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D G Pelli

Publications and source records attributed to D G Pelli.

14 recordsLinked to original sources
PubMed1994

The visual filter mediating letter identification.

We hear periodic sounds, or tones, by means of parallel auditory filters, each tuned to a band of temporal frequency, and we see periodic patterns, or gratings, by means of parallel visual filters, each tuned to a band of spatial frequency. Beyond helping us to see gratings, do these visual filters participate in everyday tasks such as reading and object recognition? After all, grafting visibility only requires the distinguishing of pattern from blank, whereas object recognition, for example letter identification, requires classification by the observer into one of many learned categories. Here we make use of results from hearing research, applying to vision a noise-masking paradigm that reveals the filter(s) mediating any threshold task. We find that letter-identification and grating-detection filters are identical, showing that the recognition of these objects at one size is mediated and constrained by a single visual filter, or 'channel'.

Humans↗
PubMed1994

The scale bandwidth of visual search.

Observers were asked to locate a target in a brief, two-scale display. Accuracy of locating the target was measured as a function of the ratio between the two scales. At each scale ratio, the probability of locating the target as a function of the number of elements is well fit by the idea that the observer accurately monitors only a "critical" number of elements. The dependence of critical number on scale ratio is well accounted for by a model that assumes that the observer's decision is based on an evenly spaced array of samples. The sample spacing is under attentional control, but is always uniform.

Adult↗
PubMed1992

The information capacity of visual attention.

Is visual attention mediated by a general-purpose processor with a small data capacity? Such an attentive processor could perform a wide range of transformations upon a small amount of image data. We suggest that this limited capacity corresponds to a fixed amount of information, measured in bits. We measure how much information an observer's attention can handle by measuring how much we can restrict display information without impairing the observer's performance. The attentive visual tasks we study are the detection of a stationary dot in a field of moving dots, and the detection of a static square in a field of flashing squares. Performance of these tasks is perfect up to a critical number of elements (the span of attention) and then falls as the number of elements increases beyond this critical number. The display information required for unimpaired performance in each of these tasks is low; the results indicate that visual attention processes only 30 to 60 bits of display information.

Adult↗
PubMed1992

Effects of noise on detection of amplitude increments of sinusoidal vibration of the skin.

Vibrotactile thresholds for detecting a 300-Hz signal in the presence of both a 300-Hz sinusoidal pedestal and a background noise were measured as a function of the amplitudes of the pedestal and noise. Threshold increased monotonically as a function of the amplitude of the noise, but was a nonmonotonic function of the amplitude of the sinusoidal pedestal. Negative masking, in which the pedestal facilitated detection of the test stimulus, was observed in the absence of background noise and in the presence of subthreshold background noise when the pedestal was near or below threshold. Negative masking disappeared when the experiment was conducted in the presence of moderately intense to intense background noise. The results are consistent with a peripheral high-energy threshold for taction.

Adult↗
PubMed1991

Accurate control of contrast on microcomputer displays.

Off-the-shelf microcomputers can now display arbitrary 8-bit images, but accurate control of these images requires dealing with several undesirable properties of real digital to analog converters (DACs) and analog video monitors. The limitations of DACs and video monitors are presented in the form of a model for their calibration and use in vision experiments. Low contrasts can be accurately rendered by summing a small accurate a.c. signal and a large less-accurate d.c. signal (Watson et al., 1986; Behavior Research, Method, Instrument and Computers, 18, 587-594). Exploiting that idea, this note presents an easy-to-build passive resistor network, a video attenuator, that combines the outputs of three 8-bit DACs to render low contrasts with 12-bit accuracy at the display. Measurements confirm the 12-bit accuracy.

Analog-Digital Conversion↗
PubMed1987

On the relation between summation and facilitation.

Summation refers to the small reduction of threshold contrast as a signal is extended in space or time, or when multiple signals are presented simultaneously. Facilitation refers to the large reduction in the threshold contrast increment when it is superimposed on a near-threshold pedestal contrast. Separate hypotheses relate each of these effects to distinct measures of the log-log steepness of the psychometric function. This note shows that for 2-alternative forced-choice (2afc) proportion correct, the Weibull function P(c) = 1-(1-gamma) exp[-(c/alpha) beta] and the d' power law d'(c) = (c/c0.76)b are practically indistinguishable. Furthermore, their steepness parameters are proportional to each other, beta = 1.247 b. This implies a simple relation between 2afc summation and facilitation which can be tested without measuring the psychometric function.

Humans↗
PubMed1987

A device for measuring tactile spatiotemporal sensitivity.

A tactile stimulator array constructed from 88 piezoelectric ceramic plates is described. The array can produce sinusoidal traveling waves with arbitrary temporal frequency, spatial wavelength, and amplitude. Detection thresholds were measured for five temporal frequencies (1, 4, 16, 64, and 256 Hz) and five spatial wavelengths (1.81, 3.62, 7.23, 14.5, and infinity mm), and were plotted as a three-dimensional, spatiotemporal threshold surface. Detection thresholds were also measured using a conventional vibrator and a large circular contactor simulating the infinite wavelength stimulus produced by the array. The results describe the spatial and temporal frequency sensitivity of the P and NP I cutaneous receptor populations.

Humans↗
PubMed1987

Probe tone thresholds in the auditory nerve measured by two-interval forced-choice procedures.

