Dynamics of a divisive gain control in human vision

Evidence for a divisive contrast gain control in human vision was obtained using a contrast version of the probe-on-flash technique that has been employed in the light adaptation literature. Thresholds were measured for a briefly flashed (30 ms), vertical test pattern superimposed on a cosine mask as a function of time after mask onset (SOA). Threshold elevations declined monotonically for SOAs up to 150 ms. and exhibited an exponential time course with an average time constant of 51 ms. Increment thresholds for the test as a function of mask contrast provide direct evidence that these effects are due to operation of a divisive gain control within the first 150 ms after stimulus onset. Experiments to measure the spatial spread of this gain control show it to be localized to a region of no more than 45 arc min radius.     

A model of Weber and noise gain control in the retina of the toad Bufo marinus

We present several variations of a model of gain control in the retina of the toad Bufo marinus, and use them to fit the threshold-vs-intensity data of an actual toad ganglion cell [Donner et al. (1990). Journal of General Physiology, 95, 733-753]. Our models are based on a proposal by Donner et al. that the gain (neural spike per photon ratio) of toad ganglion cells is set by a sequence of two retinal gain control stages. The first stage consists of a Weber gain control mechanism at the level of the red rods. The second is a more proximal "noise gain" stage, which multiplies the (incremental) input signal by a factor that is inversely proportional to the standard deviation of the random ganglion cell input and, under conditions that produce the de Vries-Rose threshold law, is also proportional to the standard deviation of the photon fluctuations within the ganglion cell receptive field. We demonstrate that noise gain control arises naturally from modeling ganglion cell spike generation with either of two common types of spike generation models: integrate-and-fire models or threshold accommodation models. We simulate the process of spike generation in both types of models and show that either model can account for the basic overall shape of the toad t.v.i. curve. However, although integrate-and-fire models appropriately generate noise gain control, they cannot quantitatively fit the threshold data with realistic retinal parameters. Integrate-and-fire models also fail to account for the observed relationship between the generator potential of the ganglion cell and its spiking probability. A threshold accommodation model with realistic retinal parameters, on the other hand, can account for both the threshold data and the generator potential-spike probability relationship. When a Weber gain stage is added to the model at the photoreceptor level, the resulting two-stage gain control model is shown to account quantitatively for the ganglion cell t.v.i. curve of Bufo marinus over the full range of background levels studied by Donner et al.     

A model of the cerebellum in adaptive control of saccadic gain. I. The model and its biological substrate

Muscle tissue (1.1 +/- 0.1 grams) was obtained from seven healthy individuals (3 males, 4 females) using an open incision approach before and after ingestion of either 75 grams of dextrose (N=5) or water (N=2). Purified sarcolemmal membranes from the muscle were prepared using a sucrose step gradient. A polyclonal antibody raised against the purified (99%) rat hepatocyte 40 KD membrane fatty acid binding protein (mFABP-L) was used to probe for this putative transporter in the muscle membranes using Western blot. A single band at the 40 KD MW band was identified which reacted antigenically with the protein purified from rat livers. These response of Berk's protein 60-75 minutes after dextrose ingestion (or water) was erratic and no specific trend could be identified. Our data demonstrate that the 40 KD mFABP-L originally isolated from rat liver is also present in human skeletal muscle membrane. This protein may be involved in transport of fatty acids across the membrane of skeletal muscle, however its physiological role in human fatty acid metabolism remains to be established.     

Induction of species-specific host accommodation in the hamster-to-rat xenotransplantation model

The combination of two immunosuppressants, leflunomide and cyclosporin A (CsA), completely inhibits immune xenoreactions in the hamster-to-Lewis rat xenotransplantation model. In addition, the control of acute xenograft rejection with this combination of immunosuppressants subdues early T-independent xenoreactivity and uncovers a late immune response that can be controlled by CsA alone. We attribute this acquired responsiveness to CsA to a modification in the recipient's humoral response to the xenograft, and refer to this change as host accommodation. Host accommodation can be induced in Lewis rats receiving hamster hearts by the combination of leflunomide and CsA. A 7-day treatment with leflunomide and CsA was able to convert xenoreactivity from one that was resistant to CsA treatment into one that was controlled by CsA. The presence of the hamster xenograft was critical for the induction of host accommodation since the immunosuppressive regimen, either alone or in combination with a transfusion with donor-specific spleen cells, was unable to modify the anti-hamster reactivity in Lewis rats. When accommodation was induced in the presence of hamster hearts, these accommodated rats were able to acutely reject third-party mouse hearts while under CsA therapy, thus indicating that the host accommodation is species specific. Finally, we demonstrate that host accommodation is associated with a loss in the ability to produce species-specific, T-independent xenoantibodies. These novel observations suggest that xenoreactive T-independent humoral responses can be deleted selectively without significant loss of other innate, Ag-specific T-independent humoral responses.     

