Biomedical subjects
Publications and source records attributed to R W Bowen.
Recent behavioral evidence supports the idea that some individuals with reading and language disorders are impaired in their perception of nonlinguistic auditory and visual information. More sophisticated measurement paradigms and analysis techniques are leading to a clearer understanding of these deficits and to possibilities for their remediation.
The perceived contrast of a pulsed grating varies markedly with the exposure duration and spatial frequency of the grating. We studied dynamic changes in perceived grating contrast with a pattern-masking paradigm. We measured masking of a brief, localized test pattern (a D6 stimulus, 30 ms in duration) by fixed-contrast cosine grating patterns of varying duration (50-500 ms). The cosine mask pattern had spatial frequency of either 1 or 6 cycles per degree (cpd) at a contrast of 0.3. The D6 test pattern was centered on a light bar of the mask and was either positive peak contrast (same-polarity test and mask) or negative peak contrast (opposite-polarity test and mask). In Experiment 1, the test and mask had simultaneous onset. With a 6-cpd mask, the same-polarity test-threshold elevation versus mask-duration function increases monotonically. For a 1-cpd mask, the same-polarity threshold-mask-duration function is nonmonotonic, with peak masking effect produced by a grating pulse of 80-100 ms. These masking effects are closely congruent with known dynamic contrast effects. With negative tests, masking-duration functions are elevated from same-polarity functions and are essentially similar in shape for 1- and 6-cpd masks. The elevated thresholds suggest inhibitory interaction between ON and OFF pathways, with a similar time course across spatial frequency. In Experiment 2, the D6 test was delayed from mask onset by 33 ms. Positive contrasts only were employed. For 1-cpd stimuli, the delay of test greatly reduced masking at all mask durations and eliminated the nonmonotonic function. This suggests that for low-spatial-frequency patterns, perceived contrast is determined by an early peak component of the neural response. But for 6-cpd stimuli, masking of the delayed test was somewhat greater at all mask durations, consistent with a gradually increasing underlying neural response to the grating. Finally, in Experiment 3, same-polarity masking effects at both spatial frequencies were replicated with negative-contrast test and mask (OFF pathway mediation). This indicates that the ON and OFF pathways have similar response dynamics.
Pattern contrast discrimination is typically studied with simultaneous onset of the base contrast (C) and added contrast (delta C) patterns. I measured contrast discrimination functions at pattern offset. A brief (30 msec) localized, spatially narrow-band D6 test stimulus was delta C. The onset of delta C was simultaneous with the offset of a large, 500 msec cosine pattern (the base contrast C). The D6 was either positive or negative contrast, and was masked by either positive or negative contrast, i.e., a light or dark bar of the cosine pattern. Stimuli were 3 cpd. Discrimination of negative delta C at the offset of positive contrast followed a "dipper" function, as if the OFF pathway were isolated. A dipper function was also obtained for a positive delta C at the offset of negative contrast (ON pathway isolation). But same-polarity delta C and C yield a monotonic discrimination function ("bumper" function) at the offset of C, suggesting inhibitory interaction. These discrimination functions for same-and opposite-polarity delta C and C are the reverse of functions obtained at pattern onset. Manipulations of temporal asynchrony between patterns and manipulations of pattern polarity are thus functionally equivalent in determining the form of the contrast discrimination function. In a second experiment, I determined delta C at times before and after the offset of a high-contrast C and manipulated pattern polarity. The time course of threshold change is different for same vs opposite-polarity test and mask. The results suggest that interaction between ON and OFF pathways is delayed relative to the masking process within a pathway. Interaction between pathways may function to improve temporal resolution by suppressing persistence of neural response in the complementary pathway. The present pattern polarity and temporal asynchrony effects on the contrast discrimination function also decisively falsify the "uncertainty" hypothesis for low-contrast threshold facilitation (the dipper).
