion the fact that, with the exception of the field of vision, we know little or nothing about the initial phase of the series of events which occur in the sensory receptors during excitation. I have a strong feeling that this gap in our knowledge will gradually be filled in the near future and will be one of the issues which will be treated in subsequent volumes of Progress in Sensory Physiology. In November, 1925, when Adrian and I succeeded in record ing the impulses in a single sensory nerve fiber when its endings were excited by natural stimulation, we were im mediately aware that this opened a new field of research which would enable us to progressively narrow the gap be- VI Foreword tween the physical events in our nervous system, which can be recorded by physical means, and the sensation aroused in our consciousness. In 1843 Justus Liebig faced this problem when he wrote: "We know well the mechanisms of the eye, but neither anatomy and still less chemistry will be able to inform us how a ray of light enters into our consciousness. Natural sciences have their natural limits beyond which you shall not pass. The gravitation like the light for the blindborn are just words." In the preface to his book The Basis of Sensation Adrian in 1927 tackled this matter in the following way: "Perhaps some drastic revision of our systems of knowledge will explain how a pattern of nervous impulses can cause a thought or show that the two events are really the same thing looked at from a different point of view." Let us, like Adrian, hope that such a revision will be made and, further, that we then may be able to understand it. After all, we are gradually approaching that point. With these words I wish the editors of Progress in Sensory Physiology and the future authors luck in their endeavors. They are all involved in one of the hardest and therefore the biggest game in the world. Progress in Sensory Physiology 1 Editors: H. Autrum D. Ottoson E. R. Perl R. F. Schmidt Editor-in-Chief: D. Ottoson With Contributions by P. Gouras E. R. Kandel M. Klein H. W. Kosterlitz A. T. McKnight E. Shapiro G. Westheimer E. Zrenner With 72 Figures and 6 Tables Springer-Verlag Berlin Heidelberg New York 1981 Editor-in-Chief' Professor Dr. David Ottoson Karolinska Institutet, Fysiologiska Institutionen II Solnavagen 1, S-10401 Stockholm 60 Editors: Professor Dr. Hansjochem Autrum Zoologisches Institut der Universitat Mtinchen Luisenstra!3e 14, 0-8000 Mtinchen 2 Professor Dr. Eduard Roy Perl University of Northern Carolina at Chapel Hill, Department of Physiology Chapel Hill, NC 27514 (USA) Professor Dr. Robert F. Schmidt Physiologisches Institut der Universitat Olshausenstra!3e 40 - 60, 0-2300 Kiel ISBN-I 3: 978-3-642-66746-6 e-ISBN-I 3: 978-3-642-66744-2 001: 10. I 007/978-3-642-66744-2 Library of Congress Cataloging in Publication Data. Main entry under title: Progress in sensory physiology. Bibliography: p. Includes index. 1. Vision-Physiological aspects. 2. Senses and sensation. I. Autrum. Hansjochem. II. Ottoson, David, 1918 - . III. Gouras, P. Q475.P89 612' .8481-4430 AACRl This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illus trations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under § 54 of the German Copyright Law, where copies are made for other than private use, a fee is payable to "Verwertungsgesellschaft Wort", Munich. © by Springer-Verlag Berlin Heidelberg 1981 Softcover reprint of the hardcover I st edition 1981 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. 2121/3140-543210 Contents G. Westheimer Visual Hyperacuity ............................ . H. W. Kosteflitz and A. T. McKnight Opioid Peptides and Sensory Function. . . . . . . . . . . . . 31 E. Shapiro, M. Klein, and E. Kandel Ionic Mechanisms and Behavioral Functions of Presynaptic Facilitation and Presynaptic Inhibition in Aplysia: A Model System for Studying the Modulation of Signal Transmission in Sensory Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 P. Gouras and E. Zrenner Color Vision: A Review from a Neurophysiological Perspective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Visual Hyperacuity* G. Westheimer Department of Physiology-Anatomy, University of California, Berkeley, CA 94720 (USA) Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 The Minimum Angle of Resolution and the Minimum Visible .......... . 3 2.1 The Minimum Angle of Resolution ................................ . 3 2.2 The Minimum Visible ........................................... . 4 3 Psychophysical and Electrophysiological Parallels ................... . 5 4 Hyperacuity ................................................... . 6 5 Displacement Detection ......................................... . 6 6 Spatial Interval Detection ........................................ . 9 7 Vernier Acuity and Orientation Discrimination ...................... . 11 8 Oblique Effect ................................................. . 