Learning a grating discrimination task broadens human spatial frequency tuning

 The effect of spatial frequency discrimination learning on spatial frequency detection tuning curves, obtained by a summation to threshold paradigm, has been investigated. Three human observers were exposed to a grating discrimination task for longer than two weeks, and their detection thresholds for compound Gabor gratings were measured before and after this time interval. Discrimination thresholds decreased continuously and substantially during the course of learning, while the spatial frequency detection tuning curves show significant broadening in the posttest. Calculating the discrimination resolution of an ensemble of sensory coding units shows that larger bandwidths lead to better spatial frequency discrimination performance if pattern discrimination rests on multidimensional comparison or one-dimensional scaling of the spatial frequency parameter. Further, it is shown that a multiple-mechanism nonlinear pooling model is capable of explaining the results if plasticity of coding unit bandwidth or adaptive weights of the coding unit responses at the stage of response integration is assumed. The alternative sources of plasticity and the consequences of the findings for psychophysical modeling are discussed. Received: 8 September 1999 / Accepted in revised form: 16 October 2000     

The frequency response function and sinusoidal threshold properties of the Hodgkin-Huxley model of action potential encoding

The behaviour of the space-clamped Hodgkin-Huxley model has been studied using bandlimited white noise (0–50 Hz) as the input membrane current and taking the output as a point process in time given by the peaks of the action potentials. The frequency response and coherence functions were measured by use of the Fourier transform and digital filtering of the spike train. The results obtained are in good agreement with those already published for the simple integrator and leaky integrator models of neuronal encoding, as well as the earlier studies on the response of the Hodgkin-Huxley model to steady currents. In addition, the threshold of the model to sinusoidal membrane currents has been measured as a function of frequency over the range of 0.1–100 Hz. This shows a relatively constant level up to 2 Hz and then a clear minimum at 60 Hz, in agreement with measured thresholds of squid axons. These results are discussed in terms of the possible contributions of action potential encoding mechanisms to the frequency responses and sinusoidal thresholds which have been measured for rapidly adapting receptors.     

Modelling spatial vision at the threshold level.

Some available single- and multiple-channel models are reviewed. Multichannel models are generalized and tested against threshold data on various stimulus sets. Without using the explicit assumption of spatial probability summation, simple multichannel models are shown to provide good simultaneous predictions of threshold curves of sinewave gratings and other gratings. They fail in predicting threshold curves of disk-shaped stimuli. If global or local spatial probability summation within channels is incorporated into the models, correctly shaped threshold curves of disks can be predicted. However, the predicted curves appear still too low if compared to measured curves. The same holds for noise gratings. Possible extensions of the models, based on local summation between channel responses and/or models consisting of initial isotropic channels (retina) followed by anisotropic channels (cortex), are discussed.     

Orientation selectivity and spatial frequency characteristics of receptive fields of occipital cortical neurons in cats

Spatial frequency characteristics of receptive fields of occipital cortical neurons were investigated in cats during presentation of visual stimuli consisting of gratings in four or eight standard orientations. The maximal increase in discharge frequency of the neurons was observed when the grating was presented in one particular orientation, which was taken to be optimal for those particular neurons. Responses of some neurons to presentation of gratings in nonoptimal orientations were less than optimal; inhibition of activity below the spontaneous discharge level was observed in other cells in this case. Maximal inhibition was observed to the orientation perpendicular to optimal. Inhibition of unit activity evoked by presentation of gratings in the nonoptimal orientation was shown to be a function of spatial frequency.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 227–232, May–June, 1981.     

Study on the limitation for detecting anaerobic threshold by respiratory frequency]

It has been reported that respiratory frequency (F) serves to determine anaerobic threshold (AT). The purpose of this study was to investigate whether the method detecting AT by using F is influenced by the subject's condition such as the existence of sport experiences. Ten healthy adults volunteered to perform progressive cycle ergometer exercise with workloads increased by 30-W (female:20-W) every 2 min at 60 rpm. VO2 at AT were determined by four different methods, which detect the point of 1)nonlinear increase in VE, VCO2, and increase in VE/VO2 without increasing in VE/VCO2 (AT-v), 2) nonlinear increase in F (visual estimation: AT-VF), 3) inflection in F by multisegment linear regression (AT-CF), 4) inflection with omitting above RC point as with 3) (AT-CF2). The mean VO2 at AT-VF (40.8 +/- 9.2 ml/kg/min) and AT-CF (42.7 +/- 9.9 ml/kg/min) was significantly higher compared with AT-V (28.2 +/- 10.4 ml/kg/min) and not RC (42.3 +/- 10.0 ml/kg/min). It would be possible that AT-VF and AT-CF indicated RC, but not AT. There were no significant differences between AT-CF2 (28.2 +/- 10.9 ml/kg/min) and AT-V, and a highly positive correlation (r = 0.79, p less than 0.05) was observed between them. It was recognized that F reached a plateau at AT in four of the subjects. The error between AT-V and AT-CF2 was observed individual variations and the error between them within 5% was observed in only one subject. These results suggest that F is inadequate as an indicator of the AT, because F may be influenced by entrainment of breathing and pedalling frequency.     

