Lawrence G. Brown

ADDITIONAL RULES FOR THE TRANSFORMED UP-DOWN METHOD IN PSYCHOPHYSICS

In a classic paper, Levitt (1971) described an adaptive procedure for estimating points on the psychometric function known as thetransformed up-down method. Levitt discussed the assumptions of the method and presented a brief table with simple rules that converge to a few different points on the psychometric function. Levitt’s original table contains only the simplest rules, and sparsely covers the range of the psychometric function. This paper provides a table with previously unpublished rules which cover the range of the psychometric function at 5% intervals. There is a brief review of the major issues in adaptive testing. Technical issues such as the mean length and logical construction of the new rules are discussed.

Noise adaptation in integrate-and-fire neurons

The statistical spiking response of an ensemble of identically prepared stochastic integrate-and-fire neurons to a rectangular input current plus gaussian white noise is analyzed. It is shown that, on average, integrate-and-fire neurons adapt to the root-mean-square noise level of their input. This phenomenon is referred to as noise adaptation. Noise adaptation is characterized by a decrease in the average neural firing rate and an accompanying decrease in the average value of the generator potential, both of which can be attributed to noise-induced resets of the generator potential mediated by the integrate-and-fire mechanism. A quantitative theory of noise adaptation in stochastic integrate-and-fire neurons is developed. It is shown that integrate-and-fire neurons, on average, produce transient spiking activity whenever there is an increase in the level of their input noise. This transient noise response is either reduced or eliminated over time, depending on the parameters of the model neuron. Analytical methods are used to prove that nonleaky integrate-and-fire neurons totally adapt to any constant input noise level, in the sense that their asymptotic spiking rates are independent of the magnitude of their input noise. For leaky integrate-and-fire neurons, the long-run noise adaptation is not total, but the response to noise is partially eliminated. Expressions for the probability density function of the generator potential and the first two moments of the potential distribution are derived for the particular case of a nonleaky neuron driven by gaussian white noise of mean zero and constant variance. The functional significance of noise adaptation for the performance of networks comprising integrate-and-fire neurons is discussed.

Evidence for a noise gain control mechanism in human vision.

For small, brief targets incremental threshold is known to obey the de Vries-Rose law: threshold rises in direct proportion to the square-root of background intensity. We present data demonstrating a square-root law for brightness matching as well. The square-root law for brightness is obtained over the full range of scotopic vision, and the low intensity end of photopic vision. The classic theory of de Vries and Rose explains the square-root law on the basis of increased variability of the photon count as the background increases. Our brightness matching data instead indicates that the mean signal level is reduced by a factor which is inversely proportional to the standard deviation of the photon count. This result is consistent with the idea that in the retina there exists a gain control mechanism that is sensitive to the variance in the photon input, rather than to the mean illuminance. The importance of this idea to the modelling of retinal gain controls is discussed.

8.1: Invited Paper: Applications of the Sensics Panoramic HMD

The Sensics HMD is a panoramic, wearable display based on OLED microdisplays. Using a tiled design, it simultaneously provides both wide field-of-view and high resolution. The display has found application around the world, especially in design/prototyping applications and research into human performance. This paper describes the design and operation of the HMD and some of the applications of the HMD to date.

37.1: Invited Paper: Full-Field High-Resolution Binocular HMD

We describe a new high resolution, wide-field of view, binocular HMD. Sixteen miniature flat panel emissive SVGA color displays are tiled in a spherical array and viewed through a spherical faceted lens array. The HMD has a 150° × 100° binocular field of view with 70° of binocular overlap and a resolution of 3 arcmin. per pixel.