The effects of vision and task complexity on Hoffmann reflex gain

Previous research demonstrates modulation of the Hoffmann reflex amplitude and gain during changes in environmental conditions. H-reflex gain (defined in this study as the ratio of H-reflex amplitude to average soleus background EMG) is considered a functional measure of reflex modulation. In this study the effects of manipulating visual input and surface stability were to investigated in 17 subjects under four experimental conditions: (1) vision-stable surface, (2) no vision-stable surface, (3) vision-unstable surface, and (4) no vision-unstable surface. In each condition, subjects performed fifteen trials of a single leg stance for 7 s. The H-reflex was electrically elicited at the end of each trial by delivering a 1 ms square wave stimulation to the tibial nerve in the popliteal online for each trial (sampling rate = 2 kHz). An analysis of variance revealed significant decreases in H-reflex gain for the visual (F1.16 = 4.71, P < 0.05) and, surface conditions (F1.16 = 7.67, P < 0.05), however there was no interaction (F1.16 = 0.48, P < 0.05), between these variables. These results suggest that supraspinal mechanisms, possibly presynaptic inhibition, modulate H-reflex gain across environmental conditions. We conclude that visual and possibly cutaneous inputs were responsible for driving presynaptic inhibition and thus decreasing H-reflex gain.     

Contrast gain control in psychophysical contrast discrimination

Contrast masking may in part reflect the operation of contrast gain control mechanisms in the visual system. Four experiments were conducted in order to examine contrast discrimination under conditions in which the base contrast was interrupted with various maskers. Interrupting the base contrast grating at 8 Hz--with a uniform field, with a randomly phase-shifted version of the same grating, or with a higher contrast version of the same grating--interfered with contrast discrimination. These effects occurred only when the background contrast was above its own threshold. These results may be explained by assuming that the networks responsible for contrast gain control operate by temporally averaging contrast over a period greater than 62.5 msec. When this time-averaged contrast is different from the base contrast, contrast discrimination suffers.     

Contrast gain control and fine spatial discriminations

The effect of contrast gain control mechanisms on discrimination between highly similar simple and complex stimuli is examined, with a focus on how discrimination accuracy changes as a function of the contrast of stimulus components. Two models of contrast gain control are evaluated. In both, the response of each pathway is attenuated by a factor determined by the total activity in a large pool of pathways. One model bases attenuation on the sum of linear filter responses within this pool; the other, based on Heeger's contrast energy-driven model [J. Neurophysiol. 70, 1985 (1993)], uses squared filter response. Predictions generated from the models are compared with data from experiments reported here and from the literature. Predictions are made for simple grafting stimuli of different sizes and for stimuli to which a second grafting component is added either as a second cue or as a mask. With one exception, predictions of the models agree closely with each other and with the data. The exception is a masking study that differentiates the models and supports the filter-driven model over the energy-driven model.     

3G语音链路中自动增益控制模块的实现和改进

针对3G终端通话时语音不够平滑圆润的问题,提出在3G上行和下行语音链路中增加自动增益控制(automatic gain control,AGC)模块.AGC模块通过自适应更新增益值算法能够防止通话过程中的语音幅度过大,从而避免语音数据经过数字增益后发生溢出,同时能对小信号进行有效地放大,对时域语音质量进行均衡,使通话双方感到语音质量自然流畅.但是如果不对现有AGC算法进行改进,处理后的语音中会产生非线性失真.改进后的AGC不仅可以使输出语音的幅值在一个预先规定的范围内,根据预先设定的门限值,放大小信号,限制强信号,防止通话过程中的语音信号过载,同时能避免非线性失真.经过理论评估和实际验证,该模块能够有效地放大小信号,限制强信号,从而增强移动终端通话质量.     