14 9 Temporal Factors ............................................... . 14 10 Stereoscopic Acuity ............................................. . 16 11 Blur and Retinal Eccentricity ..................................... . 17 12 Fourier Theory ................................................. . 18 13 Space and Geometry ............................................ . 20 14 Spatial Interference with Hyperacuity .............................. . 23 15 Two Different Concepts ......................................... . 24 16 Can "Channels" Be of Help? ..................................... . 26 References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 * The experiments on which the review is based were supported by the National Eye Institute, U.S. Public Health Service, under Grant EY00220. Most were carried out with the collaboration of Dr. Suzanne P. McKee, to whom the author is also indebted for valuable discussion and the preparation of the tables 2 G. Westheimer 1 Introduction This review concerns itself with the fine grain of visual space, i.e., with the smallest distance modules into which space can be dissected. At the outset it needs to be recognized that the visual spatial sense can be analyz ed only by means of signals that address the light or color sense. At the mini mum, then, the limitations associated with the creation, detection and processing of retinal images apply. The first of these, optical imaging in the eye, is one of the most important factors in this connection but also, fortunately, one of the best understood. In the intact animal, the optics of the eye are interposed between the world of light sources and the retinal receptors. We must, therefore, start with knowledge of the spatial changes introduced in light stimuli by the process of optical imagery in the eye. Though a complex subject, for the purposes of the present discussion it can be satisfactorily reduced to consideration of the eye's optical "impulse function," i.e., the retinal light distribution in the image of a point object of light. Because the discussion here deals almost entirely with lines rather than points of light, and because there is a straightforward relationship between the image distribution for a point and that for a line of light, we can without loss of generality limit attention to the eye's line-spread function, which is the light distribution in a cross section of the image on the retina of a line object of infinitesimal width (Westheimer 1972). Figure 1a illustrates its current best estimate for a well-corrected focused human eye. For comparison, the diameter of an individual cone in the center of the fovea is also shown. These dimensions may, of course, differ for other conditions, viz., dark-adapted vision, vision in the retinal periphery, other species, etc. a b 0------; D 1m in of arc Receptor diameter Fig. 1. a Light distribution in the retinal image of a single very thin line object (line-spread function). Its width at half height is at least twice the diameter of a receptor in the human fovea. Under good conditions the position of this image can be located with a precision of one-tenth of a receptor diameter. The term "hyperacuity" is used for this class of localizing ability. b Ordinary visual acuity as exemplified by the minimum angle of resolution. Two line objects are moved apart until they can just be detected as separate. The retinal light stimulus is the sum of the two line-spread functions. The depth of the central trough has to be sufficient for a ~I discrimination and the spatial compartment alization must be fine enough to enable the qualitative decision to be made that there are two separate peaks Visual Hyperacuity 3 2 The Minimum Angle of Resolution and the Minimum Visible 2.1 The Minimum Angle of Resolution The traditional mode of analysis of the limits of visual spatial differentiation is the resolving capacity, probed by the usual visual acuity tests. Typically, the spacing of a pair of points or lines is increased until they can be detected as separate (Fig. 1b ). The threshold distance, expressed in angular measure at the eye's entrance pupil, is called the minimum angle of resolution (MAR). A repre sentative value for this threshold is 1 min. of arc. It compares well with predic tions from diffraction theory, from the width of experimentally determined line spread functions, and from the size of retinal receptors. The evolutionary convergence of the various optical and anatomical components of visual acuity is underlined by special tests in which the optics of the eye are bypassed. When interference fringes are created directly on the retina (Westheimer 1960), without the aid of focusing by the cornea and crystalline lens, the detectable limit of their spacing is about the same as that of gratings imaged in the standard way. This shows that the anatomical and physiological factors in visual acuity cannot ordinarily outperform the optical ones. It would