An important goal of auditory physiology is to relate the coding of signals in the auditory nerve to behavioral sensitivity. A useful step towards that goal is to measure physiological thresholds for the detection of tones in the neural spike train that are comparable to psychophysical thresholds. Detectability depends on the variability as well as the mean value of the response. A two-interval forced-choice task provides a criterion-free measure of detectability. On each trial of our experiments a probe tone was taken to be correctly detected if the number of spikes in response to the tone exceeded the number of spikes in an otherwise identical interval that did not contain the probe tone. (Analysis of the pulse-number distributions also allowed construction of ROC curves directly comparable to psychophysical ROC curves.) The proportion of trials that yielded correct detections was measured as a function of stimulus intensity to form a neurometric function, directly comparable to a psychophysical psychometric function. Threshold was defined as the intensity that produced a given proportion correct. The threshold intensity was also measured by an up-down procedure. Agreement between the two measures of threshold was excellent. Using the up-down procedure we could measure threshold in about 1 min, making it practical to measure the thresholds of a single neuron for many conditions. Comparisons of physiological and psychophysical ROC curves and neurometric and psychometric functions show systematic differences indicating that the animal makes its decisions inefficiently, perhaps by basing its decision on the maximum response among many neurons, rather than just the activity of the single most sensitive neuron.

Acoustic Stimulation↗
PubMed1985

Psychophysics of reading--I. Normal vision.

This paper is about the visual requirements for reading with normal vision. It is the first in a series devoted to the psychophysics of reading with normal and low vision. We have measured reading rates for text scanned across the face of a TV monitor while varying parameters that are important in current theories of pattern vision. Our results provide estimates of the stimulus parameters required for optimal reading of scanned text. We have found that maximum reading rates are achieved for characters subtending 0.3 degree to 2 degrees. Contrast polarity (black-on-white vs white-on-black text) has no effect. Reading rate increases with field size, but only up to 4 characters, independent of character size. When text is low-pass spatial-frequency filtered, reading rate increases with bandwidth, but only up to two cycles/character, independent of character size. When text is matrix sampled, reading rate increases with sample density, but only up to a critical sample density which depends on character size. The critical sample density increases from about 4 X 4 samples/character for 0.1 degree characters to more than 20 X 20 samples/character for 24 degrees characters. We suggest that one spatial-frequency channel suffices for reading.

Humans↗
PubMed1985

Psychophysics of reading--II. Low vision.

Very little is known about the effects of visual impairment on reading. We used psychophysical methods to study reading by 16 low-vision observers. Reading rates were measured for text scanned across the face of a TV monitor while varying parameters that are likely to be important in low vision: angular character size, number of characters in the field, number of dots composing each character, contrast polarity (white-on-black vs black-on-white text), and character spacing. Despite diverse pathologies and degrees of vision loss in our sample, several major generalizations emerged. There is a wide variation in peak reading rates among low-vision observers, but 64% of the variance can be accounted for by two major distinctions: intact central fields vs central-field loss and cloudy vs clear ocular media. Peak reading rates for observers with central-field loss were very low (median 25 words/minute), while peak reading rates for observers with intact central fields were at least 90 words/minute (median 130 words/minute). Most low-vision readers require magnification to obtain characters of optimal size. Sloan M acuity was a better predictor of optimal character size than Snellen acuity, accounting for 72% of the variance. Low-vision reading is similar to normal reading in several respects. For example, both show the same dependence on the number of characters in the field. Our results provide estimates of the best reading performance to be expected from low-vision observers with characteristic forms of vision loss, and the stimulus parameters necessary for optimal performance. These results will be useful in the development of clinical tests of low vision, and in the design of low-vision reading aids.

Adolescent↗
PubMed1985

Uncertainty explains many aspects of visual contrast detection and discrimination.

More than 20 years ago, Tanner [Ann. N.Y. Acad. Sci. 89, 752 (1961)] noted that observers asked to detect a signal act as though they are uncertain about the physical characteristics of the signal to be detected. The popular assumptions of probability summation and decision variable, taken together, imply this uncertainty. This paper defines and uncertainty model of visual detection that assumes that the observer is uncertain among many signals and chooses the likeliest. With only four parameters, the uncertainty model explains why d' is approximately a power function of contrast ("nonlinear transduction") and accurately predicts effects of summation, facilitation, noise, subjective criterion, and task for near-threshold contrast. Thus the uncertainty model offers a synthesis of much of our current understanding of visual contrast detection and discrimination.

Attention↗
PubMed1985

Psychophysics of reading. III. A fiberscope low-vision reading aid.

Existing reading aids providing magnification in excess of 6X (24 diopters) are awkward to use, expensive, or not portable. The authors have developed a new type of reading aid, a low-resolution fiberscope, that provides magnification up to 40X (160 diopters), and is easy to use, inexpensive, and portable. The fiberscope consists of an objective-lens assembly, a flexible bundle of optical fibers, and an eyepiece that together transmit an image from the page to the reader's eye. Complete construction details are included. The design is based on the authors' previous findings that reading requires a field of only four characters and a resolution of only 2 cycles/character. After only half an hour of training, a diverse group of low-vision readers read with the fiberscope at rates between 12 and 95 words/minute. Low-vision readers with central field loss had a median reading rate of only 18 words/min, whereas those with intact central fields had a median reading rate of 67 words/min, not far short of the median rate of 73 words/min for normal readers using the fiberscope. This is consistent with the authors' previous findings that low-vision readers with central field loss read much more slowly than those with intact central fields, even when optimal magnification is provided. The authors conclude that the fiberscope may be very useful as an inexpensive portable reading aid for people requiring high magnification.

Adult↗