To activate selectively cortical ON and OFF pathways, I measured pattern contrast discrimination functions and manipulated contrast polarity (positive and negative) of base contrast (C) and added contrast (delta C). C was a large, long-duration cosine mask and delta C was a brief, localized, spatially narrow-band "D6" pattern. For same polarity C and delta C, contrast discrimination followed a "dipper" pattern: threshold facilitation at low C and a power relation (exponent < 1.0) at high C. The facilitation is predicted from the low-contrast response of cortical neurons and seems to represent isolation of an ON or OFF pathway. Opposite polarity C and delta C give a monotonic function. delta C increases at low base C and remaining higher than the same-polarity function at higher C values. This represents interaction between ON and OFF pathways. Pathway isolation also occurs: a positive test is detected as a contrast increment if masked by negative contrast and a negative test is detected as a contrast decrement if masked by positive contrast. Quantitative aspects of the data suggest a subtractive interaction at low C values and a divisive interaction between pathways at high C values. Test contrast thresholds upon uniform fields of varying luminance show that both the dipper effect and most of the rise in delta C with C are mediated in pattern-selective pathways rather than at a site of luminance adaptation. The pattern-polarity effects on contrast discrimination rule out the "channel uncertainty" explanation for the facilitation dipper. My results suggest that parallel ON and OFF pathways evolved because stimulus-produced decreases in the response of a single pathway are potentially confounded with the effects of contrast adaptation. Thus transient decreases in response in either pathway are not processed and both decreases and increases in contrast are expressed as response increases in separate pathways.
By comparing the masking effects of cosine gratings and uniform fields on spatially narrow-band test patterns, we obtain evidence that pattern masking is mediated by two stages of visual processing: an early process of point-wise luminance adaptation and a late process of spatial-frequency and orientation-selective filtering. The early (presumably receptoral) component of masking is not affected by the polarity (incremental or decremental) or spatial frequency of test patterns, and may either increase or decrease pattern sensitivity based on local light level. The late masking process is orientation and spatial-frequency dependent, implying a cortical origin. For this cortical process, we find competitive interaction between parallel ON and OFF visual mechanisms: on a bright bar of a mask, threshold shift is greater for decremental than incremental tests, and the opposite is true on a dark bar of the mask. We suggest that ON-OFF interactions in pattern masking serve to normalize the gain of ON and OFF mechanisms simultaneously in order to preserve the relative contrast in the image.
Temporal contrast sensitivity was measured for mirror-image sawtooth (rapid-on and rapid-off) and sine waveforms for a 1.8 deg foveal target. In one experiment, contrast sensitivity was measured for 2-26 Hz stimuli at target mean illuminance levels of 5-1260 td. At 5 td, contrast sensitivity functions for sawtooth and sine waveforms, expressed in terms of the Fourier fundamental amplitude, are equivalent. At higher light levels, sawtooth sensitivity increasingly exceeds sine sensitivity and rapid-off (decremental) sawtooths show progressively greater sensitivity than rapid-on (incremental) sawtooths. This pattern of results was obtained for two color-normal observers and for a deuteranopic observer. In a second experiment, sawtooth and sine sensitivity was tested at 500 td with an extended low-frequency range, to 0.5 Hz. Rapid-off and rapid-on sensitivities declined only slightly at low temporal frequencies in contrast with sine sensitivity. To interpret our data, we evaluate two single-pathway models (last-stage asymmetric detector and compressive response-intensity non-linearity) and a dual-pathway model in which incremental and decremental waveforms are detected by separate ON and OFF visual mechanisms.
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If a single brief light pulse follows the offset of a light field by 0.1-0.3 sec, the pulse is seen as a double flash. This "double flash effect" is a suprathreshold phenomenon: the pulse must exceed detection threshold by 10 times or more for this temporal illusion to occur. A special case of this effect is demonstrated here: two brief, high-luminance pulses separated by 0.1 sec appear as three flashes. In a superposition analysis, hypothetical impulse response functions were added together with various delays to model flash perception. A biphasic impulse response (congruent with threshold flicker and pulse sensitivity) fails to predict perception of three flashes from two pulses. The analysis instead suggests that the visual response to a suprathreshold pulse has several alternating phases of excitation and inhibition.