Effects of flicker adaptation and temporal gain control on the flicker ERG

The flicker electroretinogram (ERG) to stimuli varying in temporal frequency and modulation depth was recorded to investigate retinal gain control. With increasing modulation of a sinusoidal flickering stimulus, the flicker ERG shows an amplitude compression and a phase retardation (of the fundamental component) at 16 Hz, an amplitude expansion and a phase advance around 40-48 Hz, and an approximately linear response at 72 Hz. With sum-of-two-sinusoids stimuli, the second stimulus enhances the fundamental response to a 40 or 48 Hz test stimulus at low modulations, and reduces the variation in phase with modulation. This interaction depends primarily on the amplitude of the response to the second stimulus, but not its frequency. With temporally alternating stimuli, a similar but smaller interaction effect is measured. The results suggest that there is an active nonlinear gain control mechanism in the outer retina and this gain control works by adjusting the phase delay of the retinal response. The phase control mechanism is set by the amplitude of the outer retinal response integrated over time.     

Local control assessment in clinical oncology

The true significance of achieving local control has been a matter for debate among clinical oncologists for many years and still is for those tumors where distant metastases are the major cause of failure and mortality. The assessment of local control appears elusive as it may be conceived as the local eradication of neoplastic cells while this should be confirmed by no risk for recurrence. Tools and procedures are available to assess separately the local response after treatment and at long term. Direct methods are quite different according to the different clinical and pathologic conditions. Complete local responses may be followed by recurrence leading to a different approach: to wait for the appearance of recurrence rather than to pursue a complete response. The diagnostic clinical and instrumental methods while easy in superficial tumors, are complex, expensive and poorly reliable in deep-seated tumors. Thus, in general, any assessment of local control in terms of diagnostic work-up can frequently be difficult and misleading.     

Role of peripheral vision in the directional control of rapid aiming movements.

This study addresses the question of whether peripheral vision is involved in the control of hand trajectory direction during an aiming task performed at high speed. Ten adult subjects were required to aim at targets in various positions by making a punching movement with their hand. The experimental conditions were varied so that different parts of hand trajectory could be seen in the peripheral field. Two movement times were applied:     

Visuomotor control: where does vision end and action begin?

A new understanding of how visual information is transformed into motor acts has emerged from recent studies of the organization of the visuomotor pathways in the primate brain. This work suggests that it may no longer be useful, or even correct, to categorize different brain areas as sensory or motor.     

Effects of practice and learner control on short- and long-term gain and efficiency

This study investigated the effects of practice opportunities and learner control on short- and long-term learning from a computer-based introductory statistics curriculum. In all, 380 participants were assigned to one of five conditions. The first four conditions differed in terms of the number of problems to solve per problem set. The fifth condition allowed learners to choose the amount of practice. A subset (n = 120) of the original participants returned for testing following a six-month interval. Overall, the fixed-practice conditions showed learning gains that varied in relation to the amount of practice (i.e., more was better). The data from the learner-control condition was unexpected, showing learning gains comparable with the most extended practice condition yet the fastest tutor-completion times. We discuss implications of these findings in relation to the design of efficacious instruction. Actual or potential applications of this research include the modification of computer-based instruction that can enhance individuals' learning efficiency and outcome scores.     

A cultural accommodation model for cross-cultural psychotherapy: Illustrated with the case of Asian Americans.

As an extension of F. T. L. Leong's (1996) integrative model, this article presents the cultural accommodation model (CAM), an enhanced theoretical guide to effective cross-cultural clinical practice and research. Whereas F. T. L. Leong's model identifies the importance of integrating the universal, group, and individual dimensions, the CAM takes the next step by providing a theoretical guide to effective psychotherapy with culturally different clients by means of a cultural accommodation process. This model argues for the importance of selecting and applying culture-specific constructs when working with culturally diverse groups. The first step of the CAM is to identify cultural disparities that are often ignored and then accommodate them by using current culturally specific concepts. In this article, several different cultural "gaps" or culture-specific constructs of relevance to Asian Americans with strong scientific foundations are selected and discussed as they pertain to providing effective psychotherapy to this ethnic minority group. Finally, a case study is incorporated to illustrate application of the CAM. (PsycINFO Database Record (c) 2010 APA, all rights reserved)