be rash, however, to draw the conclusion that the subject of visual acuity is closed. Important questions remain. For instance, even the best measures of receptive field diameters of single neural units in the visual pathway seem to be too large to account for visual acuity by a factor of about two (Table 1). The answer may be that units with the smallest fields, presumably in the unanesthetized primate, have yet to be recorded. But perhaps more sophisticated processing is at work, a possibility to which recent psychophysical experiments also point. Visual acuity in the human fovea is unaffected when Snellen letter targets move across the retina at rates of up to 2 or more degrees per second (Westheimer and McKee 1975). The significance of this observation is that feature components separated by the width of a couple of receptors can cover a range of about a dozen individual receptors during a single temporal integration period, yet be resolved. Ordinary measures of summation on the retina do not appear to hold for resolution. Table 1. Estimates of minimum diameter of receptive field centers in single units of monkey visual system Investigators Animal Location Minimum size Hubel & Wiesel (1960) Spider monkey Optic nerve 4' Wiesel & Hubel (1966) Macaque LGN 2' Dow & Gouras (1973) Macaque Striate cortex 4' de Monasterio & Gouras Macaque Ganglion cell 2' (1975) Poggio, Doty & Talbot Macaque Striate cortex 2' (1977) 4 G. Westheimer If we were interested in the finest dissection of space that can be achieved by our visual sense, and we rested our case here, we would be mistaken, for spatial visual thresholds exist that are smaller by at least one order of magnitude than the minimum angle of resolution. 2.2 The Minimum Visible Before coming to these, we need to examine briefly a kind of visual threshold that exhibits exceedingly low spatial values but does not really fit into this discussion. Suppose one is looking at a black line against a bright background. How narrow can it be made before it is no longer detectable? A telegraph wire 1 cm thick can be seen against the sky at a distance of about 1 km (Hecht and Mintz 1939). This represents a visual angle of just a few seconds of arc, certainly much less than the I' or so of arc minimum angle of resolution. What sets this kind of threshold apart from the topic of this review is the fact that it concerns merely the detection of the presence or absence of a light stimulus. A narrow dark line against a uniform background creates a retinal light distribution that may be described as a dimple or groove in a uniform field. The shape of the dis tribution is essentially the same for all target widths up to 1 or 2 minutes of arc, only its depth will vary. Therefore whether a narrow line can be detected depends merely on whether the light decrement in its image exceeds the luminance difference threshold of the visual system for the prevailing state of adaptation. Although we are measuring a distance - line width - we are asking a simple detection decision of the visual system. This becomes even more apparent when considering the visibility of a single star in the sky: the object dimension is infinitesimal and visibility depends only on intensity and background. The distinction between the task of detecting the presence of a target without enquiring about its nature, and the task of making a judgment based on spatial criteria about clearly visible targets is fundamental. An early and most instructive example of the latter class is an experiment performed by Fechner and Volkmann in 1857 (Fechner 1860): Three vertical lines are shown to a subject who had to judge whether the right outer line was a far from the middle line as the left outer line. That is, the subject had to decide whether the two spatial intervals, marked off by the three lines, were equal. As was shown by Fechner and Volkmann, the use of a spatial rather than a simple detection criterion did not make the experiment any less precise or less capable of being executed with all the finesse of the scientific method. Yet this very early experiment has a setting apart from and beyond all the measurements of visual threshold based on the subject's indication regarding whether a field is uniformly lit or not, i.e., experiments where single points, lines, disks, or gratings have their contrast manipulated to find out under what conditions their presence can be detected. The contrast detection paradigm is, incidentally, not confined to psychophysical investiga tions but also constitutes a major procedure of analyzing single unit responses, recorded electrophysiologically in animals (Albrecht et al. 1980). But, whether a behavioral index or the discharge pattern of a single nerve cell is being used, associating a signal with the mere presence of a target does not exhaust the spatial differentiation capabilities of the visual system.
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