Variability in the Firing of Nerve Impulses in Eccentric Cells of the Limulus Eye

Thesis is concerned with the source and characteristics of variability in the discharge of impulses by neurons. The neuron in which variability was studied is the eccentric cell in the compound eye of the horseshoe crab, Limulus polyphemus. In Part I a theory is presented which accounts for the variability in the response of an eccentric cell to light. The main idea of this theory is that the source of randomness in the impulse rate is noise in the generator potential. Another essential aspect of the theory is the view that the process which codes the generator potential into the impulse rate may be treated as a linear filter. These ideas lead directly to Fourier analysis of the fluctuations of the generator potential and fluctuations of the impulse rate. Experimental verification of theoretical predictions was obtained by measurement of the fluctuations and calculation of their variance spectrum. The variance spectrum (or power spectrum) of the impulse rate is shown to be the filtered variance spectrum of the generator potential. Another verification of the theory is the finding that in many cells the signal-to-noise ratio is constant for responses to sinusoidally modulated light, at all modulation frequencies. Inhibition from neighboring eccentric cells will have an effect on the variability of firing of a given eccentric cell. The effects of inhibition are discussed in Part II. The reduction in the average impulse rate which is caused by inhibition decreases the variance of the impulse rate. However, this reduction of the average impulse rate increases the coefficient of variation of the impulse rate. Inhibitory synaptic noise adds to the low frequency portion of the variance spectrum of the impulse rate. This occurs because of the Ill slow time course of the inhibitory synaptic potentials. As a consequence, inhibition decreases the signal-to-noise ratio for low frequency modulated stimuli. The net effect of inhibition is to increase the coefficient of variation of the impulse rate. This effect is predicted by the linear model of the eccentric cell. The same qualitative effect is predicted by other theories of neuronal variability, although its importance is stressed here for the first time

Linking appearance to neural activity through the study of the perception of lightness in naturalistic contexts

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.The present paper deals with the classical question how a psychological experience, in this case apparent lightness, is linked by intervening neural processing to physical variables. We address two methodological issues: (a) how does one know the appropriate physical variable (what is the right x ?) to look at, and (b) how can behavioral measurements be used to probe the internal transformation that leads to psychological experience. We measured so-called lightness transfer functions (LTFs), that is the functions that describe the mapping between retinal luminance and perceived lightness for naturalistic checkerboard stimuli. The LTFs were measured for different illumination situations: plain view, a cast shadow, and an intervening transparent medium. Observers adjusted the luminance of a comparison patch such that it had the same lightness as each of the test patches. When the data were plotted in luminance–luminance space, we found qualitative differences between mapping functions in different contexts. These differences were greatly diminished when the data were plotted in terms of contrast. On contrast–contrast coordinates, the data were compatible with a single linear generative model. This result is an indication that, for the naturalistic scenes used here, lightness perception depends mostly on local contrast. We further discuss that, in addition to the mean adjustments, one may fi nd it useful to consider also the variability of an observer’s adjustments in order to infer the true luminance-to-lightness mapping function.DFG, MA5127/1-1, Die Bestimmung der Beziehung zwischen subjektiver Empfindung und Diskriminationsvermögen durch eine Kombination aus Psychophysik, Computationaler Modellierung und der Messung neuronaler Antworte

Probing the Ionization State of the Universe at z>6

We present high signal-to-noise ratio Keck ESI spectra of the two quasars known to have Gunn-Peterson absorption troughs, SDSS J1030+0524 (z=6.28) and SDSS J1148+5251 (z=6.37). The Ly alpha and Ly beta troughs for SDSS J1030+0524 are very black and show no evidence for any emission over a redshift interval of ~0.2 starting at z=6. On the other hand, SDSS J1148+5251 shows a number of emission peaks in the Ly beta Gunn-Peterson trough along with a single weak peak in the Ly alpha trough. The Ly alpha emission has corresponding Ly beta emission, suggesting that it is indeed a region of lower optical depth in the intergalactic medium at z=6.08. The stronger Ly beta peaks in the spectrum of SDSS J1148+5251 could conceivably also be the result of "leaks" in the IGM, but we suggest that they are instead Ly alpha emission from an intervening galaxy at z=4.9. This hypothesis gains credence from a strong complex of C IV absorption at the same redshift and from the detection of continuum emission in the Ly alpha trough at the expected brightness. If this proposal is correct, the quasar light has probably been magnified through gravitational lensing by the intervening galaxy. The Stromgren sphere observed in the absorption spectrum of SDSS J1148+5251 is significantly smaller than expected based on its brightness, which is consistent with the hypothesis that the quasar is lensed. If our argument for lensing is correct, the optical depths derived from the troughs of SDSS J1148+5251 are only lower limits (albeit still quite strong, with tau(LyA)>16 inferred from the Ly beta trough.) The Ly beta absorption trough of SDSS J1030+0524 gives the single best measurement of the IGM transmission at z>6, with an inferred optical depth tau(LyA)>22.Comment: To appear in July 2003 AJ, 34 pages, 11 figures; minor changes/typos fixe

Correlation between spatial frequency and orientation selectivity in V1 cortex: Implications of a network model

AbstractWe addressed how spatial frequency and orientation selectivity coexist and co-vary in Macaque primary visual cortex (V1) by simulating cortical layer 4Cα of V1 with a large-scale network model and then comparing the model’s behavior with a population of cells we recorded in layer 4Cα. We compared the distributions of orientation and spatial frequency selectivity, as well as the correlation between the two, in the model with what we observed in the 4Cα population. We found that (1) in the model, both spatial frequency and orientation selectivity of neuronal firing are greater and more diverse than the LGN inputs to model neurons; (2) orientation and spatial frequency selectivity co-vary in the model in a way very similar to what we observed in layer 4Cα neurons; (3) in the model, orientation and spatial frequency selectivity co-vary because of intra-cortical inhibition. The results suggest that cortical inhibition provides a common mechanism for selectivity in multiple dimensions