Mirror-image sawtooth waveforms (rapid-on and rapid-off) were used to test for differences in sensitivity to incremental and decremental stimuli. Temporal contrast sensitivity functions were measured for rapid-on and rapid-off sawtooths and for sine wave stimuli (for 2-26 Hz, mean retinal illuminance of 500 td, circular target of 1.8 deg, foveal). Rapid-off sawtooths yielded higher sensitivity than rapid-on sawtooths at low and middle frequencies. This advantage for detection of decremental lights was confirmed in an experiment in which contrast sensitivity was determined for the sum of a rapid-on and a rapid-off waveform added over the full range of phase angles. Our data, based on periodic rather than pulsed stimuli, broaden and strengthen the evidence that the visual system is more sensitive to decrements than to increments in light level.
When a field of light (an inducing field) is extinguished, and a subsequent probe pulse is presented (5-msec duration, interstimulus interval of 80-320 msec), the single probe pulse appears to be a double flash. We measured the frequency of seeing this double-flash illusion when it was induced by high-contrast flickering fields. The illusion is dependent on inducer-field frequency, showing a major peak and, for some interstimulus intervals, subpeaks. There are notable interobserver differences in the frequency location of the peaks. We evaluated whether the results represented the transient response of a mechanism analogous to a single-harmonic oscillator. A second experiment compares the threshold illuminance for detecting the probe pulse with the illuminance above threshold required to see the double-flash illusion. The illuminance required for a double-flash threshold is high, being 7-20 times the illuminance required for pulse detection.
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We measured colorimetric purity thresholds as a function of stimulus duration for seven wavelengths between 430 and 650 nm. Purity thresholds were measured in a hue substitution mode. The purity-duration function showed decreasing purity as duration was increased to about 640 msec. Functions for different wavelengths could be fit by a fixed chromatic template displaced on the purity axis. Increment thresholds as a function of duration were measured for white and chromatic lights added to a homogeneous white stimulus field. The "white" function showed no integration beyond 160 msec and was fit by an achromatic template. The wavelength functions were not parallel; wavelengths at the spectral extremes showed longer integration times, similar to the purity-duration functions. Increment data could be fit by a vector sum of chromatic and achromatic templates.
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The offset of a pulsed conditioning field of light has recently been shown to produce enhancements of temporal resolution and brightness discrimination. These enhancements are similar to those that are due to imposing a high level of light adaptation on the visual system. Here we analyze whether there is a true equivalence of adaptive state between some level of steady light adaptation and the offset of a conditioning field. The enhancements of visual function at field offset were replicated by using a suprathreshold two-pulse discrimination task and a task requiring detection of an incremental probe stimulus superimposed upon a suprathreshold pulse. These effects are shown to be qualitatively but not quantitatively similar to those produced by an equivalent background selected on the basis of its ability to raise threshold to the same degree as field offset. We conclude that the equivalent-background principle cannot be supported for our measures of foveal visual function.
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Temporal resolution during rapid light adaptation was studied using a suprathreshold two-pulse discrimination paradigm with stimulus conditions modeled after the classic increment threshold experiments of B. H. Crawford. The two-pulse stimulus (1 degree diam, two 20-ms pulses separated by 40 ms) was presented before, during, and after a 6 degree diam, 500 ms background conditioning field, and discriminability was measured as d' using signal detection methods. Discriminability is depressed at background field onset and offset, but increases during the exposure time of the background. Immediately following background offset, two-pulse discriminability is dramatically enhanced: two pulses are discriminated from one pulse with a d' of 4 or more. When the two-pulse stimulus follows background offset by 80 to 250 ms, a novel temporal illusion occurs: a single pulse consistently appears as a double pulse. Several hypotheses for understanding these results are discussed.