Single units and sensation: a retrospect

Behaviour and neural responses in crucian carp to skin odours from cross-order species

Many teleost species respond with fright reactions to olfactory cues from injured skin of conspecifics, but they may also display responses to skin extracts of heterospecific fish. In the present study, we exposed crucian carp to skin extracts of conspecifics and three cross-order species of fish (brown trout, pike, and perch). Behavioural experiments showed that conspecific skin extracts induced fright reactions in crucian carp; extracts of brown trout induced such behaviour less frequently, while extracts of perch and pike were poor inducers of fright responses. The olfactory bulb is chemo-topically organized, and different sub-sets of neurons respond to functionally related odorants that mediate distinct behaviours. Accordingly, behavioural responses to an alarm signal should be reflected by activation of the neurons mediating fright reaction. Extracellular recordings from single units in the olfactory bulb showed that the relay neurons activated by conspecific skin extracts were also activated by extracts of brown trout, whereas extracts of perch and pike less frequently activated these units. Thus, the difference in behavioural responses matched the differences in the neural responses, indicating that skin extracts of heterospecific fish are more likely to induce fright behaviour when the responding sub-set of olfactory neurons is similar to the sub-set responding to conspecific extracts. Our results suggest that responses to injured heterospecific fish rely on chemical resemblance between odorants from heterospecific and conspecific skin, and need not be based on any form of associative learning.

Statistics of optical coherence tomography data from human retina

Optical coherence tomography (OCT) has recently become one of the primary methods for noninvasive probing of the human retina. The pseudoimage formed by OCT (the so-called B-scan) varies probabilistically across pixels due to complexities in the measurement technique. Hence, sensitive automatic procedures of diagnosis using OCT may exploit statistical analysis of the spatial distribution of reflectance. In this paper, we perform a statistical study of retinal OCT data. We find that the stretched exponential probability density function can model well the distribution of intensities in OCT pseudoimages. Moreover, we show a small, but significant correlation between neighbor pixels when measuring OCT intensities with pixels of about 5 microm. We then develop a simple joint probability model for the OCT data consistent with known retinal features. This model fits well the stretched exponential distribution of intensities and their spatial correlation. In normal retinas, fit parameters of this model are relatively constant along retinal layers, but varies across layers. However, in retinas with diabetic retinopathy, large spikes of parameter modulation interrupt the constancy within layers, exactly where pathologies are visible. We argue that these results give hope for improvement in statistical pathology-detection methods even when the disease is in its early stages.

Optimal Neuronal Tuning for Finite Stimulus Spaces

The efficiency of neuronal encoding in sensory and motor systems has been proposed as a first principle governing response properties within the central nervous system. We present a continuation of a theoretical study presented by Zhang and Sejnowski, where the influence of neuronal tuning properties on encoding accuracy is analyzed using information theory. When a finite stimulus space is considered, we show that the encoding accuracy improves with narrow tuning for one- and two-dimensional stimuli. For three dimensions and higher, there is an optimal tuning width.

Electrical stimulation of isolated retina with microwire glass electrodes

The development of high-resolution retinal prostheses fabricated from silicon wafers presents an interesting problem: how to electrically bridge the space between the flat silicon wafer and the curved retinal surface. One potential "bridge" is a microwire glass electrode. In this paper we present our results in evaluating microwire glass electrodes. We stimulated isolated rabbit retina (n = 5) with a 0.0256 cm(2) microwire electrode. The current and pulse duration were varied from 498 to 1660 microA and 0.1 to 3 ms, respectively. We found that short pulses produced more spikes per coulomb and longer pulses produced more spikes per milliamp. The optimal pulse duration range of 0.7-1 ms was identified as a compromise between the advantages of short and long pulses. Stimulation of isolated rabbit retina with microwire glass results in consistent neuronal spike formation at safe charge density, 20.7 +/- 4.3 microC/cm(2). We also examined the response of retinas (n = 6) to stimulation with a smaller microwire electrode, 0.0002 cm(2). We found that less current was required (15 microA versus 756 microA) for a 1 ms pulse, but at the expense of greater charge density (75 microC/cm(2) versus 29.5 microC/cm(2)). Nonetheless, a 128-fold reduction in area resulted in only a 2.7-fold increase in charge density required for a 1 ms pulse duration. The results presented here indicate that microwire glass can be used as a neural stimulating electrode to bridge the gap between flat microelectronic stimulator chips and curved neuronal tissue.

Unique hues: an old problem for a new generation

The practical success of the classical theories of colour vision, such as that of Young-Helmholtz when applied to the measurement and reproduction of colour stimuli, and that of Hering's in art and architecture, has overshadowed the fact that neither theory achieved its main goal, namely to explain colour qualities. Neither the three types of cone, nor the first opponent stages of neural processing in the retina and the lateral geniculate nucleus can serve as direct correlates to the perception of elementary, or unique colours, such as red, green, yellow and blue. While our subjective experiences of these qualities do not submit to measurement, physiological conditions that are required to perceive colours of a constant hue can be identified. For instance, a constant ratio of responses of different types of opponent cells in the retina and the lateral geniculate nucleus of primates may serve as a neurophysiological correlate of a constant hue. This is, however, not the correlate for seeing a particular hue quality, say unique red. This latter correlate, if it exists as a separable entity, must be associated with yet unidentified, higher-level neural activities. The fundamental problems encountered in relating colour qualities to neural activities are discussed and references are made to the current debate about phenomenal consciousness.

Paradigm shifts: new techniques to answer new questions

The advent of a multiple-channels approach to spatial vision 20 years ago raised important questions that were difficult to approach empirically, given the technology and analytic tools of the time. These questions concerned the interaction or combination of different components of a stimulus--questions that have recently resurfaced in more complex form. Classical psychophysical methods for assessing whether two stimulus aspects are coded independently (e.g., masking, adaptation, and cue-summation) provide only limited information about the nature of whatever interactions are discovered. In both older work in detection and recent work in complex pattern discrimination, we have used a double-judgment paradigm in which the observer rates two aspects of a stimulus simultaneously. The paradigm provides a rich source of information about the codes underlying each psychophysical decision and which are unique in permitting us to psychophysically investigate effects resulting from neural noise in the system. Our analyses draw on theories of dimensional interaction in signal detection theory and in information theory, and on methods from several branches of statistics, including categorical data analysis and structural equation modeling. We review the theoretical, technological, methodological, and personal influences that led us to develop this approach.