Dependence of calcium influx in squid axons on rhythmic excitation frequency

Calcium-45 influx was measured in squid axons during excitation. Different stimulation frequencies changed this influx and general concentration of calcium ions in squid axons. The maximum influx was recorded at the frequency 10 imp/s and 30 imp/s. Calcium influx and general content of calcium ions in axoplasma during excitation was independent of the number of excitation impulses. The role of sodium and calcium channels during excitation is discussed.     

Chromosome displacement and spindle tubule polymerization: 1. The effect of alterations in pH on displacement frequency

In order to test whether 'chromosome displacement' (Ford and Lester, 1982) is related to spindle function, the phenomenon was examined under conditions known to alter spindle tubule polymerization, namely alterations in pH within the range 6.8-8.0. The following observations were made: (a) the frequency of cells showing chromosome displacement was not altered by variations in pH, but the number of displaced chromosomes per cell was markedly changed. Minimum numbers of chromosomes were displaced at pH 7.6. (b) For any chromosome, the extent of the response to pH change, was positively correlated with the basal displacement rate for that chromosome. (c) Chromosomes which have a 'stable' evolutionary history have a more predictable response to pH than those which have an 'unstable' history. Since displacement is significantly influenced by pH, it is concluded that the phenomenon is related to spindle structure.     

Acute effects of reducing vertical displacement and step frequency on running economy

This work studies the immediate effects of altering the vertical displacement of the center of mass (VD) and step frequency (SF) on the metabolic cost of level treadmill running at 16 km · h(-1) on 16 male runners. Alterations of VD, SF, and the product VD × SF was induced using a novel feedback system, which presents target and current values to the runner by visual or auditory display. Target values were set to 5 and 10% reductions from individual baseline values. The results were expressed as relative changes from baseline values. Alterations led to an increase in metabolic cost in most cases, measured as V(O2) uptake per minute and kilogram of body mass. Correlations were weak. Still, linear multiple regression revealed a positive coefficient (0.28) for the relationship between VD × SF and V(O2). Separate rank correlation tests showed negative correlation (τ = -0.19) between SF and V(O2) and positive correlation (τ = 0.16) between VD and V(O2). There is a coupling between VD and SF caused by the mechanics of running; hence, isolated reduction of either factor was hard to achieve. The linear model also showed a negative coefficient for the relationship between the height of the center of mass above the ground (CoMh) and V(O2). The effect size was small (multiple R(2) 0.07 and 0.12). Still the results indicate that reducing VD × SF by reducing the vertical displacement can have a positive effect on running economy, but a concurrent reduction in CoMh may diminish the positive effect. Midterm and long-term effects of altering the technique should also be studied.     

Dependence of responses of central auditory neurons on the depth and rate of frequency modulation

Unit responses of the inferior colliculi of anesthetized albino rats to frequency-modulated stimuli were studied. The number of spikes and the pattern and duration of the unit discharges in response to frequency modulation in different directions were shown to depend on the depth and rate of modulation. With a change in the rate of frequency modulation the number of spikes in the discharge of individual neurons could increase, decrease, or remain constant. Neurons of the inferior colliculi give their material discharge at different rates of frequency modulation. A change in the depth and rate of modulation did not change the sensitivity of most neurons to the direction of the frequency change in the stimulus within the frequency range corresponding to the center of the excitatory zone of the unit response.     

Dependence of transdetermination frequency on the developmental stage of cultured imaginal discs of Drosophila melanogaster

Male foreleg tissue from prepupal stages of Drosophila melanogaster was tested for its capacity to grow when cultured in the adult fly hemocoel and for its capacity, after culture, to produce adult cuticular structures when differentiated in a metamorphosing larva. Evaginated, segmented leg tissue from 8-hr-old prepupae (at 25°C), still retained the capacity to grow well in culture. Growth was, however, restricted to cells of the proximal half of the leg. Tissue from 11- and 24-hr stages (pupal ecdysis at 11 hr) was not successfully cultured. Cultured proximal halves of 8 hr prepupal legs frequently differentiated not only proximal structures, but also distal structures, such as sex combs and claws, indicating regeneration of missing leg structures during the culture period. Transdetermination to wing tissue occurred only rarely (once in 90 implants) whereas third-instar leg tissue in culture transdetermined frequently (50% of the implants) to wing, even though growth of tissue of the two stages was equivalent. Identical results were obtained with third-instar foreleg discs evaginated in vitro with β-ecdysone. This is the first in vitro treatment reported to reduce transdetermination frequency, without affecting growth proportionately. These results indicate that cell proliferation in culture, while probably a necessary condition for transdetermination, is not a sufficient condition. The developmental stage of the cultured tissue strongly affects the frequency of transdetermination.